About Turbochargers.

[Mark S.]

Picking up where we left off from this thread:

The combustion that happens in the cylinder goes toward driving the pistons. It does not do anything for the turbo. You are correct about the turbine engine requiring the combustion.

The pressure delivered to the turbine side of the turbo is delivered by the pistons. The air happens to be hot, but it doesn’t add anything to the work coming out of the turbo. If the hot air was adding work it would have to lose a significant amount heat to add work to the turbo.

Your thought experiment on turning the engine with an outside source, at the same rpm the engine would be spinning using combustion, is exactly what I was proposing. The turbo would spin at the same rate, whether that air is hot coming from the engine using combustion or cold coming from the engine being spun by an outside force.

Essentially, what happens on the intake side of the turbo is the exact opposite of what is happening on the exhaust side. Yes, there is a change in temperature in both cases but that change in temperature is driven by the change in pressure. That temperature change is also relatively small, not the kind of change needed to do any real work.

Since we are starting to annoy the community with what I would think is an educational conversation, we can take this offline or open a new thread.

Ahhh. I think I see the disconnect. You'll often hear people say that an internal combustion engine is nothing more than an air pump, but that's categorically wrong. It's not the piston moving up during the exhaust stroke that causes burnt exhaust gases to exit the cylinder. If that were the case modern engines would be even more inefficient than they already are owing to pumping losses. Pumping losses are really quite low, especially at or near wide open throttle where the turbocharger is generating the greatest boost. So, it's not the piston's motion that generates the pressure in the exhaust system, it's the pressure differential between the hot gas in the cylinder and the relatively cooler exhaust manifold.

For proof, watch this slow-motion video of a 4-stroke engine in operation. The cylinder is made of acrylic allowing us a peek at the processes—suck, squeeze, bang, blow—as they occur. Pay special attention starting at around 18:10, where each of the processes is labeled to make it clear what's happening. Following the combustion stroke you'll see the exhaust gases rush out of the cylinder as soon as the exhaust valve opens. It's the pressure differential due to the combustion process, not the piston motion, that results in movement of the exhaust gasses.

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[Meanderthal]

Picking up where we left off from this thread:



Ahhh. I think I see the disconnect. You'll often hear people say that an internal combustion engine is nothing more than an air pump, but that's categorically wrong. It's not the piston moving up during the exhaust stroke that causes burnt exhaust gases to exit the cylinder. If that were the case modern engines would be even more inefficient than they already are owing to pumping losses. Pumping losses are really quite low, especially at or near wide open throttle where the turbocharger is generating the greatest boost. So, it's not the piston's motion that generates the pressure in the exhaust system, it's the pressure differential between the hot gas in the cylinder and the relatively cooler exhaust manifold.

For proof, watch this slow-motion video of a 4-stroke engine in operation. The cylinder is made of acrylic allowing us a peek at the processes—suck, squeeze, bang, blow—as they occur. Pay special attention starting at around 18:10, where each of the processes is labeled to make it clear what's happening. Following the combustion stroke you'll see the exhaust gases rush out of the cylinder as soon as the exhaust valve opens. It's the pressure differential due to the combustion process, not the piston motion, that results in movement of the exhaust gasses.

You forgot to link to the video.

 

[Mark S.]

You forgot to link to the video.

Derp.

 

[Meanderthal]

In the article you linked near the beginning is this:

During the exhaust stroke, early opening (somewhat before Bottom Dead Center) of the exhaust valve(s) releases most of the residual combustion pressure, so the work done by the piston on the exhaust stroke is again to push exhaust gas out against the atmosphere.​

So, you are correct that there is some expansion of gasses from the cylinder into the exhaust manifold, but it is this “residual combustion pressure”. The last of that quote is what is really driving the turbo, “push exhaust gas out against the atmosphere”.

 

[Mark S.]

I think you skipped over the important part of that statement.

During the exhaust stroke, early opening (somewhat before Bottom Dead Center) of the exhaust valve(s) releases most of the residual combustion pressure...

