Mixture Preparation and Hydrocarbon Emissions Behaviors in the First Cycle of SI Engine Cranking

The mixture preparation and hydrocarbon (HC) emissions behaviors for a single-cylinder port-fuel-injection SI engine were examined in an engine/dynamometer set up that simulated the first cycle of cranking. The engine was motored continuously at a fixed low speed with the ignition on, and fuel was injected every 8 cycles. Unlike the real engine cranking process, the set up provided a well controlled and repeatable environment to study the cranking process. The parameters were the Engine Coolant Temperature (ECT), speed, and the fuel injection pulse width. The in-cylinder and exhaust HC were measured simultaneously with two Fast-response Flame Ionization Detectors. A large amount of injected fuel (an order of magnitude larger than the normal amount that would produce a stoichiometric mixture in a warm-up engine) was required to form a combustible mixture at low temperatures. That was because fuel delivery efficiency (the fraction of the injected fuel that constituted the combustible charge) decreased significantly with temperature, and that this efficiency also decreased with an increase in the injection amount. A thermodynamic model for mixture preparation based on representing the fuel by its major components, and on an equilibrium between the fuel liquid, vapor and part of the charge air was developed. The model agreed well with the observed dependence of the delivery efficiency on temperature and on the amount of fuel injection. The first cycle HC emissions as a function of the in-cylinder fuel equivalence ratio were bracketed by incomplete combustion on both the lean and the rich side. At stoichiometric condition, the HC emissions increased with lower ECT and with lower speed.