Heat energy extraction in turbocharger

[Tom Rauji]

I have a question I've found asked, but never found answered. I'm an EE, but I have a car as a hobby. I changed from a belt drive inlet air compressor (supercharger) to a turbocharger.

People commonly say the turbo is "much more efficient" because it "runs free on otherwise wasted exhaust heat".

I see almost the same fuel consumption per horsepower and almost the same horsepower per atmosphere of boost. For example, if I normalize boost pressure so my engine makes ~800 HP with a supercharger and 800 with a turbo with the same combustion air/fuel ratio, the turbo system fuel consumption is about 7% less.

Isn't this a reasonably accurate way of determining the actual wasted heat energy recovered? Wouldn't efficiency change most accurately show as pounds per hour fuel for a given power output? I thought about measuring temperatures and pressures, but it seems to me the real answer is just the very simple answer. It looks like the turbo only recovers about 7% of wasted energy.

I'm just try to decide if that is accurate.

 

[sophiecentaur]

The turbo is not consuming much energy when the car is doing low revs and it only contributes much after that threshold engine speed. If your consumption figures are over a whole range of driving conditions, the 7% may be due to the times when the engine is at low speed. The supercharger is doing work, compressing the input air all the time. I guess you could only tell for sure if you ran the engine on a test bed - like the big boys do.
It strikes me (I live in the UK, with lunatic fuel prices) that 7% is well worth having. For someone in the US (with equally lunatic fuel prices - but in the other direction) then 7% is probably not a big deal. But no one runs a high performance car with a view to fuel economy - do they?

 

[Tom Rauji]

The turbo is not consuming much energy when the car is doing low revs and it only contributes much after that threshold engine speed. If your consumption figures are over a whole range of driving conditions, the 7% may be due to the times when the engine is at low speed. The supercharger is doing work, compressing the input air all the time. I guess you could only tell for sure if you ran the engine on a test bed - like the big boys do.
It strikes me (I live in the UK, with lunatic fuel prices) that 7% is well worth having. For someone in the US (with equally lunatic fuel prices - but in the other direction) then 7% is probably not a big deal. But no one runs a high performance car with a view to fuel economy - do they?

I probably was not clear. The test is at wide open throttle on a drive wheel dynamometer, corrected for temperature and barometric pressure. The car's computer logs injector on-time and fuel pressure to calculate fuel flow. The dyno calculates horsepower based on acceleration of a very heavy roller, so it not measuring brake horsepower directly. It is calculating HP from the roller acceleration rate.

This is not street driven. It is simply the wide open throttle efficiency through the power peak I am looking at. This seems to tell me most of the turbine energy actually comes from the pressure drop across the turbine, rather than from temperature drop. I know the exhaust back pressure averages near zero at the cylinder exhaust ports with the supercharger, but is around 20 psi with the turbo. The turbine piles up a great deal of pressure against the exhaust ports.

Perhaps real numbers are lacking because of difficulties in obtaining meaningful data for estimates.

 

[sophiecentaur]

Oh - right. A proper experiment then.

I know the exhaust back pressure averages near zero at the cylinder exhaust ports with the supercharger, but is around 20 psi with the turbo.

Does that say something about the exhaust, downstream? How much do the increases in the inlet pressure from the two compressors compare and could you measure the power taken by the supercharger? Despite the increased back pressure, you still get 7% better fuel consumption. Is that with the same output engine power?
I'm not the person to give any useful input. Let's look for someopne who knows a bit more. Perhaps you could try posting a question of the Mechanical Engineering Forum. Many contributors only look on a very few forums and someone could be missing your question.
Try searching for terms of interest and you may find another forum with similar topics.

 

[Tom Rauji]

Does that say something about the exhaust, downstream? How much do the increases in the inlet pressure from the two compressors compare and could you measure the power taken by the supercharger? Despite the increased back pressure, you still get 7% better fuel consumption. Is that with the same output engine power?

