Fuel Puddle Model and AFR Compensator for Gasoline-Ethanol Blends in Flex-Fuel Engines

Ethanol is being increasingly used as an alternative fuel to petroleum-based gasoline and diesel derivatives. Currently available flexible fuel vehicles (FFVs) can operate on a blend of gasoline and ethanol in any concentration of up to 85% ethanol (93% in Brazil) with minimum hardware modifications. This flexibility if partly achieved through the closed-loop air-to-fuel ratio (AFR) control which maintains automatically operation around the stoichiometric ratio. Precise transient AFR control depends however on a feedforward compensator that reduces the transient effects of fuel puddle dynamics. An accurate and tunable model of the fuel puddle dynamics for gasoline-ethanol blends is, thus, necessary for the purpose of air-to-fuel ratio control. In this paper, we propose a physics-based fuel puddle model that may be used for control purposes in FFVs. In particular, the gasoline-ethanol blend is modeled using several chemical compounds and is parameterized by ethanol content. The model consists of a droplet evaporation model and a single-puddle vaporization model. The droplet evaporation model is simulated off-line to generate port wall-impacting factors of injected fuel to be used in a single-puddle vaporization model. The single-puddle vaporization model is a cycle-based model that may be simulated on-line to characterize fuel puddle dynamics in port fuel injected engines. To verify the validity of the model, simulation results are compared with limited experimental data. A transient fuel compensator based on the proposed model is also formulated.