Have you ever opened the hood of your car and wondered what was going on in there? Car engines can look like a big confusing jumble of metal, tubes and wires to the uninitiated. You might want to know what's going on in there simply out of curiousity. After all, you ride in your car every day - wouldn't it be nice to know how it works? Or maybe you are tired of going to the mechanic and having him say things that are totally meaningless to you and then charging you $750 for it. Or perhaps you are buying a new car, and you hear funny words like "3.0 liter V6" and "dual overhead cams" and "tuned port fuel injection". What does all of that mean?

If you have ever wondered about this kind of stuff, then How Stuff Works is glad to help out with this complete guide to how gasoline engines work. Enjoy!

The Beginning
A car is one of the most complicated objects that a person sees during a normal day. Cars have thousands of parts, all of them functioning reliably together day in and day out. However, the basic principles behind all car engines are very simple and once you understand them many different things about cars make a lot more sense.

So let's start at the very beginning: Why do you have a car? Answer: To move your body and your stuff from one place to another. That is what a car does. If you could step into a phone booth, dial a number and somehow instantly transmit your body to another place (like they do in Star Trek with the transporter room) would you have a car? No way. That's why no one on Star Trek has a car. [Of course it forces you to ask why Captain Kirk needs a ship - why didn't everyone just stay on planet Earth and beam themselves to all these different places instantaneously rather than messing with the Starship Enterprise? Maybe transporter beams can only travel so far...]

So you have a car to get around until someone invents the transporter room. The next question is: Why is nearly every car, motorcycle, moped and even lawn mower powered by gasoline? Because gasoline has an extremely high energy density, because it is cheap (relative to the alternatives) and because it is easy and relatively safe to move around. For comparison, it takes about 1,000 pounds of lead-acid batteries to store the same amount of energy that one gallon (7 pounds) of gas contains. It will take you several hours to recharge the batteries but it takes about 15 seconds to pump one gallon of gas. That is why you do not see very many electric cars right now - Gas is a lot easier.

The purpose of a gasoline car engine, therefore, is to convert gasoline into motion so that the 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 generally referred to as an internal combustion engine. Two things to note:

• There is such a thing as an external combustion engine. Steam engines in old-fashioned trains and steam boats are the best examples of external combustion engines. 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. It turns out internal combustion is a lot more efficient (takes less fuel per mile) than external combustion, plus internal combustion engines are a lot smaller than an equivilent external combustion engine. This explains why we don't see any cars from Ford and GM using steam engines.
• There are also different kinds of internal combustion engines. The gas turbine engine is another form of internal combustion engine. Gas turbine engines have interesting advantages and disadvantages, but their main disadvantage right now is an extremely high manufacturing cost (meaning they cost more than piston engines used in cars today). Click here for more information on gas turbines.

To understand a car engine you have to start by understanding how internal combusion in a piston engine works.

Internal Combustion
To understand the basic idea behind how a reciprocating internal combusion engine works, it is helpful to have a good mental image of how "internal combustion" works. One good example is an old Revolutionary War cannon. You have probably seen these in movies, where the soldiers load the cannon with gunpower and a cannon ball and light it. That is internal combustion, but it is hard to imagine that having anything to do with engines. A more relevant example might be this. Say that you took a big piece of plastic sewer pipe, maybe 3 inches in diameter and 3 feet long, and you put a cap on one end of it. Then say that you sprayed a little WD-40 into the pipe, or put in a tiny drop of gasoline. Then say that you stuffed a potato down the pipe. Like this:

I am not recommending that you do this! But say you did... What we have here is a device commonly known as a potato cannon. When you introduce a spark, you can ignite the fuel. What is interesting, and the reason we are talking about such a device, is that a potato cannon can launch a potato about 500 feet through the air!

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 pedestrian purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and then 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 the accompanying figure. They are:

1. Intake
2. Compression
3. Combustion
4. Exhaust

Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear. In an engine the linear motion is converted into rotational motion by the crank shaft. The rotational motion is nice because we plan to turn (rotate) the car's wheels with it anyway.

It's pretty simple actually. Now let's look at all the parts that work together to make this happen.

