Experimental Evaluation of Direct and Port Fuel Injection Strategies on a Heavy-Duty Liquefied Petroleum Gas Engine

<div>Liquefied petroleum gas (LPG) is a popular alternative fuel in the transportation sector as a result of its favorable physical and chemical properties, availability, and relatively lower emissions compared to conventional fuels. However, much of its use is currently in light-duty applications, usually in manifold or port-injected configurations primarily due to their simplicity and ease of conversion. However, there are shortfalls in heavy-duty applications where decarbonization efforts are direly needed. The key reasons for this shortfall in alternative fuel adoption in the heavy-duty sector are the deficit in engine performance when compared to conventional heavy-duty diesel engines and the lack of specialized hardware to bridge this performance gap, for example, direct injectors optimized for LPG fuel operation on large-bore engines. To address this, this study evaluated the performance, emissions, and combustion characteristics of a heavy-duty single-cylinder research engine, the Cummins ISX15L, in direct injection (DI) mode with an injector designed for liquid LPG and in a baseline port fuel injection (PFI) mode using an off-the-shelf injector currently in use on commercially available LPG engines. The engine had a compression ratio of 9.3 and a fuel delivery system designed to supply LPG at 1.6 MPa and 17.2 MPa in PFI and DI modes, respectively. The influence of both injection strategies at different start of injection (SOI) timings, equivalence ratios, combustion phasings, and engine load conditions were then investigated. The DI strategy was responsible for the highest brake thermal efficiency (BTE) recorded on the engine, 36.9%, 7% higher than the BTE in PFI mode at the same lean engine condition. The DI configuration achieved a 39% reduction in bsNOx but increased bsCO emissions by 22% compared to PFI at stoichiometric conditions. The PFI strategy demonstrated an insensitivity to the SOI timing unlike the DI strategy, which was highly unstable at retarded SOI timings.</div>