Gasoline compression ignition (GCI) offers improved efficiency by harnessing gasoline’s low reactivity to induce an extended ignition delay that promotes partial premixing of air and fuel before combustion occurs. However, enabling GCI across the full engine operating load map poses several challenges. At high load, due to the elevated pressures and temperatures of the charge mixture, the ignition delay time shrinks, leading to diminished GCI efficiency benefits. At low load, insufficient temperatures and pressures can lead to combustion instability. Variable valve actuation offers a practical solution to these challenges by enabling effective compression ratio (ECR) control.
In this paper, the effects of variable intake valve closings were investigated for high load operations in a prototype heavy-duty GCI engine, using a research octane number 93 gasoline fuel. The study focused on the 50% (B50) and the 75% (B75) load conditions at 1375 RPM. Both late intake valve closing and early intake valve closing strategies were analyzed as a measure to reduce the effective compression ratio. Reducing ECR, enabled by variable intake valve closing, not only provided control over in-cylinder temperature and pressure, but also led to a reduced in-cylinder trapped charge-mass that compromised engine load. This, in turn, led to higher boost pressure requirements.
Subsequently, a detailed air-handling system analysis was conducted to identify a turbocharger capable of delivering the high boost pressure demands for high load operations at reduced ECR. Three turbocharger systems were evaluated: (a) a stock 1-Stage turbocharger, (b) an available production 2-Stage turbocharger and (c) a prototype high-efficiency 1-Stage variable geometry turbocharger. For the analysis, an approach that closely coupled 1-D engine simulations with a 3-D CFD combustion model was used.
At B50 and B75, reducing the ECR to 13 and 12, via variable intake valve closing, resulted in 1% and 1.5% increases in gross indicated efficiency, respectively. As a result, the boost pressure demand rose by approximately 0.47 bar at B50 and 0.65 bar at B75, to compensate the loss in trapped in-cylinder charge mass.
Against the elevated boost pressure demand, the stock turbocharger, due to inadequate combined efficiencies, struggled to deliver the boost pressure targets. The production two-Stage turbocharger system successfully delivered the boost targets, at the expense of relatively high pumping losses. Finally, the prototype 1-Stage variable geometry turbocharger delivered the best combined turbocharger efficiencies at B50 and B75, resulting in the lowest pumping losses and the best brake efficiencies. The combined effects of the prototype turbocharger system, high pressure exhaust gas recirculation and variable valve actuation delivered a viable recipe for high load GCI operation in heavy-duty engines.