Homogeneous charge compression ignition (HCCI) promises low NOx emission and high efficiency, though showing a limited operating range and difficult-to-control combustion timing. In recent years, spark assisted compression ignition (SACI) was shown to be an efficacious technique to extend the operating range and to control combustion timing in HCCI engines within certain limits. As an alternative to spark assist, a hot surface ignition system (HSI) was demonstrated in a previous work to enable hot surface assisted compression ignition (HSACI) featuring similar combustion characteristics compared to SACI. The scope of this work is the comparison of both types of ignition assistance at various levels of dilution and intake temperatures with regard to the ability to control combustion timing, similarities in the course of combustion, the strength of the ignition systems and the susceptibility to cycle-by-cycle variations (CCV).
Engine trials were conducted at a single-cylinder test-bench under steady state conditions at a constant engine speed of 1400 1/min. The engine operated naturally aspirated under full load conditions using natural gas as the fuel and conditioned intake pressures in the range of 993–995 mbar. Experimental conditions cover relative air-fuel ratios (λ) in the range of λ = 2.1–3.1 and intake temperatures in between 140–170°C. The earliest applicable combustion timing was used as the target variable for the evaluation of the strength of the ignition systems.
Results show similar capabilities of SACI and HSACI to control combustion timing by means of spark timing in SACI and hot surface temperature in HSACI. Heat release analyses of individual combustion cycles at same crank angle timing of center of combustion (CA50) in SACI and HSACI show high agreement of the course of heat release and point out the similarity of both combustion processes. The evaluation of the strength of the ignition systems reveals that HSACI extends the lean limit by Δλ = 0.05–0.10 and the early ignition limit by ΔMinCA50 = 1.0–4.5°CA towards earlier CA50 depending on intake temperature and provided that ringing is not of concern. Comparison of CCV in HCCI, SACI and HSACI at given levels of CA50 show highest combustion stability for HCCI, followed by SACI. HSACI evinces highest CCV due to a larger variation in the start of combustion compared to HCCI and SACI.