The Effects of Charge Preparation, Fuel Stratification, and Premixed Fuel Chemistry on Reactivity Controlled Compression Ignition (RCCI) Combustion

Engine experiments were conducted on a heavy-duty single-cylinder engine to explore the effects of charge preparation, fuel stratification, and premixed fuel chemistry on the performance and emissions of Reactivity Controlled Compression Ignition (RCCI) combustion. The experiments were conducted at a fixed total fuel energy and engine speed, and charge preparation was varied by adjusting the global equivalence ratio between 0.28 and 0.35 at intake temperatures of 40°C and 60°C. With a premixed injection of isooctane (PRF100), and a single direct-injection of n-heptane (PRF0), fuel stratification was varied with start of injection (SOI) timing. Combustion phasing advanced as SOI was retarded between -140° and -35°, then retarded as injection timing was further retarded, indicating a potential shift in combustion regime. Peak gross efficiency was achieved between -60° and -45° SOI, and NO_x emissions increased as SOI was retarded beyond -40°, peaking around -25° SOI. Optimal cases in terms of both gross efficiency and peak pressure rise rate (PPRR) were in the mid-range SOI timings centered about -50° SOI, while late SOI resulted in decreased gross efficiency, decreased combustion efficiency, and high NO_x.

To assess the effect of the premixed fuel chemistry on RCCI combustion, a representative reformed fuel referred to as syngas (50% H₂, 50% CO by volume), and methane were substituted for PRF100. A reference baseline PRF condition with an SOI timing of -50° at T_in = 40°C and ϕ = 0.30 was used for comparison purposes. Matching combustion phasing to the baseline case by adjusting the premixed percent or SOI timing resulted in reduced gross efficiency (η_g) and increased NO_x emissions for both the syngas and methane cases. Matching the bulk heat release rate (HRR) characteristics by fixing the DI SOI quantity and duration and adding a premixed injection of n-heptane was able to regain most of the lost efficiency while decreasing NO_x emissions close to the baseline level.