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.

Car Engine Image Gallery

Photo courtesy DaimlerChrysler Jeep® Grand Cherokee Engine. See more pictures of car engines.

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?

In this article, 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.

Let's look at the internal combustion process in more detail in the next section.

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Internal Combustion

The potato cannon uses the basic 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!

Figure 1

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:

1. 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)
2. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)
3. 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)
4. 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.

Now let's look at all the parts that work together to make this happen, starting with the cylinders.

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

Figure 3. V - The cylinders are arranged in two banks set at an angle to one another.

Figure 4. 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).

Next, we'll learn what can go wrong with engines.

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. We'll look at the different subsystems used in engines in the next few sections.

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.

Figure 5. The camshaft

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. See How Automobile Ignition Systems Work for more details.

Figure 6. The ignition system

We'll look at how your car's engine starts, cools and circulates air in the next section.

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. See How Car Cooling Systems Work for details.

Diagram of a cooling system showing how all the plumbing is connected

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.

Photo courtesy Garrett

See How Turbochargers Work for details.

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.

Next, we'll look at the engine subsystems that maintain what goes in (oil and fuel) and what comes out (exhaust and emissions).

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).

See How Fuel Injection Systems Work for more details.

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. See How Catalytic Converters Work for details.

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.

Now that you know all about the main engine subsystems, let's look at ways that you can boost engine performance.

Producing More Engine Power

Horsepower For a complete explanation of what horsepower is and what horsepower means, check out How Horsepower Works.

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. See How Turbochargers Work for details.

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. See How Car Cooling Systems Work for details.

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. See How Fuel Injection Systems Work for details.

In the next sections, we'll answer some common engine-related questions submitted by readers.

Engine Questions and Answers

Here is a set of engine-related questions from readers and their answers:

• 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. See How Diesel Engines Work for more information.

• 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. See How Two-stroke Engines Work for more information.

• You mentioned steam engines in this article -- are 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. See How Steam Engines Work for more information.

• Are there any other cycles besides the Otto cycle used in car engines? The two-stroke engine cycle is different, as is the diesel cycle described above. The engine in the Mazda Millenia uses a modification of the Otto cycle called the Miller cycle. Gas turbine engines use the Brayton cycle. Wankel rotary engines use the Otto cycle, but they do it in a very different way than four-stroke piston engines.

• 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.

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How Hummers Work

Introduction to How Hummers Work

In 1979, the U.S. Army issued a request for a new vehicle design that could meet demanding standards, including the ability to modify the base vehicle for different missions. Chrysler Defense, Teledyne Continental and AM General submitted design proposals, and after extensive tests and revisions, the Army awarded AM General a $1.2 billion contract to produce their High Mobility Multi-purpose Wheeled Vehicle (HMMWV), better known as the Humvee or Hummer.

Hummer Image Gallery

Photo courtesy AM General The Humvee Base Model. See more hummer pictures.

The Hummer plays an integral role in the Army's vehicle fleet. The U.S. Marines, Navy and Air Force also use Hummers in various military operations. AM General first offered a civilian version of the Hummer in 1992. In 1999, GM purchased the right to produce vehicles using the Hummer name, so now there are two lines of Hummers in production -- AM General's military vehicles and GM's civilian Hummers.

Hummers have made a huge impact on both military applications and civilian lifestyles. This versatile vehicle seems to embody complex -- and sometimes contradictory -- ideals, from utilitarian workhorse to the ultimate expression of machismo. Even as the military looks to replace HMMWVs as part of its Future Tactical Truck Systems (FTTS) program, the Hummer continues to be a symbol of the U.S. military's presence in combat zones around the world.

Video Gallery: Robotic Hummers and Special Perks Manually driving a robotic Hummer to the gas station turns a few heads, including a police officer's. Learn how to get attention at a gas station in this NOVA segment from PBS. Automakers often claim that their vehicles can do more than just get you from point A to point B, and drivers often wish they can push the speedometer on that shiny new sports car over a hundred, but few of them ever get the opportunity -- until now. See how automakers are offering exotic perks in this video from MediaLink.

In this article, we'll look at the basic military Hummer and its amazing configurations. We'll examine the civilian models -- officially known as Hummers -- currently on the market. We'll also explore the cultural impact of the Hummer.

In the next section, we'll look at the basic military Hummer.

Hummer Specs

By the 1970s, Army officials were convinced that they needed to upgrade their fleet of ground vehicles. The Jeep, which was the Army's previous general-purpose vehicle, had outlived its versatility and usefulness in the field. They needed a new, adaptable vehicle that could handle the demands of the evolving nature of combat environments.

