The novelty of the idea comes in the form of a separate injection of ethanol to avoid combustion knock. Combustion knock happens when gasoline spontaneously combusts, as opposed to the combustion initiated by the spark plug, and can cause significant engine damage. This spontaneous combustion happens most frequently when the engine is operating at a high output or running at high speeds. Turbo charging and increased compression ratios increase engine efficiency, but also contribute to increased risk of engine knock. In addition, when the engine is downsized substantially, it will be operating at high output and at higher engine speeds more frequently, again increasing the changes of engine knock. To avoid engine knock, the MIT professors propose to inject small quantities of ethanol into the combustion chamber when the engine is operating at conditions that are prone to knocking. An increase in fuel consumption of 30% is predicted. The main problem would be related to logistics, since consumers would have to fill up an ethanol tank, but the interval might be as infrequent as an oil change.
[Source: The Technology Review]