Abstract
AThe reduction of fuel consumption is a fundamental aspect of the automotive industry. This comes from customers, as well as from legal demands. While diesel engine made enormous progress during the last period and features good fuel economy, the spark ignition engine still suffers from this point of view. Variable valve actuation (VVA) offers many opportunities to improve the spark ignition engine’s performances in areas like fuel economy, emissions and it seems it will become the next industry standard on gasoline engines.
This paper presents a variable intake valve lift (ViVL) mechanism, used to enhance fuel economy. Two operational, in-line, 4 cylinders engines prototypes are working on the test benches: one is a side mounted camshaft and overhead valves (OHV) version (i.e. a pushrod engine), still being built in some countries and the other is an overhead camshaft (OHC) version. Experiments that proved also their ability for the unthrottled operation have been conducted on the engine test bench.
Our previous experimental studies revealed an improvement of fuel economy at idle operation of about 20% in spite of an increased pumping work. This gain in fuel economy was mainly generated by an increased flow velocity of the fresh mixture into the cylinders, causing an improvement of fuel-air mixing process and, in the end, a better combustion.
One idea drawn from these previous studies is that even though minimal pumping losses should be one goal to fulfil, this must not be done with impairing of other aspects such as mixing process, charge kinetic energy prior to spark.
Thus, in order to have intimate details about the phenomenon happening during the intake stroke within our prototype ViVL engine, a CFD study on the airflow was launched using the numerical code FLUENT and choosing the k-ω turbulence model.
The study was performed to simulate the airflow at an engine speed of 800 rpm, corresponding to the idle operation. Actually, for the case of still having the throttle plate, the purpose was to obtain results about air velocity, turbulence at the valve gap, for different openings of throttle plate and different valve lifts between 1 and 8.5 mm. In order to set the boundary conditions, data were taken from experiments performed in a steady-state flow within the ViVL engine fitted with in-cylinder and intake manifold pressure sensors.
KEYWORDS: fuel economy, variable intake valve lift (ViVL), CFD simulation, intake flow velocity