How the Hydrogen Economy Works

Introduction to How the Hydrogen Economy Works

It seems like every day there is a new announcement in the news about automobiles powered by fuel cells. The promises are tantalizing, since fuel cells have the potential to very quickly double the efficiency of cars while significantly reducing air pollution.

At the same time, there have been news stories for decades about the problems associated with petroleum. Everything from oil spills to ozone alerts to gl­obal warming gets blamed on our dependence on fossil fuels.

These two forces are leading the world toward what is broadly known as the hydrogen economy. If the predictions are true, over the next several decades we will all begin to see an amazing shift away from the fossil fuel economy we have today toward a much cleaner hydrogen future.



Can society actually make this shift, or will the technological, economic and political barriers keep us bound to petroleum and other fossil fuels for the next century an­d beyond? In this article, you will learn about the benefits of a hydrogen economy, along with its potential problems. We will also examine some of the technology that would make the transition possible.



Problems with the fossil fuel economy­ ­

Currently, the United States and most of the world is locked into what could be called the fossil fuel economy. Our automobiles, trains and planes are fueled almost exclusively by petroleum products like gasoline and diesel. A huge percentage of our power plants use oil, natural gas and coal for their fuel.



If the flow of fossil fuels to the United States were ever cut off, the economy would come to a halt. There would be no way to transport the products that factories produce. There would be no way for people to drive to work. The whole economy, and in fact the whole of western society, currently depends on fossil fuels.

While fossil fuels have played an important role in getting society to the point it is at today, there are four big problems that fossil fuels create:

1.Air pollution - When cars burn gasoline, they would ideally burn it perfectly and create nothing but carbon dioxide and water in their exhaust. Unfortunately, the internal combustion engine is not perfect. In the process of burning the gasoline, it also produces:
Carbon monoxide, a poisonous gas
Nitrogen oxides, the main source of urban smog
Unburned hydrocarbons, the main source of urban ozone
Catalytic converters eliminate much of this pollution, but they aren't perfect. Air pollution from cars and power plants is a real problem in big cities.



It is bad enough now that, in the summer, many cities have dangerous levels of ozone in the air.

2.Environmental pollution - The process of transporting and storing oil has a big impact on the environment whenever something goes wrong.



An oil spill, pipeline explosion or well fire can create a huge mess. The Exxon Valdez spill is the best known example of the problem, but minor spills happen constantly.

3.Global warming - When you burn a gallon of gas in your car, you emit about 5 pounds (2.3 kg) of carbon into the atmosphere. If it were solid carbon, it would be extremely noticeable -- it would be like throwing a 5-pound bag of sugar out the window of your car for every gallon of gas burned. But because the 5 pounds of carbon comes out as an invisible gas, carbon dioxide, most of us are oblivious to it. The carbon dioxide coming out of every car's tailpipe is a greenhouse gas that is slowly raising the temperature of the planet. The ultimate effects are unknown, but it is a strong possibility that, eventually, there will be dramatic climate changes that affect everyone on the planet. For example, if the ice caps melt, sea level will rise significantly, flooding and destroying all coastal cities in existence today. That's a big side effect.

4.Dependence - The United States, and most other countries, cannot produce enough oil to meet demand, so they import it from oil-rich countries. That creates an economic dependence. When Middle East oil producers decide to raise the price of oil, the rest of the world has little choice but to pay the higher price.

Advantages of the hydrogen economy

In the previous section we saw the significant, worldwide problems created by fossil fuels. The hydrogen economy promises to eliminate all of the problems that the fossil fuel economy creates. Therefore, the advantages of the hydrogen economy include:

1.The elimination of pollution caused by fossil fuels - When hydrogen is used in a fuel cell to create power, it is a completely clean technology. The only byproduct is water. There are also no environmental dangers like oil spills to worry about with hydrogen.

2.The elimination of greenhouse gases - If the hydrogen comes from the electrolysis of water, then hydrogen adds no greenhouse gases to the environment. There is a perfect cycle -- electrolysis produces hydrogen from water, and the hydrogen recombines with oxygen to create water and power in a fuel cell.

3.The elimination of economic dependence - The elimination of oil means no dependence on the Middle East and its oil reserves.

4.Distributed production - Hydrogen can be produced anywhere that you have electricity and water. People can even produce it in their homes with relatively simple technology.

The problems with the fossil fuel economy are so great, and the environmental advantages of the hydrogen economy so significant, that the push toward the hydrogen economy is very strong.

Technological Hurdles

The big question with the hydrogen economy is, "Where does the hydrogen come from?" After that comes the question of transporting, distributing and storing hydrogen. Hydrogen tends to be bulky and tricky in its natural gaseous form.

Once both of these questions are answered in an economical way, the hydrogen economy will be in place.

Where does the hydrogen come from?

­One of the more interesting problems with the hydrogen economy is the hydrogen itself. Where will it come from? With the fossil fuel economy, you simply pump the fossil fuel out of the ground and refine it . Then you burn it as an energy source.

Most of us take oil, gasoline, coal and natural gas for granted, but they are actually quite miraculous. These fossil fuels represent stored solar energy from millions of years ago. Millions of years ago, plants grew using solar energy to power their growth. They died, and eventually turned into oil, coal and natural gas. When we pump oil from the ground, we tap into that huge solar energy storehouse "for free." Whenever we burn a gallon of gasoline, we release that stored solar energy.

In the hydrogen economy, there is no storehouse to tap into. We have to actually create the e­nergy in real-time.

There are two possible sources for the hydrogen:

1.Electrolysis of water - Using electricity, it is easy to split water molecules to create pure hydrogen and oxygen. One big advantage of this process is that you can do it anywhere. For example, you could have a box in your garage producing hydrogen from tap water, and you could fuel your car with that hydrogen.

2.Reforming fossil fuels - Oil and natural gas contain hydrocarbons -- molecules consisting of hydrogen and carbon. Using a device called a fuel processor or a reformer, you can split the hydrogen off the carbon in a hydrocarbon relatively easily and then use the hydrogen. You discard the leftover carbon to the atmosphere as carbon dioxide.

­The second option is, of course, slightly perverse. You are using fossil fuel as the source of hydrogen for the hydrogen economy. This approach reduces air pollution, but it doesn't solve either the greenhouse gas problem (because there is still carbon going into the atmosphere) or the dependence problem (you still need oil). However, it may be a good temporary step to take during the transition to the hydrogen economy. When you hear about "fuel-cell-powered vehicles" being developed by the car companies right now, almost all of them plan to get the hydrogen for the fuel cells from gasoline using a reformer. The reason is because gasoline is an easily available source of hydrogen. Until there are "hydrogen stations" on every corner like we have gas stations now, this is the easiest way to obtain hydrogen to power a vehicle's fuel cell.

