Dilute Combustion Assessment in Large Bore, Low Speed Engines 2017-01-0580
The promising D-EGR gasoline engine results achieved in the test cell, and then in a vehicle demonstration have led to exploration of further possible applications. A study has been conducted to explore the use of D-EGR gasoline engines as a lower cost replacement for medium duty diesel engines in trucks and construction equipment. However, medium duty diesel engines have larger displacement, and tend to require high torque at lower engine speeds than their automobile counterparts. Transmission and final drive gearing can be utilized to operate the engine at higher speeds, but this penalizes life-to-overhaul. It is therefore important to ensure that D-EGR combustion system performance can be maintained with a larger cylinder bore, and with high specific output at relatively low engine speeds. Based on application projections studied in this work, an engine having a 107mm bore and 124mm stroke, operating at 2000 rpm at 17 bar Brake Mean Effective Pressure (BMEP) was selected as representative.
The objective of the study was to use combustion modeling to make an initial assessment of combustion performance under the conditions identified as optimal for a medium duty D-EGR gasoline engine application. Beginning from a validated 2 liter, 86mm bore by 86mm stroke D-EGR automobile engine, the objective was to scale this engine to one having a 107mm bore and 124mm stroke. Combustion performance was then to be assessed at 2000 rpm, with loads in the range of 15 to 17 bar. Higher engine speeds and further load increases were also to be assessed to further bracket the findings.
It must be emphasized that this study provides an initial exploration of engine geometries for which little test data is available. While well-validated models and careful engineering judgment were used, the results must be considered approximate at this point in time. The increased bore diameter results in longer unburned mixture residence time, and this would be expected to increase the problem of auto-ignition. However, the increased bore diameter also increases the flame travel time, reducing the rate of energy release, and resulting unburned mixture pressure and temperature.
Under the longer stroke conditions of the medium duty engine the higher piston speed at a given engine speed increases the turbulent kinetic energy and heat release rate. While this increases flame speed and reduces unburned mixture residence time it also increases pressure and temperature. The result is an increased tendency to auto-ignite, and the need for aggressive timing retard. Predicted results were that sufficient timing retard could be achieved, but with a resulting fuel efficiency penalty.