PREDICTION OF THE WALL FILM FORMATION AND PERFORMANCE OF AN ENGINE OPERATED WITH THE ETHANOL BLEND E85

摘要

Efforts will have to be made to reduce the CO_2-emission of future vehicles. For example, arnfleet emission limit of 120 g/km will be introduced in Europe by 2012. In addition a steadilyrngrowing number of vehicles faces limited oil resources. Therefore, low carbon fuels and fuelsrnfrom biomass with an improved CO_2-balance like ethanol are considered as an importantrnalternative to conventional fuels for SI engines.rnHowever, ethanol has a different stoichiometric air/fuel-ratio, a lower heating value and arnhigher heat of vaporization than conventional fuels. This has a high impact on the enginernprocess. Therefore, detailed investigations on the engine process are necessary to predict therndifferences of the gas exchange, the engine efficiency and the full load performance whenrnswitching to ethanol.rnA turbocharged SI engine with port fuel injection was investigated at the Institute forrnPowertrains and Automotive Technology of the Vienna University of Technology in order tornshow the potentials when being operated with the ethanol blend E85. Numerical studies withrnthe code GT-Power and measurements at the engine test bench have been carried out tornexplain the mixture preparation with conventional fuel and E85.rnIt could be shown with the help of detailed CFD-simulations that an extensive wall filmrnformation takes place in the inlet port during the fuel injection. Therefore, the heat ofrnvaporization is rather taken from the material than from the aspirated air. This effect decreasesrnthe evaporative cooling of the mixture and, as a result, a higher boost pressure than predictedrnby simulation is required to reach a defined torque. There is a significantly higher impactrnobservable for E85 compared to conventional fuel because of its higher heat of vaporizationrnand lower stoichiometric air/fuel-ratio. Therefore, the mixture formation for E85 must bernconsidered in more detail when analysing the engine process. In the following, a predictivernmodel of the wall film formation in the inlet ports and thermal models of the port walls werernused for the numerical studies.rnWith the improved model the gas exchange and necessary boost pressure were in goodrnagreement with measurements from the engine test bench for both fuels. The investigationsrnhave shown that particularly for alternative fuels with fluid properties that differ from those ofrnconventional fuel the standard simulation models do not remain valid and an analysis of thernwhole process, employing measurements and numerical methods, is necessary.