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Exhaust 101

Basics of Combustion

BRIEF HISTORY OF INTERNAL COMBUSTION ENGINES

In 1885 Gottlieb Daimler constructed what is generally recognized as the prototype of the modern gas engine: small and fast, with a vertical cylinder, it used gasoline injected through a carburetor. In 1889 Daimler introduced a four-stroke engine with mushroom-shaped valves and two cylinders arranged in a V, having a much higher power-to-weight ratio; all modern gasoline engines are descended from Daimler's engines.

HOW THE FOUR STROKE ENGINE WORKS

To understand the basic idea behind how a reciprocating internal combustion engine works, it is helpful to have a good mental image of how "internal combustion" works. 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. If you can create a cycle that allows you to set off explosions like this several hundred times per minute, and if you harnessed that energy in a useful way, you would have the core of a car engine!

Here's what happens as a car engine goes through its cycle:

  1. Intake Stroke
    The piston starts at the top, the intake port opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.

  2. Compression Stroke
    Then the piston moves back up to compress this fuel/air mixture. The compression stroke makes the explosion more powerful.

  3. Power Stroke
    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.

  4. Exhaust Stroke
    Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.

Now the engine is ready for the next cycle, so it intakes another charge of air and gas.

Almost all cars today use this type of reciprocating internal combustion engine because this engine is:

  • Efficient compared to an external combustion engine.
  • Relatively inexpensive compared to a gas turbine.
  • Easy to refuel compared to an electric car.

These advantages beat any other existing technology for moving a car around.

exh101_intakeStroke.gif exh101_compressionStroke.gif exh101_powerStroke.gif exh101_exhaustStroke.gif

HORSEPOWER

The engineer James Watt invented the term horsepower. Watt lived from 1736 to 1819 and is most famous for his work on improving the performance of steam engines.

The story goes that Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. In Watt's judgment, one horse can do 33,000 foot-pounds of work every minute. So, imagine a horse raising coal out of a coal mine. A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute. You can make up whatever combination of feet and pounds you like -- as long as the product is 33,000 in one minute and you have a horsepower.

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.

  1. 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.
  2. 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 gasoline prevents 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.
  3. Let more air in - If you can get more air and therefore fuel into a cylinder of a given size, you can get more power from the cylinder. Turbochargers and superchargers pressurize the incoming air to effectively get more air into a cylinder.
  4. 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.
  5. 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 airflow.
  6. Let exhaust exit more easily - Flow resistance makes it hard for exhaust to exit a cylinder. This resistance robs the engine of power. Flow 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 backpressure. Backpressure is any resistance to free flow in an exhaust system. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate backpressure 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.
  7. 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.
  8. Inject the fuel - Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy.