Why don't more cars have turbos?

[Bignate603]

^^ That's a fair point, but most turbo vehicles record much poorer fuel economy ratings than their NA counterparts. To be sure, a lot of that probably has to do with the testing techniques though. I've had many turbo vehicles, and could get excellent fuel economy by just staying out of boost and keeping the revs down. In essence : right foot = fuel economy controller.

You can't compare turbocharged engine with a NA engine of similar displacements because a turbocharged engine will flow more air. You need to compare it against an engine with similar power output. In the diesel world they've known for quite some time that they can get better efficiency out of a turbocharged engine.

Imagine the same scenario with a gas turbine engine. It's a jet engine with a shaft coming out of the front to drive the vehicle. Now put an extra turbine at the back to extract extra energy. Does that sap energy and reduce power output from the driveshaft? Of course.

If the turbine engine was designed so that it was dumping combustion gas out the back that still had elevated pressure and temperature enough to get work out of it like you do in a NA gasoline engine it would benefit from an extra turbine stage. In fact, reworking the hot end of a turbine engine to add another turbine stage is a common way of increasing power output.

FYI I design components for turbine engines for a living, I might know a thing or two about them. 😉

You still don't seem to understand that letting that exhaust gas expand freely as soon as the valve opens is letting energy literally go out the tail pipe. The turbo isn't being spun by the piston pushing the air out of the cylinder, it's spun by the combustion gas leaving the cylinder under pressure and with high temperature. If you could stop the crankshaft with the piston at bottom dead center right before the beginning of the exhaust stroke you could crack the exhaust valve the combustion gasses would still be enough to spin the turbo.

 

[jlee]

But it only delivers that torque when the turbo is spun up. The "flat torque" marketing speak is misleading.

Your n/a motor also only delivers that torque when you push the gas pedal. What's your point?

 

[Bignate603]

Actually after-burners work on much the same principle but on a more extreme scale than a turbo.

No, afterburners do not work like turbochargers. Afterburners are used to drastically increase exit velocity of a jet engine and have no rotating components. Afterburners just take in hot gas in one side, add large amounts of fuel and combust it, then eject the greatly expanded gas at incredibly high speeds. They are not used to extract power from hot gas and use it to spin a shaft to get useful work out of it.

The compressor and turbine section of a turbine engine shares many similarities to a turbocharger. In fact, quite a few people have built turbine engines out of turbochargers. They just need to build a combustor between the compressor side and the turbine side of the turbo.

 

[exdeath]

^ afterburners act like an inefficient rocket engine that burns a ridiculous amount of added fuel, they don't aid in efficiency or extract energy from waste or aid the primary engine in any way.

If anything is like an afterburner on a car, its an anti lag system that dumps raw fuel directly into the exhaust to keep the turbo in boost with no regard for fuel waste or efficiency. But even still that only benefits the primary engine and works pre turbine rather than in a rocket nozzle in your exhaust tip to provide secondary thrust, so still not the same thing :awe:

 

[IcePickFreak]

No, afterburners do not work like turbochargers. Afterburners are used to drastically increase exit velocity of a jet engine and have no rotating components. Afterburners just take in hot gas in one side, add large amounts of fuel and combust it, then eject the greatly expanded gas at incredibly high speeds. They are not used to extract power from hot gas and use it to spin a shaft to get useful work out of it.

The compressor and turbine section of a turbine engine shares many similarities to a turbocharger. In fact, quite a few people have built turbine engines out of turbochargers. They just need to build a combustor between the compressor side and the turbine side of the turbo.

Well right, I suppose I should have said "A turbine with an after-burner." I was just stating that, in principle it is similar in that turbine engines with an after-burner are using expanding gases through the turbine to increase their power output. In the example I was quoting there, he stated "have a shaft coming out the front" of the turbine engine isn't going do anything anyway since that's the compressor side, so I wasn't expecting to be checked by an industrial professional on my terminology in my reply lol. 😀

 

[Bignate603]

^ afterburners act like an inefficient rocket engine that burns a ridiculous amount of added fuel, they don't aid in efficiency or extract energy from waste or aid the primary engine in any way.

Yup, if anything afterburners greatly reduce efficiency. They chew through massive amounts of fuel and would drain a plane's tank very quickly if used for extended periods but in certain situations the large amount of thrust they create can be very useful.

