Pulse-Jet

[forty-two]

Sorry to be totally off the wall, but some thoughts have occurred to me of late about the current regs and quite where the boundaries are. Perhaps some of the esteemed members can advise?

I understand that the retarded ignition concept works when off throttle by timing the spark such that the combustion takes place as the fuel-air mix is being pushed out of the cylinder. One of the problems with this is that an amount of unburnt fuel-air is finding it's way into the exhaust system, but the benefit is that the amount of gas coming out of the end of the pipe is more regular while off throttle allowing the exhaust gas to be used to augment downforce giving devices such as the blown diffusor (please correct me if I am wrong!).

Well, it occurred to me, given the current development work going on with Renault's Forward Exhaust Exit, but more generally the idea of using exhaust gases for gain, if a suitably motivated team could find a way to actually use any unburnt fuel finding it's way into the pipe and allow it to combust there.

The title of this thread is perhaps misleading, but AFAIK, the Pulse Jet employed on the V1 bomb during WW2 consisted larely of a large bore pipe which had a combustion chamber in it's middle, and two "exhaust" pipes leading out of each end of it, with one pipe bent around to vent in the same direction as the other.

Well, what if, in an attempt to increase mass flow, perhaps only while off-throttle, a team were to design a system where lots of unburnt fuel/air was being pumped into a combustion chamber, and that combustion chamber was cleverly arranged so as to allow combustion, but the products of that combustion were vented outwards through two additional outlet pipes? A suitable glow-plug could theoretically be used to ignite any unburnt fuel arriving in the chamber, and in theory, this could produce significantly more mass flow at the crucial moments.

The technology required to acheive such a device is not "rocket science" (pardon the pun!), as it was successfully achieved on Scrapheap challenge using only bits of junk:
[youtube]http://www.youtube.com/watch?v=8QoP4zX8Gwc[/youtube]

I know it sounds off the wall, and I doubt very much if anyone would be mad enough to actually try it, but I don't know of any specific regulations which would explicitly prohibit it.

Just a stupid idea off the top of my head so don't flame me!

[Ciro Pabón]

We won't flame you, the engine will do it for us. If it works, it will be forbidden in the blink of an eye AND I ask for not being the guy raising the back of the car in pit stops. This position is reserved for my ex mother in law.

I did some quick calculations. I couldn't find how to calculate a valveless pulse jet engine, so I did it for a pulse jet with valves, with this: http://www.pulse-jets.com/download/pjc14.xls

First problem (no free lunches!): the valves. Using valves in an exhaust is not precisely what you want in a combustion engine, as they are, essentially, air pumps. You mix the air with gas for an Otto engine to work, but air pumps they are.

You want efficiency, so, restrictions in the exhaust are like Berlusconi saying hello to pitbabes in front of a TV camera: a bad combination.

You can use a low efficiency valve system. Of course you won't, your nick is 42. So, what about a high efficiency valve? (yes, yes, I want one of those). Better yet, a valve grid (even more efficient!). They restrict the flow, but... unless you can come with numbers for a valveless, I'm stuck here.

I started with a 200 kilograms thrust (desired).

Big problem. The diameter of the exhaust pipe is 30 cm (well, feasible, even if the efficiency of the combustion engine would go to FIA, erm, I mean, hell) but I get a pipe length of 2.8 meters... No, no. Combustion chamber length is a mighty 60 cm. Exit cone, the same. A pipe a foot in diameter seems a little ricer-y, if you ask me.

Now, let's try with 100 kg of thrust (you will notice that, sure).

Same shite. The length of the pipe is 2.14 meters. That's a mighty exhaust, although it could work in some drifting cars and old Cadillacs.

More modest thrust of 50 kg gives me a pipe of 1.64 m. Darn.

What about 25 kilos? Pipe shortens to 1.26 m. FIA will notice.

So, how long can the exhaust be? At 10 kgf (a misery, if you ask me) you need 89 cm. Exhaust diameter is 7 cm. Combustion chamber diameter is 10 cm for a valve grid and 13 cm for a low efficiency one. Feasible... but 10 kilos perhaps is not what you're looking for.

Provide a number for the pipe diameter and length you can give to the pulse jet engine and the thrust is determined. You already got the idea, I'm sure.

The question is: how long can the pipe be? This is related directly to thrust, no variations here, the formulas are pretty precise (or so I've heard).

You will use a pile of fuel, btw and the pipe will get HOT, like... well, like Berlusconi in front of pitbabes. Be prepared (I'm always) to increase a bit the size of the radiators and to put a mighty heat shield around the exhaust, the crash structure and the rear wing... that might weight 10 kilos.

I want my free lunch!

[ringo]

I built a valved pulse jet once. It's just 30 inches long, but it's very very loud; about 150 decibels if i remember. the FIA will definitely notice that.
For a small engine, the valves don't last very long either.

A valveless engine is probably more fitting, but it wont be as powerful as the valved, it will be less sensitive and it needs 2 unequal diameter exhaust/intake pipes.
And finaly fuel needs to dumped into them so in essence you will be carrying 2 engines on your car, or 3 if you're putting a pulsejet per bank.

[Shaddock]

I can't see any where in the reg that say that teams can't fit an injector into the exhaust manifold. It's not the thrust you (for thrust sake) want but the fast moving expanding gases over/into or to the side of the diffuser.

[forty-two]

I can't see any where in the reg that say that teams can't fit an injector into the exhaust manifold. It's not the thrust you (for thrust sake) want but the fast moving expanding gases over/into or to the side of the diffuser.

That's the lines along which I was thinking. Only the "injection" could take place by allowing unburnt fuel/air to "fall" into the pipe while off throttle to be burnt while in a part of the pipe in order to increase mass flow while off throttle.

I wasn't actually suggesting the flow be used to push the car along!

I don't know of anything in particular in the regs which would prohibit such an idea.