Transient “Single-Fuel” RCCI Operation With Customized Pistons in a Light Duty Multi-Cylinder Engine

Reactivity Controlled Compression Ignition (RCCI) combustion in a light-duty multi-cylinder engine over transient operating conditions using fast response exhaust UHC₁, NO and PM measurement instruments was investigated. RCCI has demonstrated improvements in efficiency along with low NOx and PM emissions by utilizing in-cylinder fuel blending, generally using two fuels with different reactivity in order to optimize stratification. In the present work, a “single-fuel” approach for RCCI combustion using port-injected gasoline and direct-injected gasoline mixed with a small amount of the cetane improver 2-ethylhexyl nitrate (EHN) was studied with custom designed, compression ratio of 13.75:1, pistons under transient conditions. The EHN volume percentage in the mixture for the direct-injected fuel was set at 3%. In an experimental investigation, comparisons were made to transient RCCI combustion operation with gasoline/diesel. The experiments were performed over a step load change from 1 to 4 bar brake mean effective pressure (BMEP) at constant 1,500 rev/min on a General Motors Z19DTH 1.9 liter diesel engine The transients were conducted by changing the accelerator pedal command to provide a desired torque output with a DRIVVEN engine control unit (ECU) that replaced the original Bosch ECU. All relevant engine parameters are adjusted accordingly, based on 2D-tables. Previous to the transient engine operation, 4 steady-state points were used to obtain performance and emission values. Engine calibration at these 4 points, as well as the interpolation of the intermediate points, allowed for smooth operation during the instantaneous step changes. Differences between the steady-state and transient results indicate the complexity of transient operation and show the need for additional controls to minimize undesirable effects. The steady-state points were calibrated by modifying the fuel injection strategy (actual Start of Injection (aSOI) timing, port-fuel injection (PFI) fraction, etc.), EGR and rail pressure in order to obtain predefined values for the crank angle at 50% of total heat release (CA50). Furthermore, emission targets (HC₁ < 1500ppmC3, NO < 10ppm, FSN < 0.1 with a maximum pressure rise rate < 10bar/deg) and noise level targets (<95dB) for RCCI combustion were maintained during the calibration and mapping. The tests were performed with a closed-loop (CL) calibration by using a next-cycle (NC) controller to adjust the PFI ratio of each cycle in order to reach the steady-state CA50 values in the table. The results show that single-fuel RCCI operation can be achieved, but requires significant alteration of the operating conditions, and NOx emissions were significantly elevated for gasoline/gasoline-EHN operation. While combustion phasing could not be matched, UHC₁ emissions were at a similar level as for gasoline/diesel combustion. It is expected that the implementation of different injection strategies and boosted operation, combined with use of higher compression ratio pistons in order to compensate for the lower reactivity direct injection (DI) fuel, could raise the potential for improved performance.