 

[Bignate603]

Well right, I suppose I should have said "A turbine with an after-burner." I was just stating that, in principle it is similar in that turbine engines with an after-burner are using expanding gases through the turbine to increase their power output. In the example I was quoting there, he stated "have a shaft coming out the front" of the turbine engine isn't going do anything anyway since that's the compressor side, so I wasn't expecting to be checked by an industrial professional on my terminology in my reply lol. 😀

Turbine engines in general have components that perform similar functions to parts of a turbocharger but an afterburner is not one of them. Exdeath gave example of what a component similar to an afterburner in a turbocharger would be.

What he described as a turbine engine "with a shaft coming out the front" is typically called a turboshaft engine. They're found in things like helicopters and even the Abrams tank. Most of the engines I do things for are actually turboshaft engines for helicopters. In theory they could have the shaft come out the turbine end rather than the compressor but this is complicated by a few things, one of the most noticeable being the temperature of the exhaust.

 

[IcePickFreak]

Turbine engines in general have components that perform similar functions to parts of a turbocharger but an afterburner is not one of them. Exdeath gave example of what a component similar to an afterburner in a turbocharger would be.

What he described as a turbine engine "with a shaft coming out the front" is typically called a turboshaft engine. They're found in things like helicopters and even the Abrams tank. Most of the engines I do things for are actually turboshaft engines for helicopters. In theory they could have the shaft come out the turbine end rather than the compressor but this is complicated by a few things, one of the most noticeable being the temperature of the exhaust.

I think we're on the same page, just misunderstanding.. or maybe not. I wasn't implying an afterburner has any equivalent component in a turbo ICE engine. Just the principle under which they operate.

The afterburner adds to the expanding gases exiting through the turbine by adding fuel (and an ignition source) to burn the remaining oxygen, correct?

This in turns spins the turbine faster (than when the turbine engine is not running the afterburner) which in turns spins the compressor side faster, correct?

Please don't take those questions the wrong way, as they're more to validate my own understanding, I'm not asking them in a degrading matter by any means. I said "same principle" because a turbine with an afterburner is using increased expanding gases to power the turbine, which in turn powers the compressor. That's all I was getting at, in a one sentence post no less. 😉

 

[exdeath]

That's where you're misunderstanding, the afterburner occurs in the nozzle well beyond the jet engine and creates thrust directly like a liquid fuel rocket, its post turbine and has no effect on the jet engine operation itself. Its a completely separate auxiliary system in the nozzle of the exhaust and independent of the turbine.

The automotive equivalent would be dumping gallons of fuel into your exhaust tip way after the turbo and moving forward from the rocket thrust alone.

 

[Bignate603]

Exdeath explained what an afterburner is well. IcePickFreak is describing the component that's called the combustor. Every turbine engine has one, it's where the fuel is mixed with the air and ignited. In most engines the combustor can heat the air passing through the engine to temperatures high enough that the turbine sections would essentially burn up. While designing the engine they set the fuel map to limit the temperatures to reasonable levels. A way to upgrade engines to get more power is to increase the temperature that the turbine section can withstand.

Afterburners are pretty much only found on military jets that need extra bursts of speed. Like the name sounds, an afterburner is placed after the rest of the engine and the majority of the time they don't do anything, it just acts like a normal part of the exhaust duct.

 

[IcePickFreak]

Yup you guys are absolutely right, I was the one off there dunno what I was thinking - all those years of flight sims gone to waste.

Do I at least get a prize for being the first person to be wrong on the internet? At least I've never seen one before. 😀 If not I'll just blame it on the cold medicine. :awe:

 

[gorobei]

@throckmorton
read up on exhaust scavenging. As long as the system is properly designed, the piston should never be facing any backpressure from the turbine. An exhaust designed for a turbo will take advantage of the negative pressure at the cylinder developed by the expanding gas volume.
http://en.wikipedia.org/wiki/Exhaust_manifold#Exhaust_Scavenging

 

[gorobei]

the whole backpressure issue came from all the old school car guys complaining when catalytic converters became a requirement. Exhaust tuning can compensate for all kinds of stuff.

 

[Bignate603]

Yup you guys are absolutely right, I was the one off there dunno what I was thinking - all those years of flight sims gone to waste.

Do I at least get a prize for being the first person to be wrong on the internet? At least I've never seen one before. 😀 If not I'll just blame it on the cold medicine. :awe:

There are plenty of people that are wrong on the internet but you may well be the first one that's admitted it. 😉

 

[exdeath]

Imagine the same scenario with a gas turbine engine. It's a jet engine with a shaft coming out of the front to drive the vehicle. Now put an extra turbine at the back to extract extra energy. Does that sap energy and reduce power output from the driveshaft? Of course.