The interesting thing about the first option is that it is the core of the real hydrogen economy. To have a pure hydrogen economy, the hydrogen must be derived from renewable sources rather than fossil fuels so that we stop releasing carbon into the atmosphere. Having enough electricity to separate hydrogen from water, and generating that electricity without using fossil fuels, will be the biggest change that we see in creating the hydrogen economy.



Where will the electricity for the electrolysis of water come from? Right now, about 68 percent (reference) of the electricity produced in the United States comes from coal or natural gas. All of that generating capacity will have to be replaced by renewable sources in the hydrogen economy. In addition, all of the fossil fuel energy now used for transportation (in cars, trucks, trains, boats, planes) will have to convert to hydrogen, and that hydrogen will be created with electricity, as well. In other words, the electrical generating capacity in the country will have to double in order to take on the demands of transportation, and then it will all have to convert from fossil fuels to renewable sources. At that point, and only at that point, will the flow of carbon into the atmosphere stop.

Right now there are several different ways to create electricity that do not use fossil fuels:

Nuclear power
Hydroelectric dams
Solar cells
Wind turbines
Geothermal power
Wave and tidal power
Co-generation (For example, a sawmill might burn bark to create power, or a landfill might burn methane that the rotting trash produces.)



In the United States, about 20 percent of the power currently comes from nuclear and 7 percent comes from hydroelectric. Solar, wind, geothermal and other sources generate only 5 percent of the power -- hardly enough to matter.

In the future, barring some technological breakthrough, it seems likely that one of two things will happen to create the hydrogen economy: Either nuclear-power or solar-power generating capacity will increase dramatically. Remember that, in a pure hydrogen economy, the electrical generating capacity will have to approximately double because all of the energy for transportation that currently comes from oil will have to be replaced with electrically generated hydrogen. So the number of power plants will double, and all of the fossil fuel plants will be replaced.

The electrical-generation problem is probably the biggest barrier to the hydrogen economy. Once the technology is refined and becomes inexpensive, fuel-cell vehicles powered by hydrogen could replace gasoline internal combustion engines over the course of a decade or two. But changing the power plants over to nuclear and solar may not be so easy. Nuclear power has political and environmental problems, and solar power currently has cost and location problems.

How do you store and transport the hydrogen?

­At this moment, the problem with putting pure-hydrogen vehicles on the road is the storage/transportation problem. Hydrogen is a bulky gas, and it is not nearly as easy to work with as gasoline. Compressing the gas requires energ­y, and compressed hydrogen contains far less energy than the same volume of gasoline. However, solutions to the hydrogen storage problem are surfacing.

For example, hydrogen can be stored in a solid form in a chemical called sodium borohydride, and this technology has appeared in the news recently because Chrysler is testing it. This chemical is created from borax (a common ingredient in some detergents). As sodium borohydride releases its hydrogen, it turns back into borax so it can be recycled.

Once the storage problem is solved and standardized, then a network of hydrogen stations and the transportation infrastructure will have to develop around it. The main barrier to this might be the technological sorting-out process. Stations will not develop quickly until there is a storage technology that clearly dominates the marketplace. For instance, if all hydrogen-powered cars from all manufacturers used sodium borohydride, then a station network could develop quickly; that sort of standardization is unlikely to happen rapidly, if history is any guide.

There might also be a technological breakthrough that could rapidly change the playing field. For example, if someone could develop an inexpensive rechargeable battery with high capacity and a quick recharge time, electric cars would not need fuel cells and there would be no need for hydrogen on the road. Cars would recharge using electricity directly.

Prospects for the future

­ You will hear more and more about the hydrogen economy in the news in the co­ming months, because the drumbeat is growing louder. The environmental problems of the fossil fuel economy are combining with breakthroughs in fuel-cell technology, and the pairing will allow us to take the first steps.

The most obvious step we will see is the marketing of fuel-cell-powered vehicles. Although they will be powered initially by gasoline and reformers, fuel cells embody two major improvements over the internal combustion engine:

They are about twice as efficient.
They can significantly reduce air pollution in cities.
Gasoline-powered fuel-cell vehicles are an excellent transitional step because of those advantages.

Moving to a pure hydrogen economy will be harder. The power-generating plants will have to switch over to renewable sources of energy, and the marketplace will have to agree on ways to store and transport hydrogen. These hurdles will likely cause the transition to the hydrogen economy to be a rather long process.

BMW H2R Model

Introduction to How the BMW H2R Works

It's sleek. It's aerodynamic. It's environmentally friendly. BMW's H2R (Hydrogen Record Car) is powered entirely by the clean-burning process of liquid-hydrogen combustion, and this next-generation racecar has already set speed records in its class.


So why is BMW expending copious amounts of time, money and energy to spearhead the production of hydrogen-powered vehicles? According to the BMW Web site, the company's long-term goal is to eventually replace the cars that run on atmosphere-polluting fossil fuels with vehicles that use clean-burning, liquid-hydrogen fuel. In this article, you'll learn about the H2R and its unique, hydrogen combustion engine.

With its smooth lines, aerodynamic design and gleaming silver body, the H2R appears to have sprung from the pages of a science fiction novel: It looks like a cross between a thoroughbred sports car, a sleek zeppelin and a futuristic rocket ship.

With a lightweight aluminum chassis, a monocoque aluminum space frame and an outer skin composed of carbon-fiber-reinforced plastic, the H2R is designed for high speed and maximum stability under extreme driving conditions. Tires measuring 245/40/19 ensure maximum road contact.





The H2R's suspension system comprises a double-wishbone, spring-strut front axle, rack-and-pinion steering, forged-aluminum track control arms (with two ball joints for superior wheel guidance and directional stability), a tie bar and an anti-roll bar. The result is a stable skeleton that results in a smooth and vibrationless driving experience.

The H2R's race-car-like appearance and high-speed performance belie its extreme environmental friendliness. Conceived and developed in only 10 months, the H2R features a 6.0-liter, V-12 hydrogen-powered engine. Whereas other carmakers have gone the route of hydrogen fuel cells, BMW has opted for a more traditional-style engine that utilizes a nontraditional fuel. The hydrogen combustion engine operates on the same principle as other internal combustion engines except that liquid hydrogen is used as the fuel source rather than gasoline or diesel oil.

BMW H2R Specifications
6.0-liter, V-12 liquid-hydrogen-powered engine
232 horsepower
Top speed over 187 mph (301 kph)
Aluminum space-frame chassis (generates almost no interior vibration)
Carbon-fiber-reinforced, Formula 1-style body
0 to 62 mph (100 kph) in 6 seconds
3,440 pounds (1,560 kg) with full tank and driver
5.4 meters (17.7 feet) long, 2 meters (6.6 feet) wide

Q: What is the BMW H7?

