In enhancing the performance of automotive internal combustion engines, increasing the compression ratio offers an effective means of improving engine thermal efficiency.
If the compression ratio is increased, however, the problem of knock occurs in exchange for improvement in engine thermal efficiency. In other words, an increase in compression ratio causes in-cylinder compressive end gas temperature to rise, resulting in the occurrence of knock. This in turn requires ignition timing retard to combat the knock. This trade-off makes it difficult to achieve the theoretical maximum combustion efficiency.
In this paper, we clarify the feasibility of suppressing the occurrence of knock by increasing the burn rate. Specifically, we increase the burn rate by injecting high-pressure air directly into the combustion chamber, causing highly turbulent in-cylinder flow. To optimize the generation of this turbulent flow, we examined the following parameters of “direct high pressure air injection” (DHPAI): injection timing and the arrangement and quantity of injector nozzles. After modifying a commercially available, 2.0-liter 4-cylinder gasoline engine, we made performance comparisons at full load between running with and without DHPAI. The actual ignition timing advance comes close to MBT (minimum advance for best torque) by selectively improving the burn rate in the later stages of the combustion period. As a result, the output torque is increased 10% in the low and medium engine speed conditions.