Thermal efficiency improvement of lean burn high compression ratio engine coupled with water direct injection

The turbocharged downsizing gasoline direct injection (GDI) engines aiming for lower fuel consumption, lower emissions, and higher performance have been regarded as the future hybrid electrical vehicle (HEV) pathway. To improve the thermal efficiency of the HEV engine under lean burn operation, a water direct injection (WDI) system and a high-energy ignition (500 mJ) system were proposed in an Atkinson cycle GDI engine with a high compression ratio of 17. The effect of homogeneous lean burn, water injection ratio (WIR), and spark timing (ST) on engine knock, combustion process, and indicated thermal efficiency (ITE) improvement was studied. The result shows WDI at an earlier stage of the compression stroke (100° CA BTDC) could mitigate knock without significantly increasing combustion instability. When WIR increased to 50%, the engine knock decreased 82.7%, the indicated specific fuel consumption (ISFC) dropped 9.9% as the ST advanced from 1.6° to 9° CA BTDC. The maximum brake torque spark timing (MBT) could be further advanced by extending the lean burn limit (LBL). The wider LBL was achieved with a larger WIR. At λ = 1.1, CO emission reduced by 5 ∼ 7 times and reached 570 ppm, and unburned hydrocarbons were reduced by 9%∼13%. Less than 500 ppm of NOx was noted when λ ≥ 1.5 under each WIR case. The IMEP was extended from test basic 8.3 bar to 13 bar with an increase of 47.73%. The remarkably high ITE of 47.09% can be achieved with a controllable knock level at lean burn condition (λ = 1.9), with an increase of 7.84% compared with the stoichiometric test basic IMEP of 8.3 bar.