Abstract
Keywords - Spark ignition engine, Lean burn, Knocking, Supercharging, Numerical analysis
Abstract - Improvement of fuel economy and specific torque of a gasoline engine up to those of recent turbo-charged DI diesel engines is a challenging task. In order to achieve a high compression ratio under the high load, knock suppression is the biggest issue. For this challenge, a boosted homogeneous lean combustion concept has been proposed. It aims to reduce octane requirement by lean combustion and simultaneously to achieve a high engine output by increased air flow. According to this concept, significant improvement of knock limit is obtained if a charging system providing sufficient air flow is available. However, both the elevated pressure (gas density) and the temperature through the increased specific heat ratio promote auto-ignition of the unburned mixture, so that there would be a limitation of lean boost combustion concept. Thus, in this paper, the potential of lean boost combustion was experimentally and theoretically investigated in terms of the knock suppression.
In the experiment, the effect of equivalence ratio of homogeneous mixture on knock was studied. The result showed that the lean combustion with increasing air flow did not suppress knock and increased the time loss compared with stoichiometry at same fuel flow. The reason was investigated by a numerical analysis using an in-house developed detailed chemical kinetic model combined with the Livengood-Wu integral. This method could save the time-consuming of predicting knock and good validation was achieved for various engine operating conditions. The theoretical work showed that lean mixture did not always suppress knock because knock tendency depends not only on the equivalence ratio but also on the pressure and temperature history of the unburned mixture. In case of lean boost combustion, increased in-cylinder pressure promotes the auto-ignition rather than suppresses the chemical reaction by the dilution. Therefore, significant reduction of octane requirement at high load condition can not be achieved only by boosting lean mixture. Within a limited output range, the gain of increased specific heat ratio overcomes the time loss with ignition timing retard, so that the lean boost combustion can improve the fuel economy. In order to expand the advantage into the higher output, the concept requires some extra solutions for knock suppression.