<div class="section abstract"><div class="htmlview paragraph">The effects of physical factors on ignition delay have been studied on a motored research engine using a single injection technique. The fuels used included a high cetane number reference fuel, gas oil and M. T. 80 petrol.</div><div class="htmlview paragraph">The primary factors investigated are those pertaining to the fuel spray, such as injection timing, quantity, and pressure (affecting drop size, velocity and injection rate); hole diameter (affecting drop size and injection rate) and spray form (nozzle type); and those pertaining to the engine, such as temperature, pressure and air velocity.</div><div class="htmlview paragraph">Engine operating variables such as speed and load affect the ignition delay because they change the primary factors such as injection pressure, compression temperature, pressure and air velocity.</div><div class="htmlview paragraph">It has been found that under normal running conditions, compression temperature and pressure are the major factors. All other factors have only secondary effects.</div><div class="htmlview paragraph">Under starting conditions, when ignition is marginal, mixture formation becomes as important as compression temperature and pressure. Such factors as air velocity and spray form which affect the mixing pattern can have a very pronounced effect on ignition delay.</div><div class="htmlview paragraph">Published data on ignition delay are compared with those obtained in the present investigation and a generalization of the data is recommended for engine design and computational work.</div></div>