Converting a diesel engine to natural gas fuel has the potential to reduce Greenhouse Gas Emissions by as much as 25%, if the efficiency is maintained.&nbsp; However, even small amounts of unburned methane which leave the engine in the exhaust gas, due to its 28 times greater GHG effect, can deteriorate this net GHG benefit.&nbsp; This unburned exhaust methane is usually referred to as Methane Slip.&nbsp;The current approach for converting a diesel engine to gas fuel is via Dual Fuel technology where a gas admission valve is added to the port of the engine and the diesel injection quantity is reduced to pilot (&lt;15% diesel) or micro-pilot quantities (&lt;2% diesel) and the diesel injection acts as the ignition source.&nbsp; Like the original diesel, the engine will run lean burn combustion to reduce NOx emissions to below the regulated levels or to within a comfortable margin for input to an SCR system.&nbsp; The challenge, especially when the diesel engine is not overly modified for gas, there are three pathways for methane slip (1) the methane and air are premixed and this mixture goes into the crevice regions, especially the gap between the piston and liner above the top ring where it is shielded by from the flame by the quench surfaces; (2)&nbsp; methane burns relatively slow in the lean environment, and (3) a third source of unburned methane short circuited from intake to exhaust port during the valve overlap period.&nbsp; There are of course obvious mechanical remedies which can be deployed, such as reducing valve overlap and moving the tip piston ring up to minimize the crevice volume.&nbsp; These can and should be deployed.&nbsp; The remaining remedies will be related to combustion.&nbsp; This report highlights combustion methods to improve the combustion of methane in such an engine.A known solution, HPDF (High Pressure Dual Fuel) injection has been demonstrated to avoid the primary methane slip pathways, but it requires high pressure gas and a dual function injector and maintains the relatively slow burning of a diffusion flame.&nbsp; This report looks to explore additional methods to reduce methane slip.&nbsp; The methods explored are (A) lean burn RCCI - Reactivity Controlled Compression Ignition - combustion with active combustion phasing control, (B) stoichiometric/EGR and FT-AI - Flame Triggered Auto-Ignition and (C) MPDI - Medium Pressure Direct Injection.&nbsp; All of these methods are well controlled with active combustion control using the Woodward LECM and RT-CDC (Real Time Combustion Diagnostics and Control) system.It is shown that each method improves combustion quality, reduced NOx, reduced methane slip and improves efficiency relative to conventional diesel combustion (CDC) and conventional dual fuel combustion (CDF) in all or most of these metrics. And each alternative method, having achieved the reduced methane slip targets, can be evaluated for practicality on an engine type by engine type basis, but this work demonstrates the feasibility of each methane slip reduction strategy, thus given engine makers options to mitigate methane slip, while reducing NOx emissions and improving efficiency, thus enabling diesel to gas conversions at 25% or better GHG reduction. &nbsp;

Strategies to Reduce Methane Slip in Natural Gas Diesel Dual Fuel Marine and Power Generation Engines

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2021/volume/85512

ASME 2021 Internal Combustion Engine Division Fall Technical Conference (ICEF2021)

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