Effect of fuel properties on the first cycle fuel delivery in a Port Fuel Injected Spark Ignition Engine

Abstract

Achieving robust combustion while also yielding low hydrocarbon (HC) emissions is difficult for the first cycle of cranking during the cold start of a Port Fuel Injected (PFI) Spark Ignition (SI) engine. Cold intake port wall and valve temperatures, near-atmospheric manifold pressure, and low port air velocity combine to create an adverse environment for fuel delivery - the process of injecting and vaporizing liquid fuel to create a combustible air-fuel mixture. As a result, only a small fraction of the injected fuel mass contributes to the combustible mixture; the fraction is less than 10% at cold ambient temperatures. With fast light off catalysts, the first cycle produces a significant portion of the total trip emissions. The low fuel delivery fraction results in high residual liquid fuel in both the port and cylinder; this fuel contributes significantly to the exhaust HC emissions. Since the first cycle engine control is open-loop, the Engine Control Unit (ECU) must determine how much fuel to inject under given conditions - temperature, pressure, and for a given fuel. Fuel properties play a significant role in first cycle fuel delivery, since the energy available for vaporization is a limiting factor in fuel delivery. The effect of fuel properties on fuel delivery for the first cycle was quantified at a wide range of cold start temperatures by using a skip-firing strategy to simulate the first cycle of cranking on a production PFI engine. Four fuels between 1083 and 1257 Driveability Index (DI) were tested, and the fuel delivery results have been correlated to properties of the ASTM distillation curve. The fractional distillation point that correlates to fuel delivery is a function of temperature - at colder temperatures, the results

(cont.) correlate with the more volatile end of the distillation curve. Fuel delivery results for the fuels were also simulated with a thermodynamics-based fuel delivery model based on partial equilibrium with the charge air.