Reported in the current paper is a study into the cycle efficiency effects of utilising a complex valvetrain mechanism in order to generate variable in-cylinder charge motion and therefore alter the dilution tolerance of a Direct Injection Spark Ignition (DISI) engine.
A Jaguar Land Rover Single Cylinder Research Engine (SCRE) was operated at a number of engine speeds and loads with the dilution fraction varied accordingly (excess air (lean), external Exhaust Gas Residuals (EGR) or some combination of both). For each engine speed, load and dilution fraction, the engine was operated with either both intake valves fully open - Dual Valve Actuation (DVA) - or one valve completely closed - Single Valve Actuation (SVA) mode.
The engine was operated in DVA and SVA modes with EGR fractions up to 20% with the excess air dilution (Lambda) increased (to approximately 1.8) until combustion stability was duly compromised. At 1500 Revolutions Per Minute (RPM), 3.6 bar and 7.9 bar Gross Mean effective Pressure (GMEP), the dilution tolerance of the engine was significantly increased for a given combustion stability limit utilising SVA. This resulted in fuel consumption reductions of up to 3.8% and 3.1% respectively for these two engine speed and load conditions as a result of being able to operate the engine with more thermodynamically attractive mixtures when adopting SVA. At 2000RPM, 9.8 bar GMEP, the dilution tolerance was only marginally increased which resulted in a fuel consumption reduction of 1.3% when adopting SVA over DVA (for the same reasons outlined above).
Increased dilution tolerance in all cases was achieved as a result of significant enhancement in charge motion when adopting SVA. By enhancing the in-cylinder charge motion (confirmed using Computational Fluid Dynamics (CFD)), ignition to 10% Mass Fraction Burned (MFB) and 10-90% MFB durations for equivalent levels of dilution were significantly shorter when adopting SVA. This therefore allowed greater dilution tolerance (and ultimately an increase in the thermal efficiency of the working cycle) when adopting SVA over DVA without detrimental increases in the burn duration metrics that would ordinarily result in misfire and partial burn and a significant detriment to combustion stability.
Conversely, for equivalent levels of dilution, there was little, if any difference in fuel consumption between DVA and SVA even though burn duration metrics were significantly shorter when adopting SVA over DVA. In combination with CFD, the polytropic coefficient of compression was calculated to be lower in all cases for SVA compared to DVA for a given level of dilution. This indicated greater heat transfer when adopting SVA over DVA for equivalent trapped mass (confirmed using CFD). As such, this detrimental increased heat transfer (again confirmed with CFD) attributed to the increased in-cylinder activity with SVA offset the favourably faster combustion; thus resulting in little, if any reduction in fuel consumption for equivalent levels of dilution when implementing SVA over DVA. This was particularly pertinent at the higher engine speed and load where the significantly increased heat transfer for SVA resulted in an increase in fuel consumption for SVA over DVA for equivalent levels of dilution.