Decarbonizing the transport sector requires solutions that reduce CO₂ emissions while improving the efficiency of existing engine platforms. This study explores a retrofit strategy in which a heavy-duty diesel engine is converted to Otto-cycle operation and equipped with a passive pre-chamber combustion (PPCC) system. Methanol was used as the fuel due to its high octane number, low carbon intensity, and favourable combustion properties.
The performance of the PPCC system is experimentally compared to conventional spark ignition (SI) across varying engine speeds, loads, and exhaust gas recirculation (EGR) levels. A dual-dilution strategy, combining lean operation (λ = 1.6) with EGR, was applied to extend dilution tolerance and assess the feasibility of operating near stoichiometry. All tests were conducted under steady-state conditions with fixed spark timing.
Results show that PPCC consistently delivers faster combustion than SI across all conditions, with greater stability and reduced sensitivity to operating variations. Increasing engine speed shortened combustion duration, while EGR extended it. At higher loads, PPCC performance improved due to increased pressure differential between the pre-chamber and main chamber. Unlike SI, which became unstable beyond an EGR-diluted λ = 1.15, PPCC maintained stable operation up to stoichiometry at 36% EGR. This enabled potential compatibility with a three-way catalyst (TWC), with less than a 1% efficiency penalty and no degradation in combustion stability.
However, SI outperformed PPCC in terms of overall efficiency, largely due to higher combustion completeness. The lower combustion efficiency of PPCC, linked to increased crevice volume and surface losses, also led to significantly higher total hydrocarbon (THC) emissions. These findings highlight the trade-offs between ignition stability and efficiency demonstrating that with proper calibration, PPCC can support high-dilution operation and conventional aftertreatment in retrofitted heavy-duty engines running on low-carbon fuels such as methanol.