About Turbochargers.

[Meanderthal]

Okay, I remembered last night that I read something 5+ years ago about an anti-lag system in rally cars which does something similar to what you are suggesting happens in the BS. Maybe this is enough to convince you that it is rarified and not the norm.

Here is the article, and here is an excerpt:

But the engine's electronic control unit modifies the engine management. What it does is to delay the spark plug ignition point as much as possible. Thus, when the engine piston is already in its expansion stroke is when the mixture is set on fire. This causes the vast majority of the energy from that combustion to be propelled through the cylinder on the exhaust stroke, which sends the burning mixture and all that energy to the gas turbine. In addition, those gases are mixed with the high-pressure air coming into the exhaust manifold from the intake turbine.​

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In this way, the turbo still has energy in the form of high-speed, high-temperature exhaust gases, so it continues to compress air at high pressure.​

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[Mark S.]

Okay well here is the issue, my engineering education and everything I can find says that turbos are driven by exhaust gas flow. Essentially like this site:

The compressor (3) and the turbine (15) are mechanically connected through a rigid shaft. At the end of the exhaust cycle, the exhaust gases are pushed out from the cylinder (12) through the exhaust manifold (11) and through the turbine (15). The exhaust gas flow (kinetic energy) will hit the blades of the turbine, forcing it to spin. In the same time, having a fixed connection, the compressor (3) will spin, compressing the intake air in to the intake boost pipe (5).​

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I do not see any reference in any outside source that definitively states that the turbo uses the left over combustion from the cylinder to drive the turbo. Everything that I see refers to exhaust gas flow. Nothing I have ever seen references combustion in the exhaust manifold as a driver for the turbo.

The thermal efficiency numbers you quote are correct, other than you seemed to gloss over the 50% of that energy that goes out to the oil/coolant/block. You didn‘t really gloss over it but just did not acknowledge that it was such a high proportion of the overall thermal energy. Basically 80% of the energy is wasted in the form of heat and only 20% is converted to mechanical energy to push the piston down in the cylinder. I think we are in agreement here.

Because intake air charge is pressurized by the turbo, that pressure is part of the pressure that is pushing the gases out of the cylinder when the exhaust valve is opened. It is a smaller part than the pressure that has been created by the combustion (which is somewhere around 90% complete when the exhaust valve opens based on your video). But if we assume that the engine has a 10:1 compression ratio and the exhaust valve is opening roughly 48 degrees before BDC (number I found that is much lower than what you quoted as 90 degrees, video from same source), then we have used most of that pressure to push the piston. If the 48 degree number is accurate, then that is about 80% (got to use some geometry here) of the pistons travel before the exhaust valve opens. I know, your video of a lawn mower engine shows something higher and the graph you posted seems to suggest something higher as well. We all know that lawn mower engines aren’t exactly known for high efficiency, so at least for me that seems like a stretch given other sources about multi-cylinder engines. All that to really say that more of the combustion energy than you have been suggesting is going in to moving the piston.

Sorry, there is a lot of thinking out loud in that last paragraph and putting together multiple thoughts and data from multiple sources. I’m not sure that it actually proves anything.

You say that the exhaust manifold is relatively cool. Relative to what, I guess, is the question. I’m sure that it varies somewhat, but exhaust gasses are around 750-800 F. I would assume that the exhaust manifold temperature is close to that figure. I don’t know that the exhaust manifold could withstand the full temperature of combustion (1900 F) if there was much of it really happening within the manifold. The shape of a turbine engine is very carefully designed so that combustion and expansion happen in an ever expanding cross-section to avoid melting components. Essentially, in the turbine engine the combustion is not so much contained as it is directed.

We know that there is significant pressure in the cylinder that has to be released by the exhaust valve opening. The exhaust valve opens before BDC and release most/all of that pressure so that the piston is not working against it. We also know that in a turbo engine, the pressure in the exhaust manifold is very high (2-3X boost pressure). So, as much as we have tried to relieve pressure in the exhaust so that the piston isn’t working against it, the turbo has backed up the pressure and the piston does have to work against a fair amount of pressure (not to mention frictional forces of pushing the gases through the valve opening).

Now I think I have a compelling argument against your assertion that the continuation of combustion is the thing that drives the turbo. If that were the case, then that continued combustion would create a large pumping loss. The combustion would create pressure in all directions, not just in the direction of the turbo. The velocity in the exhaust manifold would counteract this to some small extent, but until you start to reach the speed of sound, that affect would be minimal. I know pistons are moving fast but really not that fast, about 50 mph at high rpm.

Okay, I’ve racked my brain on this subject enough for today. Thanks for sticking with me, I am enjoying this conversation, even though you have yet to convince me of your hypothesis (nor I you).

Good stuff. I've got a lot on my plate today, but I will get back to this. We are getting closer to agreement...

 

[Mark S.]

Ok, @Meanderthal, it seems to me we are in complete agreement that the exhaust manifold is pressurized by a combination of hot exhaust gas and movement of the pistons during the exhaust stroke, and that it's this pressurized gas seeking lower pressure that causes it to move through and power the turbocharger turbine before escaping to atmosphere through the remainder of the exhaust system. Where we differ is the percentages. I contend that pressure in the exhaust manifold results primarily from the expansion of gases due to the heat and pressure of combustion. I believe your argument is that the exhaust manifold is pressurized primarily due to the pumping action of the piston during the exhaust stroke.

