Another way gasoline engines are improving is by modifying the way fuel reaches the combustion chamber. In a gasoline engine, fuel and air use a spark to ignite in the combustion chamber. But, researchers have found that by heating and pressurizing the fuel before it's injected into the combustion chamber they can ignite the fuel without a spark -- sort of how a diesel engine operates. This process helps it burn cleaner and more efficiently, reducing the amount of fuel the engine needs to operate.
A normal gasoline engine has a compression ratio of about 10 to 1 (or slightly less). However, a new engine from Mazda (one that is currently on sale in Japan) uses this technology and has a compression ratio of 14 to 1. The Mazda Demio gets a reported 70 miles per gallon (29.8 kilometers per liter).
One problem with an increased compression ratio is that the fuel can ignite prematurely, causing what's known as engine knock. To help keep that from happening, direct injection systems spray a fine mist of fuel directly into the cylinder (usually, air and fuel are mixed in the port before entering the system). That helps keep the engine's temperature down and reduces knock. Direct fuel injection can improve engine efficiency by 12 percent [source: U.S. Department of Energy].