A CFD STUDY ON THE EFFECTS OF THE VALVE LIFT ON THE INTAKE PROCESS

摘要

The reduction of fuel consumption is a fundamental aspect of the automotive industry. Thisrncomes from customers, as well as from legal demands. While diesel engine made enormousrnprogress during the last period and features good fuel economy, the spark ignition engine stillrnsuffers from this point of view. Variable valve actuation (VVA) offers many opportunities tornimprove the spark ignition engine’s performances in areas like fuel economy, emissions and itrnseems it will become the next industry standard on gasoline engines.rnThis paper presents a variable intake valve lift (ViVL) mechanism, used to enhance fuelrneconomy. Two operational, in-line, 4 cylinders engines prototypes are working on the testrnbenches: one is a side mounted camshaft and overhead valves (OHV) version (i.e. a pushrodrnengine), still being built in some countries and the other is an overhead camshaft (OHC)rnversion. Experiments that proved also their ability for the unthrottled operation have beenrnconducted on the engine test bench.rnOur previous experimental studies revealed an improvement of fuel economy at idle operationrnof about 20% in spite of an increased pumping work. This gain in fuel economy was mainlyrngenerated by an increased flow velocity of the fresh mixture into the cylinders, causing anrnimprovement of fuel-air mixing process and, in the end, a better combustion.rnOne idea drawn from these previous studies is that even though minimal pumping lossesrnshould be one goal to fulfil, this must not be done with impairing of other aspects such asrnmixing process, charge kinetic energy prior to spark.rnThus, in order to have intimate details about the phenomenon happening during the intakernstroke within our prototype ViVL engine, a CFD study on the airflow was launched using thernnumerical code FLUENT and choosing the k-ω turbulence model.rnThe study was performed to simulate the airflow at an engine speed of 800 rpm,rncorresponding to the idle operation. Actually, for the case of still having the throttle plate, thernpurpose was to obtain results about air velocity, turbulence at the valve gap, for differentrnopenings of throttle plate and different valve lifts between 1 and 8.5 mm. In order to set thernboundary conditions, data were taken from experiments performed in a steady-state flowrnwithin the ViVL engine fitted with in-cylinder and intake manifold pressure sensors.