The more and more severe regulations on exhaust emissions from vehicles and the worldwide demand for fuel consumption reduction impose continuous improvements of the engine thermal efficiency. Base engine geometrical setups are important aspects which have to be taken into account to improve the engine efficiency.
This paper discusses the influence of the bore-to-stroke ratio on emissions, fuel consumption and full load performances of a Diesel engine.
The expected advantage of a reduced bore-to-stroke ratio is mainly a decrease of the thermal losses, due to a higher volume-to-surface ratio, reducing the wall surfaces, responsible for the heat losses, per volume of gas. The advantages concerning the wall heat losses are opposed to the disadvantages of lower volumetric efficiency, as a smaller bore requires smaller valve diameter. Additionally does a reduction of the bore-to-stroke ratio lead to an increase of the friction losses, as the mean piston speed increases.
Within this study, three different bore-to-stroke ratios are analyzed on a single-cylinder CR Diesel engine. The methodology used for this investigation is to set constant as much engine parameters as possible between the three configurations. Therefore, the displacement, the injection system, the in-cylinder TDC swirl and the compression ratio are fixed, in order to isolate the phenomena depending on the bore-to-stroke ratio.
Five engine operating points are investigated: three partial load points, representative of the NEDC conditions on a C-class vehicle, and two full load points.
In addition to standard analysis and combustion analysis including heat exchanges, an energy strip study using an in-house tool is performed in order to evaluate which parameters have the highest influence on the efficiency differences between the three configurations.
The combustions process is significantly impacted by the bore-to-stroke ratio due to differences in wall heat losses and piston speed. Higher gas temperatures around TDC decrease the premixed combustion phase and affect exhaust gas emissions.
The main conclusion of this study is that the smallest bore-to-stroke ratio configuration shows the best trade-off between thermal and mechanical losses, with respect to the overall engine efficiency.