Supercharger For Extracting Exhaust Gas.

[blah_blah]

Rolls is your username 'Foilage' on OCAU?

Saw the exact same topic on there

[Rolls]

Rolls is your username 'Foilage' on OCAU?

Saw the exact same topic on there

Yup thats me

[karma_syke]

Apparently not 100% of all the available fuel and oxygen is used in the combustion process, so i present to you:

[89CAL]

Never heard of EGR?

[GTSBoy]

Never heard of sarcasm?

[89CAL]

Never heard of sarcasm?

Maybe

[The Mafia]

lol I can't believe that this thread is still going

[Rolls]

lol I can't believe that this thread is still going

shouldn't you be off trolling threads?

[The Mafia]

seriously though, did you know how much horsepower is actually needed to drive a decent super charger? Some rob more than 100bhp from the engine.

Its very obvious that this won't work, but people persist.

Maybe read up a little more on engines in general and see how they work, physics of airflow vs heat vs pressure vs anything else you can think of.

Get past the whole "asking what boost someone is running before even asking what sort of turbo they are running" sort of thing first, and then you'll start to realise lots more.

This question is kind of like asking why you can't get solar panels, make them power a light, and then shine the light into the panels, make it go in circles and have free power.

[Rolls]

seriously though, did you know how much horsepower is actually needed to drive a decent super charger? Some rob more than 100bhp from the engine.

Perhaps you missed the part where I accepted that.

Maybe read up a little more on engines in general and see how they work, physics of airflow vs heat vs pressure vs anything else you can think of.

I'm not a f**king moron mafia, I design gas well heads and program the computers that do flow control for them day in day out, I understand how an engine works!

Get past the whole "asking what boost someone is running before even asking what sort of turbo they are running" sort of thing first, and then you'll start to realise lots more.

Have you even read my posts before? The amount of times I've said this same thing is astounding.

Edited September 8, 2011 by Rolls

[Legionnaire]

I don't think it would work. Not because it's impossible to create a big enough or heat-resistant supercharger.

First of all, you'll definitely lose more power driving that SC that get back with increased engine scavenging. But not only that, you NEED that backpressure, or to be more correct, the pressure differential across the turbine section to give it power to drive the compressor. So you can't increase VE in the upper RPM range by sucking gasses out of the turbo. Not by any significant amount.

You might actually achieve something similar to what you suggest if you add some cleverly shaped piping and exhaust gas cooling. This combination may (or may not) create a momentum large enough to move gasses outward and create some suction at the turbine outlet.

Or you can suggest a supercharger between engine and turbo, which will pressurise turbine inlet but will keep engine backpressure low. But this system apparently has even more penalties to it.

I'm all for innovative ideas and out of the box thinking, but IMO it should be exceptionally well thought out and calculated/modelled first.

Edited September 8, 2011 by Legionnaire

[hypergear]

I normally break ideas into bits, think every stage of what is likely to happy. This leads to more accurate predictions.

In a fuel injected pistons engine.

1. exhaust charger is on. making the whole exhaust system ie -100 vacuum.

2. when engine runs. fuel get injected into the chamber.

3. valves seals the chamber.

4. combustion take on place

5. Exhaust valve opens. inlet value shuts.

6. -100 vacuum removes combusted hot air out of the exhaust at faster rate.

assume all valves seals 100%.The -100 vacuum:

Does not affect air flow pre inlet valve.

Does not affect combustion process.

Accelerates only combusted air exit speed.

So engine's not sucking in more air, getting more fuel, having the same level of combustion, technically there will not differences in power or torque in engine out put.

How ever in a turbocharger system having hot air out faster means cooler EGT (very slight). Could make a tinny bit of more power and torque up top.

Not against your idea, that's how I can picture it would work assume 0 power draw from running this charger.

[Rolls]

assume all valves seals 100%.The -100 vacuum:

Does not affect air flow pre inlet valve.

Does not affect combustion process.

Accelerates only combusted air exit speed.

