The Starting of High-speed Oil Engines
Page 52
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I N a contribution .to a symposium on
the starting of oil engines, arranged iecently by the Diesel Engine Users Association, Mr. J. H. Pitcbford, of Ricardo and Co., Ltd., dealt with high-speed types of engine and commented on the various factors that influence their starling abilities. Given sufficient turning power, manual or mechanical, ease of starting increases with cylinder size, because the surfacevolume ratio of cylinder and combustion chamber decreases and, as a result, the beat losses are reduced.
Similarly, the direct-injection type of engine is inherently better in respect of cold starting than the indirect injection type, for the heat lossesare lower in the former and a lower compression' ratio can be employed. In both the above cases the use of auxiliary heating devices has not been considered. Also, the indirect-injection type of engine is more easily started if, at starting speeds, atomized fuel penetrates a stagnant high-temperature zone
in the combustion chamber. This principle is used with success in the Perkins " Acrofla " and the Ricardo " Whirlpool" combustion systems.
A high-compression ratio has a beneficial effect on ease of starting and smooth transition from compression to ignition, but adversely affects the maximum load-carrying capacity. of the engine, as it increases the proportion of the total clearance volume absorbed in parasitic clearances such as valve recesses, head-to-piston clearance, etc.,
and the optimum compression ratio tor any particular engine is, therefore, a compromise. It is immediately apparent that the flywheel size, which is often arbitrarily limited in roadvehicle engines, the ignition quality of the fuel, and the viscosity of the lubricating oil, all influence starting.
Aids to cold starting include the injection, when starting, of excess fuel, and advancement of the injection timing; the latter, however, is open to abuse if manually controlled.
Heater Plugs v. Torches.
Electric heater plugs and detachable torches are well known. The latter are much more effective than the former, and a self-igniting variety of torch cartridge is now available which is particularly valuable in multicylindered engines. To be really useful, electric air-intake heaters should be of about 200 watts per litre of cylinder-swept volume. Fuel or lubricating oil may be introduced into the inlet ports before starting to seal the piston rings and valves and raise the compression ratio in proportion to the volume of oil' introduced.
From a graph reproduced in the paper it appears that the addition of 10 c.c. to 20 c.c, of off: per litre of cylinder volume is very, effective as an aid to starting. It is stated also that this practice reduces very considerably the bore wear that results from corrosion under cold-starting conditions.