I'm not the person to give any useful input. Let's look for someopne who knows a bit more. Perhaps you could try posting a question of the Mechanical Engineering Forum. Many contributors only look on a very few forums and someone could be missing your question.
Try searching for terms of interest and you may find another forum with similar topics.

I have searched it extensively. I have yet to find anyone put typical or general numbers on it. I'm unable to find a venue to obtain a good answer or discussion, so I don't know where to go. I have found a few places that acknowledge the pressure differential build across the turbine, which is considerable, but none that give even a rough idea of how much wasted energy is recovered.

I think I understand what is happening, but it is never good to just go off thinking one is right without critical review.

It looks like, since I used the same engine and only changed the supercharger, my system recovers about 7% that would have otherwise been wasted out the exhaust. The turbo makes 7% more power for the same fuel usage. I'm not sure if that is reasonable, or I have changed something else I have not considered. It may not mean a thing for other similar systems.

 

[sophiecentaur]

@Tom. You have a problem here because you can't get much useful info from the vast number of home enthusiasts who work on the suck it and see principle and the highly commercial organisations will not publish their findings. You are sitting right in between them.
When I suggested searching, I was meaning the PF Mechanical Engineering Forum. I would bet that some of the Engineers who frequent that forum might well have some ideas for sources of info, at least.
Good luck with it.

 

[russ_watters]

We do have some serious gearheads who hang out in the ME section. I'll move the thread...

 

[JBA]

According to the test method you describe are restricting the testing to a full throttle acceleration test mode that is never steady state and therefore may not reflect the fuel usage reduction obtained for a steady state engine speed condition. The relationship between fuel savings and increased horsepower ratio is determined by the amount of power produced for a given unit of fuel energy vs unit of engine energy output; therefore, for an equal fuel consumption you are in fact gaining horsepower proportionally to fuel energy consumption,. i.e., in steady state, you can open the throttle further to create more horsepower for an equal amount of fuel consumption.

 

[CWatters]

Google found..

http://training.sae.org/webseminars/wb1018/

In gasoline engines, turbocharging enables downsizing which improves fuel economy by 5-20%.

Some web sites say that it's not the turbo that saves fuel but a turbo allows you to use a smaller displacement engine to achieve the same power and it's the smaller displacement part that saves the most fuel.

 

[JBA]

The SAE Seminar is focusing upon the current primary industry focus of reducing engine size to reduce emissions and increase fuel economy with a minimum loss of vehicle horsepower; at the a same time, the basic fact that turbochargers increase power for smaller engines also applies to their much longer prior application on large engines strictly as a method to increase their horsepower.

 

[Randy Beikmann]

This seems to tell me most of the turbine energy actually comes from the pressure drop across the turbine, rather than from temperature drop.

Actually temperature drops and pressure drops are intimately linked. For example, after you've burnt the air/fuel mixture in an IC engine, the resulting gas is near TDC at a high pressure and temperature, with a small volume. Once the piston starts moving down the cylinder, the gas does work on the piston (by its pressure), and the energy came from the drop in temperature of the gases. Simultaneously, the pressure drops during expansion. The collisions between the gas molecules, and a piston that is moving away from them, causes the molecules to slow down. This drops both their temperature AND pressure.

While the process is a little more involved in a turbocharger, the same reasoning applies.

 

[jack action]

For the supercharger to work, you must take the power required to pump the air from the crankshaft output. The fact that overall power is increased is because the combustion of the compressed air creates more power than what is required to compress the air.

For the turbo, it runs on exhaust pressure which is usually lost to atmosphere once the exhaust valve opens. The pressure differential is used to pressurize the incoming air. So - in theory - the crankshaft is not affected.

But, by adding the turbo, the pressure has to be increased in the cylinder during the exhaust process; otherwise the turbo wouldn't work. This is a loss compared to the supercharged engine which can have a tuned exhaust system that will harvest the exhaust gases, sometimes even creating a slight vacuum that pulls the exhaust gases out. That increased pressure is pushing down on the piston, going against its motion and, thus, also taking power away from the crankshaft output.

