Industrial gas engine exhaust backpressure minimum?

[Bribyk]

Any idea why an engine manufacturer would specify a minimum exhaust backpressure on both NA and turbo'd models?

I need an exhaust system that will work for a wide variety of engines and outputs. The system needs to be large to keep the pressure below the maximum spec for the biggest engines but results in little backpressure (below manufacturer's specs) for the smallest engines.

 

[MikeHalloran]

I am aware of one Swedish engine manufacturer who specifies a (shockingly high) minimum backpressure for their marine engines. Nobody in their US office claims to know why... and questioning Mother Sweden is not tolerated, so they don't ask.

I speculate it's because somebody mis-read or otherwise misinterpreted a normalized graph, and nobody else has enough juice to question the edict.

Make the system big, and insert an orifice plate somewhere to restrict the engines that require a minimum.





Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

The orifice plate was my solution too but I'm hoping not to have to use it (have to change it out for a different power range and I'm trying minimize setup time for each test).

I've asked the manufacturer's applications engineering department about the minimum backpressure requirement and am awaiting a response.

Competing manufacturer's explicitly state there is NO minimum backpressure in their applications guides.

I've heard the burnt valve folklore but haven't seen anything to back this up. I could see how running without headers could burn a valve but I would think it has more to do with temperature fluctuations and has zero relation to backpressure.

 

[gr2vessels]

I recall the Caterpillar "bible" aka technical book said 2-4 kPag backpressure is required on all engines exhaust, except some 1-2 kPag for the low pressure gas engines. Why? Don't ask, just do it!
I could not find any technical reasoning for that statement...
cheers,
gr2vessels

 

[Bribyk]

I was referring to Caterpillar as the competing manufacturer and their Applications & Installation Guides for Natural gas engines explicitly state there is "no minimum backpressure".

 

[gr2vessels]

Bribyk,
My Caterpillar manual is "Engine Installation & Service Handbook" and is obviously in contradiction with your book, because on page 25, under chapter "Exhaust Systems Formulas", it says:-
"The limits for a given engine's exhaust backpressure can be located in the TMI system. In general terms the backpressure limit is 27 inches of water for all Caterpillar turbocharged engines. 34 inches of water for naturally aspired engines. The 3600 series of engines have a limit of 10 inches of water.......Remember that the closer you get to the limit the more affect the exhaust backpressure will have on the performance of the engine."
I am however, surprised that Caterpillar has published apparently contradicting information on their handbooks.
cheers,
gr2vessels

 

[ivymike]

gr2 - is the limit you mention not the maximum allowable value?

 

[MikeHalloran]

Yeah, the limits mentioned by gr2 are maxima.

Mike Halloran
Pembroke Pines, FL, USA

 

[IceStationZebra]

It may have to due with the fact that engine emissions were never verified at that low back pressure. And the few occasions where it comes up don't justify the effort? ISZ

 

[ivymike]

that's the one thing I can think of for a n/a engine - you'd potentially have excessive flow through the cylinder during overlap. Doesn't seem like it would be much... but if the mfr hasn't done emissions work for it, then they probably can't legally sell into that app.

 

[Bribyk]

I wasn't thinking about the big picture, it could be an emissions rating issue. If that's the case, it shouldn't matter for our application (short-term mechanical testing and break-in). I'll post what, if anything, I hear back from the manufacturer.

 

[Lou Scannon]

I can think of a couple of possible reasons for a minimum EBP specification. One would be if the turbo is free-floating and would tend to overspeed/overboost with too little backpressure. The other would be if an exhaust oxygen sensor is used for closed loop A/F control; the sensor signal is affected by the pressure differential between its sensing cell and the exterior. In that case the A/F calibration would only be valid between minimum and maximum EBP limits.

 

[Bribyk]

There are no O2 sensors on these engines, they still use carbs or MAP-based open-loop fuel control (mechanical or electronic). Some models are lean-burn and run up to 10% O2 in the exhaust (double stoic AFR); which doesn't work well with even a wide-band O2.

The turbos are also wastegated based on MAP (mechanically or electronically) so they shouldn't overspeed.

 

[Bribyk]

The manufacturer's response is that the minimum exhaust backpressure was set to ensure that components were sized properly and if the piping was too large it would be difficult for the gases to evacuate the engine and could condense.

I don't see how creating a restriction makes it easier for the exhaust to escape and my calculations show the temps to remain above the dewpoint.

However, heat transfer requires a lot of assumptions and they've been doing this for a very long time.

 

[R1100S]

Why do you want one exhaust system to work over such a wide range of displacements and powers? It seems that the system would be too big for the little engines and too little for the big engines?

As for back pressure, the engine was designed to work with a certain level of back pressure. If you change the back pressure, you need to change other variables, cam timing, fuel flow rate, etc., for the engine to run properly.

 

[franzh]

Regardless of what I have read or listened to in lectures, I have always felt that if an engine ran better with some exhaust backpressure, there was a problem with calculating the proper camshaft valve timing events.

For example: Top Fuel Dragsters. They run 8 open individual exhaust stacks with no backpressure. 4 to 6 thousand horsepower from 500 cubic inches. All modern high performance exhaust systems tout "reduced backpressure" or claims to that effect.

Just IMHO.

Franz



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[Bribyk]

The exhaust system is for a dyno cell to test run and break-in overhauled engines. They range in power from 100 to 3000+ bhp; which is why the system needs to work for as much of this range as possible. We can keep backpressure below manufacturers' maximum recommended limits by sizing the system for the largest engines, but then a minimum backpressure recommended by a sole manufacturer makes it more difficult to cover the range. The less setup and changes required between tests, the more engines we can put through, the more $$$...

A lot of turbo-diesel test cells run zero backpressure because they use a fumehood-type exhaust extractor. Natural gas is too hot and NA is too loud though (and a hood interferes with the overhead crane).

Chevrolet's NASCAR engine program runs a blower to reduce backpressure below on-car levels to increase the test sensitivity when making cam/valvetrain changes.

Top fuel engines only have to last 3 minutes or so but I haven't seen anything substantiating that backpressure is good, either.

 

[GregLocock]

Nothing wrong with a butterfly valve in the exhaust for an engine dyno.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Lou Scannon]

That's pretty standard practice for any test cell that has to deal with more than one engine model.

 

[anyoldname]

Depending on what emissions rating the engine is designed for and its emssions strategy, you can need a minimum back pressure to drive EGR flow (hence back pressure valves on some engines).