1972-02-01

New Aspects of Preignition in European Automotive Engines 720114

Research has been done on preignition in automotive engines using a new experimental method enabling the phenomenon to be followed from its onset to its final phase. The technique consists of interrupting conventional spark ignition for a brief period and detecting any uncontrolled ignition signal at no matter what instant in the cycle it may occur.
With regard to the danger of preignition, it was demonstrated that fuel effect depends on the following two properties: specific resistance to hot-point ignition and capacity to heat up a possible ignition source. The latter characteristic is linked to both combustion speed and fuel flame temperature.
The source of preignition is usually the exhaust valve covered with mineral deposits coming from fuel and lubricant additives. The most harmful deposits are made of barium and calcium phosphates coming from detergent additives in lubricants; whereas lead compounds coming from antiknock additives in fuels have no appreciable effect.
The effect of thermal parameters concerning engine design and operating conditions was also examined. In particular, it was demonstrated that preignition may appear no matter what the prior engine operating conditions may have been, provided that the engine is using large amounts of a harmful lubricant.
Physicochemical transformations were also examined inside exhaust valve deposits during preignition.
A synthesis of the results obtained reveals the most effective ways of eliminating or reducing the danger of preignition in automotive engines. Such ways consist mainly of decreasing the harmful influence of various metallic lubricant additives on exhaust valve deposit properties.

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