Abstract: Test bench has high value applications, which can evaluate performance of catalytic converter quickly and effectively. During bench test process, gasoline engine cycle variation leads to speed fluctuations that will reduce control precision of A/F and the credibility of bench test. Twostage injections is adopted to reduce cycle variation. The first injection is before intake stroke , which forms homogeneous gas mixture.The other fuel is injected in intake stroke, which will enhance gas mixture concentration around spark plug during ignition. The impact of twostage injections on cycle variation is studied by comparing the speed fluctuations with different injection timing and injection proportion. The A/F characteristic of the catalytic converter is evaluated by adopting twostage injections. Result shows that speed fluctuation can be reduced by optimizing the injection parameter, and A/F control accuracy can also be enhanced.

Key words: bench test, speed fluctuation, twostage injection , catalytic converter