Potential Benefits in Heavy Duty Diesel Engine Performance and Emissions from the Use of Variable Compression Ratio 2006-01-0081

Worldwide demand for reduction of automotive fuel consumption and carbon dioxide emissions results in the introduction of new diesel engine technologies. A promising technique for increasing the power density of reciprocating engines, improving fuel economy and curtailing engine exhaust emissions is the use of variable compression ratio (VCR) technology. Several automotive manufacturers have developed prototype vehicles equipped with VCR gasoline engines. The constructive pattern followed to alter the compression ratio varies with the manufacturer. The implementation of VCR technology offers two main advantages: the reduction of CO₂ emissions due to optimal combustion efficiency in the entire range of engine operating conditions and the increase of power concentration due to high boosting of a small engine displacement (i.e., engine downsizing). However, the aforementioned benefits concern mainly spark ignition engines, where the increase of compression ratio during part-throttle operation results in a noticeable reduction of brake specific fuel consumption (bsfc) because of their thermodynamic operating cycle and the relatively low CR used. On the other hand, the application of VCR technology in heavy-duty diesel engine, if any, is extremely limited whereas is not expected to provide a similar benefit since their compression ratio is already higher than spark-ignition engines. In addition, high values of compression ratio are necessary in diesel engines to ensure proper auto-ignition of injected diesel fuel. The variation of compression ratio in heavy-duty diesel engines confronts with the excessive increase of peak combustion pressure (Pmax) mainly at high engine loads. Specifically, studies conducted in the past by the present research group dealing with the application of “internal measures” in diesel engines, revealed the possibility for reducing bsfc and NOx emissions without significant penalty on soot using advanced injection timing and high EGR rates. However, this resulted in a considerable increase of peak combustion pressure, which generates serious structural problems at high engine load. Therefore, the use of a variable compression ratio in heavy-duty diesel engines can be a promising technique to control peak combustion pressure. In this case, a high compression ratio can be maintained at part load while a lower one can be used at full load where peak firing pressure is a problem. However, it is still doubtful if the expected improvement in fuel economy will be overwhelmed by the manufacturing complexity and the subsequent cost aggravation and reliability implications, which will accompany the application of this technology in diesel engines. For this reason, in the present study, an engine simulation model is used to investigate potential benefits in specific fuel consumption of a heavy duty DI diesel engine by varying its compression ratio. Additionally, the implications on engine exhaust emissions and peak combustion pressure from the implementation of VCR technology at both part and full load conditions are examined. The main objective of the study is to identify the necessary variation of compression ratio with engine operating conditions to achieve optimum fuel consumption, avoid excessive peak combustion pressures, and assess its repercussions on engine out emissions with regard to future emission standards. This analysis will facilitate the determination of a possible “optimized” variation law of compression ratio with engine speed and load for heavy-duty diesel engines. The results of this theoretical examination in conjunction with serious technical issues accompanying the variation of compression ratio will judge the applicability of this technique in heavy-duty diesel engines.