The basic Hummer vehicle is 6 feet tall, 7 feet wide, 15 feet long and weighs 5,200 pounds (2,340 kilograms). AM General used a steel frame with five cross members to support the weight of a vehicle with a payload of up to 2,500 pounds (1,125 kilograms), allowing a gross vehicle weight (GVW) -- the weight of the vehicle, passengers and maximum payload -- of 7,700 pounds (3,465 kilograms). In order to keep the vehicle's total weight at a manageable level, AM General used aluminum to construct the body of the car. The aluminum is strong enough to support heavy armaments or carry troops, yet is able to flex when the Hummer travels over rough ground.

Joe Raedle/Getty Images Some military Humvees are used as troop transport vehicles.

A double A-arm independent suspension in the front and rear with hydraulic shock absorbers allow the Hummer to tackle unforgiving terrain. It also has torque-biasing differentials and four-wheel disc brakes, which give the Hummer the ability to continue moving even when some of the wheels lose contact with the ground.

The Hummer is a fully four-wheel drive vehicle -- the engine powers all four wheels at all times. It also has open-differential gears with Torsen differentials. When one wheel begins to slip, it loses torque. The Torsen differential system senses the loss of torque and increases torque to the other wheels. Coupled with the brake traction control system, the Humvee's Torsen differentials give the vehicle incredible off-road capabilities.

AM General outfitted the Hummer with military tires, and some Hummers included a central tire inflation system (CTIS). With this system, a driver can adjust tire pressure without leaving his seat. By lowering the pressure in the tires, the driver could increase the Hummer's grip on rough surfaces, which can be handy if you're trying to drive up a steep hill covered in rocks. Higher pressure is better when driving on even surfaces -- it helps maintain a smooth ride.

Hummers have a 25-gallon (95-liter) fuel tank and can go about 300 miles (480 kilometers) before needing to refuel -- meaning a basic Hummer gets about 12 miles to the gallon, though heavier Hummer variations are somewhat less efficient. As per the Army's request, all military Hummers run on diesel fuel and have an automatic transmission -- the Army wanted all its vehicles to run on the same fuel system and felt that automatic transmissions would be easier for new trainees to learn quickly.

When It Has To Be There Overnight AM General designed Hummers for rapid deployment, including dropping them from low-flying aircraft. A C-130 Hercules transport aircraft can carry three Hummers. The C-5A Galaxy aircraft can carry up to 15. Helicopters can even carry them and deliver the vehicle using a Low Altitude Parachute Extraction System. In theory, the Army can rapidly deploy a fully equipped Hummer to men in the field in a matter of hours.

The Hummer has power steering and includes a 12/24-volt electrical system. Either two or four soldiers can sit inside the cab of the vehicle, depending on the variant. According to AM General, the location of the seats on either side of the drive train helps give the Hummer a low center of gravity.

The vehicles have powerful combat locks on each of the doors to help keep soldiers safe. Unfortunately, some soldiers have found it impossible to open the doors if the locks are damaged out in the field. To fix this problem, AM General now includes a D-ring on all Hummer doors. The D-ring is a loop of metal attached to the outside of the Hummer's doors and functions as a place to attach a cable or chain. A winch or similar device can pull the cable, forcing the door open and letting soldiers out of the vehicle.

That's the lowdown on the basic Hummer. Using this as a starting platform, AM General produced 15 different HMMWV variants. They designed 44 interchangeable Hummer parts, allowing the Army to modify, maintain and repair vehicles with incredible efficiency, economy and flexibility.

In the next section, we'll look at some of the variants of the military Hummer.

Hummer Models

The 15 variations on the basic Hummer model allow the Army to use the same vehicle as a troop or cargo transport, weapons carrier, shelter carrier (a vehicle designed to transport electronic equipment) or ambulance. Some variations look almost identical, while others seem to be completely unrelated. All use the same frame, drive train geometry, suspension and lower body.

He Ain't Heavy, He's My Hummer You may recall from the previous section that the basic Hummer weighs a hefty 5,200 pounds (2,340 kilograms) -- that's almost 600 pounds more than a 2007 Ford F-150. Even so, it's a lightweight compared to the monster known as the M1151A1 with B1 armor, a Hummer designed as an armament carrier complete with armor designed to protect the vehicle from ballistic attacks and landmines. This little baby weighs in at a lean 10,300 pounds (4,635 kilograms), but when carrying its full payload, the GVW reaches an incredible 12,100 pounds (5,445 kilograms) -- more than 6 tons!

