A Review of Spark-Assisted Compression Ignition (SACI) Research in the Context of Realizing a Production SACI Strategy 2019-24-0027
Low temperature combustion (LTC) strategies have been a keen interest in the automotive industry for over four decades since they offer improved fuel efficiency compared to conventional spark-ignition (SI) engines. LTC strategies use high dilution to keep combustion temperatures below about 2000 K to reduce heat transfer losses while avoiding locally rich in-cylinder regions that produce high soot. High dilution also enables an efficiency improvement from reduced pumping work and improved thermodynamic properties, though it requires high ignition energy. Combustion can be achieved by triggering autoignition from compression energy. High compression ratios are typically required to produce this level of ignition energy, which further improves fuel efficiency. The timing of the autoignition event is influenced by fuel properties and mixture composition, and is exponentially sensitive to temperature. Control of autoignition timing is difficult without a direct actuator, and has been a significant obstacle for realizing LTC in production. Spark-assisted compression ignition (SACI) addresses this challenge by using a spark plug to initiate chemical reactions that trigger autoignition. The combustion chamber is slightly stratified to promote combustion radical generation near the spark plug, though the high dilution far from the spark plug inhibits combustion radical propagation. The energy released from the early spark-initiated reactions trigger autoignition that consumes the remaining charge. Early researchers viewed SACI as a way to expand the narrow range of homogeneous charge compression ignition (HCCI). However, the operating range of SACI has continued to expand, and research groups have pushed its operation to the majority of the engine operating map. Extending SACI operation has largely come from improved mixture preparation to reduce low-load cycle-to-cycle variation and high-load ringing intensity. Specifically, fuel injection and exhaust gas recirculation (EGR) strategies were significant and were extensively analyzed. This work presents a summary of the broad research of SACI, an analysis of significant findings, and comments on discrepancies. Additionally, a thorough discussion of how production-intent SACI designs extend or innovate upon previous decades of research is included. A comparison of efficiency and emissions performance between laboratory and production-intent SACI work is presented, along with a synthesis of the sensitivity of SACI combustion parameters to control actuators.
Dennis Robertson, Robert Prucka
14th International Conference on Engines & Vehicles