Calibration experiments on engine emission behaviour during the stopping-and-restarting process in a hybrid electric vehicle application

Abstract

A transient emission test bench was designed for simulating the high-speed cranking start process of a port-fuel-injected gasoline engine in a hybrid electric vehicle (HEV) application. The mechanisms of the emissions of both hydrocarbns (HCs) and nitrogen oxides during the stopping-and-restarting process were investigated. The cranking speed, the transient exhaust air-to-fuel equivalence ratio λ, the ignition timing, the idle speed, and the engine coolant temperature (ECT) during hot idle were calibrated. The experimental results showed that an extremely high cranking speed (greater than 1500 r/min) would sharply increase the coefficient of residual gas in the cylinder and lead to incomplete combustion or misfiring. The λ value should be leaner than the theoretical λ value during the cranking period to reduce the HC emission. Retarding the ignition and increasing the idle speed could shorten the light-off time of the three-way catalytic converter (TWC). Engine-off in the idling strategy depended on the ECT. Heat preservation of the TWC (above 300 °C) was very important and was highly effective in increasing the conversion efficiency of the TWC so as to reduce the emissions. After calibration, the HEV emission behaviour in the New European Driving Cycle in a drum test was greatly improved.