Complete engine leak testing is generally followed to conform quality of manufacturing and assembly of different parts which otherwise create failures related to overall performance, fluid leak, gas leak and emissions etc. Light weighting of engines and downsizing is a generally accepted and followed theme towards the future emission norms which influences the part design complexity. Objective of the study is to investigate the significant parameters related to leak in cylinder head system and countermeasures to reduce the same when designing the small compact engines. In this study a systematic approach has been followed for arriving the leak rate specifications to qualify the products.
In the design and manufacturing of small engines, features like compact combustion chamber with constraints of clamping bolts layout, gasket design, compact crank case layout and lower wall thickness of major casting parts are used to achieve weight and cost targets. During the development of one of such new engine, cylinder head leak rate at the valve – seat interface observed was significantly higher compared to existing production engines. Cause and effect analysis was prepared for the various leaks and valve interface leak has been focused further to bring it under acceptance level. Failure modes causing leak were experimentally evaluated under simulated conditions. Finite elemental analysis was done to understand the cylinder head distortion levels and compared with the bench mark models. Based on the force transfer mechanism from bolt clamping force to the cylinder head, solution was proposed to reduce distortions and leak rate. Volume effect on the pressure drop leak test was studied theoretically and validated experimentally to arrive at the optimum leak rate specification for the new engine.
Improvements which reduced valve seat distortion helped in reducing leak significantly and an optimum leak specification arrived for the new engine. Parameters influencing leak in new engine is studied experimentally in sub-system and engine level. Countermeasure was proposed to reduce distortion and leak, improve sealing; process improvements to achieve consistency are studied experimentally and suggested. Combination of design improvements and process improvements helped in achieving the acceptable leak rates which helped to meet the engine quality, cost and weight targets.
TVS Motor Co., Ltd.
SAE/JSAE Small Engine Technology Conference