Abstract
Employing a direct-injection diesel engine equipped with a common-rail type of injection system, adding formaldehyde (CH_2O) to the intake air, and changing the fuel-injection timing, the compression ratio and the intake-air temperature, a mechanism for CH_2O as a fuel additive to affect auto-ignition was discussed. Unlike an HCCI type of engine, the diesel engine can expose an air-fuel mixture only to a limited range of the in-cylinder temperature before the ignition, and separate low- and high-temperature parts of the mechanism. When low-temperature oxidation starts above 900 K, there are cases where the CH_2O advances the ignition timing. Below 900 K, to the contrary, it always retards the timing. It is because above 900 K, a part of the CH_2O is changed into CO and H_2O_2 as an ignition promoter. Below 900 K, on the other hand, the CH_2O itself acts as an OH radical scavenger against cool-flame reaction, from the beginning of low-temperature oxidation. Then, the engine was modified for its extraordinary function as a gasoline-knocking generator, in order that effect of CH_2O on knocking could be discussed. The CH_2O retards the onset of auto-ignition of an end gas. Judging from a large degree of the retardation, the ignition is probably triggered below 900 K.
