Effects of Compression Ratio and Water Vapor Induction on the Achievable Load Limits of a Light Duty Diesel Engine Operated in HCCI Mode 2019-01-0962

Among the various Low Temperature Combustion (LTC) strategies, Homogeneous Charge Compression Ignition (HCCI) is most promising to achieve near zero oxides of nitrogen (NO_x) and particulate matter emissions owing to higher degree of homogeneity and elimination of diffusion phase combustion. However, one of its major limitations include a very narrow operating load range owing to misfire at low loads and knocking at high loads. Implementing HCCI in small light duty air cooled diesel engines pose challenges to eliminate misfire and knocking problems owing to lower power output and air cooled operation, respectively. In the present work, experimental investigations are done in HCCI mode in one such light duty production diesel engine most widely used in agricultural water pumping applications. An external mixture preparation based diesel HCCI is implemented in the test engine by utilizing a high-pressure port fuel injection system, a fuel vaporizer and an air preheater. With an existing compression ratio of 17.5, the engine could not be operated beyond 20% of rated BMEP owing to severe knocking. The existing bowl shaped piston is modified into a flat piston which is justified by the fact that fuel-air mixing has minimal or negligible role in HCCI combustion. In order to examine the effects of compression ratio on the achievable load range in HCCI, the geometric compression ratio is reduced in incremental steps from 17.5 to 11.5 with an interval of 2.5 by reducing the piston crown thickness. However, the compression ratio could not be reduced below 11.5 with the existing piston design. The results obtained show that the load range could be extended up to 57% by reducing the compression ratio to 11.5 without utilizing exhaust gas recirculation. It is also intended to examine the effects of water vapor induction on the achievable load range in HCCI. For this purpose, 20 ultrasonic atomizers with 1.7 MHz vibration frequency are utilized to produce water vapor which is inducted along with diesel vapor and air during the engine suction stroke. The homogeneous mixture of diesel vapor, water vapor and air is ignited during the engine compression stroke at a fixed compression ratio of 15. The water vapor concentration is varied from 0.8 mg/cycle to 2.4 mg/cycle by using a control valve. The results obtained shows that the load range could be extended up to 50% in HCCI by utilizing water vapor induction. At a fixed load condition, water vapor induction reduces NO_x and smoke emissions, while unburned emissions are higher compared to the results obtained with reducing compression ratio because of lower temperatures and displacement of intake oxygen. Overall, the present work shows that either by reducing the geometric compression ratio or by utilizing water vapor induction, there is a greater potential to increase the load range of diesel HCCI engines.