Fact #940: August 29, 2016 Diverging Trends of Engine Compression Ratio and Gasoline Octane Rating

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From the 1920s to the 1970s, the evolution of engines (measured by compression ratio) and the evolution of fuels (measured by octane rating) occurred in tandem. Gasoline octane improvement during that period (red markers in the graph below) was likely due to refinery technology improvement and the addition of lead, which guards against engine knocking. In 1973, the Environmental Protection Agency (EPA) mandated a reduction in the lead content of gasoline and eventually banned the use of lead in fuel for on-road vehicles. Since that time, other sources have been used as fuel oxygenates to control engine knock and the average octane rating of gasoline has been fairly constant at about 88-90 AKI (anti-knock index).

The engine compression ratio of new cars and light trucks (black markers below) improved along a similar course as octane rating from the 1920s to the 1970s. After that time, the average compression ratio continued to improve due to advanced engine design and controls, diverging from the octane trend. There is some concern that in the future, auto manufacturers will reach the limit of technological increases in compression ratios without further increases in the octane of the fuel.

Average Engine Compression Ratio Compared to Average Gasoline Octane Rating, 1925-2015

Average engine compression ratio compared to average gasoline octane rating from 1925 to 2015

Note: AKI = anti-knock index.

Fact #940 Dataset

Supporting Information

Average Engine Compression Ratio Compared to Average Gasoline Octane Rating, 1925-2015
YearAverage Compression Ratio for New Light VehiclesAverage Octane Rating (AKI) YearAverage Compression Ratio for New Light VehiclesAverage Octane Rating (AKI)
1925not availablenot available 19718.6490.08
1926not availablenot available 19728.4690.25
19274.44not available 19738.1390.13
19284.53not available 19748.3489.67
19294.57not available 19758.3289.71
19304.6361.44 19768.2789.62
19314.7261.46 19778.2889.63
19324.8762.10 19788.2989.43
19335.1064.46 19798.3089.49
19345.3568.47 19808.4088.97
19355.6670.46 19818.5089.01
19365.9870.46 19828.5888.80
19376.1371.02 19838.6688.04
19386.2272.16 19848.6988.25
19396.2872.76 19858.8188.25
19406.2874.05 19868.9588.10
19416.2677.32 19878.9888.22
19426.3876.53 19889.0288.40
1943not available75.01 19899.0488.45
1944not available74.11 19909.0088.27
1945not available72.27 19919.0088.19
19466.4777.83 19929.1088.24
19476.4977.54 19939.1088.25
19486.4977.79 19949.3088.26
19496.4778.17 19959.3088.26
19506.8679.81 19969.3088.10
19516.9081.19 19979.3088.05
19527.0480.52 19989.3588.10
19537.3481.54 19999.3988.04
19547.5282.33 20009.4287.87
19557.9283.48 20019.5387.86
19568.4985.15 20029.5887.88
19578.9885.88 20039.6487.82
19589.2486.61 20049.7087.75
19599.0687.02 20059.7687.66
19608.9187.81 20069.8787.61
19618.8488.04 20079.9487.59
19629.0788.26 200810.0487.54
19638.9188.46 200910.0987.55
19648.7988.72 201010.2287.53
19659.0289.02 201110.2687.52
19669.2089.24 201210.3487.57
19679.2689.77 201310.3987.59
19689.4389.84 201410.5087.60
19699.4890.02 201510.5287.65
19709.5290.05    

Note: Average octane rating based on refiner sales volumes.
Sources:
Frontiers in Mechanical Engineering, "A Historical Analysis of the Co-evolution of Gasoline Octane Number and Spark-Ignition Engines," January 6, 2016.
2014-15 Average octane rating calculated from Energy Information Administration, Refiner Motor Gasoline Sales Volumes, accessed June 29, 2016.
2015 Average compression ratio calculated from Ward's Auto, "North America Light Vehicle Engines Availability & Specifications, 2014," accessed June 29, 2016.

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