Lowering the exhaust back pressure by sucking it out

dd-ardvark · Post by **dd-ardvark** » Tue Jan 11, 2005 9:21 pm

I'm sure this hot rod trick has been tried before: Lowering the exhaust back pressure by sucking it out...! In other words, a geared turbo connected to the exhaust system, to suck the exhaust back pressure down to a negative back pressure..., or at least much lower.

Right after WWII my father at the time was a pilot with the flight test division at Republic Aviation out of New York around 1945-54. In later years he told me of some of the tests that were done, all in the name of horse power and speed with an unlimited budget because of the fear that the Russians would get ahead of the USA in whatever. Anyway, He knew of at least two tests that were preformed by Airplane engine manufactures Rolls-Royce and Pratt-Whitney in and around 1943-46 respectively. The results (1943-46) of both were that it took more power to run the system than it gave out.

-Some of the highlights with these engines were that they were good flame throwers and had good healthy noises to them.
With the valve overlap of the cam keeping the intake and exhaust valves open, it would be pulling a hot mixture (unburned) right through the engine and it would burn on the down steam side turbo, this would cause and oscillation in RPM. Seems that caming this engine was one thing that wasn't pushed real hard, Dad always thought that if a cam change had been pushed more, it may have had potential. Although, World War II was long before my time, He told me that when he was stationed in England, the German Buzz booms (Pulse-Jet-Engines) could be heard for up to six miles away, He compared the engine noise to this,... Absolutely Deafening. -Kind of makes me want to build one!

In the day and age we live in now, we've designed multiple valved heads, cams and elaborate exhaust systems with exhaust back pressure in mind, and not the other way around. SO MY QUESTION TO YOU IS: Has this been tried, with what we know now?

Just thinking and rambling on outside of the box. dd-ardvark

fortyeye · Post by **fortyeye** » Tue Jan 11, 2005 9:39 pm

Its interesting that you mention the buzz bomb (pulse jet) ... the following looks like more fun than re-inventing an exhaust system ...

http://www.aardvark.co.nz/pjet/

AARDVARK eh? ... I thought you were from Florida.

lorddoom · Post by **lorddoom** » Tue Jan 11, 2005 9:47 pm

It wont work, you will always have the problem of the mixture being sucked out. The only way I can see it working is if you can create such a vacuum in the chamber before the intake opens that it will suck more charge in, but the work will take more than any gain you can possibly realize.

Post by **Piledriver** » Tue Jan 11, 2005 10:35 pm

OTOH, one could go Miller cycle...
Requires a supercharger though.

I MUST build one of those valveless pulsejets...

lorddoom · Post by **lorddoom** » Tue Jan 11, 2005 11:25 pm

Miller cycle? Mazda allready has done it: http://www.mazda.com.au/articleZone.asp ... eZoneID=92

fortyeye · Post by **fortyeye** » Wed Jan 12, 2005 8:02 am

Piledriver wrote:
I MUST build one of those valveless pulsejets...

I especially like this one from the above (aardvark/new zealand site):

The $5,000 Cruise Missile
Visitors to this site might also be interested in another project (a spin-off from my X-Jet development work) that is now underway. See the Interesting Projects website for more info.

raygreenwood · Post by **raygreenwood** » Thu Jan 13, 2005 6:23 pm

Heck...you guys laugh...but with the ind of tooling that is available used and for cheap tehse days (it does not take much)...a pulse jet would besimple to build. Have you een the original plans for these things? you could build them in the average garage these days. I'm truely surprised that some terrorist group has not been building buzz bombs. Though....you need forward velocity to start them. But I have always thought that the V-1 was one of the most elegant and simple weapons. We are quite lucky that no real guidance system was available at the time.
I have read, that it is still common in the areas where these were launched along the coast...to find in the dirt and sand.... steam catapult pistons for the V-1. they look like dumbells with o-rings.

I wonder...with some mods to the opening bell shape and volume, the fuel type, and the shutter springs....if a buzz-bomb pulse jet mounted on top of my 412 could be started with the speeds I can hit

just kidding...but it would look cooool! Ray

Post by **Piledriver** » Thu Jan 13, 2005 10:19 pm

The need for the catapult was only a function of the V1 not having a landing gear (for obvious reasons)---
Pulsejets require no "push start", and fire off just fine sitting still.

