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Cooling of the RPJ Engine

The cooling of the Rotary Pulse Jet engine is the next critical factor that has to be considered. The fact that the engine rotates means that air-cooling would be far more efficient than that provided by fins on a stationary engine, such as a two stroke motorbike. However, the piston engine enjoys a considerable advantage in the cooling process in that the pistons are constantly being drenched in oil. To offset this one has to consider the tremendous frictional heat that is generated in a piston engine by the constant up and down movement of the pistons in the cylinder. In an engine functioning at high rpm say 7000rpm to 8000 rpm it is possible that the heat generated by friction is almost equal to that of the actual combustion process. So the Rotary Pulse Jet enjoys a distinct advantage in this respect, in that there are no moving parts to give rise to frictional heat. Secondly, the extremely short time for which the heat is present in the combustion chambers of the RPJ, something on the order of 1/400 of a second from the start of combustion to the ejection of the hot gases at speed, means that the engine should be considerably cooler while running. By contrast a piston engine tends to retain the heat in the cylinder for considerably longer times 1/50 of a second being typical. This being so it is possible that cooling fins on the RPJ should carry away heat efficiently enough for the engine to function without undue stress. If this is not the case and water cooling is required, than it can be achieved with a slightly more complicated design. This would be a pity because it is the extreme simplicity of the RPJ engine design that is most appealing. The melting point of steel is 1370 ⁰ C ( on average.) Yet typical maximum temperatures within the IC piston engine combustion chamber are as high as 2000 ⁰C . How is this possible? Again, even though the maximum temperatures inside the combustion chamber can rise to as high as 2000⁰ C , the pistons are often made of aluminium, which has a melting point of about 850 ⁰ C. How is this possible why don't the pistons melt ?. There are many factors involved in the explanation, the most important of which are the phenomenon of quenching, and the transfer of heat. The phenomenon of quenching takes place when the walls of the combustion chamber, which are at a lower temperature than the combustion gas try to absorb some of the heat of combustion, this results in a layer of gas a few molecules thick forming a layer on the combustion chamber walls preventing the heat of the combustion gases directly affecting the walls. The phenomenon of quenching is especially important when dealing with combustion chambers in rockets. Secondly a large amount of the heat of the combustion gases is transferred to the outer walls. However the most important factor is the time for which the heat is present, since in most cases this is a few fiftieths of a second appreciable damage from high temperature is avoided.

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