Improved Engine Design Concepts Using the Second Law of Thermodynamics

Abstract

This project was aimed at developing and using numerical tools which incorporate the second law of thermodynamics to better understand engine operation and particularly the combustion process. A major activity of this project was the continual enhancement and use of an existing engine cycle simulation to investigate a wide range of engine parameters and concepts. The major motivation of these investigations was to improve engine efficiency. These improvements were examined from both the first law and second law perspective. One of the most important aspects of this work was the identification of the combustion irreversibilities as functions of engine design and operating parameters. The combustion irreversibility may be quantified in a number of ways but one especially useful way is by determining the destruction of exergy (availability) during the combustion process. This destruction is the penalty due to converting the fuel exergy to thermal energy for producing work. The engine cycle simulation was used to examine the performance of an automotive (5.7 liter), V-8 spark-ignition engine. A base case was defined for operation at 1400 rpm, stoichiometric, MBT spark timing with a bmep of 325 kPa. For this condition, the destruction of exergy during the combustion process was 21.0%. Variations of many engine parameters (including speed, load, and spark timing) did not alter the level of destruction very much (with these variations, the exergy destruction was within the range of 20.5-21.5%). Also, the use of turbocharging or the use of an over-expanded engine design did not significantly change the exergy destruction. The exergy destruction during combustion was most affected by increased inlet oxygen concentration (which reduced the destruction due to the higher combustion temperatures) and by the use of cooled EGR (which increased the destruction). This work has demonstrated that, in general, the exergy destruction for conventional engines is fairly constant ({approx}21%) for a range of operating and design parameters. Further, to achieve high efficiency engines requires that the exergy be managed and not necessarily reduced. The overall thermodynamics is the final discriminator regarding high efficiency engines.