Parts of an Engine
The figure at the left identifies all of the different parts in a simple 4-cycle engine. Here's a quick description of each one, along with a lot of vocabulary that will help you understand what all the car ads are talking about.

• Cylinder - The core of the engine is the cylinder. The piston moves up and down inside the cylinder. The engine described here has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (4, 6 and 8 cylinders are common). In multi-cylinder engines the cylinders usually are arranged in one of three ways: inline, V or horizontally opposed, as shown here:
  
  Different configurations have different smoothness, manufacturing-cost and shape characteristics that make them more suitable in some vehicles.
  
• 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: 1) They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion, and 2) 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.
• Combustion chamber - The combustion chamber is the area where compression and combustion take place. As the piston moves up and down, you can see that the size of the combustion chamber changes. It has some maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement, and is measured in liters or CCs (Cubic Centimeters, where 1,000 cubic centimeters equals a liter). So if you have a four cylinder engine and each cylinder displaces half a liter, then the entire engine is a "2.0 liter engine". If each cylinder displaces half a liter and there are 6 cylinders arranged in a V configuration you have a "3.0 liter V6". Generally the displacement tells you something about how much power an engine has. A cylinder that displaces half a liter can hold twice as much fuel/air mixture as a cylinder that displaces a quarter of a liter, and therefore you would expect about twice as much power from the larger cylinder (if everything else is equal). So a 2.0 liter engine is roughly half as powerful as a 4.0 liter engine. You can get more displacement either by increasing the number of cylinders or by making the combustion chambers of all the cylinders bigger (or both).
• 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.
• Crank Shaft - The crank shaft 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).

What Can Go Wrong
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. There are three fundamental things that can keep an engine from running: a bad fuel mix, lack of compression or lack of spark. Beyond that there are thousands of minor things that can have problems, but these are the "big three". Based on the simple engine we have been discussing, here is a quick run-down on how these problems affect your engine:

• Bad Fuel Mix - A bad fuel mix can occur in several ways. Perhaps you are out of gas, so the engine is getting air but no fuel. Or the air intake might be clogged so there is fuel but not enough air. Or the fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly. Or 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, then the combustion process will not work like it should. Lack of compression might occur if 1) your piston rings are worn (allowing air/fuel to leak past the piston during compression), or 2) if the intake or exhaust valves are not sealing properly, again allowing a leak during compression, or 3) if 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) (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 then small holes develop between the cylinder and the cylinder head and these holes cause leaks.
• Lack of spark - The spark might be non-existant or weak for a number of reasons. If your spark plug or the wire leading to it is worn out then 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) then the fuel will not ignite at the right time and this can cause all sorts of problems.

In a properly running engine all of these factors are within tolerance.

Engine Subsystems
As you can see in the above descriptions, an engine has a number of systems that help it do its job of converting fuel into motion. Most of these subsystems can be implemented using different technologies, and better technologies can improve the performance of the engine. Here's a look at all of the different subsystems used in modern engines:

• Lubrication system - The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main things needing oil are the pistons (so they can slide easily in their cylinders) and any bearings that allow things like the crankshaft and cam shafts 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.
• Fuel system - The 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. There are three common ways to deliver the fuel: 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 fuel injected engines the right amount of fuel is injected individually into each cylinder either right above the intake valve (port injection) or directly into the cylinder (direct injection).
• Exhaust system - 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.
• 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 cam shaft. The cam shaft has lobes on it that move the valves up and down, as shown in the figure.
  Most modern engines have what are called overhead cams. This means that the cam shaft is located above the valves (as shown in the figure). 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 actuation of the valve and the valve's subsequent motion. A timing belt or timing chain links the crankshaft to the cam shaft so that the valves are in sync with the pistons. The cam shaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have 4 valves per cylinder (two for intake, two for exhaust) and this arrangement requires two cam shafts per bank of cylinders, hence the phrase "dual overhead cams".
• Ignition system - The ignition system 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 4, 6 or 8 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.
• Cooling system - 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 instead air-cooled (you can tell an air-cooled engine by the fins adorning the outside of each cylinder to help disipate heat). Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance.
• Air intake system - Most cars are normally aspirated, meaning that air flows through an air filter and directly into the cylinders. High-performance engines are either turbo charged or super charged, meaning 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. Turbo chargers use a small turbine attached to the exhaust pipe to spin a compressing turbine in the incoming air stream. Super chargers are attached directly to the engine to spin the compressor.
• Starting system - 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: 1) all of the internal friction caused by the piston rings, 2) the compression pressure of any cylinder(s) that happen to be in the compression stroke, 3) the energy needed to open and close valves with the cam shaft, and 4) 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 start 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.
• Emission control system - The emission control system in modern cars consists of a catalytic convertor, a collection of sensors and actuators, and a computer to monitor and adjust everything. For example, the catalytic convertor 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.
• Electrical system - 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 (ignition system, radio, headlights, windshield wipers, power windows and seats, computers, etc.) through the vehicle's wiring.

