CYCLE EFFICIENCY The pistonlike action of the expanding hot gases is in some engines
capable of producing rather powerful shock waves. There are cases in which the shock wave that is generated
by each explosion is forced to reverberate through the engine so that it may be utilize to precompress
the charge for the next or later explosion. The effect of precompression is to increase the initial temperature
of the combustion process therefore to decrease of the entropy rise associated with combustion. Pressure
exchange may be a very effective means of increasing the cyclic efficiency. However once account is taken
for the entropy increment produced by the shock waves in the pressure exchange process, the best cycle
efficiency is found to be obtained with a relative moderate precompression ratio.
Despite the mechanical simplicity of the pulse jet a great deal of information is still lacking about
such important details as its operating cycle. The full behavior through intake and at the discharge.The
timing and mechanism of ignition and the combustion process. This lack of basic information has necessitated
the use of quite arbitrary assumptions in all theoretical investigations of the pulse combustion. It
has been assumed to take place a constant volume or in the mode of a plane propagating flame layer or
has been approximated by an exponential change of temperature with time in the combustion chamber.A high
central peak pressure rise occurrs simultaneously and instantaneously over the whole calm of the fresh
charge. Changes of state of the gases are the results of this analysis are significant and that the
help to explain the experimentally observed effects of assigned parameters and operating conditions
on performance of the pulse jet. The design criteria for the improvement of the pulse jet performance
requires a more adequate account of periodic phenomena than the method of characteristics is capable
providing. A good deal of qualitative information concerning the gross effects of some factors can be
obtained from the inspection of the wave diagrams. For example the wave diagrams suggest the following;
INCREASING THE COMBUSTION CHAMBER LENGTH Increasing the combustion chamber length without changing
the total length of the engine will increase the path of expansion wave therefore delaying the formation
of the shock wave and increasing duration of the intake. And of the cycle. INCREASING THE OVERALL
LENGTH Increasing the overall length without changing the combustion chamber length increases the
duration of the cycle and of the intake. But the increases is nonlinear because the deflection of the
wave pass in the region of their mutual interaction.
CYCLE EFFICIENCY The effect of precompression is to increase the initial temperature of the combustion
products and also to decrease the entropy rise associated with combustion. Therefore pressure exchange
may be a very effective means of increasing the cycle efficiency pressure. However one account is taken
of the entropy increment produced by the shock waves in the pressure exchange process. The best cycle
efficiency is found to be obtained with a relatively moderate precompression ratio. Moderate compression
ratios are found to maximize efficiency. This likely efficiency of the pulse jet engine may be adversely
affected by flow fluctuations occurring in certain areas of the engine. The pulse engine has an advantage
over continuous flow types. Maximum allowable gas temperature is often considerably higher in non-steady
flow type engines that in steady flow type engines. This is possible because no parts of the engine are
exposed to continuous hot gas flow. The shock motion in the diffuser or intake section decreases considerably
when the frequency of these pressure ways are generated closer in succession. Their neutralizing interaction
takes place over a shorter distance therefore the intake duct required to prevent the back pressure disturbances
from reaching the intake is inversely proportional to the disturbance frequency. Losses associated with
the oscillation at the diffuser and their shocks may be greatly reduced by increasing the diffuser length.
In multi-unit configurations with a large number of intermittent flow chambers operating out of phase
it should be possible to achieve big increases of efficiency. Compared to those in steady flow diffusers
shock losses in reservoir type diffusers may also reduce shock losses.
