The Influence of In-Cylinder Flows on Emissions and Heat Transfer from Methane-Diesel Dual Fuel Combustion 2013-01-2509

In order for premixed methane diesel dual fuel engines to meet current and future legislation, the emissions of unburned hydrocarbons must be reduced while high efficiency and high methane utilization is maintained.

This paper presents an experimental investigation into the effects of in cylinder air motion, swirl and tumble, on the emissions, heat transfer and combustion characteristics of dual fuel combustion at different air excess ratios. Measurements have been carried out on a single cylinder engine equipped with a fully variable valve train, Lotus AVT. By applying different valve lift profiles for the intake valves, the swirl was varied between 0.5 and 6.5 at BDC and the tumble between 0.5 and 4 at BDC. A commercial 1D engine simulation tool was used to calculate swirl number and tumble for the different valve profiles. Input data for the simulation software was generated using a steady-state flow rig with honeycomb torque measurements. To measure heat transfer, thermocouples were fitted in the cylinder head and heat exchangers on the coolant circuit and the engine oil.

The study shows that swirl has a strong effect on the heat transfer; increasing the swirl from 0.5 to 6.5 increases the heat transfer to the coolant by 50%. With regards to emissions; swirl has the effect of increasing oxidation of hydrocarbons returning from crevices. For this reason a 20% reduction of hydrocarbon emissions can be achieved by increasing the swirl from 0.4 to 3. At high λ of 1.9, combustion is very sensitive to mixing between the gas and the air. The mixing is affected by the turbulence generated over the intake valves. A difference in engine out HC emissions by a factor of two can be achieved by varying the valve lift curve and hence varying the turbulence generated during the intake event. The timing of the gas injection can also improve mixing and achieve similar results. Compared to SI, dual fuel combustion is relatively insensitive to tumble.