Take another look at the video, especially the area to the right of the exhaust port as the exhaust valve opens. The smoke floating there helps you visualize gas movement. When the exhaust valve opens the pressurized gas in the cylinder blasts out of the exhaust port. This occurs even as the piston is still moving downward. How much gas movement do you see as the piston begins moving up during the exhaust stroke? How does that compare to the blast of gas we see when the valve opens?

The reason you see so little gas movement from the piston motion is because there is almost no pressure differential between the gas in the cylinder and atmospheric pressure. We know that gas flow is caused by pressure differential, with the speed of the flow directly related to the magnitude of the differential. Just before the exhaust valve opens the pressure in the cylinder is many times that of atmospheric, which is why the gas explodes from the port.

The graph below shows cylinder pressures in a typical four-cycle engine at different crankshaft positions throughout the four engine cycles. The exhaust valve opens near the end of the combustion stroke after almost all of the work has been extracted from the expanding gases. This is nearly 90 degrees prior to the piston reaching BDC. It's that time between exhaust valve opening and the piston reaching BDC when the pressure differential between gas in the cylinder and atmospheric pressure is greatest, and we see that directly reflected in the gas movement in the video.

Take a look the area from BDC to TDC, after the exhaust valve has been open for 90+ degrees of crankshaft rotation, where the piston is moving upward during the exhaust stroke. Pressure in the cylinder at this point is within a few PSI of atmospheric, meaning there is almost no differential to drive flow. Granted, the piston pushes half-a-liter of gas from the cylinder into the exhaust system, but which do you think affects overall exhaust system pressure and flow more, the rapid expulsion of hot, pressurized gas when the exhaust valve opens, or the piston pushing a half liter of relatively cool gas through the exhaust port?

BTW, the pressure wave generated when the exhaust valve opens is what makes exhaust noise, and it's what exhaust system manufacturers take advantage of when designing components such as headers. You can read more about the design process here. An excerpt:

As the exhaust valve opens, the relatively high cylinder pressure (70 – 90 psi), initiates exhaust blowdown and a large pressure wave travels down the exhaust pipe. As the valve continues to open, the exhaust gases begin flowing through the valve seat. The exhaust gases flow at an average speed of over 350 ft/sec, while the pressure wave travels at the speed of sound of around 1,700 ft/sec.

In our engines, exhaust pressure waves are largely absorbed by the turbocharger rather than traveling down the exhaust pipe. Ford's EcoBoost design incorporates the exhaust manifold into the head, reducing the distance between the cylinder exhaust ports and the turbocharger. This is great for reducing the turbo lag (less distance for pressure pulses to get to the turbo), but terrible for getting decent noises out of the exhaust system.

Here's a good article discussing some other unique design features of Ford's EcoBoost line of engines.

 

[Meanderthal]

I agree that the pulse of pressure that is released when the exhaust valve opens contributes to driving the turbo. So, my initial statement that driving the engine with an outside power source would result in the same turbo output is most like off a little.

I think that you are neglecting the volume of gasses that is pushed out by the piston. The pressure is lower but the volume is much higher. I don't know for sure, but I suspect the graph you posted is showing results from a naturally aspirated engine. Meaning, the exhaust manifold pressure is a lot lower (nearly atmospheric, as shown in your graph).

Here is a quote from an article on exhaust manifold pressure (EMAP):

While it’s not a common input in most street cars, turbine inlet pressure (TIP) or exhaust manifold pressure (EMAP) is a useful input when it comes to analysing your turbo sizing and performance in high end race engines. It’s not uncommon with factory turbocharged engines to find that the EMAP is double the boost pressure seen in the intake manifold (sometimes even more!).​

​

So, the exhaust valve is opening while there is still some pressure from combustion, but nearly 90% of that pressure has gone into moving the piston/turning the crank. Then the full volume of the cylinder is pumped out by the piston rising up from BDC to TDC. Let's not forget that while the exhaust valve opens, the piston is still moving down in the cylinder, so some of the pressure drop from the time the valve opens until BDC is also because the volume of the cylinder is increasing. This happens even more so at higher rpms, meaning the push from the pressure released from the cylinder will (maybe could) actually drop as rpms increase.

