Combustion chamber area compared to whole engine area ?

[Rocket Man]

Does anyone know how combustion chamber volume effects the performance of a pulse jet engine?

The combustion chamber volume of my new engine is 23.78% of the whole engine volume. The fuel is extremely critical on this engine, if it is off plus or minus about 1% the engine won't run.

What I am wondering is, what is the combustion chamber volume compared to the whole engine volume? Bruce Simpson says 20% and he claims it is not critical. Is there any research date that compares different combustion chamber volume to the whole engine volume that shows a certain area is best???

I was reading German V1 research data plus several other pulse jet research data sheets where they compare physical changes in the engine, 10% longer exhaust tube makes the engine easer to start and extends the life if the valves. The longer exhaust lowers the pulse rate and produces more thrust. Smaller combustion chamber cross sectional area extends the life of the valves. Nothing in the information about the actualy combustion chamber total volume.

Another question, is the cone volume considered to be part of the combustion chamber volume?

This is what I know. How close is this to being correct?

Thrust = 3.6 x tail pipe cross sectional area.

Combustion chamber cross sectional area = 2 x tail pipe cross sectional area.

Tail pipe length = tail pipe diameter x 15.

Combustion chamber volume = .2 x whole engine volume.

Cone length = combustion chamber length.

Combustion chamber length = combustion chamber diameter.

Air intake cross sectional area = .5 x combustion chamber cross sectional area.

EXAMPLE: The Pulso 1 engine.

Exhaust pipe volume = 58.573 cu in.

Cone volume = 30.88585 cu in.

Combustion chamber volume = 43.2512 cu in.

Exhaust pipe = 44.136% of total engine volume.

Cone = 23.273% of total engine volume.

Combustion chamber = 32.590% of total engine volume.

If cone is part of combustion chamber then total combustion chamber volume = 74.13705 cu in.

Then combustion chamber = 55.863% of the total engine volume.

[SR71Fan]

Your description is sort of confusing; first you talk only in ratios, then you talk
only in volumes, no ratios. Is this engine some sort of secret design?

In any case, here is my opinion:

a) consider the cone to be part of the combustion chamber.
In this case, Pulso 1's CC of 56% is much larger than the 20%
value of Bruce Simpson, which I believe is a good target.

b) your air inlet is probably two times too large. That is probably why
this engine will not run well. When the air inlet is too large, the engine
cannot generate a proper vacuum for the exhaaust back-flow required
to ignite the next fuel/air mixture.

I would suggest that if you reduce the diameter of your exhaust pipe,
and reduce the air inlet area, your engine will run better.

SR71

[Rocket Man]

I see several typing errors in my original post but I no longer have an EDIT option so I can not correct it.

I made a change in the demensions of the combustion chamber of my engine today. I increased the combustion chamber length by 4" longer. Now the engine starts easy and runs excellent. I ran it on gasoline first time then propane second time. I did a static test run 5 minutes each time. The reed valves are showing no signs of damage and they show very little signs of getting hot.

Combustion chamber length = 2 X combustion chamber diameter.

[Viv]

Sorry but I have never really been a great believer in this sort of calculated ratio/relationship thingy that goes on with pulse jets, to me its a bit like trying to calculate the predicted length of an elephants trunk based on the combined area of its feet, and I know I am probably leaving myself wide open on that as there probably is a relationship between an elephants trunk and its feet just waiting to be calculated

Thats not to minimize or denigrate all the work and thought thats gone in to this over the years but it just does not sit easy with me as an actual empirical method but rather an accidental relationship that some times gives a near enough to correct answer to work some times, its like the old electrical analogies we worked with at the beginning of the forum they were close but also had an annoying error that could not be corrected.

Any way just my ten pfennig

Viv