Gasoline Blending Plus

Testing Diesel in Octane Engines

    Reasonably accurate predictions of diesel fuel Cetane number are available using correlations with density and boiling point.  However those correlations do not take into account effects of Cetane improvers.

    If a knock test laboratory does not have a Cetane test engine, a comparison of ignition quality can be made using a Motor octane test engine.  Diesel engine knock and spark ignition engine knock are opposite phenomena, and consequently high Cetane number corresponds with low octane number.

    In a diesel engine it is important for the fuel to undergo spontaneous ignition early so that the whole charge does not go off at once.  With spark ignition, the spark starts the combustion and the fuel should resist spontaneous ignition so that it does not occur and cause knock before all the fuel is burned.

    Diesel fuels have very low octane, and are expected to cause combustion chamber deposits in the Motor octane engine.  To minimize such problems, 50:50 blends of diesel fuel and isooctane were tested.  Octanes were still quite low, but a good correlation with Cetane number was obtained.  Effects of Cetane improvers were positive but not as large as in Cetane test engines.  No adverse effects on the test engine were found, but prolonged operation on such blends might cause problems.

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