All in all, the turbo is still more efficient, but maybe not as much as one might expect.

 

[JBA]

For the supercharger to work, you must take the power required to pump the air from the crankshaft output. The fact that overall power is increased is because the combustion of the compressed air creates more power than what is required to compress the air.

For the turbo, it runs on exhaust pressure which is usually lost to atmosphere once the exhaust valve opens. The pressure differential is used to pressurize the incoming air. So - in theory - the crankshaft is not affected.

But, by adding the turbo, the pressure has to be increased in the cylinder during the exhaust process; otherwise the turbo wouldn't work. This is a loss compared to the supercharged engine which can have a tuned exhaust system that will harvest the exhaust gases, sometimes even creating a slight vacuum that pulls the exhaust gases out. That increased pressure is pushing down on the piston, going against its motion and, thus, also taking power away from the crankshaft output.

All in all, the turbo is still more efficient, but maybe not as much as one might expect.

With regard to your "tuned exhaust" note: A tuned exhaust is only effective in a very narrow engine rpm range as opposed to a turbocharger which provides a power boost at all rpm levels.

 

[Ketch22]

A couple quick observations.
One is on the tuned exhaust item. The differences are quite subtle however the experimenting I have done seems to point to tuning working better on a turbo than on other motor styles ( at least on 4 cyl motors). The best explanation is that the increased back pressure seems to matter little but regardless of engine speed the pressure pulse still arrives in a synchronized fashion slightly increasing turbine efficiency.

The other item is in an addendum to Jack action's comment. The crankshaft parasitic loss is close to a wash with the back pressure. The Supercharger shows better increase in the lower engine rpm ranges and as the volumetric requirements increase they have more trouble keeping up. Without careful planning the top end often declines before engine redline. On a turbo the low rpm operation suffers but up top the boost continues to increase. This being in the same area where the Hp is peaking I tend to see more increase with the turbo.

 

[JBA]

That is a really interesting note about exhaust scavenging with a turbocharger; but, I have to ask, where the performance boost cycling is occurring, on exhaust scavenging side, between the exhaust valve and the turbine inlet or the turbine outlet and the exhaust discharge point or; alternatively, on the engine intake system side between the compressor outlet and the intake valve?

In a standard inlet/exhaust configuration there are very distinct separate regions, inlet and exhaust where tuning can be performed with clear wave reflection points; but, on the other hand, I would have expected the turbo gas flow entry and exit velocity flow points and the compressor discharge flow point to be very poor wave reflection points.

 

[Ketch22]

I do not believe that it is truly a scavenging item. Exhaust scavenging is a function of a negative pressure wave ( generated by an adjacent cylinder) reflecting and assisting removal of spent gasses. The increased pressure in the manifold strongly reduces or eliminates the scavenging.

What I believe I am seeing is that with the unequal length primary tubes from different cylinders the exhaust pulse is arriving at the turbo in a syncopated fashion. This uneven drive reduces the efficiency of the turbine. With an equal length manifold the turbine works more efficient and thus the compressor is more effective.

I guess one would call that between exhaust valve and turbine but more correct it is an item of turbine efficiency.

 

[JBA]

Thank you for your response and that may be what is the case but I think it is an error to simply designate it as an element of turbocharger efficiency because that tends to mask the fact that it would appear that the rpm region in which an additional boost might be attained is a potentially selectable by variations in exhaust header design independent upon the efficiency of the installed turbocharger.

( generated by an adjacent cylinder)

One other minor point regarding the above part of your about exhaust scavenging statement. Exhaust scavenging, while being able to be achieved by timing of multi-cylinder exhaust waves, can also be achieved by the correct exhaust pipe length tuning on both single cylinder engines and multi-cylinder engines with individual cylinder exhaust pipes. The same being true for engine inlet tuning (which is the area of this subject where I have some background).

