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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.