HCCI Load Expansion Opportunities Using a Fully Variable HVA Research Engine to Guide Development of a Production Intent Cam-Based VVA Engine: The Low Load Limit

While the potential emissions and efficiency benefits of HCCIcombustion are well known, realizing the potentials on a productionintent engine presents numerous challenges. In this study we focuson identifying challenges and opportunities associated with aproduction intent cam-based variable valve actuation (VVA) systemon a multi-cylinder engine in comparison to a fully flexible,naturally aspirated, hydraulic valve actuation (HVA) system on asingle-cylinder engine, with both platforms sharing the same GDIfueling system and engine geometry. The multi-cylinder productionintent VVA system uses a 2-step cam technology with wide authoritycam phasing, allowing adjustments to be made to the negative valveoverlap (NVO) duration but not the valve opening durations. On thesingle-cylinder HVA engine, the valve opening duration and lift arevariable in addition to the NVO duration. The content of this paper is limited to the low-medium operatingload region at 2000 rpm. Using different injection strategies,including the NVO pilot injection approach, the single-cylinderengine is operated over a load range from 160-390 kPa net IMEP at2000 rpm. Changes to valve opening duration on the single-cylinderHVA engine illustrate opportunities for load expansion andefficiency improvement at certain conditions. For instance, the lowload limit can be extended on the HVA engine by reducing breathingand operating closer to a stoichiometric air-fuel ratio (AFR) byusing valve deactivation. The naturally aspirated engine used here without external EGRconfirmed that as operating load increases the emissions ofNO_xincreases due to combustion temperature.NO_xemissions are found to be one limitation to themaximum load limitation, the other being high pressure rise rate.It is found that the configuration of the production intentcam-based system represents a good compromise between valve liftand duration in the low to medium load region. Changing the extentof charge motion and breathing via valve deactivation provebeneficial at moderating the pressure rise rate and combustionstability and extending the low load limit at 2000 rpm on the HVAengine. It also confirms that strategies using a pilot fuelinjection are beneficial at low operating loads but that asoperating load is increased, the benefits of multiple injectiondiminish to the point where a single injection offers the bestperformance.