So engine's not sucking in more air, getting more fuel, having the same level of combustion,

Not quite. Two scenarios, cams with zero overlap. Lets say the charger sucks all the exhaust out and leaves a -100 vacuum when the exhaust stroke is over. Without the exhaust charger there would be a very slight positive pressure in the chamber, maybe a few psi max, if we have a -100 vacuum this means when the intake valve opens we have a larger pressure differential between the combustion chamber and the intake manifold, a greater differential pressure will mean the intake air will rush in faster and hence fill the cylinder more. Without the charger the volumetric efficiency would be say 80% or whatever a typical NA engine makes, with the charger it will be slightly higher.

Also without the exhaust charger not all exhaust will be pushed out of the combustion chamber, so if not all exhaust leaves, that means there is less room for oxygen hence less bang. So via two methods does it allow more oxygen and fuel in the cylinder.

1) the vacuum causing a greater DP which sucks air faster into the cylinder

2) less exhaust gas dilution so more physical space for air + fuel.

both of these effects will increase volumetric efficiency as there is now more air in the cylinder and hence it is closer to 100% VE.

Scenario 2.

Camshafts with overlap. If you have camshaft overlap the same as above will happen but you can now suck the intake charge into the cylinder via the exhaust, some air+fuel will be sucked out the exhaust but this will dramatically increase the velocity of the air flowing into the cylinder. Greater intake velocity means greater cylinder filling hence greater volumetric efficiency hence more bang.

Basically think of this exhaust charger as a mechanical extractor, it works the same way that extractors cause an increase in torque, extractors remove the restriction in the exhaust so the cylinder has less work to do on the exhaust stroke, they also increase the vacuum in the cylinder which increases volumetric efficiency.

Both of these scenarios ignore the fact that the charger will suck a large amount of power, so whilst you get a small torque increase from greater volumetric efficiency, you'd still almost certainly have a net loss in torque.

Edited September 11, 2011 by Rolls

[hypergear]

I think its a good idea to set it up for a small scale test. It shouldn't be expansive. You can get a steel duction fan with external power supply, make some pipes and connect it on to the exhaust for few dyno run and see if that makes any differences based on a small NA engine. Shouldn't burn any thing.

[Rolls]

I think its a good idea to set it up for a small scale test. It shouldn't be expansive. You can get a steel duction fan with external power supply, make some pipes and connect it on to the exhaust for few dyno run and see if that makes any differences based on a small NA engine. Shouldn't burn any thing.

I think the amount of power you'd need to remove the gas fast enough to make a vacuum would be massive, I doubt it would work. Was just explaining how the effect would delivery more torque like an extractor system does.

[UWAM]

Wouldn't work, for the reasons mentioned before.

I usually only deal with NA engines, but I want to add a couple of things to think about.

First, mass flow rate. The idea does not increase the mass flow rate through the engine (directly anyway, beyond a greater pressure differential during valve overlap, and possibly slightly better turbo performance, both of which would be tiny). The mass flow rate of the air going into the engine governs how much air (thus fuel) there is to combust, and so how much power you make. It's why we strap turbos and superchargers to our cars. Also, mass flow rate is a big factor in how much work your compressor does.

Two, backpressure. Backpressure is undesirable, but it is a result of something that is needed, which is exhaust velocity. Fast flow exhaust gases help scavenging and VE, and prevent things like reversion. It's the reason why cars don't just run 6" exhausts, or race engines with no exhaust at all.

[GTSBoy]

Guys,

There is no scavenging effect post turbine on a turbo exhaust system. As such, the biggest, shortest, lowest restriction exhaust system is the best. Also, as such, any device that you could put into the exhaust system that would further reduce the pressure immediately downstream of the turbine can only have a beneficial effect in nearly every (probably actually every) aspect of turbo operation. Which is as I said in an earlier post.

The problem is only that the power spent driving such a device, which is only sucking hot, spent gases out of the chamber, would probably (almost certainly) yield a greater increase in engine output if you spent it on making more boost (and of course intercooling that air back down).