A: In 2006, BMW introduced the world's first hydrogen-powered production vehicle -- the H7. BMW claims that the H7 will world-class luxury and performance along with the clean-air benefits of hydrogen technology when it debuts. The H7 will feature a combustion engine that can run on either liquid hydrogen or gasoline. BMW's Double-VANOS and Valvetronic engine technologies will allow the engine to adjust to burning hydrogen/air or gasoline/air fuel mixtures.

A Hydrogen-Fueled Car

The use of hydrogen as a fuel in motor vehicles offers several advantages over traditional fossil fuels:

1.There exists an unlimited supply of hydrogen -- hydrogen is the most abundant element in the universe and the tenth most abundant element on Earth.

2.Hydrogen is renewable -- When hydrogen reacts with oxygen, the by-product is water (H2O), which can then be hydrolyzed (broken up into its component parts) to yield more hydrogen.

3.Hydrogen is clean-burning -- Unlike the burning of fossil fuels, hydrogen combustion does not produce any destructive environmental pollutants.

4.Hydrogen weighs less and generates more power than hydrocarbon-based fuels.

5.Hydrogen burns faster (and at a lower temperature) than conventional gasoline.




The H2R's hydrogen combustion engine produces record speeds on clean-burning fuel.

But carmakers and the general public have yet to declare hydrogen power safe for consumer use.In addition to running on hydrogen instead of fossil fuels, the internal components of the H2R's engine are unique in two significant ways: the hydrogen-injection valve and the materials used for the combustion chambers. In the H2R, the injection valves have been integrated into the intake manifolds, as opposed to injecting fuel directly into the combustion chambers.




Liquid hydrogen does not lubricate the way gasoline does, so the H2R uses altered valve seat rings that compensate for this. To maximize power and efficiency, hydrogen is injected into the intake manifold as late as possible, so the injection valves have been redesigned, as well.

The H2R Fuel Tank

Pure hydrogen is highly flammable and produces a great deal of energy when it reacts with oxygen, so safety is of primary importance in the design of any hydrogen-powered vehicle. The H2R's fuel tank is vacuum-insulated and double-walled, and it's equipped with three active safety valves.



To prevent possible leaks in the jacket around the fuel tank, which helps maintain the liquid hydrogen at a sufficiently low temperature (hydrogen takes its liquid form at -423F/-253C), the H2R features a double-redundant safety system: If the pressure within the tank ever exceeds 5 bar, two additional safety valves open up immediately. As an additional safety precaution, the combustion chambers are cooled by air before the hydrogen/air mixture flows into the cylinders to ensure that it won't ignite in an uncontrolled manner.

Refueling the H2R

Aside from the notable scarcity of hydrogen filling stations, refueling a hydrogen-powered vehicle requires no more effort than refueling a gasoline-powered one.

Hydrogen is added to the H2R's tank at a mobile hydrogen filling station through a manual tank coupling. Because of an interesting safety setup, it is impossible for hydrogen gas to leak into the air during the refueling process. In a liquid-hydrogen-powered BMW, the hydrogen left in the tank has returned to a gaseous state by the time the driver needs more fuel. This gaseous hydrogen exerts a higher pressure inside the tank. At the refueling station, when super-cold liquid hydrogen is pumped into the tank, the gaseous hydrogen already there condenses. The condensation of the gaseous hydrogen reduces the partial pressure inside the tank, so no hydrogen escapes while the tank is being filled.

The Evolution of the BMW H2R

The BMW H2R, which came out in 2004, was the gleaming, high-tech fruit of over 25 years of experimentation and innovation.

In 1979, BMW developed the 520, a prototype vehicle featuring an engine that ran on either hydrogen or gasoline. Building on the potential of the 520, BMW produced and road tested three generations of hydrogen-powered cars from 1984 to 1996 and in 2000 introduced the 5.0-liter V-12 750hL, the company's fifth-generation hydrogen car.

In 2001, BMW produced its sixth-generation hydrogen concept car, the 4.4-liter V-8 745h. It had two fuel tanks -- one for hydrogen and one for gasoline. When running on hydrogen, the 745h generated 182 horsepower, reached 62 miles per hour (100 kph) in 9.9 seconds and had a top speed of 134 mph (216 kph).

In 2004, BMW unveiled the H2R hydrogen-powered concept racecar, which went on to set nine speed records for hydrogen-combustion vehicles at the Miramas Proving Grounds in France.

Hydrogen Energy in the H2R Engine

Pure hydrogen gas rarely occurs in nature. As there are no reserves of pure hydrogen on the planet, hydrogen must be extracted from other compounds if it is to be used a fuel source. For example, in the process of hydrolysis, electrical current is passed through water to break it down into hydrogen and oxygen according to the following reaction: 2H2O + electricity --> 2H2 + O2. The reverse reaction -- the combustion (oxidation) of hydrogen -- is the process by which energy is created in the H2R's engine: 2H2 + O2 --> 2H2O + energy.



As you can see, the only by-product of this reaction is water, which makes the combustion of liquid hydrogen a clean-burning alternative to the combustion of fossil fuels. Unfortunately, as hydrogen does not occur naturally in its pure state, an initial input of energy is required to separate pure hydrogen from other naturally occurring compounds. Essentially, we need to use "dirty" energy to produce "clean" energy. The BMW Group is researching ways to generate the initial energy input in environmentally friendly ways, such as through wind, solar or hydroelectric power.

The use of liquid hydrogen as a fuel source is not a new concept. The aerospace industry already uses liquid hydrogen in rockets and spacecraft, and liquid hydrogen is being considered for use in airplanes because of its low density. Hydrocarbon-based fuels are very heavy; an equal volume of liquid hydrogen weighs less and produces nearly three times more power than gasoline.

The BMW H7

In 2006, BMW introduced the world's first hydrogen-powered production vehicle -- the H7. BMW claims that the H7 will offer world-class luxury and performance along with the clean-air benefits of hydrogen technology. When it debuts, the H7 will be available only in select markets.

Because of the present scarcity of hydrogen refueling stations, the H7 will feature a combustion engine that can run on either liquid hydrogen or gasoline. BMW's Double-VANOS and Valvetronic engine technologies will allow the engine to adjust to burning hydrogen/air or gasoline/air fuel mixtures. These sophisticated systems also help prevent nitrogen oxide (NOx) emissions in the combustion process. NOx emissions contribute to the formation of ozone and acid rain.

FUTURE MOTORS

How light will cars be in the future?

­In the p­ast decade, automobiles in the United States have followed the same trend as Americans' waistlines, growing bigger and bigger. We're not just talking about enormous models such as the Hummer and other full-size SUVs. Even sedans and mid-sized cars have ballooned. For instance, the 2007 Honda Accord outweighs its 1988 counterpart by almost 600 pounds.

Since fuel prices have risen as well, car companies and consumers alike are scrambling to find more efficient alternatives. Much of the mainstream attention has focused on different ways to power cars, rather than altering the body. But one promising solution that researchers and manufacturers have tossed around since the mid-1990s is the concept of an ultralight car.




