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THE DAVIS DIESEL CONVERSION. The Davis diesel conversion is not a foreign product (it is manufactured by Davis Diesel Development, Box 141. Milford. Connecticut 06460) but it is so interesting that we could not resist commenting on it in this column. If any other excuse for doing so is necessary. Let's say that, as the model diesel motor is European in origin, this ingenious new American development of a foreign idea should not be ignored.
Naturally, since model diesels depend on good compression to generate the heat necessary for self-ignition of the special fuel mixtures used, an engine with good piston seal is essential for trouble-free starting and good performance. This, no doubt, was one reason for choosing Cox engines for the initial application of the DDD system. Robert Davis has, in fact, experimented with similar conversions on all the Cox motors, including the TDOIO. He reports that they all run well and he is now working on heads for other popular motors, the largest of which is a 40. One would expect that the ABC type engine, its ringless piston and tapered bore normally pros. idi very good cold compression, would be among wie most easy-starting conversions.
There is on other significant advantage with the DDD type head, compared with the usual tight-fitting diesel contra-piston. This is the fact that the DDD compression control (a knob, rather than a lever) moves more freely; sufficiently freely, in fact, for the manufacturer to suggest that, by fixing an arm on the compression knob, this can be linked to a servo and used to control engine speed.
Here. you may ask: What's wrong with a conventional carburetor throttle?" The fact is, as any reader who has tried to throttle a diesel will know, intake throttles do not work too well on model compression-ignition engines. If a diesel is throttled down, it naturally begins to cool off since less fuel is being burned in the combustion chamber and excess heat in the surrounding metal is quickly dissipated. With the DDD fluorocarbon seal, heat loss should be less rapid but if a conventional diesel is throttled down for more than. say, twenty seconds, it may not recover when called upon to deliver full power again. This is because, in a relatively cool combustion chamber, the fuel mixture will ignite too late resulting in the engine misfiring or cutting out altogether. To restore the engine to full power, one would need to be able to increase compression temporarily to advance the ignition timing, returning the compression setting to its original position when the engine had regained its normal full throttle working temperature. Remote control of the compression setting, either on its own or in conjunction with a carburetor throttle (or coupled to the needle-valve to enrich the mixture for smoother low speed running at reduced compression settings) would therefore be a very worthwhile refinement in a diesel intended for R C use.
The first completely new approach to the problem is. in fact, to be seen in Robert Davis' diesel conversion for the Cox .049/.051 engines, an idea which he plans to extend to other glow motors of larger displacement. U.S. and foreign patents_are currently pending on this invention and it is possible that it may also be licensed to manufacturers of existing diesel motors overseas.
Unlike the D-E Arden conversion, the Davis head screws straight into the cylinder in place of the standard glow head without the insertion of a separate steel liner for the contra-piston. The diesel head, machined from aluminum, itself contains the contra-piston which is also of aluminum but has an o.d. that is .001" smaller than the i.d. of the head so that a working clearance is maintained at all temperatures. Now comes the important bit. Before the head and contra-piston are fitted, a plastic disc of fluorocarbon material is located in the cylinder so that it rests on the head gasket seat and is held in position by the head when this is screwed home.
The disc has two purposes. First it seals the combustion chamber, obviating the need for a precision contra-piston fit, yet remains flexible enough to follow the small vertical movements of the contra-piston as compression is adjusted. Secondly, the fluorocarbon material of which the disc is made acts as a reflective insulator to reduce heat loss and thereby raise thermal efficiency,