Cummins Engine Company

Introduction

The concept of a low heat rejection or "adiabatic" engine has recently generated a great deal of interest among diesel engine researchers. Many studies have been reported, addressing the concept in a variety of ways, yet often with conflicting results. A cursory look at the published literature reveals a wide range of conclusions concerning the ultimate potential of the low heat rejection engine concept.

The purpose of this paper is to present a recommendation for future research direction, based on the interpretation of engine data and analysis in the published literature. The objective of this paper is threefold; first, to review recent published work on the low heat rejection engine concept. This is not an exhaustive review, but one that identifies some of the most significant work, either in terms of technical contribution. or because of their impact on the research community - unfortunately these are not necessarily the same criteria.

The second objective is to demonstrate that consistent conclusions-can be drawn from the seemingly wide range of results which have been reported.

* Numbers in brackets designate references found at the end of this report.

Finally, based on the preceding conclusions, directions are identified for further research.

Background

Although receiving some consideration since the very inception of the diesel engine, the concept of a low heat rejection engine did not gain popularity until the mid-1970s. At that time several analyses were reported based on crude cycle simulations [1-4]*. The most notable of these analyses, in terms of the attention it received, was that reported by Kamo and Bryzik in 1978 [3]. Their paper showed an improvement in fuel consumption of over 20 percent, and for this reason generated a great deal of interest. Although hindsight judges this early work guite harshly, it must be remembered that an experimental database with which to validate such analyses was virtually nonexistent. Furthermore. analytical capability was far less advanced than it is today.

As time went on, and experimental data started becoming available, several more sophisticated analyses have been conducted. Some of these have drawn far more conservative conclusions. while others have remained optimistic. Thus, if anything, the range of predictions has become wider, and seemingly less conclusive. At the same time. the results of several experiments demonstrating the low heat rejection engine concept have been reported. These too have shown a wide range of results.

Bases of Comparison The first step in drawing meaningful and consistent conclusions from such a wide range of results involves the identification of meaningful engine constraints constraints which allow an accurate. direct comparison of an insulated engine's performance to its conventionally cooled counterpart. The first such constraint is peak cylinder pressure. In general, increases in cylinder pressure result in improved thermal efficiency. but the majority of diesel engines in production today are run at or near their mechanical design limits. Reducing the heat rejection of an engine results in an increase in cylinder pressure, which must be addressed for accurate assessment of comparative performance. As the heat rejection of an engine is reduced, the cylinder pressure must be maintained constant, either through retarding the injection timing. or more favorably. but more difficult to achieve experimentally. maintaining optimum timing and reducing the compression ratio.

Clearly a further constraint is that imposed by emission regulations. It is hardly worth discussing potential fuel consumption improvements without consideration of the fact that exhaust emissions cannot be compromised, yet comparatively few research efforts have addressed this important issue.

A final engine constraint which must be imposed is that of air-fuel ratio. This has been especially noteworthy in experimental work, where the insulated engine has resulted in a reduction in volumetric efficiency, which is often manifested as a reduction in air-fuel ratio. In making an accurate assessment of low heat rejection engine performance, the air-fuel ratio must be restored to the same level as the conventionally cooled engine.

The next step in correctly discerning the impact of reduced heat rejection on engine performance involves its effect on heat release. When reviewing the literature on low heat rejection engine experiments, two scenarios emerge. The first is that in which the engine is operated at high load, or otherwise has such a combustion system that ignition delay is very short. In this case the vast majority of energy release occurs in diffusion burning. The rate of combustion is controlled by the rate of mixing between the air and the fuel. Raising the wall temperature has little if any impact on this mixing rate, and thus the rate of heat release remains virtually unchanged by insulation. The second scenario emerges from experiments in which the ignition delay of the base, or conventionally cooled engine is very long. A significant portion of the fuel mixes with air prior to the onset of combustion. thus causing a large energy release early in the combustion process due to the ignition of the pre-mixed charge, followed by energy release in diffusion burning. In this case the effect of insulation is to significantly reduce the ignition delay. thereby reducing the pre-mixed portion of the heat release process, and correspondingly increasing the diffusion portion. In several experiments this has been reported to result in significantly improved thermal efficiency. Although brought about by insulation, it must be recognized that this is a secondary effect. Insulation has caused a change in engine operation which could similarly have been brought about by other means - a revised fuel injection system for example. In these cases the question must be asked whether insulation provides the most cost-effective of several alternatives in accomplishing the same end.

Each of the factors mentioned in this section must be kept in mind as the various reported efforts are reviewed in the

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