The term started popping up in many places in 1993 with the kick-off of the Partnership for a New Generation of Vehicles. This agreement brokered by then-Vice President Al Gore and the executives of the Big Three auto makers (Ford, DaimlerChrysler and General Motors) laid out a goal to build an 80-mile-per-gallon car by 2003. One cornerstone of the alliance was researching ultralight materials, such as carbon composites and lighter steels and plastics as a way to jolt fuel efficiency.

­Just how light are we talking? German car startup company, Loremo, which stands for low resistance mobile, will soon put its first models on sale in Europe that weigh around 1,200 pounds (544 kilograms). To put that in perspective, that 2007 Honda Accord we mentioned earlier is 3,197 pounds (1,450 kilograms). And it's even lighter than the pint-sized, 1800-pound (816-kilogram) Smart Fortwo model. In exchange for the more svelte body, the Loremo LS gets around 120 miles to the gallon.

Why does weight make such a difference? And how safe are these fitter models?

Size and Safety of Lighter Cars

The average car uses only 15 percent of its energy to actually move the vehicle and drive . Most cars on the road today lose all but that sliver of power to braking, friction, idling and having to haul piles of accessories . By slightly altering the design, prioritizing function over luxury and using lighter materials, those issues can be minimized. In fact, according to the U.S. Department of Energy, every 10 percent of weight reduction translates to a 7 percent increase in fuel economy.

Thanks to that significant exchange, one of the goals outlined by the Partnership for a New Generation of Vehicles was to reduce the weight of cars by 40 percent, or around 1,200 pounds (554 kilograms). But when thinking about ultralight cars like these, it's important to differentiate between mass and weight. The mass, or size, of ultralight prototypes car companies have revealed aren't smaller than the average compact. Rather, the weight of the materials involved makes the difference.

Cars weigh so much because many of them are made from steel parts and sizeable engines. Ultralight future cars will be made of lightweight products including plastics, aluminum and metal composites and other hybrid materials. By adjusting the weight of the car parts, companies don't have to sacrifice size and can also use smaller engines.

But what about safety? If an ultralight car rams into the side of a building, won't it crumple like a ball of paper? Well, yes, it may crumble, but its impact absorption will protect the people inside. According to the State Department , "carbon-fiber composites can absorb 12 times the energy per kilogram as steel." Researchers at MIT also have discovered clay nanotech particles that could be used to make ultralight, yet ultra-strong auto parts. By adding these tiny particles to materials, it reinforces them and forms a strengthened network for incredible durability.

For Loremo, addressing safety concerns involved creating an entirely new type of car frame. While most cars are designed to distribute impact around the passenger, the Loremo passes it under them. It does so by using a longer chassis, the frame on which the car sits, that runs along the length of the car to spread the force impact linearly. The company compares it to the same effect as striking a nail. When hit, the nail doesn't break because its impact is directed along its length rather than at an angle. Also, by keeping the body close to the ground and adding air shafts that funnel air through the bottom of the car, it adds driving stability in the same way as featherweight racecars .

When will consumers start to see these ultralight phenomena on the car lot? The Loremo goes into production in 2010, and depending on its reception, it could set off a domino effect.

Car Engines

Introduction to How Car Engines Work

­Have you ever opened the hood of your car and wondered what was going on in there? A car engine can look like a big confusing jumble of metal, tubes and wires to the uninitiated.

You might want to know what's going on simply out of curiosity. Or perhaps you are buying a new car, and you hear things like "3.0 liter V-6" and "dual overhead cams" and "tuned port fuel injection." What does all ­of that mean?



we'll discuss the basic idea behind an engine a­nd then go into detail about how all the pieces fit together, what can go wrong and how to increase performance.

­The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine -- combustion takes place internally.

Two things to note:

There are different kinds of internal combustion engines. Diesel engines are one form and gas turbine engines are another. See also the articles on HEMI engines, rotary engines and two-stroke engines. Each has its own advantages and disadvantages.

There is such a thing as an external combustion engine. A steam engine in old-fashioned trains and steam boats is the best example of an external combustion engine. The fuel (coal, wood, oil, whatever) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine. Internal combustion is a lot more efficient (takes less fuel per mile) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine. This explains why we don't see any cars from Ford and GM using steam engines.

Internal Combustion

The ­principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 1. They are:

Intake stroke
Compression stroke
Combustion stroke
Exhaust stroke
You can see in the figure that a device called a piston replaces the potato in the potato cannon. The piston is connected to the crankshaft by a connecting rod. As the crankshaft revolves, it has the effect of "resetting the cannon." Here's what happens as the engine goes through its cycle:

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)
Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)
When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)
Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)
Now the engine is ready for the next cycle, so it intakes another charge of air and gas.
Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear (straight line). In an engine the linear motion of the pistons is converted into rotational motion by the crankshaft. The rotational motion is nice because we plan to turn (rotate) the car's wheels with it anyway.

Basic Engine Parts

The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. The engine described above has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer), as shown in the following figures.




Figure 1. Inline - The cylinders are arranged in a line in a single bank.




Figure 2. V - The cylinders are arranged in two banks set at an angle to one another.




Figure 3. Flat - The cylinders are arranged in two banks on opposite sides of the engine.

Different configurations have different advantages and disadvantages in terms of smoothness, manufacturing cost and shape characteristics. These advantages and disadvantages make them more suitable for certain vehicles.

Let's look at some key engine parts in more detail.

Spark plug
The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly.

Valves
The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

Piston
A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings
Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.
They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.
Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.

Connecting rod
The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates.

Crankshaft
The crankshaft turns the piston's up and down motion into circular motion just like a crank on a jack-in-the-box does.

Sump
The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump (the oil pan).


Engine Problems

So you go out one morning and your engine will turn over but it won't start... What could be wrong? Now that you know how an engine works, you can understand the basic things that can keep an engine from running. Three fundamental things can happen: a bad fuel mix, lack of compression or lack of spark. Beyond that, thousands of minor things can create problems, but these are the "big three." Based on the simple engine we have been discussing, here is a quick rundown on how these problems affect your engine:

Bad fuel mix - A bad fuel mix can occur in several ways:

You are out of gas, so the engine is getting air but no fuel.
The air intake might be clogged, so there is fuel but not enough air.
The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly.
There might be an impurity in the fuel (like water in your gas tank) that makes the fuel not burn.
Lack of compression - If the charge of air and fuel cannot be compressed properly, the combustion process will not work like it should. Lack of compression might occur for these reasons:

Your piston rings are worn (allowing air/fuel to leak past the piston during compression).
The intake or exhaust valves are not sealing properly, again allowing a leak during compression.
There is a hole in the cylinder.
The most common "hole" in a cylinder occurs where the top of the cylinder (holding the valves and spark plug and also known as the cylinder head) attaches to the cylinder itself. Generally, the cylinder and the cylinder head bolt together with a thin gasket pressed between them to ensure a good seal. If the gasket breaks down, small holes develop between the cylinder and the cylinder head, and these holes cause leaks.



