The control of an unthrottled homogeneous DISI engine through reduced intake valve lift and duration : a study of the in-cylinder flows and charge formation

摘要

This research investigated a novel combustion system for gasoline direct injection spark ignition (DISI) engines. This combustion system burned an unthrottled, stoichiometric, homogenous charge at part load, in comparison to the unthrottled,lean, stratified charge burned by conventional DISI engines. Unthrottled homogeneous operation, enabled by the use of variable valve timing. allowed high fuel efficiencies to be achieved while addressing the particulate emissions, poor combustion stabilities and NOx after-treatment issues associated with stratified charge DISI engines, when compared to the port fuel injection (PFI) engines they are replacing. Experiments were performed to quantifY the bulk in-cylinder air motions, determine their effect on the fuel spray, and examine the resulting air-fuel mixture preparation of various early inlet valve closing (EIVC) and late inlet valve opening (LIVO) strategies that were suitable for controlling engine load under homogeneous engine conditions. A broad matrix of engine conditions has been investigated, with engine speeds ranging from idle (750 rpm) to 5000 rpm, and engine loads ranging from 2.7 bar indicated mean effective pressure (!MEP) to wide open throttle (WOT). Particle Image Velocimetry (PIV) was used to record mean in-cylinder flow fields in the tumble and swirl planes for a range of engine conditions and valve profiles. This included measurements at higher engine speeds (3500rpm) than previously published. Air flows in the difficult-to-access cylinder head were measured with Laser Doppler Anemometry (LDA) and the effect of these air flows on the fuel spray produced by a latest generation multi-stream fuel injector was investigated with Mie imaging. The resulting mixture preparation was then investigated over a crank angle period ranging from the start ofinjection (SOl) to the time of spark with Laser Induced Exciplex Fluorescence (LIE F). Supporting data from a thermodynamic sister engine with identical combustion chamber geometry was recorded at University College London. Unthrottled, homogeneous operation with low lift EIVC valve profiles improved engine fuel consumption by up to 20% compared to throttled operation with conventional, full-lift profiles. This was a consequence of a reduction in the throttling losses and improvements in air-fuel mixing. The intake air momentum was more significant than the fuel spray momentum from the injection system in determining the air-fuel mixing process. This resulted in engine performance being strongly affected by engine speed, intake valve lift and injection timing. The greatest benefits in ISFC occurred when only one of the two inlet valves was operated. This was attributed to an overall increase in the level ofin-cylinder swirl. However, the choice of which inlet valve was opened was critical, with greater gains occurring if the fuel spray from the centrally mounted injector was directed towards the spark plug than when the spray was directed away from the plug. EIVC combustion also exhibited significantly longer burn times than throttled operation. This was due to lower cylinder pressures that reduced the laminar flame speed and lower levels of turbulence around the spark plug at the time of ignition. Flame front measurements on the optical engine showed that during the longer early heat release phase (0-10% mass fraction burned), the flame kernel was transported away from the spark plug and towards the combustion chamber wall beneath the inlet valves. Investigations into the fuel mixture preparation using Laser Induced Exciplex Fluorescence (LIE F) demonstrated that, under high load conditions, a source of particulate emissions from PFI engines was large droplets in the vicinity of the spark plug around the time of ignition. These fuel rich regions were precursors in the generation of soot and were all but eliminated with direct injection fuelling strategies. Late Intake Valve Opening (LIVO) valve strategies generated a sub-atmospheric cylinder pressure of between 0.5 to 0.3bar (absolute). Spray images obtained under these conditions showed greater penetration of the fuel spray and a poorly defined spray cone boundary. Due to the increased momentum and increased shear forces of the inducted air, and the cylinder pressure falling below the saturation vapour pressure of some components of the gasoline fuel at the temperature of the mixture, flash evaporation of those components was seen to occur. The improvement in atomisation and faster burn rate with LIVO compensated to some extent for the increase in irrecoverable pumping work of this operating strategy over conventional EIVC. However, a practical disadvantage of LIVO was poor control of the trapped air mass, arising from the intake air momentum supercharging the engine cylinder at the conditions tested.