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    Fact of the Week

    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.Plot chart showing the Average Engine Compression Ratio Compared to Average Gasoline Octane Rating, 1925-2015Note: AKI = anti-knock index.

    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)
    1925 not available not available   1971 8.64 90.08
    1926 not available not available   1972 8.46 90.25
    1927 4.44 not available   1973 8.13 90.13
    1928 4.53 not available   1974 8.34 89.67
    1929 4.57 not available   1975 8.32 89.71
    1930 4.63 61.44   1976 8.27 89.62
    1931 4.72 61.46   1977 8.28 89.63
    1932 4.87 62.10   1978 8.29 89.43
    1933 5.10 64.46   1979 8.30 89.49
    1934 5.35 68.47   1980 8.40 88.97
    1935 5.66 70.46   1981 8.50 89.01
    1936 5.98 70.46   1982 8.58 88.80
    1937 6.13 71.02   1983 8.66 88.04
    1938 6.22 72.16   1984 8.69 88.25
    1939 6.28 72.76   1985 8.81 88.25
    1940 6.28 74.05   1986 8.95 88.10
    1941 6.26 77.32   1987 8.98 88.22
    1942 6.38 76.53   1988 9.02 88.40
    1943 not available 75.01   1989 9.04 88.45
    1944 not available 74.11   1990 9.00 88.27
    1945 not available 72.27   1991 9.00 88.19
    1946 6.47 77.83   1992 9.10 88.24
    1947 6.49 77.54   1993 9.10 88.25
    1948 6.49 77.79   1994 9.30 88.26
    1949 6.47 78.17   1995 9.30 88.26
    1950 6.86 79.81   1996 9.30 88.10
    1951 6.90 81.19   1997 9.30 88.05
    1952 7.04 80.52   1998 9.35 88.10
    1953 7.34 81.54   1999 9.39 88.04
    1954 7.52 82.33   2000 9.42 87.87
    1955 7.92 83.48   2001 9.53 87.86
    1956 8.49 85.15   2002 9.58 87.88
    1957 8.98 85.88   2003 9.64 87.82
    1958 9.24 86.61   2004 9.70 87.75
    1959 9.06 87.02   2005 9.76 87.66
    1960 8.91 87.81   2006 9.87 87.61
    1961 8.84 88.04   2007 9.94 87.59
    1962 9.07 88.26   2008 10.04 87.54
    1963 8.91 88.46   2009 10.09 87.55
    1964 8.79 88.72   2010 10.22 87.53
    1965 9.02 89.02   2011 10.26 87.52
    1966 9.20 89.24   2012 10.34 87.57
    1967 9.26 89.77   2013 10.39 87.59
    1968 9.43 89.84   2014 10.50 87.60
    1969 9.48 90.02   2015 10.52 87.65
    1970 9.52 90.05        

    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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