Methanol Diesel Dual Fuel Direct Injection in a HD-CI Engine

Licentiate thesis, 2018

Laws concerning emissions from heavy duty (HD) internal combustion engines are becoming increasingly stringent. New engine technologies are needed to satisfy these new requirements and to reduce fossil fuel dependency. One way to achieve both objectives can be to partially replace fossil fuels with alternatives that are sustainable with respect to emissions of greenhouse gases, particulates and nitrogen oxides (NOx). Suitable candidates are ethanol or methanol. The aim of the studies presented here was to investigate the possible advantages of combusting methanol in a heavy duty Diesel engine - in particular, the potential to greatly reduce particulate emissions and thereby bypass the soot-NOx tradeoff. To better understand the fuel-air mixing process in alcohol fuel sprays, ethanol sprays were studied in a high pressure/temperature spray chamber with optical access. The experiments were performed at a gas density of 27kg/m3 at 550 C and 60 bar, representing typical operating conditions for a HD engine at low loads. High speed video images of the developing sprays were recorded, enabling measurement of spray parameters such as the liquid cone angle, liquid penetration length and vapor penetration at injection pressures between 500 and 2200 bar. The results obtained provide insight into the fuel-air mixing process. Having characterized the behavior of alcohol sprays, a second study was conducted to investigate the combustion of alcohol fuels in a Diesel engine. Because of methanol’s poor auto-ignition properties, a pilot Diesel injection was used to initiate the combustion process. The two fuels were injected directly but separately, necessitating the use of two separate standard common rail Diesel injection systems together with a newly designed cylinder head and adapted injection nozzles. The studies serve as a proof-of-concept showing that methanol and ethanol can successfully be used in a high pressure Diesel injection system. The dual fuel system's combustion properties were compared to those of pure Diesel with the same dual injection strategy. Methanol offered comparable combustion efficiencies to conventional Diesel with lower emissions of NOx and significantly lower soot emissions. A design of experiments study was performed to characterize the methanol-diesel system’s behavior in detail at a single speed-load point, and a sweet spot analysis revealed that it may be possible to optimize the tested setup to improve its gross efficiency while maintaining very low soot emissions and low emissions of NOx.