Post-Combustion In-Cylinder Vaporization During Cranking and Startup in a Port-Fuel–Injected Spark Ignition Engine

Abstract

During port-fuel–injected (PFI) spark-ignition (SI) engine startup and warm-up fuel accounting continues to be a challenge. Excess fuel must be injected for a near stoichiometric combustion charge. The “extra” fuel that does not contribute to the combustion process may stay in the intake port or as liquid films on the combustion chamber walls. Some of this combustion chamber wall liquid fuel is transported to the engine’s oil sump and some of this liquid fuel escapes combustion and evolves during the expansion and exhaust strokes. Experiments were performed to investigate and quantify this emerging in-cylinder fuel vapor post-combustion cycle by cycle during engine startup. It is believed that this fuel vapor is evaporating from cylinder surfaces and emerging from cylinder crevices. A fast in-cylinder diagnostic, the fast flame ionization detector, was used to measure this behavior. Substantial post-combustion fuel vapor was measured during engine startup. The amount of post-combustion fuel vapor that develops relative to the in-cylinder precombustion fuel charge is on the order of one for cold starting (0 °C) and decreases to ∼13 for hot starting engine cycles. Fuel accounting suggests that the intake port puddle forms quickly, over the first few engine cranking cycles. Analysis suggests that sufficient charge temperature and crevice oxygen exists to at least partially oxidize the majority of this post-combustion fuel vapor such that engine out hydrocarbons are not excessive.