 

[Ketch22]

JBA, You are exactly right in your comments. It is possible to tune single cylinder or multi for better scavenging and induction by adjusting size and length of the runners. My hesitation in calling it scavenging and leaning towards turbo efficiency lies in the effect. When I have been able to invest time in the issue it appears to happen much broader range then the tuned length. What I believe is occurring is that the impulses are arriving in a structured fashion. The cylinders fire on a regular timing and by nature of the equal length primary tubes they arrive in sequence regardless of rpm. For most of the range they are not scavenging though as the tubes on most turbo manifolds are far to short for the rpm.

Fascinating discussion, I may start a new thread for this item as I don't want to hijack someone else's.

 

[Tom Rauji]

With regard to your "tuned exhaust" note: A tuned exhaust is only effective in a very narrow engine rpm range as opposed to a turbocharger which provides a power boost at all rpm levels.

I think his explanation is good. This is only a race car, so low RPM cruise is unimportant. The tuned exhaust with the belt driven SC worked well, because the engine spent it's life between 6000 and 7800 RPM. The efficiency of the SC exhaust is probably why the lbs/hr fuel doesn't change much with the turbine driven compressor. The belt driven SC exits exhaust through large tuned individual pipes to open air at a collector point. The pressure at exhaust ports averages near zero psig.

The turbo routes all exhaust to one point, where it goes through the turbine and out a single pipe. The pressure at the exhaust port is almost 30 psig average. So while it may harvest some wasted heat energy, it also increases pumping losses.

Most "car people" make blanket statements implying a turbo system compresses air for "free", running off wasted energy. I know that extreme is not true, because I can measure a very dramatic exhaust back pressure increase. I was trying to find a simple but accurate way to express what the typical system actually does, even if somewhat vague. For example, "for the same power, volumetric efficiency increases 5-20% using a turbo".

I was hoping something is documented somewhere, because looking at one system is not the best way to form an opinion. I can only measure a few things. I can measure rear wheel power, fuel consumption, temperatures, and pressures. There are errors in measurement, also.

 

[JBA]

I did a bit of web searching and did not find anything specifically addressing your question but I did find the below rather simple post that gives a very straight forward and brief discussion of the relative merits of SC's vs TC's (abbreviations I found prevalent). From what is presented there, the benefits of SC vs TC are largely dependent upon the total desired operating envelope of the engine.

http://tomak3.tripod.com/page10.html

At the same time, another site I visited discussed the subject of the inclusion of a boost pressure limiting controlled exhaust bypass; and, (conceding my basic lack of actual turbocharger applications) specifically related to your measured manifold pressure, the mention of the inclusion of an included exhaust bypass raises the question as to whether a bypass on your system would allow your desired inlet boost to be achieved even if the exhaust manifold / turbo inlet pressure were reduced by an installed properly sized bypass.

 

[Tom Rauji]

Thanks, I just read the link you posted. My centrifugal SC was a more modern high efficiency design, producing 15 psi over a fairly wide RPM range. The engine used more air as the blower speed increased, pressure actually stayed reasonably constant as long as a certain minimum engine speed was reached. Since the engine never sees less than 5000 or so, the lack of low RPM boost was not an issue.

A roots style blower has positive displacement, so it has the same proportion of air as the engine can pump as speed changes. If the engine is 350 cu inches and the blower displaces 350 cu inches per two engine crankshaft turns, it will make about 15 psig over the range where the engine has a VE of 1. It has boost at all speeds.

My turbo has a waste gate, or it would damage the turbo and engine. The waste gate is operated by the outlet pressure. Mine can be set to anything from 5 psig to 30 psig (1/3 to around two atmospheres boost). The exhaust back pressure runs proportional to the boost, probably just a coincidence of the turbine back pressure required to make a certain boost happening to work out that way by turbine and compressor characteristics. The engine does not have enough exhaust volume to have full boost at slower speeds, but I'm calling for 15-25 psig of pressure. That's pretty high.

With similar horsepower, the turbo uses about 7% less lbs/hr fuel. Of course measurements have tolerances, but it looks like I'm only getting 7% free power. Out of 750 nominal, that's 50 HP worth of power with the same fuel consumption. That seems reasonable to me, given what I read. :)