Reduction of Fuel Consumption By Thermodynamical Optimization of the Otto-Engine

  • D. Gruden
  • R. Hahn
  • H. Lörcher
Conference paper


By the example of the PORSCHE 924 2-liter Otto engine it was demonstrated that the optimization of the compression ratio, combustion chamber shape, air/fuel ratio, and ignition timing is a means to reduce fuel consumption in the overall load and speed range by 5 to 30 % as compared to the conventional variant, and this without affecting maximum power output. Thus, the thermodynamically optimized Otto-engine achieves consumption values normally returned by Diesel engines with similar displacements under steady state conditions. The compression ratio of the new engine is CR = 13.0. In the partial load range the engine is operated with a lean fuel air mixture (λ ≈ 1.2).

Performance tests carried out with the PORSCHE 924 car have shown that substantially improved results are achieved with the optimized Otto engine compared to the Diesel engine. The acceleration time from 0 to 100 km/h is 9.7 seconds for the Otto engine and 24.7 seconds for the Diesel power plant.

Under mixed traffic conditions (city, road, and highway) the fuel consumption of both variants is about identical. Only in city driving (ECE-test) the Diesel engine consumes 8 % less fuel than the Otto engine due to its lower idling consumption. At high speeds (90 and 120 km/h) the fuel economy of the thermodynamically optimized Otto engine is 2 to 8 % higher.


Fuel Consumption Diesel Engine Fuel Economy Ignition Timing Brake Thermal Efficiency 
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Copyright information

© ECSC, EEC, EAEC, Brussels and Luxemburg 1980

Authors and Affiliations

  • D. Gruden
    • 1
  • R. Hahn
    • 1
  • H. Lörcher
    • 1
  1. 1.Research and Development of Propulsion Systems R & D Center in Weissach Dr.-Ing. h.c. F. Porsche AGGermany

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