In recent years gasoline compression ignition (GCI) has been shown to offer an attractive combination of low criteria pollutants and high efficiency. However, enabling GCI across the full engine load map poses several challenges. At high load, the promotion of partial premixing of air and fuel is challenging due to the diminished ignition-delay characteristics at high temperatures, while under low load operations, maintaining combustion robustness is problematic due to the low reactivity of gasoline. Variable valve actuation (VVA) offers a means of addressing these challenges by providing flexibility in effective compression ratio.
In this paper, the effects of VVA were studied at high loads in a prototype heavy-duty GCI engine using a gasoline research octane number (RON) 93 at a geometric compression ratio (CR) of 15.7. Both late intake valve closing (LIVC) and early intake valve closing (EIVC) strategies were analyzed as a measure to reduce the effective compression ratio. For the analysis, a close-coupled GT-Power based 1-D engine simulation with a 3-D CFD modeling methodology was used. The analysis was performed on the B-speed engine load conditions B25, B50 and B75, derived from the heavy-duty Supplemental Emissions Test (SET) cycle.
EIVC was shown to have a greater impact on reducing effective compression ratio than LIVC, but at the expense of higher gas exchange, or pumping, losses. With increasing load, the effect of the LIVC strategy on compression ratio gradually diminished, because the reverse flow through the intake valve was reduced. Overall, the charge temperature and pressure were reduced linearly with EIVC, whereas retarding LIVC showed an exponential rise in temperature and pressure.
The simulated combined efficiency for the stock turbocharger deteriorated with both the EIVC and LIVC approach.