Lack of spark - The spark might be nonexistent or weak for a number of reasons:

If your spark plug or the wire leading to it is worn out, the spark will be weak.
If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark.
If the spark occurs either too early or too late in the cycle (i.e. if the ignition timing is off), the fuel will not ignite at the right time, and this can cause all sorts of problems.
Many other things can go wrong. For example:

If the battery is dead, you cannot turn over the engine to start it.
If the bearings that allow the crankshaft to turn freely are worn out, the crankshaft cannot turn so the engine cannot run.
If the valves do not open and close at the right time or at all, air cannot get in and exhaust cannot get out, so the engine cannot run.
If someone sticks a potato up your tailpipe, exhaust cannot exit the cylinder so the engine will not run.
If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize.
In a properly running engine, all of these factors are within tolerance.

As you can see, an engine has a number of systems that help it do its job of converting fuel into motion.

Engine Valve Train and Ignition Systems

Most engine subsystems can be implemented using different technologies, and better technologies can improve the performance of the engine. Let's look at all of the different subsystems used in modern engines, beginning with the valve train.

The valve train consists of the valves and a mechanism that opens and closes them. The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down, as shown in Figure 5.



Most modern engines have what are called overhead cams. This means that the camshaft is located above the valves, as you see in Figure 5. The cams on the shaft activate the valves directly or through a very short linkage. Older engines used a camshaft located in the sump near the crankshaft. Rods linked the cam below to valve lifters above the valves. This approach has more moving parts and also causes more lag between the cam's activation of the valve and the valve's subsequent motion. A timing belt or timing chain links the crankshaft to the camshaft so that the valves are in sync with the pistons. The camshaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have four valves per cylinder (two for intake, two for exhaust), and this arrangement requires two camshafts per bank of cylinders, hence the phrase "dual overhead cams." See How Camshafts Work for details.

The ignition system (Figure 6) produces a high-voltage electrical charge and transmits it to the spark plugs via ignition wires. The charge first flows to a distributor, which you can easily find under the hood of most cars. The distributor has one wire going in the center and four, six, or eight wires (depending on the number of cylinders) coming out of it. These ignition wires send the charge to each spark plug. The engine is timed so that only one cylinder receives a spark from the distributor at a time. This approach provides maximum smoothness.

Engine Cooling, Air-intake and Starting Systems

The cooling system in most cars consists of the radiator and water pump. Water circulates through passages around the cylinders and then travels through the radiator to cool it off. In a few cars (most notably Volkswagen Beetles), as well as most motorcycles and lawn mowers, the engine is air-cooled instead (You can tell an air-cooled engine by the fins adorning the outside of each cylinder to help dissipate heat.). Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance.



So now you know how and why your engine stays cool. But why is air circulation so important? Most cars are normally aspirated, which means that air flows through an air filter and directly into the cylinders. High-performance engines are either turbocharged or supercharged, which means that air coming into the engine is first pressurized (so that more air/fuel mixture can be squeezed into each cylinder) to increase performance. The amount of pressurization is called boost. A turbocharger uses a small turbine attached to the exhaust pipe to spin a compressing turbine in the incoming air stream. A supercharger is attached directly to the engine to spin the compressor.




Increasing your engine's performance is great, but what exactly happens when you turn the key to start it? The starting system consists of an electric starter motor and a starter solenoid. When you turn the ignition key, the starter motor spins the engine a few revolutions so that the combustion process can start. It takes a powerful motor to spin a cold engine. The starter motor must overcome:

All of the internal friction caused by the piston rings
The compression pressure of any cylinder(s) that happens to be in the compression stroke
The energy needed to open and close valves with the camshaft
All of the "other" things directly attached to the engine, like the water pump, oil pump, alternator, etc.
Because so much energy is needed and because a car uses a 12-volt electrical system, hundreds of amps of electricity must flow into the starter motor. The starter solenoid is essentially a large electronic switch that can handle that much current. When you turn the ignition key, it activates the solenoid to power the motor.

Engine Lubrication, Fuel, Exhaust and Electrical Systems

When it comes to day-to-day car maintenance, your first concern is probably the amount of gas in your car. How does the gas that you put in power the cylinders? The engine's fuel system pumps gas from the gas tank and mixes it with air so that the proper air/fuel mixture can flow into the cylinders. Fuel is delivered in three common ways: carburetion, port fuel injection and direct fuel injection.

In carburetion, a device called a carburetor mixes gas into air as the air flows into the engine.
In a fuel-injected engine, the right amount of fuel is injected individually into each cylinder either right above the intake valve (port fuel injection) or directly into the cylinder (direct fuel injection).

Oil also plays an important part. The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing oil are the pistons (so they can slide easily in their cylinders) and any bearings that allow things like the crankshaft and camshafts to rotate freely. In most cars, oil is sucked out of the oil pan by the oil pump, run through the oil filter to remove any grit, and then squirted under high pressure onto bearings and the cylinder walls. The oil then trickles down into the sump, where it is collected again and the cycle repeats.



Now that you know about some of the stuff that you put in your car, let's look at some of the stuff that comes out of it. The exhaust system includes the exhaust pipe and the muffler. Without a muffler, what you would hear is the sound of thousands of small explosions coming out your tailpipe. A muffler dampens the sound. The exhaust system also includes a catalytic converter.

The emission control system in modern cars consists of a catalytic converter, a collection of sensors and actuators, and a computer to monitor and adjust everything. For example, the catalytic converter uses a catalyst and oxygen to burn off any unused fuel and certain other chemicals in the exhaust. An oxygen sensor in the exhaust stream makes sure there is enough oxygen available for the catalyst to work and adjusts things if necessary.

Besides gas, what else powers your car? The electrical system consists of a battery and an alternator. The alternator is connected to the engine by a belt and generates electricity to recharge the battery. The battery makes 12-volt power available to everything in the car needing electricity (the ignition system, radio, headlights, windshield wipers, power windows and seats, computers, etc.) through the vehicle's wiring.


Producing More Engine Power

Using all of this information, you can begin to see that there are lots of different ways to make an engine perform better. Car manufacturers are constantly playing with all of the following variables to make an engine more powerful and/or more fuel efficient.

Increase displacement - More displacement means more power because you can burn more gas during each revolution of the engine. You can increase displacement by making the cylinders bigger or by adding more cylinders. Twelve cylinders seems to be the practical limit.

Increase the compression ratio - Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (before the spark plug ignites it). Higher-octane gasolines prevent this sort of early combustion. That is why high-performance cars generally need high-octane gasoline -- their engines are using higher compression ratios to get more power.