You don't get it, it's ok. Your question has been answered numerous times, and asking the same question different ways isn't going to change the answer. Turbochargers work by recovering energy from still expanding gases in the exhaust, not by the piston pushing the exhaust through them.

PS: if that extra turbine is connected to a 15 foot turbofan, the jet engine becomes 10x more powerful and 10x more efficient for the 1% more effort required by the exhaust to pass by that extra turbine.

 

[exdeath]

the whole backpressure issue came from all the old school car guys complaining when catalytic converters became a requirement. Exhaust tuning can compensate for all kinds of stuff.

Actually it came from people noticing a loss in power when the exhaust was too big or not present at all, so they reasoned that engines need some backpressure to run optimally.

Really what is happening is that with an exhaust too big, you lose velocity. The pressure/vacuum waves dissipate because the exhaust piping is too big for the volume it needs to flow and the exhaust cools too quickly and loses velocity and the scavenging effect is lost.

In simple terms, think that the exhaust can't "seal" inside the piping in discrete packets and form pressure/vacuum waves and maintain velocity because the piping is too big for the amount of exhaust flow. It's like capillary action not working if the tube is too big.

What is required for optimal power is the smallest exhaust possible (or slightly smaller even) that can flow the maximum amount of gas produced by the engine; this will result in the greatest exhaust velocity and greatest scavenging while not choking the engine at maximum flow.


For a turbo car, it doesn't matter, the exhaust pulses are scrambled by the turbo and you don't have scavenging anyway, so you want the largest possible exhaust post turbo you can get to provide the largest possible pressure drop across the turbine. This is why you have 4" exhausts on cars like Supras, but ricers mimic this on their 1.5L n/a engines and end up losing power and burning valves.

No amount of tuning can overcome a genuine physical restriction. That's what high flow single piece cats, etc, are made for.

 

[Throckmorton]

You can't compare turbocharged engine with a NA engine of similar displacements because a turbocharged engine will flow more air. You need to compare it against an engine with similar power output. In the diesel world they've known for quite some time that they can get better efficiency out of a turbocharged engine.



If the turbine engine was designed so that it was dumping combustion gas out the back that still had elevated pressure and temperature enough to get work out of it like you do in a NA gasoline engine it would benefit from an extra turbine stage. In fact, reworking the hot end of a turbine engine to add another turbine stage is a common way of increasing power output.

FYI I design components for turbine engines for a living, I might know a thing or two about them. 😉

You still don't seem to understand that letting that exhaust gas expand freely as soon as the valve opens is letting energy literally go out the tail pipe. The turbo isn't being spun by the piston pushing the air out of the cylinder, it's spun by the combustion gas leaving the cylinder under pressure and with high temperature. If you could stop the crankshaft with the piston at bottom dead center right before the beginning of the exhaust stroke you could crack the exhaust valve the combustion gasses would still be enough to spin the turbo.

I know that it's spun by expanding gas, but there has to be an equal and opposite force to the pressure on the turbine, and that force is applied to the piston. If it was cold, non pressurized gas, the backpressure of the turbine would be working against the piston. That doesn't suddenly change because the gas is at high pressure and expanding.

If what you guys are saying is true, then putting a restrictor plate into the exhaust pipe would have no effect on the piston "because the gas is expanding".

 

[Howard]

Yes, there is an increase in backpressure. Is it significant? No.

 

[exdeath]

If what you guys are saying is true, then putting a restrictor plate into the exhaust pipe would have no effect on the piston "because the gas is expanding".

If the restrictor plate is sized such that it's in a portion of the exhaust that is bigger around than the rest of the piping, then restricted to the size that the rest of the exhaust is, then yes it will have little effect.

The size of the turbine housing and wheel are choosen to so as to offset and compensate for the restriction that they will impose.

It's the turbine housing that is the biggest restriction if it's not sized correctly for the engine demands. The mass of the turbine wheel itself has little effect. Any effect that it does have is greatly outweighed by the greater airflow being pumped into the engine by the compressor, which in turn becomes a greater exhaust flow that makes the turbine wheel even less of a problem. The momentum of the turbine is <<<<<<<<<<<<<< momentum of the piston, the piston won't feel a thing.

