The combination of super-lean burn spark ignition (SI) engine with high compression ratio (CR) and in-cylinder water injection has a thermal efficiency potential exceeding 50%. The objective of this study is to obtain guidelines for optimizing the combination between fuel, water injection, and operating conditions in lean burn SI engine. For that, the effect of in-cylinder water injection on SI engine performance is comprehensively investigated at excess air ratio of 1.85 and CR of 17 and 19 using premium gasoline (PG), 5-components surrogate fuels for premium gasoline (S5H), and for regular gasoline (S5R). At CR of 17 and IMEP of 1.0 MPa using PG and S5H, water injection with water/fuel weight ratio (W/F) of 57.7%–62.9% enables minimum spark advance for best torque (MBT) operation, and gross indicated thermal efficiency (gITE) improves by 2.7–3.2 pt compared to no-water injection. In S5R, a strong knock forces a large spark retard, and the spark timing cannot be advanced to the region where stable lean combustion can be achieved even with water injection up to W/F = 75.4%. Furthermore, increasing CR from 17 to 19 in PG results in a 42.5% increase in the required water injection amount, limiting the improvement in gITE to 0.33 pt. Zero-dimensional chemical kinetic simulations using detailed reaction mechanisms for S5H and S5R well reproduce the experimental trends of water injection with respect to fuel and CR, and thermodynamic and chemical effects of water addition on auto-ignition is clarified. Finally, at CR = 19 in PG, reducing the IMEP to 0.85 MPa decreases W/F and gITE reaches its maximum. This study demonstrates that water injection can improve the thermal efficiency of lean-burn SI engines by using fuel with high knock resistance and increasing CR and load within a range that avoids excessive W/F increase.
Nagasawa et al. (Tue,) studied this question.