"Comprex" pressure-wave supercharger 1

[Deividas]

Hello everyone, i wanna ask some questions about this device, it's very interesting method of air charging (to me), first question is: why this device was only used in diesel engines? (except Ferrari formula one experiments)?

 

[dgallup]

I remember seeing one of those decades ago. At that time, it only operated effectively over a very narrow RPM band. It was essentially limited to constant speed engines like a genset. I have not even heard the name in years, don't know if they have overcome that limitation.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Deividas]

Mazda used it in late 80's/ early 90's diesel engines, Wikipedia says, that other brands also used it

 

[BrianPetersen]

The device depends on pressure waves both to charge the intake (transferring a pressure wave from the exhaust side to the intake side) and to empty itself out. If you did that to a gasoline engine, how do you handle part-throttle operation? Obviously the throttle has to be after the Comprex, but there's a significant area of the possible speed and load map that will have weak, nonexistent, or even wrong-direction pressure waves in the exhaust, and then the Comprex won't have left-over pressure inside itself to purge the exhaust and draw in fresh air.

The Comprex is an interesting device but there are a whole lot of side effects.

Pressure waves operate at the local speed of sound, the Comprex is reliant on careful port timing, but the operating speed of the engine could vary by a factor of 10, so what do you set the port timing to??

Since it relies on pressure waves to purge each chamber out, what happens when the air filter gets clogged ... or someone backs up into something with the exhaust pipe and partially blocks it? A normal engine will shoot a potato out of the exhaust pipe pretty hard. What happens with one of these? A turbocharger doesn't have this problem.

The Comprex does not keep the intake and exhaust streams fully separated. There will inevitably be some gas exchange. Combine that with modern emission control systems that rely on very accurate air/fuel ratio control and don't like "unmetered air". Doesn't matter gasoline or diesel. A turbocharger doesn't have this problem.

 

[gruntguru]

Ferrari dabbled with it during the F1 turbo era.

je suis charlie

 

[Deividas]

BrianPetersen - can You explain more about part-throttle operation and how it affect exhaust pressure waves? I don't understand why at part-throttle exhaust pressure waves is different compared to at WOT conditions, it's because vacuum at intake side and valve overlap?

 

[Panther140]

Where would a comprex supercharged 4 stroke SI engine have an advantage over a modern 2 stroke?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[BrianPetersen]

With a throttled spark ignition engine, if the pressure in the cylinder at the end of the intake stroke is 1 atmosphere at full load it will be 3 atmospheres at the end of the power stroke when the exhaust valve opens. Lots of potential pressure-wave energy there.

If the engine is running at light load (throttle almost shut) the pressure at the end of the intake stroke might be 1/3 of an atmosphere. Then it will be about 1 atmosphere at the end of the power stroke. There's no pressure-wave energy left.

 

[LadaTrouble]

I was a bit puzzled when I first came across these on Mazdas. Belt brand and tension was critical...you knew it was slipping because the backside of the filter was black with soot.

 

[RodRico]

There's a great paper from "Journal of Engineering for Gas Turbines and Power" (

http://www.egr.msu.edu/mueller/NMRe...viewOfWaveRotorTechnologyAndApplications.pdf)

summarizing development of wave rotors from the 1920's through the Comprex and Hyprex devices right up to the Wave Disk Engine. The paper cites a vast body of existing research (as of 2006) in its reference section and concludes by saying, "Continued research on sealing and thermal expansion control are needed to solve these persistent challenges." Beyond sealing and thermal expansion, I think contamination of intake charge with exhaust gas and the size of the device render it less useful to the automotive engineer than the compact and reliable turbo charger.

 

[Deividas]

Maybe someone can explain in detail how those shock wave reflection pockets help to extend "Comprex" operating range? Because without those pockets, operating range is quite limited

 

[RodRico]

From the paper I linked to in my earlier post... "performance over the engine speed range was improved by using pockets in the end plates to control wave
reflections." I strongly suggest you read the paper I linked and chase the references until your need is satisfied.

 

[tbuelna]

These compression devices like Comprex that use phased ports and ducts in a rotating drum to utilize dynamic exhaust gas pressure waves to compress an intake air charge, look very attractive on paper due to their high efficiency, good throttle response, and high speed capability.

If you look at the paper linked by RodRico, the most attractive use of the device is as a HP compressor stage in a turbine engine, due to its capability to operate at relatively high speed, handle high air flows, give a high PR from its stage at good efficiency, and add minimal weight. In reality, there are very difficult problems to resolve, such as sealing of the rotor end faces. If someone can figure out a reliable method to solve these problems, this device could have a big market for use in turbine engines as a way to significantly increase PR, and thus improve TE/SFC.

 

[Deividas]

RodRico - i read that paper in your link, but there is no explanation, how exactly those pockets work

 

[gruntguru]

Wave action I would think.

je suis charlie

 

[RodRico]

Deividas, there are several places in the linked article where those pockets are mentioned. Each is referenced to another paper. You will need to search for and read those other papers to find a good answer to your question as I doubt there are any wave rotor experts posting here.