Fuel injection rate shaping and its effect on exhaust emissions in a direct-injection diesel engine using a spool acceleration type injection system

摘要

In diesel engines, pilot injections and injections at a reduced initial injection rate with high-pressure fuel injection have a potential to reduce particulate, NO_x and noise emissions simultaneously. For this reason, various shaping methods of injection rate waveform have been proposed. The present authors also propose such a high-pressure injection system with variable injection rate that relies on spool acceleration and oil-hammering in the injection pipeline. This paper first describes the injection rate shaping, including injections with pilot and reduced initial injection rate, and elucidates their effects on reducing exhaust and noise emissions. A pilot injection can be achieved by either installing a fuel spill path in a plunger body or elongating prelift of the spool. Computer simulations and bench tests of such injection systems show that pilot injection quantity is small enough and the pilot injection pressure is much lower than that of the main part of the injection. To reveal the effect of fuel injection rate on exhaust and noise emissions, experiments were carried out on a high-speed direct-injection diesel engine having a high-swirl deep-bowl combustion chamber. The results show that reduction of initial injection rate and pilot injection lowers both the exhaust NO_x concentration and the noise emission, and that smoke is significantly reduced by increasing the average injection rate. This characteristic is remarkable at lower engine speeds and scarcely depends on a nozzle orifice diameter.