The current 15 variations fall into two different broad categories -- the A2 model series and the Expanded Capacity Vehicle (ECV) models. Nine models are in the A2 series, including the M1097A2 Cargo/Troop Carrier, the M1097A2 Shelter Carrier, the M997A2 Maxi-Ambulance with Basic Armor and the M1045A2 Armament/TOW Missile Carrier with Supplemental Armor. The TOW missile is a tube-launched, optically tracked, wire-guided missile designed to neutralize enemy tanks and aircraft.

Photo courtesy AM General Helicopters can airlift Humvees and transport them into the field.

The Army has lots of options when it comes to arming A2 Hummers. Some of the weapons you might find on one include:

• A .50 caliber M2HB Machine Gun
• The MK 19 40 mm Grenade Machine Gun
• A 106mm Recoilless Rife
• A Giat 30, M781 Cannon
• TOW and TOW II Anti-Aircraft Missile Systems
• Milan Anti-Armor Missile System

The Army also wanted some vehicles that could carry heavier payloads without a detrimental effect on the cars' mobility. AM General's response was the ECV series. The M1113 ECV is used in special operations missions and as a communications shelter carrier. The M1114 has upgraded ballistic protection and is used in military police and explosive disposal missions. The M1151 is an armament carrier and the M1152 can either be used as a troop carrier or shelter carrier. The M1165 is used as a command and control vehicle. The M1116 is the U.S. Air Force's variation on the Hummer, with a larger cargo area and turret gunner armor.

Just like a commercial vehicle, there's some optional equipment you can get on a military Hummer. The options available to both the Hummer and civilian vehicles are air conditioning, special paints, a hard top, central tire inflation system and a winch. Here are a few of the options the military can include that are definitely not available on your average set of wheels:

• Troop seats for up to eight soldiers
• A deep water (up to 5 feet) fording kit with snorkel
• An Arctic kit
• A desert filtration package
• Pedestal weapons mount
• Special Ops configuration

International Hummers AM General partners with international defense contractors to develop Hummer-based vehicles like the Eagle and the Cobra for other nations. The Eagle modification is used in surveillance, reconnaissance, escort and police missions. The Cobra is a heavily armored vehicle that can carry up to 11 crewmembers depending on the variant. Both the Eagle and the Cobra can have manually or remotely operated weapons turrets. Photo courtesy AM General The Eagle Photo courtesy AM General The Cobra

In the next section, we'll look at how the Hummer invaded the civilian automobile market.

Civilian Hummers

The American public's awareness of the Hummer really blossomed in 1991 as Operation Desert Storm dominated daily headlines and newscasts. For most Americans, it was the first chance to see the Army's Hummer in action. Many were intrigued by its wide, powerful design, and before long people began to ask AM General if it planned to produce a commercial version of the Hummer. AM General's answer was "yes."

Daniel T. Yara, morguefile An H2 Hummer

In 1992, AM General began to produce a four-wheel drive vehicle based off the military Hummer. They marketed it as "the world's most serious 4x4." The Hummer shared many of its military cousin’s features, including the brake traction control system that gives the Hummer the ability to adjust torque even when a wheel is completely off the ground. Off-road enthusiasts were overjoyed at the prospect of getting behind the wheel of a car that could tackle courses that would scare a Jeep driver. The car's safety rankings were very high -- but you'd probably expect that from a car originally designed for combat missions.

Come With Me If You Want to Live One person who was particularly interested in Hummers was future California governor and former Terminator, Arnold Schwarzenegger. Schwarzenegger's fascination with the vehicle made the news when he first asked AM General to sell him a Hummer. AM General refused to sell a military vehicle to a civilian, but eventually conceded by designing and producing the Hummer. In 1992, Schwarzenegger became the first owner of the civilian Hummer.

Civilian Hummers use the same chassis as the military Hummer, and AM General even uses the same manufacturing facilities to build them. From 1992 to 1995, the Hummer used a diesel engine (either a 6.2 or 6.5-liter engine, depending on the year). From 1995 to 1997, AM General experimented by producing a model that used a 5.7-liter gasoline V-8 engine. Unfortunately, the Hummer just weighed too much -- early Hummers weighed in at around 7,000 pounds (3,150 kilograms) -- and AM General went back to using diesel engines after 1997.

In 1999, General Motors purchased the right to produce vehicles under the Hummer brand name. AM General would continue to produce the original Hummer, renamed as the H1, under the GM brand name. GM named later models the H2 and H3. Although GM designed the H2, AM General builds the cars in its facilities. General Motors manufactures the Hummer H3 using its own facilities.

