Performance Scaling of Spark-Ignition Engines: Correlation and Historical Analysis of Production Engine Data 2000-01-0565
This study examines the scaling between engine performance, engine configuration, and engine size and geometry, for modern spark-ignition engines. It focuses especially on design features that impact engine breathing. We also analyze historical trends to illustrate how changes in technology have improved engine performance. Different geometric parameters such as cylinder displacement, piston area, number of cylinders, number of valves per cylinder, bore to stroke ratio, and compression ratio, in appropriate combinations, are correlated to engine performance parameters, namely maximum torque, power and brake mean effective pressure, to determine the relationships or scaling laws that best fit the data.
Engine specifications from 1999 model year vehicles sold in the United States were compiled into a database and separated into two-, three-, and four-valves-per-cylinder engine categories. The results for four-valves-per-cylinder engines were further divided into fixed and variable-valve-timing valvetrains to examine the effects of improved breathing on engine performance. Turbocharged and supercharged engines were also grouped together and compared to naturally aspirated engine performance. Historical data from the prior 15 years was also stored in a database and used to provide information on engine performance trends and chart technical changes in the industry over the period 1984 to 1999.
The results from the scaling law analysis demonstrate an excellent correlation between maximum torque and engine displacement. However, a linear relationship between maximum power and engine displacement did not correlate as well. A better relationship was obtained by dividing maximum power by the maximum mean piston speed and correlating that ratio to total piston area. Improvements in performance of about 20 percent are attainable through engine breathing improvements of 4-valve (and 3-valve) engines over 2-valve engines in normally aspirated engines, and about 10 percent for variable compared to fixed valve timing. Larger increases are realized in supercharged and turbocharged engines. Historically, the transition from two- to four-valves-per-cylinder engines, the introduction of variable valve timing, improvements in friction, improved intake and exhaust system design, and other changes, contribute to a steadily improving maximum bmep (or normalized torque) of about 1.5 percent per year in spark ignition engines over the past 15 years.