The valveless ones really get me... Basically a tuned pipe with a 180 bend and expansion/combustion chamber. No moving parts... except the fuel injection or (however) you set it up.

It's ramjets that need a push... But a good pulsejet might be suitible for gettin a ramjet to sufficient speed...

Then it would be possibly capable of supersonic speeds, and again, no moving parts other than fuel delivery.

raygreenwood · Post by **raygreenwood** » Thu Jan 13, 2005 11:50 pm

Nope! the pulse jet on the V-1 had a spring loaded shutter valve/inlet. In order for there to be a second pulse quickly after the first, there needed to be considerable static pressure of air against it.....which opened the shutters...which causes the compressed air pressurized fuel in the tank, to inject. After that....ignition. The chamber pressure then slammed the shutters shut from the inside. They would not reopen until forward motion in air forced them open to repeat the cycle. There was never a ground start version of the V-1. The catapult ramps were first fixed...then mobile...after we regularly bombed them. The only other way (other than in a static chamber with a wind tunnel) to start one...was dropping it off a plane. DO-17's and JU-88's were the primary movers for that. It took between to 80-100 mph from what I have read.

When the engine ran for a calculated amount of time...and the barometric altitude meter said it was at the correct altitude....there was a mechanism that literally severed the fuel line...when it kicked it into a dive.
The ones you are thinking of...are the valveless variety. They must carry compressed air to feed the chamber along with fuel to start. Ray

Post by **Piledriver** » Fri Jan 14, 2005 12:37 am

Hmmm... I've seem model pulse jets in RC aircraft (not mine, sadly) that rely on the initial vacuum from the initial explosion >exhaust---sort of a tuned scavenging effect-- to open the valves and suck in the next gulp of air.... And had them run stationary.

RC pulse jets are very impressive, but did't (to me) seem like much fun, as they moved too fast, and had issues taking off.

Big circles>>out of gas, land, refuel. Not for me.
(at least as these were designed)

OTOH being shaped like an SR71 probably didn't help.

There is a reason they have that long tailpipe--- It's tuned to set the resonant frequency of the engine, and it fires at that rate.
Bang>suck>bang>suck....

I understood the V1s fired with a compressed air "starter" . It makes for a sure start to have the assist, but I've seen it work without it, at least on "baby" models..

Perhaps I'm thinking of postwar testing. I must do some more research... I sort of grew up around stuff related to this...

My father was a tech working for Reaction Motors... Has some pictures and stories ... Like one of their motors on a test stand, under development... They used to throw old tires in the exhaust to see how far they would fly for fun.. It later went into the X1.

Smaller "coke bottle" engines were tested on ice sleds, basically external gas tanks with outriggers & skis to run on the frozen lakes.

Trivia--- The X1 engine used Chevy valves to control the LOX and kerosene.

The valveless pulsejets apparently CAN be started WO compressed air, but the extra air must help, as the fellow in NZ almost set himself on fire while using a leaf blower to get his big one started, after he tried an unassisted start. (the wind changed, and the things must throw huge fireballs on startup)

Must not require much pressure, just volume.

ZoSo914 · Post by **ZoSo914** » Sun Jan 16, 2005 8:41 pm

Ive been looking at pulse jet stuff for awhile and even wanted to build one for my engineering class. they offer a great research platfrom, because they are so similer to ICE but really havnt been developed very far. would make for a very cool toy to show off. as far as pumping air out of a cylinder goes, i dont see how you could use a turbo as a pump....because its running off of exhust gases...it would be like picking yourself up by your own boot straps....maybe im wrong. so you would need a super charger, and in that case... It would probaly cause more drag on the system then any gains thay you might see from it. Its a neat thought though...and that story about your grandpa is also very cool. I like stuff like that.

dd-ardvark · Post by **dd-ardvark** » Wed Jan 19, 2005 11:18 am

ZoSo914 WROTE:
(as far as pumping air out of a cylinder goes, i dont see how you could use a turbo as a pump....because its running off of exhust gases...it would be like picking yourself up by your own boot straps....maybe im wrong. so you would need a super charger, and in that case... It would probaly cause more drag on the system then any gains thay you might see from it. Its a neat thought though...)

The last part of this statement was true in the 40's. Again I say: In the day and age we live in now, we've designed multiple valved heads, cams and elaborate exhaust systems with exhaust back pressure in mind, and not the other way around. Remember their was light before the light bulb. Do you remember the guy who refined fire into what we know as a lamp..., me neither, But I do remember the guy who created the bulb, because he thought differently. You want to make a motor run faster, longer..., think outside of the box.