Stuff more into each cylinder - If you can cram more air (and therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would by increasing the size of the cylinder). Turbochargers and superchargers pressurize the incoming air to effectively cram more air into a cylinder.



Cool the incoming air - Compressing air raises its temperature. However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand when combustion takes place. Therefore, many turbocharged and supercharged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder.

Let air come in more easily - As a piston moves down in the intake stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves in each cylinder. Some newer cars are also using polished intake manifolds to eliminate air resistance there. Bigger air filters can also improve air flow.

Let exhaust exit more easily - If air resistance makes it hard for exhaust to exit a cylinder, it robs the engine of power. Air resistance can be lessened by adding a second exhaust valve to each cylinder (a car with two intake and two exhaust valves has four valves per cylinder, which improves performance -- when you hear a car ad tell you the car has four cylinders and 16 valves, what the ad is saying is that the engine has four valves per cylinder). If the exhaust pipe is too small or the muffler has a lot of air resistance, this can cause back-pressure, which has the same effect. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system. When you hear that a car has "dual exhaust," the goal is to improve the flow of exhaust by having two exhaust pipes instead of one.

Make everything lighter - Lightweight parts help the engine perform better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes.

Inject the fuel - Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy

Engine Questions and Answers

1.What is the difference between a gasoline engine and a diesel engine?

In a diesel engine, there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine gets better mileage.

2.What is the difference between a two-stroke and a four-stroke engine?

Most chain saws and boat motors use two-stroke engines. A two-stroke engine has no moving valves, and the spark plug fires each time the piston hits the top of its cycle. A hole in the lower part of the cylinder wall lets in gas and air. As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another hole in the cylinder. You have to mix oil into the gas in a two-stroke engine because the holes in the cylinder wall prevent the use of rings to seal the combustion chamber. Generally, a two-stroke engine produces a lot of power for its size because there are twice as many combustion cycles occurring per rotation. However, a two-stroke engine uses more gasoline and burns lots of oil, so it is far more polluting.

3.Is there any advantages to steam engines and other external combustion engines?

The main advantage of a steam engine is that you can use anything that burns as the fuel. For example, a steam engine can use coal, newspaper or wood for the fuel, while an internal combustion engine needs pure, high-quality liquid or gaseous fuel.

4.Why have eight cylinders in an engine?

Why not have one big cylinder of the same displacement of the eight cylinders instead? There are a couple of reasons why a big 4.0-liter engine has eight half-liter cylinders rather than one big 4-liter cylinder. The main reason is smoothness. A V-8 engine is much smoother because it has eight evenly spaced explosions instead of one big explosion. Another reason is starting torque. When you start a V-8 engine, you are only driving two cylinders (1 liter) through their compression strokes, but with one big cylinder you would have to compress 4 liters instead.

How are 4-cylinder and V6 engines different?

The number of cylinders that an engine contains is an important factor in the overall performance of the engine. Each cylinder contains a piston that pumps inside of it and those pistons connect to and turn the crankshaft. The more pistons there are pumping, the more combustive events are taking place during any given moment. That means that more power can be generated in less time.

4-Cylinder engines commonly come in “straight” or “inline” configurations while 6-cylinder engines are usually configured in the more compact “V” shape, and thus are referred to as V6 engines. V6 engines have been the engine of choice for American automakers because they’re powerful and quiet but still light and compact enough to fit into most car designs.



Historically, American auto consumers turned their noses up at 4-cylinder engines, believing them to be slow, weak, unbalanced and short on acceleration. However, when Japanese auto makers, such as Honda and Toyota, began installing highly-efficient 4-cylinder engines in their cars in the 1980s and 90s, Americans found a new appreciation for the compact engine. Even though Japanese models, such as the Toyota Camry, began quickly outselling comparable American models, U.S. automakers, believing that American drivers were more concerned with power and performance, continued to produce cars with V6 engines. Today, with rising gas prices and greater public environmental awareness, Detroit seems to be reevaluating the 4-cylinder engine for its fuel efficiency and lower emissions.

As for the future of the V6, in recent years the disparity between 4-cylinder and V6 engines has lessened considerably. In order to keep up with the demand for high gas-mileage and lower emission levels, automakers have worked diligently to improve the overall performance of V6 engines. Many current V6 models come close to matching the gas-mileage and emissions standards of the smaller, 4-cylinder engines. So, with the performance and efficiency gaps between the two engines lessening, the decision to buy a 4-cylinder or V6 may just come down to cost. In models that are available with either type of engine, the 4-cylinder version can run up to $1000 cheaper than the V6. So, regardless of what kind of performance you’re looking to get out of your car, the 4-cylinder will always be the budget buy.

One final note: It’s not a good idea to try to install a V6 engine into a car model that comes with a standard 4-cylinder. Retrofitting a 4-cylinder car to handle a V6 engine could cost more than simply buying a new car.

Quartz Watches

Introduction to How Quartz Watches Work

During the 1970s, the "quartz watch" burst onto the scene as the newest high-tech gadget. Initially, these watches had red LED displays and they cost around $500 in the United States. Since then, the quartz watch has evolved so that either an LCD or a traditional mechanical (hour and minute hand) movement displays the time, and the price has fallen dramatically. It is not uncommon to find quartz watches given away for free in boxes of cereal!



Have you ever wondered why it is called a quartz watch? Or why quartz watches are so much more accurate than wind-up watches?

Before Quartz

The wind-up watch is an amazing piece of technology itself! It is part of a continuous research-and-development effort that started at the end of the 14th Century. Over the years, different innovations made wind-up watches smaller, thinner, more reliable, more accurate and even self-winding!

The components that you find in today's wind-up watches have been around for centuries:

A spring to provide the power
Some sort of oscillating mass to provide a timebase
Two or more hands
An enumerated dial on the face of the watch
Gears to slow down from the ticking rate of the oscillating mass and connect the mass and spring to the hands on the dial
See How Pendulum Clocks Work for a description of these different parts.

By the end of the 1960s, the Bulova watch company made the first step away from the oscillating balance wheel -- it used a transistor oscillator that maintained a tuning fork. This watch hummed at some hundreds of hertz (Hz, cycles per second) rather than ticking! Cogs and wheels still converted the mechanical movement of the tuning fork to movement of the hands, but two major steps had been taken:

The replacement of the balance wheel and spring with a single-material resonator: the tuning fork
The replacement of the wind-up main spring with a battery
A watch-making company in the late 1960s was bound to look for the next step -- a technology that would give even better time keeping than the tuning fork. Integrated circuits were very new at the time, but the price was dropping rapidly and the number of transistors was growing. LEDs were also new on the scene. There were still a couple of problems to be solved: finding a new timing element and designing an integrated circuit that would use very little power to allow the watch to run on a tiny internal battery.

