Car engine Compression Ratio

Have you noticed the wave of cars with great gas mileage hitting the market? The Mazda3 with the SkyActiv engine can get 42 miles per gallon (17.9 kilometers per liter). The Chevrolet Cruze Eco can get 40 miles per gallon (17 kilometers per liter), and the Hyundai Elantra can too. And get this: Even though these cars get some of the best gas mileage in the industry, they're not using gasoline-electric hybrid technology, alternative fuels or other green tricks. They're powered by the old-fashion internal combustion engine. So what makes their fuel economy so good? Their engines are super-efficient, thanks to their engineers playing with a little thing called compression ratio.

Your basic car engine works by turning chemical energy from a controlled explosion of the mixture of air, gasoline and a spark, into mechanical motion. For a more detailed look at this process, check out how a car engine works. But, the basic story is that each car engine has a set number of cylinders that house pistons. The controlled explosion makes the piston move up and down, which turns the engine's crankshaft (that's the conversion of chemical to mechanical energy), which in turn, powers the wheels.

The compression ratio is the ratio of the volume of the cylinder and the combustion chamber when the piston is at the bottom, and the volume of the combustion chamber when the piston is at the top. Automotive engineers can improve fuel efficiency and fuel economy by designing engines with high compression ratios. The higher the ratio, the more compressed the air in the cylinder is. When the air is compressed, you get a more powerful explosion from the air-fuel mixture, and more of the fuel gets used. Think about it this way: If you had to be near an explosion, you'd probably choose to be near one somewhere outside, because the force of the explosion would dissipate, and it wouldn't seem as powerful. In a small room, however, the force would be contained, making it feel much more powerful. It's the same thing with compression ratios. By keeping the explosion in a smaller space, more of its power can be harnessed. By increasing the compression ration from 8:1 to 9:1, for example, you can improve fuel economy by about 5 to 6 percent.

The type of compression ratio we've just learned about is what's known as a static compression ratio. It's called static because it's only measured when the intake valve is closed. There's another type of compression ratio that takes the opening and closing of the intake valve into account. We'll talk about that on the next page.