Effect of three-hole nozzle orientations on sprays and combustion in methanol-diesel dual direct injection engines

Abstract

This study achieves combustion applications of methanol-diesel dual-direct injection in a retrofitted diesel engine by investigating methanol sprays and engine performance/emissions. Methanol draws high attention due to its green production potential and ease of adaptation to existing combustors and supply infrastructure. One of the most promising methods for utilising methanol in diesel engines is the dual direct injection, which provides a wide operating range and flexible injection control. This study provides an effective solution for dual direct injection by implementing a nozzle cap idea, which can mount a conventional direct injector for methanol delivery in an existing diesel engine. The custom-made nozzle cap can provide hole orientation variations, which effectively controls the methanol-air mixture distributions within the piston bowl. To this end, methanol sprays formed through the three-hole nozzle cap are analysed for varied injector pressure of 15 ∼ 35 MPa. High-speed schlieren imaging confirmed working of the new nozzle for methanol injection with expected results of increased liquid penetration length and cone angle for higher injection pressure. The spray images also helped understand how the mixtures would be distributed within the piston bowl due to direct injection. Experiments performed on a 1-litre single-cylinder common-rail diesel engine operating at 1400 rpm, up to 70 % methanol energy fraction and a broad range of methanol injection timings of BDC to TDC, showed that the methanol-diesel dual direct injection combustion produces overall lower power output than the diesel baseline due to lower calorific value and flame temperature of methanol but significantly reduced CO₂ emissions by up to 16 % and very low smoke emissions. The results showed high sensitivity to methanol injection timings and energy fraction in terms of the measured pressure, derived heat release rate and produced power due to an increase in mixture homogeneity for earlier injection timings and stratified charge conditions for later injection timings. However, the nozzle orientation change and expected methanol-air mixture distributions showed no measurable impact on pressure and heat release rate as well as engine power output and combustion stability. The most significant impact of the methanol three-hole nozzle orientation and resulting mixture distributions was found from uHC and NO_x emissions because a nozzle orientation directing methanol more towards the corner of the piston bowl opposite side of the injector led to increased liquid wall wetting and methanol in crevice volumes and thereby causing less complete combustion for higher uHC and lower NO_x. The low sensitivity of methanol mixture distributions to the in-cylinder pressure and engine power output but the measurable impact found on uHC and NO_x emissions empathies the required optimisation of methanol direct injector nozzle depending on the combustion chamber design of the base diesel engine.