Experimental Investigation into the Influence of Piston Crevices on Engine-Out Hydrocarbon Emissions from a Diesel Dual Fuel Engine
In today’s society, when the fuel prices are increasing and the climate changes are becoming more and more noticeable, alternative fuels for combustion engines are becoming an important topic for the manufacturers. Two interesting fuels in those perspectives are Biogas and Compressed Natural Gas, CNG. The main constituent of these two fuels is methane. Currently methane is mostly used in spark ignited engines but can also be used in diesel engines since it has a high resistance to knock. An engine concept where methane can be used as fuel in a diesel engine is the Diesel Dual Fuel concept, DDF. In this concept a diesel engine is run with two fuels, diesel and methane, where the methane is injected into the intake runners and the diesel is direct injected into the cylinder. The engine is mainly run on methane and uses a diesel pilot as a liquid spark plug to ignite the homogenous air/methane mixture. The biggest challenge when it comes to emissions for the DDF concept is the HC emissions since the combustion chamber in a diesel engine is not optimized for homogeneous charges, especially noticeable for the piston ring pack crevices. Engine tests are therefore carried out to study the contribution from the piston ring pack crevices to the engine-out HC emissions of a DDF engine. The results show that the flame is not able to burn down into the top land volume and consume the air/fuel mixture there when the standard piston with a top land clearance of 0.6 mm is used. Increasing this clearance to 2.1 mm makes the flame able to burn down into this volume and consume most of the air/fuel mixture there. The contribution to the engine-out HC emissions from the top land volume varies for different lambda values and engine loads. The same trend could be seen for both the light and middle engine loads tested with regards to lambda; however a larger amount of the HC emissions is expected to originate from the top land volume at the higher load. The contribution from the second land volume shows the opposite trend with lambda if compared with the top land volume.
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