Analysis On Combustion And Emission Behavior Of GDI Engine Direct-Start Process

Faced with the increasing serious issues of energy and environment, the recent fuelconsumption regulations and emission regulations become more stringent and comprehensive to the internal combustion engines. The idling start-stop technology could sharply cut down the idling condition in urban driving cycle, which could help reducing fuel consumption and exhaust emission. The direct start mode used on GDI engines could start the engine without starter, and it’s becoming popular because of its low cost, high start speed and NVH performance.This paper has built a Direct Start Process Transient Observe and Control Platform on a Mitsubishi 4G15 GDI engine, and studied the influence of control parameters(excess air ratio, injection timing, ignition timing), state parameters(initial piston position, coolant temperature, injection pressure) and shut-down strategy(shut-down speed, throttle open percentage) to the rotation, combustion and emission performance during forward direct start process. The results show that:1. The most important control parameter of direct start process is the excess air ratio, the mixture for the first two cycles should be richer than the theoretical power mixture to achieve higher start speed; as well, the injection and ignition parameter should be chose basing on the benefit of mixture formation; the separate injection strategy could help improving rotation and combustion performance of first cycle under same conditions.2. The initial piston position range which could ensure the start reliability is limited, the initial piston position should be chose within this range to balance the mixture amount between the first two cycles; higher rotation speed and in-cylinder pressure could be reached under lower coolant temperature, but higher coolant temperature could extend the lean burn limit, which has positive significance for improving emission performance; appropriately increasing the injection pressure helps improving mixture quality, but too high injection pressure will lead to fuel spray impingement problem.3. Increasing shut down speed and throttle open percentage during shut down process could both affect the piston stop position and improve in-cylinder scavenging quality, however, increasing throttle open percentage has an obvious advantage on sweeping residual gas, at the same time the probability that the piston in expansion cylinder stops at the middle of the expansion stroke is significantly increased.