Look guys, we've all heard of tuned exhaust systems. For those not understanding this basic exhaust scavenging principal. It loosely goes like this: Exhaust Pulse-Vacuum-EP-V-EP-V-EP-V-EP-V-EP-V and so... The theory is that the proceeding Exhaust Pulse creates a Vacuum just behind that high pressure pulse going down the exhaust pipe, and pulls the next one to it, thus is the reason it's so important to make the exhaust runners all the same length, so each pulse well pull the last one out, one after another. Theirs a lot more to it than this, but this is the jist of it.

As far as a Turbo, and the confusion of it as a pump...Well, it it it it...is a pump! and so is a super charger, and they both use power to run them. My point was to simply use the exhaust side of the turbine on a turbo and gear it to the crank. Use an accumulator in the exhaust system to give the turbo working room, (vacuum pressure) and YES..., obviously their would be problems as far as the logistics of how the exhaust system is made. I've been kicking around this idea since the onset of the Variable Lobed Cam came into it's popularity. What I'm getting at here, is to try and get you to think outside of the standard exhaust system.

Exhaust Trivia: The aeronautical field, discovered in the mid 1940's, that for every 90 deg. bend in an exhaust system, it gave a 10% rise in back pressure. Piston engine fighters subsequently just had a stub pipes aimed straight out the side, just long enough to keep the valves from getting bent from the cold air.
It was later discovered that by aiming the stubs back in order to keep rain water out while sitting on the tarmac, that as an example the P-51 mustang gained 37knots of speed by doing this.

dd-ardvark

ZoSo914 · Post by **ZoSo914** » Wed Jan 19, 2005 7:19 pm

this got me thinking heavily on putting a turbo onto a pulse jet. like "wow think how much more power you could get out of a pulse jet" then im like DUH thats a turbine....oh well.
I always think its funny how companys make a big deal out of things like the "HEMI" and people think turbos are cutting edge technology. hemi heads have existed in airplanes since before WWI. I recently visited an Air musem and saw a pre WWI engine. It was a V12 DOHC with turbo charger. its amazing to think how fast the aviation industry advances.

I found this link on a different site. Its a company that makes rotaory valves http://www.coatesengine.com/csrv.html I dont know if its ever been succesfully used before. Im sure the idea has been around awhile.

raygreenwood · Post by **raygreenwood** » Wed Jan 19, 2005 8:37 pm

One of the problems though ...with tuned exhausts....is that even with exact lengths, you still have the timing of the exhaust valves to deal with. Even with perfect collector design, the art of dropping each succeeding pulse in behind each other...will only "perfectly"work at a given range of rpm. But, if the whole system has very low back pressure...AND its timing/resonance point is right in the rpm sweet spot of the engine...thats very nice indeed. Otherwise, generally...but not always, at least having a whole lot less back pressure helps anything.
With those thoughts in mind, on my highly tuned 1.7....I thought about possibly making ....a 4 into 4..

. Yes....4 seperate equal length pipes.....including the intial header pipes, exiting at 4 seprate tips. I thought of useing a primary and secondary resonator...on each pipe. All fabbed from Stainless. Thats 8 resonators total. Two high speed and two slow speed. With no reversals...and a venturi re-entry at the tail end of each resonator At the very least, if the resonators are correctly done, and 1" and 5/8" pipe is used with smooth mandrel bends, I should end up with a system that is relatively quiet...and have less tha 2% backpressure anywhere in the power band. This is a HUGE reduction from the stock unit...which for a 1.7 ernst muffler...is not that bad anyway. The really cool thing is, that is can be built very cleanly and simply...and all four pipes will be identical in almost every way, save for a slight bend up at the starting end for 2 and 4 and a slight bend down for 1 and 3. Ray

ZoSo914 · Post by **ZoSo914** » Thu Jan 20, 2005 4:53 pm

well Ive always thought about how that would work. manufactures have been making mulitiple stanged intakes forever...so why not an exhust right? how are you going to control it? some kind of valve thats activated by rpm? or manifold pressure or somthing? It would be really cool if you could do one like Porsches intake that was continuesly variable. have some kind of slider at the tail pipe that moved in and out.