Effect of free-radical release on diesel ignition delay under simulated cold-starting conditions

Peak compression temperatures typical of those achieved when cranking a cold diesel engine can be produced by running a gasoline engine of compression ratio 7.5, with a block temperature of 100°C. Under the chosen operating conditions, diesel fuel of cetane index 43 would ignite only very feebly, with ignition delays in excess of about 9 ms, but in the presence of various additives, iso-octyl nitrate, di-tert-butyl peroxide, tert-butyl perbenzoate, 1,1-di-(tert-butylperoxy) cyclohexane, and 1,1-di-(tert-butylperoxy) 3,3,5-trimethyl cyclohexane, ignition delays of between 3 and 7 ms were observed; although the thermal decomposition rates of these additives span approximately a factor of 60, the ignition delays do not correlate with these rates. Some measurements were made on the effect of diethyl ether, either blended with the fuel or bled into the air intake: its efficacy as a cold-starting aid stems from the fact that diethyl etherair mixtures ignite spontaneously during the compression stroke, thereby causing the fuel to ignite as well.