A Modified Cetane Scale for Low Ignition Quality Fuels

Detailed analysis was performed on the Cetane Scale used to rate the auto ignition quality of fuels; the effect of an increase in temperature and pressure, due to increasing the compression ratio, on ignition delay was theoretically and experimentally evaluated. The CFR cetane variable compression engine used in the investigation had a combustion chamber divided into two parts connected by a passage tangential to the prechamber. Cylinder gas pressure, injector needle lift, fuel line pressure, fuel flow rate, air flow rate, intake air heater performance, and intake air surge tank performance were measured. Experimental work was conducted in four phases: engine certification; analysis of cetane number auto ignition parameters; techniques to modify the Cetan Scale to rate low ignition quality fuels; and comparison between fuel ratings on 150-degree F and 350-degree F scales. Results showed ignition delay was more sensitive to air temperature than air pressure. Sensitivity increased with a drop in fuel cetane number. A modified cetane scale was developed by raising the inlet temperature from 150 to 350 degrees F without changing the rest of the rating technique; this extended the scale to zero cetane number and enabled the rating of low ignition quality fuels. Increasing intake air temperature was effective in reducing the compression ratio required to rate low ignition quality fuels. The cycle-to-cycle variation in the length of the ignition delay decreased with a raise in intake air temperature. Fuel blends with low cetane numbers exhibited sensitivity and resulted in lower cetane ratings at higher inlet temperatures. It is recommended that a study be made of the factors affecting the cycle-to-cycle variation in the length of ignition delay of low ignition quality fuels (e.g. gasoline fuels), of cetane scales at intake temperatures other than 350 degrees F, and of the problem of fuel sensitivity of low ignition quality fuels when the intake temperature is increased. Engine specifications and information on the computer model for the compression process in the CFR cetane engine are appended.