There just isn't enough time and/or volume of combustion gasses escaping when the exhaust valve opens to drive the turbo primarily by that means.

I don't have the time right now to read all that you have linked, but I will get back to it later.

 

[Barry S.]

The whooshing sound you hear is this entire subject going over my head but I do appreciate you guys and the knowledge you bring to this forum.

 

[ScareCrow]

The whooshing sound you hear is this entire subject going over my head but I do appreciate you guys and the knowledge you bring to this forum.

My knowledge of turbochargers......

 

[Mark S.]

So, the exhaust valve is opening while there is still some pressure from combustion, but nearly 90% of that pressure has gone into moving the piston/turning the crank.

But due to heat and the volume of gas, the remaining pressure is still many times that of atmospheric.

Then the full volume of the cylinder is pumped out by the piston rising up from BDC to TDC.

But that volume of gas is far less than the volume before the exhaust valve opens. During the intake stroke the compressor has pumped air into the cylinder under boost. That means the amount of air in the cylinder following the intake stroke is a multiple of the cylinder's volume. It's then compressed and ignited, further increasing the volume. Following combustion the gas cools reducing the volume somewhat, but the heat is still many times higher than the intake air charge, so its volume must be many multiples of the cylinder's displacement. The graphs I posted show the pressure in the cylinder equalizes (or nearly so) with the pressure on the other side of the exhaust valve long before the piston begins moving upward on its exhaust stroke, meaning the overwhelming volume of gas has already exited the cylinder.

Let's not forget that while the exhaust valve opens, the piston is still moving down in the cylinder, so some of the pressure drop from the time the valve opens until BDC is also because the volume of the cylinder is increasing.

True, but how much? We know the volume of gas in the cylinder prior to the exhaust valve opening is many multiples of the cylinder's displacement. What is the increase in cylinder volume due to piston movement following exhaust valve opening? 15%? 20%? Let's say it's 20% just for the sake of argument. That's a .1 liter increase in cylinder volume. Let's assume the volume of gas in the cylinder, due to all the factors listed above, is four times the cylinder's displacement. It's probably more than that, but let's keep the math simple. A tenth of a liter is 5% of 2 liters, so the amount of pressure drop attributable to piston movement would be five percent. That seems like an insignificant factor in this discussion.

It’s not uncommon with factory turbocharged engines to find that the EMAP is double the boost pressure seen in the intake manifold (sometimes even more!).

This doesn't surprise me, mainly because the volume of gas in the cylinder increases due to combustion—there's more gas going out than coming in.

Here's a thought experiment for you: Why do engine designers set the exhaust valve timing such that it opens while the piston is still traveling downward on the combustion stroke?

 

[Meanderthal]

But due to heat and the volume of gas, the remaining pressure is still many times that of atmospheric.


But that volume of gas is far less than the volume before the exhaust valve opens. During the intake stroke the compressor has pumped air into the cylinder under boost. That means the amount of air in the cylinder following the intake stroke is a multiple of the cylinder's volume. It's then compressed and ignited, further increasing the volume. Following combustion the gas cools reducing the volume somewhat, but the heat is still many times higher than the intake air charge, so its volume must be many multiples of the cylinder's displacement. The graphs I posted show the pressure in the cylinder equalizes (or nearly so) with the pressure on the other side of the exhaust valve long before the piston begins moving upward on its exhaust stroke, meaning the overwhelming volume of gas has already exited the cylinder.



True, but how much? We know the volume of gas in the cylinder prior to the exhaust valve opening is many multiples of the cylinder's displacement. What is the increase in cylinder volume due to piston movement following exhaust valve opening? 15%? 20%? Let's say it's 20% just for the sake of argument. That's a .1 liter increase in cylinder volume. Let's assume the volume of gas in the cylinder, due to all the factors listed above, is four times the cylinder's displacement. It's probably more than that, but let's keep the math simple. A tenth of a liter is 5% of 2 liters, so the amount of pressure drop attributable to piston movement would be five percent. That seems like an insignificant factor in this discussion.



This doesn't surprise me, mainly because the volume of gas in the cylinder increases due to combustion—there's more gas going out than coming in.

