It has long been recognized that ignition of the fuel-air charge in Otto cycle engines can be and sometimes is induced by sources other than the electric spark. These additional sources of ignition may be any hot surface to which the fuel-air mixture is exposed. However, since the sources of ignition, other than the spark, which prevail are quite often transient in nature and difficult to reproduce, a systematic study of their effects on the performance of engines actually operating in service is a difficult and time consuming task. This difficulty can be circumvented, however, by employing controlled additional sources of ignition such as extra spark plugs or artificial and controllable hot spots.
This paper presents the results of a series of tests made to study the influence of a controlled second source of ignition on performance in a single cylinder CFR overhead valve engine. An independently timed second spark plug was used to simulate surface ignition.
It was found that surface ignition simulated in this manner can either increase or decrease the power output of the engine. The magnitude and even the direction of the change are largely dependent upon the basic spark timing of the engine as well as the time at which the second ignition takes place.
As was to be expected, surface ignition can increase the level of fuel octane quality needed to eliminate noise (knock). When the engine was operated with spark timing retarded from peak power, simulated surface ignition was observed to be capable of causing substantially greater changes in octane requirement than when the engine was timed for peak power development. It was also observed that the no-noise octane requirement of the engine could be increased by a second ignition occurring either before or after the firing of the basic spark plug. The effects exerted by simulated surface ignition on power output and octane requirement are consistent with the rates of pressure development in the engine cylinder and the peak pressures reached.


Subscribers can view annotate, and download all of SAE's content. Learn More »


Members save up to 40% off list price.
Login to see discount.
Special Offer: With TechSelect, you decide what SAE Technical Papers you need, when you need them, and how much you want to pay.