Abstract
Combustion chamber leakage, caused mainly by blowby, results in a reduced engine performance and higher fuel consumption. The blowby rate is, to some extent, determined by the design of the piston-ring-cylinder assembly (PRC) and the blowby rate varies throughout the life of an engine due to wear of the said assembly. The paper presents a quantitative evaluation of the influence of the combustion chamber blowby on the engine performance and fuel consumption on the example of two diesel engines: older generation naturally aspirated indirect injection diesel engine and a modern turbocharged direct injection engine. The assessment was made based on a simulation research using the AVL Boost software and the input data for the calculations were ascertained based on measurements performed on actual objects. The results have shown that a reduction of the blowby by half compared to the values occurring in engines of good technical condition would increase the maximum torque and power by approx. 0.5% for both investigated engines. The results of the simulation have also shown that increases in the blowby occurring in engines after long service lead to increased fuel consumption from 1% to 7% and the lower the engine speed and load the greater theses values.
Highlights
The air tightness of a combustion chamber is one of the features influencing the overall engine efficiency and is decisive of its technical condition
The influence of blowby on the mean indicated pressure at a constant fuel dose at a given engine speed for the two tested engines has been shown in figures 3 and 4
The fuel doses for the given engine speed were calculated based on the fuel consumption at full engine load
Summary
The air tightness of a combustion chamber is one of the features influencing the overall engine efficiency and is decisive of its technical condition. The piston inside the cylinder liner, aside from its sealing function plays other roles and any design attempts to reduce the blowby very often lead to an increased oil consumption or mechanical resistance of the friction pairs. Designers must reach a compromise that involves accepting a certain level of blowby. A greater blowby level will result in a lower engine overall efficiency i.e. increased fuel consumption. During the service life, following a wear of the cylinder liner, the piston rings and the ring grooves, the gaps in the ring pack increase, which results in a reduced air tightness of the combustion chamber
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