The Combustion Chambers:

This was just a preliminary design but I later actually manufactured a combustion chamber out of teflon, although the original will be in high tensile steel. The main reason for manufacturing the model of the combustion chamber was to ascertain whether the CDN valve would work as postulated. It worked perfectly and should be no problem to implement. Here are some pictures of the model.

The above picture shows the slide valve and the CDN nozzle

The picture above shows how the combustion chamber is made up in three units, enabling the proper semi-circular shape leading to the CDN valve. See below:

Since the combustion chamber is cylindrical in shape the two critical stresses to deal with are (a) the circumferential or hoop stress and the (b) longitudinal stress.

            The hoop stress may be calculated by using the formula P x r / t  where :

P = Internal Pressure

r = radius of cylinder

t =  thickness of the wall of the cylinder.

            The longitudinal stress within the cylinder may be calculated using the formula:

P x r / (2 x t). It can therefore be seen that in all cases dealing with cylindrical shapes the longitudinal stress is half that of the hoop stress or circumferential stress. The hoop stress is therefore normally taken to be the critical stress. 

It is possible to work out the minimum wall thickness of the pressure chamber if the Tensile stress of the metal being used in the manufacture of the combustion chamber is known. Typical values for the tensile strength of steel are 17, 500 p.s.i.

            If the critical stress of the material being used and the dimensions of the pressure chamber are known it is possible to calculate the minimum wall thickness using the formula :   P x r /  TS   Where:

P = Internal Pressure.

r =  radius of cylinder

TS = Tensile strength of the material being used..

            Given that the measurements for the combustion chamber are as follows:

Radius = 1”.

P = Maximum internal pressure = 500 p.s.i.

T.S = 17,500 p.s.i.

Then the wall thickness needed is  500 x 1 / 17500 =  0.028”

            Thus it can be seen that even a relatively thin walled steel container can  hold a pressure of 500 p.s.i.  The hoop stress in a vessel with walls of  0.03in would be

500 x 1/ 0.03 = 16,666p.s.i, which is just below the ultimate tensile strength of the material being used.  Since safety measure require that the thickness of the wall possess a margin of at least seventy five percent of the tensile strength. A thickness of  0.25 ins. is sufficient. 500 x1 / 0.25 = 2000 p.s.i  circumferential or hoop stress, which gives a safe margin given that the tensile strength of steel is 17,500 p.s.i.

            The longitudinal strength would then be half the hoop strength as can be seen by the calculation 500 x 1 / ( 2 x 0.25 ) = 1000 p.s.i.

            Similarly it is possible to calculate the critical stress of the combustion chamber i.e., the point at which it would fail by using the formula:  TS/ ((r /(2xt))

17,500/ ((1/(2 x 0.25)) = 8750 p.s.i. Thus the vessel would burst if a pressure of 8750 p.s.i were present.

The great advantage of such a design is that it is easy to implement, carry out servicing and once it is clamped into position in the rotor it will be further strengthened. The walls of the combustion chamber are 1cm thick and are made of high tensile steel with a tensile limiting stress of 17,600 lbsf. The CDN valve is practically the only part of the engine, apart from the main shaft bearings and the poppet valve to require oil lubrication.  The combustion chamber can easily withstand the maximum pressure of 500 psi that is present after ignition. The question of cooling of the engine will be dealt with next.

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