Researchers at MIT are designing The Incredible Shrinking Engine:
Overall, higher compression will lead to a more efficient engine and more power per stroke. But increasing the pressure too much causes the fuel to heat up and explode independently of the spark, leading to poorly timed ignition. That’s knock, and it can damage the engine.To avoid knock, engine designers must limit the extent to which the piston compresses the fuel and air in the cylinder. They also have to limit the use of turbo?charging, in which an exhaust-driven turbine compresses the air before it enters the combustion chamber, increasing the amount of oxygen in the chamber so that more fuel can be burned per stroke. Turning on a car’s turbocharger will provide an added boost when the car is accelerating or climbing hills. But too much turbocharging, like too much compression, leads to knock.
An alternative way to prevent knock is to use a fuel other than gasoline; although gasoline packs a large amount of energy into a small volume, other fuels, such as ethanol, resist knock far better. But a vehicle using ethanol gets fewer miles per gallon than one using gasoline, because its fuel has a lower energy density. Cohn and his colleagues say they’ve found a way to use both fuels that takes advantage of each one’s strengths while avoiding its weaknesses.
The MIT researchers focused on a key property of ethanol: when it vaporizes, it has a pronounced cooling effect, much like rubbing alcohol evaporating from skin. Increased turbocharging and cylinder compression raise the temperature in the cylinder, which is why they lead to knock. But Cohn and his colleagues found that if ethanol is introduced into the combustion chamber at just the right moment through the relatively new technology of direct injection, it keeps the temperature down, preventing spontaneous combustion. Similar approaches, some of which used water to cool the cylinder, had been tried before. But the combination of direct injection and ethanol, Cohn says, had much more dramatic results.
The researchers devised a system in which gasoline would be injected into the combustion chamber by conventional means. Ethanol would be stored in its own tank or compartment and would be introduced by a separate direct-injection system. The ethanol would have to be replenished only once every few months, roughly as often as the oil is changed. A vehicle that used this approach would operate around 25 percent more efficiently than a vehicle with a conventional engine.