How to Help an Engine Produce More Power

Horsepower For a very interesting and complete explanation of what horsepower is and what horsepower means, please see How Horsepower Works!

Using all of this information you can begin to see that there are lots of different ways to make engines perform better. Car manufacturers are constantly playing with all of the following variables to make engines 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. 12 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 (prior to the spark plug igniting 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). Turbo chargers and super chargers pressurize the incoming air to effectively cram more air into a cylinder.
• Cool the incoming air - Compressing air raises its temperature. 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 turbo charged and super charged 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 2 intake and 2 exhaust values has 4 valves per cylinder, which improves performance - when you hear a car ad tell you the car has 4 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 then 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.

Q and A
Here's a set of questions from readers:

• 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 causes the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so diesel engines get better milage.
• What is the difference between a two-stroke and a four-stroke engine? Most chain saws and boat motors use two-stroke engines. In a two-stroke engine there are 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 two-stroke engines produce a lot of power for their size because there are twice as many combustion cycles occuring per rotation. However, two-stroke engines use more gasoline and burn lots of oil, so they are far more polluting.
• 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, steam engines can use coal, newspaper or wood for the fuel, while an internal combustion engine needs pure, high-quality liquid or gaseous fuel.
• Are there any other cycles besides the Otto cycle used in car engines? The 2-stroke engine cycle is different, as is the diesel cycle described above. There is also the Wankle cycle used in rotary engines (Mazda once used Wankle rotary engines extensively but no longer does - rotary engines are light for the power they produce but use a lot of gas and do not last as long). The engine in the Mazda Millenia uses a modification of the Otto cycle called the Miller cycle. Gas turbine engines use the Brayton cycle.
• Why have 8 cylinders in an engine? Why not have one big cylinder of the same displacement of the 8 cylinders instead? There are a couple of reasons why a big 4.0 liter engine has 8 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 8 evenly-spaced explosions instead of one big explosion. Another reason is starting torque. When you start a V-8 engine you are only driving 2 cylinders (1 liter) through their compression strokes, while with one big cylinder you would have to compress 4 liters instead.

Books!

Great Books about Engines and Racing - Learn more about the engines and racing with this great list of books!

Links
Here are links to several interesting sites related to engines:

• How Gas Turbine Engines Work - Gas turbines are another form of internal combustion engine that you may want to learn about.
• How Diesel Engines Work
• How Diesel 2-stroke Engines Work
• How Two-Stroke Engines Work
• How Manual Transmissions Work
• Inner Auto - A great set of figures and animations showing different engine types and the parts inside an engine.
• Under the hood
• How Things Work: Automobiles - Questions and answers about cars
• Erbman's Engine Emporium - a great article describing factors affecting engine performance
• The Power to Please - great article on how different changes (cam shaft, heads, intake manifold, etc.) can affect performance.
• Analysis and Optimization of Pontiac's Ram Air System
• Engine Science
• Yahoo Engine links
• Antique Gas Engine Homepage - Hundreds of interesting photos.
• Harry's Old Engine Homepage - Good list of shows.
• Manufacturers of Performance Parts - A detailed list of manufacturers of parts for improving your car's performance.
• Understanding Your Car
• Reference page to The Cooling System - a lot of detail on your engine's cooling system.