Here's a thought experiment for you: Why do engine designers set the exhaust valve timing such that it opens while the piston is still traveling downward on the combustion stroke?

Just a quick reply to part of your post that stands out to me.

The change in volume in the cylinder from the time the exhaust valve opens is approximately double. Since the exhaust valve is opening with about 90° of crank rotation before BDC.

I think the discussion here is possibly veering away from what we started discussing. Is the turbo driven by expanding hot gasses? So, there is still a little bit of combustion happening when the exhaust valve opens, as the video you posted showed. This, in it's entirety, is how a turbine engine functions. The combustion drives expansion and all of that expansion goes into driving the fans in the outlet.

In the internal combustion engine, with or without a turbo, 90% the combustion is used to drive the piston and something less than 10% of the combustion is pushed out into the exhaust manifold. What I am hearing from you, is that you believe this last 10% of the combustion is providing all (or most) of the drive to the turbo? This is where I am willing to say this extra bit of combustion is adding to the drive the turbo receives but it is not the primary mode of drive.

More of a response than I intended but it's there.

 

[Mark S.]

I think the discussion here is possibly veering away from what we started discussing. Is the turbo driven by expanding hot gasses?

Yes, that's the question.

What I am hearing from you, is that you believe this last 10% of the combustion is providing all (or most) of the drive to the turbo? This is where I am willing to say this extra bit of combustion is adding to the drive the turbo receives but it is not the primary mode of drive.

No, that's not what I'm saying. I'm saying that nothing operates without the heat of combustion and the resultant expansion of hot gasses. It's combustion heat which causes expansion of gasses in the combustion chamber, and generates the work of pushing the piston down to turn the crankshaft. All the heat generated by thousands upon thousands of combustion events occurring every minute the engine is running doesn't just disappear after its used to push the piston; it has to go somewhere.

Most internal combustion engines average around 20% thermal efficiency. In other words, only some 20% of the heat generated by combustion goes to generating the work of pushing down on the piston. The rest is essentially wasted heat energy. A portion goes toward heating the engine oil, coolant, and the engine itself. The remainder of the heat energy—about 30% for the average car—goes out the exhaust pipe. I agree that a portion of exhaust gas is forced out of the exhaust valve by piston movement, but that portion is insignificant compared the movement of gas resulting from the massive pressure differential between the hot, expanded gases in the cylinder and the relatively cooler exhaust system. It's this massive, sudden rush of hot, pressurized gas into the exhaust system that generates pressure waves traveling at the speed of sound and makes those wonderful exhaust noises gearheads love. It's also a motive force that can be used for other things, like spinning turbochargers.

 

[Idahobro]

 

[Meanderthal]

Yes, that's the question.


No, that's not what I'm saying. I'm saying that nothing operates without the heat of combustion and the resultant expansion of hot gasses. It's combustion heat which causes expansion of gasses in the combustion chamber, and generates the work of pushing the piston down to turn the crankshaft. All the heat generated by thousands upon thousands of combustion events occurring every minute the engine is running doesn't just disappear after its used to push the piston; it has to go somewhere.

Most internal combustion engines average around 20% thermal efficiency. In other words, only some 20% of the heat generated by combustion goes to generating the work of pushing down on the piston. The rest is essentially wasted heat energy. A portion goes toward heating the engine oil, coolant, and the engine itself. The remainder of the heat energy—about 30% for the average car—goes out the exhaust pipe. I agree that a portion of exhaust gas is forced out of the exhaust valve by piston movement, but that portion is insignificant compared the movement of gas resulting from the massive pressure differential between the hot, expanded gases in the cylinder and the relatively cooler exhaust system. It's this massive, sudden rush of hot, pressurized gas into the exhaust system that generates pressure waves traveling at the speed of sound and makes those wonderful exhaust noises gearheads love. It's also a motive force that can be used for other things, like spinning turbochargers.

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).​

​

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).

 

[Dude]

What will you guys talk about when EVs are pervasive and turbochargers exist in museums

 

[Meanderthal]

What will you guys talk about when EVs are pervasive and turbochargers exist in museums

Overclocking!

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