The Quantitative Effect of Engine Carbon on Detonation 270031

METHODS adopted and results obtained in an investigation of the quantitative effect of engine carbon on detonation are described, together with the standard methods of detonation and carbon-deposition measurement that were used. It is stated that carbon deposition is believed to influence detonation in proportion to the greatest thickness of deposit over any considerable area of the combustion-chamber surface. Since this is indicated, it is suggested that detonation tests should supersede gravimetric carbon-deposition tests, inasmuch as the objection to carbon is because of its detonation-inducing characteristics, which are governed by the character and the thickness of the deposit.

The tests were made with a four-cylinder, model DU-8 Waukesha motor-truck engine of 4½-in. bore and 6¼-in. stroke, direct-coupled to a Sprague Electric dynamometer. A crankcase jacket permitted control of the oil temperature. The temperature of the jacket water at the inlet was maintained at 100 deg. fahr. and that of the carbureter air at 200 deg. fahr. A mercury barometer was attached to the intake-manifold for indicating absolute intake-manifold pressure. For antiknock tests, the engine was provided with high-compression cylinder-heads and was operated at a speed of 400 r.p.m. The throttle was opened gradually and the load was increased simultaneously until a point was reached at which a certain definite standard amount of detonation occurred. The absolute intake-manifold pressure was then recorded. A calibration was then made by using a standard fuel and observing the manifold pressure at which it detonated.

Standard fuels were prepared by adding various quantities of “ethyl fluid” to Texaco spirits, the latter being a cut distilling at a temperature of from 300 to 425 deg. fahr. The carbon-deposit results, excepting the carbon-remover tests, were obtained in the course of routine antiknock test-work; that is, the deposit was allowed to accumulate for a period while routine tests were being made. Hence the carbon was deposited under non-uniform operating-conditions as regards fuel and load.

To measure the effect of the carbon on various parts of the combustion-chamber surfaces of the engine in increasing detonation, the engine was operated for about 40 hr. under the standard conditions. It was then dismantled and the deposit was scraped from the cylinder-head only, leaving the cylinder-block, the valves and the pistons with a considerable depth of carbon. The engine was then reassembled and operated, with frequent calibrations, for another period of about 40 hr. Later, the same procedure was used, scraping the cylinder-block, the valves and the pistons, instead of scraping the cylinder-head.

A test was made of a popular carbon remover, not on the basis of the comparative weight of the carbon deposit but on the basis of the comparative detonation-inducing tendency of the deposit. Fresh oil was added every 15 min. The engine was equipped with a Stromberg carbureter having a No. 57 orifice but no needle-valve or idling-jet.

Points brought out in the discussion relate to whether various amounts of carbon accumulation can be used to determine whether some one combustion-chamber is a better type than some other; the effect of turbulence; the effects of additions of nitro-benzene and of aniline; and the desirable characteristics of oils.