The Quartz Crystal

There was no problem with the choice of a timing element. The quartz crystal is possibly thousands of times better for timing than the tuning fork, and quartz crystals had been around for many years. Only the type and the frequency of the crystal needed to be chosen. The difficulty was in the selection of the integrated circuit technology that would function at sufficiently low power.

Quartz crystals have been in regular use for many years to give an accurate frequency for all radio transmitters, radio receivers and computers. Their accuracy comes from an amazing set of coincidences: Quartz -- which is silicon dioxide like most sand -- is unaffected by most solvents and remains crystalline to hundreds of degrees Fahrenheit. The property that makes it an electronic miracle is the fact that, when compressed or bent, it generates a charge or voltage on its surface. This is a fairly common phenomenon called the Piezoelectric effect. In the same way, if a voltage is applied, quartz will bend or change its shape very slightly.

If a bell were shaped by grinding a single crystal of quartz, it would ring for minutes after being tapped. Almost no energy is lost in the material. A quartz bell -- if shaped in the right direction to the crystalline axis -- will have an oscillating voltage on its surface, and the rate of oscillation is unaffected by temperature. If the surface voltage on the crystal is picked off with plated electrodes and amplified by a transistor or integrated circuit, it can be re-applied to the bell to keep it ringing.

A quartz bell could be made, but it is not the best shape because too much energy is coupled to the air. The best shapes are a straight bar or a disk. A bar has the advantage of keeping the same frequency provided the ratio of length to width remains the same. A quartz bar can be tiny and oscillate at a relatively low frequency -- 32 kilohertz (KHz) is usually chosen for watches not only for size, but also because the circuits that divide down from the crystal frequency to the few pulses per second for the display need more power for higher frequencies. Power was a big problem for early watches, and the Swiss spent millions trying to bring forward integrated-circuit technology to divide down from the 1 to 2 MHz the more stable disk crystals generate.

Modern quartz watches now use a low-frequency bar or tuning-fork-shaped crystal. Often, these crystals are made from thin sheets of quartz plated like an integrated circuit and etched chemically to shape. The major difference between good and indifferent time keeping is the initial frequency accuracy and the precision of the angle of cut of the quartz sheet with respect to the crystalline axis. The amount of contamination that is allowed to get through the encapsulation to the crystal surface inside the watch can also affect the accuracy.

The electronics of the watch initially amplifies noise at the crystal frequency. This builds or regenerates into oscillation -- it starts the crystal ringing. The output of the watch crystal oscillator is then converted to pulses suitable for the digital circuits. These divide the crystal's frequency down and then translate it into the proper format for the display,in a quartz watch with hands, the dividers create one-second pulses that drive a tiny electric motor, and this motor is connected to standard gears to drive the hands.

Digital Clocks

Introduction to How Digital Clocks Work

Chances are that in your bedroom you have a digital clock beside your bed. Have you ever looked at it in the morning and wondered how it works?



In this Post, you're going to learn exactly how a digital clock (or wristwatch) works. In fact, you're even going to learn how to build your own!

The Basics

If you have read How Pendulum Clocks Work, you know that all clocks (regardless of technology) have a few required components:
A source of power to run the clock
In a pendulum clock, the weights or the springs handle this role.

An accurate timebase that acts as the clock's heartbeat
In a pendulum clock, the pendulum and escapement handle this role.

A way to gear down the timebase to extract different components of time (hours, minutes, seconds)
In a pendulum clock, gears serve this role.

A way to display the time
In a pendulum clock, the hands and face serve this role.
A digital clock is no different. It simply handles these functions electronically rather than mechanically. So in a digital clock, there is an electrical power supply (either a battery or 120-volt AC power from the wall). There is an electronic timebase that "ticks" at some known and accurate rate. There is an electronic "gearing mechanism" of some sort -- generally a digital clock handles gearing with a component called a "counter." And there is a display, usually either LEDs (light emitting diodes) or an LCD (liquid crystal display).

High-Level View

Here is a quick overview of the components of a digital clock at a high level.
At the heart of the clock there is a piece that can generate an accurate 60-hertz (Hz, oscillations per second) signal. There are two ways to generate this signal:

The signal can be extracted from the 60-Hz oscillations in a normal power line. Many clocks that get their power from a wall socket use this technique because it is cheap and easy. The 60-Hz signal on the power line is reasonably accurate for this purpose.

The signal can be generated using a crystal oscillator. Obviously, any battery-operated clock or wristwatch will use this technique instead. It takes more parts, but is generally much more accurate.
The 60-Hz signal is divided down using a counter. When building your own clock, a typical TTL part to use is a 7490 decade counter. This part can be configured to divide by any number between 2 and 10, and generates a binary number as output. So you take your 60-Hz time base, divide it by 10, divide it by 6 and now you have a 1-Hz (1 oscillation per second) signal. This 1-Hz signal is perfect for driving the "second hand" portion of the display. So far, the clock looks like this in a block diagram:




To actually see the seconds, then the output of the counters needs to drive a display. The two counters produce binary numbers. The divide-by-10 counter is producing a 0-1-2-3-4-5-6-7-8-9 sequence on its outputs, while the divide-by-6 counter is producing a 0-1-2-3-4-5 sequence on its outputs. We want to display these binary numbers on something called a 7-segment display. A 7-segment display has seven bars on it, and by turning on different bars you can display different numbers:



To convert a binary number between 0 and 9 to the appropriate signals to drive a 7-segment display, you use a (appropriately named) "binary number to 7-segment display converter." This chip looks at the binary number coming in and turns on the appropriate bars in the 7-segment LED to display that number. If we are displaying the seconds, then the seconds part of our clock looks like this:




The output from this stage oscillates at a frequency of one-cycle-per-minute. You can imagine that the minutes section of the clock looks exactly the same. Finally, the hours section looks almost the same except that the divide-by-6 counter is replaced by a divide-by-2 counter.

Now there are two details left to figure out if you are building a real clock:

The clock as designed here does not understand that at 12:59:59 it is supposed to cycle back to 1:00. That is a messy little problem, and there are a couple of ways to solve it. One technique involves creating a little bit of logic that can detect the number 13 and reset the hour section back to 1 (not zero). Another technique involves using an adder. For our purposes, it is easier to deal in military time, because military time includes a zero hour.

We need a way to set the clock. Typically this is handled by gating higher-than-normal frequencies into the minutes section. For example, most clocks have "fast" and "slow" set buttons. When you press the "fast" button, the 60-Hz signal is driven straight into the minutes counter. When you press the "slow" button, a 1-Hz signal is driven into the minutes section. There are other possible techniques, but this one is the most common.

