LNG Fuel Differentiation: DME/LNG Blends for HPDI Engines 2020-01-2078
With increased awareness and scrutiny of greenhouse gas (GHG) emissions, the heavy-duty truck industry is on the lookout for solutions that can maximize GHG savings, through either lowering fuel consumption and lowering methane slip. This paper focuses on whether it is possible to provide a differentiated Liquefied Natural Gas (LNG) that supports the further improvement of a High-Pressure Direct Injection (HPDI) Engine. Desired improvements from this LNG blend are the lowering or substitution of the pilot Diesel use of the current HPDI engine, the lowering of the raw exhaust gas methane concentration and any additional performance improvements. Sixty-five substances were identified that could potentially be blended into cryogenic methane thus creating a differentiated LNG fuel. This paper goes through the process of additive selection and then focuses primarily on the results for using Dimethyl Ether (DME) as an LNG component, one of the candidate substances, but also showcases some of the other potential additives.
To study the autoignition properties of DME/LNG blends, autoignition delay times were simulated and then measured in a Rapid Compression Machine at engine conditions. The results were used to optimize the chemical mechanism that is used as input into a High-Pressure Direct Injection engine model. It was found that more than 5 vol% DME is required to reach a significant reduction in the autoignition delay time at typical operating conditions. The engine modelling results were also used to determine the initial conditions for HDPI engine tests using a modified 15L-Westport engine.
These engine tests showed that an LNG/DME blend could potentially be used to develop a mono-fuel HPDI engine. However, it was found that although the mono-fuel concept works for high load conditions with the existing HPDI engine, further research is needed to enable stable combustion at lower loads and idling while keeping DME proportions at levels that could be dissolved in LNG. It was also found that higher proportions of DME in the LNG could lead to a reduction of the methane slip.