Did you know that a glow engine is a semi-diesel? Hot engines have been known to start without the glow plug hooked up. To best understand the difference between glow and diesel, it’s best to understand the basic difference between the combustion process in each system.
The main ingredient in glow fuel is methanol, which ignites at a rather high temperature. The compression of the engine plays a part in raising the temperature, but not high enough to ignite it under normal conditions. The hot platinum element in the glow plug causes the mixture to ignite. The nitromethane in the fuel liberates oxygen to help alcohol burn faster. The faster the alcohol burns, the more you can burn to produce more power.
The conductive heat of the glow plug, to the head, together with the heat loss of the combustion serves to produce lots of waste heat at the top of the glow engine. This heat must be removed effectively to allow normal operation of the engine. Running your glow engine slightly rich allows the fuel itself to carry off much of this waste heat. This of course reduces the power somewhat, a small price to pay for the benefits gained in extended engine life; it is however, rather wasteful.
Exhaust back pressure also plays a part in heat balance, and thus the performance of your engine. Just as the carburetor restricts the inlet air and fuel, thus slowing your engine down, the muffler restricts the outlet air (exhaust) and also slows your engine down. The hot exhaust is rapidly expanding as it leaves the engine and enters the muffler. The muffler restricts expansion somewhat, causing a less than free flow of hot exhaust from the engine. This in turn causes the engine temperature to rise, since it can no longer freely expel its waste heat (exhaust). Mufflers, as you know, provide a measure of sound reduction and a ready source of pressure for aiding fuel flow to the engine. A very important point to remember based on these facts is that in glow operation, peak power can never be safely or reliably achieved in actual use due to the fine line between just right, and too lean.
The main ingredient in diesel fuel is kerosene, which ignites at a significantly lower temperature than alcohol. To raise the temperature of this mixture to its ignition point by compression alone would be impractical in a model diesel engine, since the means to create proper atomization cannot be achieved by stock carburetion at this time. To start the combustion of the kerosene, ether is employed. The lower ignition temperature of this combination of ingredients allows combustion to be achieved at a practical compression ratio. Ether and kerosene can be ignited by the heat increase brought about by compression alone; thus the fuel is burned faster, more completely, producing less waste heat, and thus more energy.
A diesel is a heat machine; unlike its glow cousin, the diesel can benefit from some heat buildup. The exhaust temperature leaving the diesel is much lower. This is due to the fact that less fuel is being burned more efficiently to produce more power than in a glow engine. Couple this with a more than generous muffler, designed for glow operation, and you have a condition where there is less back-pressure and thus engine operating temperatures is less than optimum for all conditions. This allows you to extend your exhaust away from your craft without fear of creating undo power loss, a desirable feature when it comes to cleanup time. Extending the exhaust also gives another side benefit, that of further reducing engine noise. Many modelers using diesel conversions have routed engine exhaust through a plastic tube through the entire length of the fuselage and out the tail, and by doing so have achieved a further degree of silencing impossible with glow operation.
|
|
Foreign News by Peter Chinn - “Expert Opinion”
In 1977, Peter Chinn referred to the Davis Diesel as “the first completely new approach to diesel conversion design in thirty years”. An “ingenious new American development of a foreign idea. . . should not be ignored.”
Again 1979, Peter Chinn, “On the basis of the dieselized OS 40 FSR tests, it has to be said that his engine makes mincemeat of most previously held convictions in regard to large diesels and, at the same time, confirms the feasibility of converting modern-ringed glow engines to diesel operation.”
|
Wow! 24x10" prop at 6000 rpm. Idles at 600.
Cox TD .049, 8x3" at 10,000+ rpm.
.40 cu. in. engine turns 12x7" prop at 10,000 rpm.
|
