Particulate Matter Reduction From a Pilot-Ignited, Direct Injection of Natural Gas Engine

Abstract

This paper reports an evaluation of various combustion strategies aiming to reduce engine-out particulate matter (PM) emissions from a natural-gas fuelled heavy-duty engine. The work is based on a Westport HPDI fuelling system, which provides direct injection of both natural gas and liquid diesel into the combustion chamber of an otherwise unmodified diesel engine. The diesel acts as a pilot to ignite the natural gas, which normally burns in a non-premixed fashion, leading to significant PM formation. The concepts to reduce PM evaluated in this work are: 1) adjusting the relative phasing of the natural gas and diesel injections to allow more premixing of the natural gas prior to ignition; 2) reducing the pilot quantity to increase the ignition delay of the gas jet; and 3) reducing the level of EGR at select modes to reduce PM formation. These strategies are evaluated at steady state using single- and multi-cylinder research engines, supported by CFD analysis. The results demonstrate that allowing limited premixing of the gas jet prior to ignition can significantly reduce PM emissions. Excessive premixing can lead to high rates of pressure rise; EGR can be used to moderate the combustion under these conditions, without causing increased PM emissions. Reducing pilot quantity is another effective technique to reduce PM, primarily by allowing more air to mix with the gas jet before ignition. These various techniques can be combined to form a new operating strategy that significantly reduces engine-out PM and NOx emissions compared to the baseline strategy without significantly impacting fuel consumption.

Characterization of Particulate Matter Morphology and Volatility from a Compression-Ignition Natural-Gas Direct-Injection Engine

“…The natural gas is then ignited by the diesel pilot flame. The natural gas combustion is predominately unmixed, which helps avoid knock and allows for a high-compression ratio and thermal efficiency, like those associated with diesel engines (McTaggart-Cowan et al 2012). In this engine, soot particles are mainly formed by incomplete combustion of natural gas (Jones 2004).…”

“…They determined that PM mass emissions were affected primarily by EGR and EQR, and PM number emissions were also affected by EGR and EQR, and less strongly by GRP and diesel pilot quantity. McTaggart-Cowan et al (2012) worked to reduce the PM mass emissions for the same engine mode used by Patychuk and Rogak (2012). They found that PM could be reduced by adjusting the relative phasing of the diesel and natural gas injections to allow for more premixing of the natural gas with air.…”

Simulation Modeling of a Dual Fuel (Natural Gas-Diesel) Engine Using Early Direct Injection Technique of Natural Gas

“…The benefits include improvements in thermal efficiency through increasing the flame speed of NG resulting from the combustion of locally stoichiometric NG-air mixture [13]. However, the main drawbacks of this method include the increased complexity of the injection system due to the high injection pressure and the increase in soot emissions due to the presence of rich mixture zones [14,15]. Engine simulations are valuable tools for evaluating the performance and emission parameters.…”