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.