Is this accurate?

 

[Meanderthal]

Ok, @Meanderthal, it seems to me we are in complete agreement that the exhaust manifold is pressurized by a combination of hot exhaust gas and movement of the pistons during the exhaust stroke, and that it's this pressurized gas seeking lower pressure that causes it to move through and power the turbocharger turbine before escaping to atmosphere through the remainder of the exhaust system. Where we differ is the percentages. I contend that pressure in the exhaust manifold results primarily from the expansion of gases due to the heat and pressure of combustion. I believe your argument is that the exhaust manifold is pressurized primarily due to the pumping action of the piston during the exhaust stroke.

Is this accurate?

I think the main difference in our thinking is that I believe that combustion is happening primarily in the cylinder and that any combustion that escapes into the exhaust manifold contributes a small amount to an increase in pressure there. I believe that escaping combustion is more a byproduct of the exhaust valve opening early to prevent pumping losses, but I concede that this combustion would increase pressure in the exhaust manifold by some amount.

The pressure in the cylinder does increase substantially during combustion. I think we both agree about that. I believe that increase in pressure in the cylinder is responsible for almost all of the pressure in the exhaust manifold. Obviously the turbo is a restriction to flow, or there would not be any (or very little) pressure above atmospheric in the exhaust manifold. In the article that you linked about exhaust that talked about the sound of the exhaust, I think the talk about the sound of the exhaust was about the release of cylinder pressure, not combustion in the exhaust manifold. I got the impression you believe that exhaust sound is caused primarily by that escaping combustion. I still think the amount of escaping combustion is going to be a lot different in the automotive engine than it is in the lawn mower engine shown in your linked video. A very cool video, by the way.

I think that this discussion has been about where the pressure in the exhaust manifold comes from. Does it come from cylinder pressure, or from combustion in the exhaust manifold. I think my contention is that primarily the pressure that drives the turbo comes from increased pressure that happens in the cylinder during combustion before the exhaust valve opens. Any combustion that happens after the exhaust valve opens will contribute some amount but it is not the primary motive force for the turbo.

 

[Mark S.]

Sorry for the long delay in responding to this.

I think the main difference in our thinking is that I believe that combustion is happening primarily in the cylinder and that any combustion that escapes into the exhaust manifold contributes a small amount to an increase in pressure there.

Agreed.

The pressure in the cylinder does increase substantially during combustion. I think we both agree about that.

Yup. Several times atmospheric pressure.

I believe that increase in pressure in the cylinder is responsible for almost all of the pressure in the exhaust manifold.

Agreed.

Obviously the turbo is a restriction to flow...

Agreed.

...or there would not be any (or very little) pressure above atmospheric in the exhaust manifold.

Not necessarily. Even in a normally aspirated engine the pressure in the exhaust manifold is sufficiently higher than atmospheric to induce flow in the system.

In the article that you linked about exhaust that talked about the sound of the exhaust, I think the talk about the sound of the exhaust was about the release of cylinder pressure, not combustion in the exhaust manifold.

Agreed.

A very cool video, by the way.

Yes!

I think that this discussion has been about where the pressure in the exhaust manifold comes from. Does it come from cylinder pressure, or from combustion in the exhaust manifold. I thought we were discussing whether pressure came from the expansion of hot gasses in the combustion chamber tha I think my contention is that primarily the pressure that drives the turbo comes from increased pressure that happens in the cylinder during combustion before the exhaust valve opens. Any combustion that happens after the exhaust valve opens will contribute some amount but it is not the primary motive force for the turbo.

Okay, I think we have found the root of our "disagreement." Unless the engine is running very rich (which it can when cold), there is very little combustion occurring in the exhaust manifold. If I said that earlier in the thread then I mistyped; that's certainly not what I meant.

Like you, I believe the overwhelming majority of pressure in the exhaust manifold is the result of combustion heat causing expansion of gases in the cylinder. That pressure is released immediately into the exhaust manifold when the exhaust valve opens. The sound results from the massive pressure release when the valve opens, not from the combustion event, which is largely contained in the cylinder.

I believed you were arguing the pressure in the exhaust manifold was largely due to movement of the piston pushing gases from the cylinder rather than the rapid expulsion of pressurized gas following exhaust valve opening. I would have to go back and re-read the thread to figure out where I got that impression, but if that's not what you're saying then I'm not going to waste time doing that.

 

[Meanderthal]

Okay, maybe we are done here. The only thing I will add is that the piston does have to push against the pressure in the exhaust manifold, so it is pushing a fair amount.

The increased pressure in a naturally aspirated engine’s exhaust, and in a turbo engine’s exhaust after the turbo, is due to restrictions like the catalytic converter, muffler, any bends in the tubing, and just general head loss of a gas/fluid flowing through a pipe. That pressure is much less than the pressure in the exhaust manifold on the turbo engine.

 

[Mark S.]

That pressure is much less than the pressure in the exhaust manifold on the turbo engine.

Agreed.

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