- The Washington Times - Wednesday, October 29, 2003

Diesel engines operate without ignition systems (heat of compression ignites the fuel) and are therefore sensitive to the rate of combustion.

Ignition quality is measured by ignition delay, the time between the start of fuel injection and the start of fuel combustion. Too long an ignition delay at high engine loads leads to an excessively rapid increase in pressure within the cylinder before combustion, which in turn leads to diesel knock, or rough operation, especially on cold starts. Excessive pressure rise rates are theoretically caused by extra chemical reactions occurring before combustion.

The products formed by the extra reactions burn rapidly. Rapid burning causes excessively rapid pressure rise in the cylinder. With a shorter ignition delay, the extra reactions don’t have a chance to occur, hence the rate of pressure rise is reduced.

Diesel fuels are measured according to their ignition quality and are given a Cetane number. Cetane, the colorless, oily hydrocarbon on which the cetane number scale is based, has extremely good ignition quality and is assigned a number of 100. Fuels with higher Cetane numbers give short ignition delays and good cold-start and driveability. Lower numbers mean a fuel doesn’t ignite as efficiently, of course, and at present Cetane numbers in North America may be as low as 37.

The EPA must address this issue as well as the sulfur content. Not doing so will result in the need to use aftermarket Cetane boosters, additives that might cause emission controls to function unpredictably.

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