Hummer Options

The Hummer H2 is longer than, but not as wide as the H1, and the H3 is the smallest vehicle under the brand name so far. Even so, you can't describe the H3 as dainty -- it's 85.5 inches (7.1 feet) wide and 186.7 inches (15.6 feet) long. Drivers who love the series find the size empowering, while other drivers often feel intimidated when they encounter such gargantuan vehicles.

The Hummer line is not known for its fuel efficiency. Various models of the Hummer H1 reportedly averaged at around 10 miles per gallon [source: Shortnews.com]. Peter Ternes, Hummer's director of global product communications, said that the H2 got 12 miles to the gallon [source: AOL Autos]. The H3 fares the best with 15 mpg city/19 mpg highway. There's also the experimental H2H, a hydrogen-powered Hummer, but because there's no hydrogen fuel infrastructure in the United States, it's not likely to be commercially available soon.

Vince Bucci/Getty Images Evander Holyfield's Hummer

As the Hummer brand has evolved over the last few years, General Motors has introduced more options catering to customer comfort. The earliest commercial Hummer vehicles were only a little more comfortable than their military Hummer counterparts were. The appeal of the vehicle was in its off-road capabilities, not its style or amenities. The biggest concession to customer comfort was air conditioning, something many military Hummers lack.

Current Hummer vehicles have more appealing options like chrome finishing, leather seating, heated windshields, remote keyless entry, power windows and cruise control. You can buy an H3 Hummer with satellite radio and heated seats if you like. Other options cater to rugged outdoorsy types -- winches, off-road hardware, tool kits and trailer hitches are available.

As amenities have increased, the price of Hummers has actually gone down. The original Hummer's price was more than $100,000. The first H2 vehicles were also expensive, though now a 2008 H2 Hummer will cost you about $55,500. The H3 is the most affordable model -- a 2008 H3 is priced between $30,000 and $39,000.

In the next section, we'll look at how Hummers have impacted our culture.

Hummer's Cultural Impact

Spc. L.B.Edgar, U.S. Army A soldier makes alterations to a Humvee toimprove safety conditions .

Images and video of Hummers appear nearly every day in reports on the United State's presence in foreign countries like Iraq and Afghanistan. Ever since Desert Storm, the Hummer has been a symbol of the U.S. military. In some cases, critics have pointed out perceived shortcomings in the Hummer's design, mainly focusing on its armor. AM General points out that the Army never demanded extensive armor for its Hummers. AM General does offer basic and supplemental armor for several Hummer models, though these packages only provided limited protection. A direct shot from a relatively powerful rifle could still pierce the armor.

Reports of the Hummer's armor limitations give rise to accusations that the equipment sent to our troops overseas isn't sufficient. Many military outfits up-armor their equipment, installing armor plating directly to the vehicle once they receive it. Adding armor to a Hummer increases its weight, which in turn can affect its performance. In some cases, a Hummer may be better armored but less maneuverable, making it a more likely target for enemy fire. Because of these limitations, critics say the Army should focus on other vehicle options like the Stryker vehicle.

On the civilian Hummer front, there are other issues. Drivers who enjoy challenging their skills and pushing a vehicle to its limits in off-road courses tend to love Hummer vehicles. A Hummer can scale hills that are insurmountable to most other cars. They can go up steep slopes, drive over objects that would obstruct other vehicles and even plow through up to 30 inches of water without a problem.

While Hummers might traverse a difficult obstacle course with relative ease, they face a much more daunting roadblock when it comes to environmentalists and other critics. The Hummers' infamous fuel efficiency -- or lack thereof -- is enough to turn a green-minded person bright red. In California, a state where environmental concerns are prominent in the minds of its citizens, Gov. Schwarzenegger faced criticism for his unabashed love of the vehicles. His involvement during the publicity tour of the H2H hybrid vehicle did little to calm his critics.

David McNew/Getty Images California governor and Humvee fan Arnold Schwarzenegger fuels up a hydrogen-powered Hummer.

Others see the Hummer as a symbol of American excess and gluttony. A few go so far as to place the blame of the country's foreign policy on the owners of Hummers and other SUVs, claiming that we're involved in conflicts in the Middle East due to our enormous rate of oil consumption. Some people just feel the cars are large, ugly, unwieldy and scary to encounter on the road. A few people even take action: Hundreds of people protested at Hummer dealerships on November 15, 2003 -- a day activists called "National Protest Day Against Hummers" in a fit of creative genius [source: CommonDreams.org].

It seems that growing numbers of Americans are trying to be environmentally conscious. In that environment, the Hummer line of vehicles faces its toughest road yet -- there may come a day when Hummer owners face more than just the scorn of critics. It's ironic that the same vehicle that was a symbol of American heroism, determination and unity in purpose in 1991 has become a divisive symbol both at home and overseas today.

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