Building Your Own Digital Clock

The best way to understand the different components of a digital clock and how they work together is to actually walk through the steps of building your own clock. Here we will build just the "seconds" part of the clock, but you can easily extend things to build a complete clock with hours, minutes and seconds. To understand these steps, you will need to have read How Boolean Logic Works and How Electronic Gates Work. In particular, the electronic gates article introduces you to TTL chips, breadboards and power supplies. If you have already played around with gates as described in that article, then the description here will make a lot more sense.
The first thing we need is a power supply. We built one in the electronic gates article. That time, we used a standard wall transformer that produced DC (direct current) power and then regulated it to 5 volts using a 7805. For our clock, we want to do things slightly differently because we are going to extract our 60-Hz timebase from the power line. That means that we want an AC rather than a DC transformer, and we will use a part called a bridge rectifier to convert the AC to DC. Therefore, we need the following parts for our power supply:

Part name Jameco part #
12-volt AC transformer 115602
Bridge rectifier 103018
7805 5-volt regulator (TO-220 case) 51262
Two 470-microfarad electrolytic capacitors 93817
5.1-volt zener diode 36097
1-K-ohm resistor 29663

A few notes on the parts used:

The difference between the AC transformer we are using here and the DC transformer we used in the article on gates is that the AC transformer preserves the 60-Hz sine wave found in 120-volt household current. If you want to use your volt-ohm meter to measure the voltage of an AC transformer, be sure you use an AC voltage range rather than a DC range.

We use the bridge rectifier to convert the AC to DC. One of the terminals on the rectifier will be marked with a "+" -- from that you can find the minus and AC inputs. There is no polarity to an AC transformer, so it does not matter which transformer lead you connect to which AC lead of the rectifier.

The resistor and the zener diode extract a 60-Hz signal from the transformer's sine wave. A diode is a one-way valve for electrons. A zener diode is also a one-way valve, but it also passes electrons in the other direction if they are above a certain voltage. The zener diode therefore turns a 10-volt sine wave into a clipped wave oscillating between 0 and 5 volts. This is perfect for clocking the TTL counters. The 1-K-ohm resistor makes sure that the current to the zener diode is limited so we do not burn out the diode. The diode will have a band painted on one end -- this band should be the end connected to the resistor.

Circuit Diagram

Here's a circuit diagram for the power supply and time base:



To create the rest of the clock you will need:

At least four 7490 or 74LS90 chips
At least two 7447 or 74LS47 binary-to-7-segment converters
At least 20 resistors for the LEDs in the 7-segment displays (330 ohms would be fine.)
Some normal LEDs
At least two common-anode (CA) 7-segment LED displays (Jameco part # 17208 is typical.)
Breadboards, wire, etc. (See this page for a complete list.)
The number of chips, resistors and LEDs you need depends on how many digits you are interested in implementing. Here we will discuss only seconds, so the "at least" numbers are correct.

7490 Pinout

Let's look at the 7490 briefly to see how it works. Here is the pinout:



The 7490 is a decade counter, meaning it is able to count from 0 to 9 cyclically, and that is its natural mode. That is, QA, QB, QC and QD are 4 bits in a binary number, and these pins cycle through 0 to 9, like this:

QD QC QB QA
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1

You can also set the chip up to count up to other maximum numbers and then return to zero. You "set it up" by changing the wiring of the R01, R02, R91 and R92 lines. If both R01 and R02 are 1 (5 volts) and either R91 or R92 are 0 (ground), then the chip will reset QA, QB, QC and QD to 0. If both R91 and R92 are 1 (5 volts), then the count on QA, QB, QC and QD goes to 1001 (5). So:

To create a divide-by-10 counter, you first connect pin 5 to +5 volts and pin 10 to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 2, 3, 6, and 7. You run the input clock signal (from the timebase or a previous counter) in on pin 14. The output appears on QA, QB, QC and QD. Use the output on pin 11 to connect to the next stage.

To create a divide-by-6 counter, you first connect pin 5 to +5 volts and pin 10 to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 6 and 7. Connect pin 2 to pin 9 and pin 3 to pin 8. Run the input clock signal (from the timebase or a previous counter) in on pin 14. The output appears on QA, QB and QC. Use pin 8 to connect to the next stage.

Creating the Second Hand

Knowing all of this, you can easily create the "second hand" of a digital clock. It looks like this:



In this diagram, the top two 7490s divide the 60-Hz signal from the power supply down by a factor of 60. The third 7490 takes a 1-hertz signal as input and divides it by 10. Its four outputs drive normal LEDs in this diagram. The fourth 7490 divides the output of the third by 6, and its three outputs drive normal LEDs as well. What you have at this point is a "second hand" for your clock, with the output of the second hand appearing in binary. If you would like to create a clock that displays the time in binary, then you are set! Here is a view of a breadboard containing a divide-by-10 counter, a divide-by-6 counter and a set of LEDs to display the output of the counters in binary:




If you pack things tighter and do a neater wiring job, you can fit up to four counters on a single breadboard.

Displaying the Time as Numerals

If you want to display the time as numerals, you need to use the 7447s. Here is the pinout of a 7447, as well as the segment labeling for a 7-segment LED:




You connect a 7447 to a 7490 like this:

Provide +5 volts on pin 16 and ground on pin 8 to power the 7447 chip.
Connect QA, QB, QC and QD from a 7490 to pins 7, 1, 2 and 6 of the 7447, respectively.
Connect 330-ohm resistors to pins 13, 12, 11, 10, 9, 15 and 14 of the 7447, and connect those resistors to the a, b, c, d, e, f, and g segments of the 7-segment LED.
Connect the common anode of the 7-segment LED to +5 volts.



You will need to have the pinout for the specific LED display that you use so that you know how to wire the outputs of the 7447 to the LEDs in the 7-segment device. (Also, note that the 7448 is equivalent to the 7447 except that it drives common-cathode displays. Ground the common cathode of the LED in that case.)

You can see that by extending the circuit, we can easily create a complete clock. To create the "minute hand" section of the clock, all that you need to do is duplicate the "second hand" portion. To create the "hour hand" portion, you are going to want to be creative. Probably the easiest solution is to create a clock that displays military time. Then you will want to use an AND gate (or the R inputs of the 7490) to recognize the binary number 24 and use the output of the recognizer to reset the hour counters to zero.

The final piece you need to create is a setting mechanism. On a breadboard, it is easy to set the clock -- just move the input wires to drive higher-frequency signals into the minute-hand section of the clock. In a real clock, you would use pushbuttons or switches and gates to do the same thing.

If you happen to take your bedside clock or watch apart, one thing you will notice is that there are probably not 15 TTL ICs inside. In fact, you may not be able to find a chip at all. In most modern clocks and watches, all of the functions of the clock (including the alarm and any other features) are all integrated into one low-power chip (in a watch, the chip and display together consume only about a millionth of a watt). That chip is probably embedded directly into the circuit board. You might be able to see a blob of black plastic protecting this chip. That one tiny chip contains all of the components we have discussed here.

Now you have a complete understanding of how digital clocks work.