You need to convince yourself how little it takes to spin up a turbo. Find someone who has one on a bench and just breath hot air into the turbine inlet and see how effortlessly and quickly it starts to spin.

http://www.youtube.com/watch?v=ug6_aetYjbE

Also the exhaust gas itself has momentum as it's going through the exhaust manifold and into the turbine housing. By the time the pressure wave backs up at the turbo, if at all, the exhaust valve is already closed. In addition, the valve overlap and incoming compressed intake charge aid in forcing exhaust gas out as the piston starts the intake stroke. Momentum reversal of the intake/exhaust flow isn't as easy as you think. It's easier for the exhaust gas to go through the turbine than it is to stop, reverse, and flow backwards toward the exhaust port.

 

[Throckmorton]

If the restrictor plate is sized such that it's in a portion of the exhaust that is bigger around than the rest of the piping, then restricted to the size that the rest of the exhaust is, then yes it will have little effect.

The size of the turbine housing and wheel are choosen to so as to offset and compensate for the restriction that they will impose.

It's the turbine housing that is the biggest restriction if it's not sized correctly for the engine demands. The mass of the turbine wheel itself has little effect. Any effect that it does have is greatly outweighed by the greater airflow being pumped into the engine by the compressor, which in turn becomes a greater exhaust flow that makes the turbine wheel even less of a problem. The momentum of the turbine is <<<<<<<<<<<<<< momentum of the piston, the piston won't feel a thing.

You need to convince yourself how little it takes to spin up a turbo. Find someone who has one on a bench and just breath hot air into the turbine inlet and see how effortlessly and quickly it starts to spin.

http://www.youtube.com/watch?v=ug6_aetYjbE

Also the exhaust gas itself has momentum as it's going through the exhaust manifold and into the turbine housing. By the time the pressure wave backs up at the turbo, if at all, the exhaust valve is already closed. In addition, the valve overlap and incoming compressed intake charge aid in forcing exhaust gas out as the piston starts the intake stroke. Momentum reversal of the intake/exhaust flow isn't as easy as you think.

But the momentum isn't the issue. It's the fact that the turbine has to turn a compressor that is pushing air into the cylinder.

 

[Throckmorton]

Yes, there is an increase in backpressure. Is it significant? No.

If it's not significant, why do turbocharged engines get worse gas mileage than they do without the turbo?

 

[exdeath]

If it's not significant, why do turbocharged engines get worse gas mileage than they do without the turbo?

Because they are flowing more air and producing more power throughout the power band, and they are tuned richer than an n/a engine in order to protect the engine from higher chance of detonation due to hotter denser air and higher cylinder pressure/temperature during the compression stroke.

Thus you can only compare a significantly smaller turbo engine making the same power curve as a larger n/a engine: the turbo engine will always come out more efficient and higher mpg.

 

[exdeath]

But the momentum isn't the issue. It's the fact that the turbine has to turn a compressor that is pushing air into the cylinder.

The ratio of added energy that the compressor provides to the engine's exhaust stream always outweighes the added effort that is required to turn the turbine against the increasing resistance of the compressor.

This is precisely the reason a wastegate is required to hold boost otherwise you'd quickly see runaway exponential rise of "infinite" boost and blow the engine to pieces. For any amount of energy x required to turn the compressor, x+n is seen at the turbine.

Actually that principal is fundamental to the operation of any heat engine period. Combustion pressure is always higher than intake and compression pressure, that's how engines work, and is how they are "self sustaining" once started. The added energy that keeps the process going indefinitely is supplied by the fuel.

 

[Zenmervolt]

If the turbine engine was designed so that it was dumping combustion gas out the back that still had elevated pressure and temperature enough to get work out of it like you do in a NA gasoline engine it would benefit from an extra turbine stage. In fact, reworking the hot end of a turbine engine to add another turbine stage is a common way of increasing power output.

I'm going to bow out and leave this to you since you clearly have more practical knowledge here than I do. :thumbsup:

ZV

 

[Zenmervolt]

If it's not significant, why do turbocharged engines get worse gas mileage than they do without the turbo?

Bignate: Sorry, I guess I lied.

Throckmorton: They get worse mileage because they make more power. Power is created by burning fuel. The way you get more power is to burn more fuel. A turbo allows you to burn more fuel by shoving more air into the engine, which means more oxygen which means more fuel. You can't get more power without burning more fuel.

Also, fuel mileage is the result of a complex interplay among aerodynamics, gearing, brake-specific-fuel-consumption curves, and other less-critical components. Fuel mileage rates the car as a complete system, not the engine itself. As such, you cannot use it as a gauge of an engine's efficiency.

ZV