Research on Super-Lean Burn Spark Ignition Engine with In-Cylinder Water Injection using Gasoline Surrogate Fuels

The combination of super-lean burn spark ignition engine (excess air ratio λ ≈ 2) and in-cylinder water injection (WI) makes it possible to achieve thermal efficiency higher than 50%. Toward future fuel diversification including carbon-neutral fuels, technologies to improve SI engine thermal efficiency applicable to various fuels are required. In this study, the effect of in-cylinder WI on SI engine performance with a compression ratio of 17 and λ = 1.85 is investigated using premium gasoline, 5 components surrogate fuels for premium gasoline (S5H), and for regular gasoline (S5R). In the case of premium gasoline and S5H, spark timing can be advanced to MBT (minimum advance for best torque) by WI and gross indicated thermal efficiency (gITE) increases to 51.2% (premium gasoline) at water/fuel weight ratio (W/F) = 57.7% and 50.8% (S5H) at W/F = 62.9%. In the case of S5R, on the other hand, a strong knock forces a large spark retard at no-water condition. Although the water injection suppresses the knock and gives spark advance, the spark timing cannot be advanced to the region where stable lean combustion can be achieved. As a result, the maximum gITE for S5R only increases to 45.1% by WI of W/F = 69.3%. These results show that the effect of thermal efficiency improvement by WI can be obtained regardless of the fuel while a fuel with high knock resistance is required to achieve compatibility with super-lean burn.