Utilization of waste heat in trucks for increased fuel economy. Final report

Abstract

Trucks currently reject up to 40% of the total fuel energy in the exhaust. Since petroleum costs are continuing to increase, there is growing interest in techniques that can utilize this waste heat to improve overall system efficiency. The report given evaluates and compares improvement in fuel economy for a broad spectrum of truck engines and waste heat utilization concepts. The engines considered are the Diesel, spark ignition, gas turbine, and Stirling. Principal emphasis is placed on the four-stroke Diesel. Because there will be a significant increase in the amount of exhaust energy, the still-to-be-developed Diesel is also examined. The waste heat utilization concepts include preheating, regeneration, turbocharging, turbocompounding, and Rankine engine compounding. Predictions are based on fuel-air cycle analyses, computer simulation, and engine test data. All options are evaluated in terms of maximum theoretical improvement, but the Diesel and adiabatic Diesel are also compared on the basis of maximum expected improvement and expected improvement over a driving cycle. The study indicates that Diesels should be turbocharged and aftercooled to the maximum possible level. At higher boost pressures, the engine power and the fuel economy can be increased, and leaning out the fuel-air mixture or aftercooling the compressor outlet air will reduce the NOx. Turbocharging also increases the potential for turbocompounding if compressor and turbine efficiencies can be maintained. The results reveal that Diesel driving cycle performance can be increased by 20% through increased turbocharging, turbocompounding, and Rankine engine compounding. The Rankine engine compounding provides about three times as much improvement as turbocompounding but also costs about three times as much. Performance for either can be approximately doubled if applied to an adiabatic Diesel.