The only really great thing that would come out of having such a device in the exhaust system would be that you would be able to reduce exhaust manifold pressure and so reduce reversion and charge contamination. Doing so would improve the amount of mixture you can jam in the cylinder without causing detonation. Would also allow you to reduce overlap because you wouldn't be relying on the fresh charge blowing into the cylinder to push out as much exhaust, which would improve the engine's manners and reduce fuel consumption a bit. But really, these are all just aspects of the same imagined benefit of putting a suck onto the exhaust system.

As has been said at least twice so far, twin charging brings these same benefits and is easier to arrange and make work properly.

[Rolls]

I was talking NA wise and yeah I agree with the twin charging comment.

Edited September 11, 2011 by Rolls

[The Mafia]

No one has commented on how almost any device will have a very short life pumping hot gas full of carbon.

This will never work. I'm still surprised that amount of people trying to entertain the idea.

[Legionnaire]

Nobody uses cams with zero overlap on n/a engines. So exhaust charger is n/a on 0 overlap n/a engine. But even if we are to use 0 overlap engine, vacuum in chamber prior to intake event wouldn't make cylinder filling good, because it's gas we're dealing with here, so when your engine opens its intake valves, gas just loses density, it won't "rush in".

On n/a engines with some overlap, vacuum does not do the major part of cylinder filling. Gas momentum does. This momentum is created because some mass of gas gets moving - it moves because of pressure differential betw. chamber full of hot burnt gasses and atmosphere. You basically suggest to lower the past-valve pressure below atmospheric, make pressure differential greater and thus increase gas momenum. Good thinking, but it comes with the penalty. On a well-engineered exhaust manifolds exhaust events are arranged in such a way that when some cylinder opens its valves and commenses exhaust stroke, there is already vacuum in the manifold - it was created by exhaust stroke of the previous cylinder and consumes no engine power. Your idea is the same in that it lowers exh. manifold pressure too - but unlike "extractors" it uses engine power.

By the way, there is no need to

suck the intake charge into the cylinder via the exhaust

because it significanlty reduces engine fuel efficiency, and SFC rises.

The whole idea is not worth the hassle IMO, because of 1 fundamental reason. You can not drop pressure below vacuum, i.e. there is no such thing as negative pressure. You're trying to increase pressure differential across the engine. In the best possible scenario there will be 1-0 pressure diff - atmosperic at the intake and 0 past the engine, and density of air trapped in chamber will be somewhere in between, i.e. below atmosperic.

On the other hand you can make the same pressure differential by adding 1 bar of boost to intake side. Same diff, this time 2-1 though, and density of air again will be somewhere in between, but now between 2 and 1, above atmosperic. Bonus - more airmass - bigger bang! And another bonus - boost pressure does not have to be 1 bar - it can be 2, 3, sky is the limit. So with n/a engine you should just add a supercharger.

And some more about this. Engine consumes air by volume, but makes power based on air mass - that's why VE is not all that important on supercharged engines. To be more precise, on SC engines VE is meaningless without pressure and density ratios. By pressurising air you make is denser - more mass per volume. By sucking air you make it less dense - less mass per volume. While you may observe some slightly increased VE, your actual cyllinder filling may drop. So in reality you should just use supercharger instead of "superscavenger" (or supersucker?)

But it's very nice to see you defending your idea like that, even it's not the best idea. Discussions like this one make people want to think and learn, and it's one of the better things an internet forum can do, so please, by all means keep posts coming.

No one has commented on how almost any device will have a very short life pumping hot gas full of carbon.

This will never work. I'm still surprised that amount of people trying to entertain the idea.

Maybe it's because most people here have cars with turbos that do exacly that - pump hot gas full of carbon?

Why, don't be so negative, it's a good discussion. Much better that one more "whats the best turbo for my RB25" thread.

Edited September 11, 2011 by Legionnaire