Why don't more cars have turbos?

[EightySix Four]

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.

At cruising RPMs this does not matter, all the engines are going to run at stoich or in lean cruise mode. The efficiency loss is due to the dramatic differences in compression ratio between boosted engines and their N/A counterparts.

 

[Throckmorton]

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

But at cruising speed, with no acceleration, a turbo and a NA engine are putting out the exact same horsepower. Why would one be less efficient than the other, except for exhaust restriction?

 

[EightySix Four]

But at cruising speed, with no acceleration, a turbo and a NA engine are putting out the exact same horsepower. Why would one be less efficient than the other, except for exhaust restriction?

Look one post up. OEM turbo engines are going to have a much lower compression ratio than their turbocharged counterpart. At cruise, this is going to affect efficiency pretty heavily.

 

[Throckmorton]

Look one post up. OEM turbo engines are going to have a much lower compression ratio than their turbocharged counterpart. At cruise, this is going to affect efficiency pretty heavily.

What about adding a turbo to an engine and not changing the compression ratio? Has anyone ever done that and not lost efficiency?

 

[exdeath]

What about adding a turbo to an engine and not changing the compression ratio? Has anyone ever done that and not lost efficiency?

You can run high compression ratio and very little boost, or you can run low compression ratio and run higher boost. You can't do both without breaking even the strongest parts available.

Typically lower static compression ratio and higher boost is the way to go, as a turbine is much more efficient at compressing air than a piston.

 

[thedarkwolf]

Isn't the big plus of the new direct injection turbo engines that they can run the high compression and still a decent amount of boost?

 

[PlasmaBomb]

Isn't the big plus of the new direct injection turbo engines that they can run the high compression and still a decent amount of boost?

The problem here is people aren't distinguishing between static compression ratio (CR) and dynamic CR - and they are different between turbo engines and normally aspirated (NA) engines. For example - NA engine with a 10.5 static CR will have dynamic CR of ~10 when running, a turbo engine may well have a static CR of 8, but during operation (or more precisely on boost) the turbo can force in more air into the cylinder - so if forces in ~50% more air you have a dynamic CR of ~ 11.5.

 

[JCH13]

But at cruising speed, with no acceleration, a turbo and a NA engine are putting out the exact same horsepower. Why would one be less efficient than the other, except for exhaust restriction?

Off the top of my head, here are a few reasons why different engines might have different efficiencies:
-Different compression ratio
-Different fuel
-Different fuel tuning
-Different fuel delivery
-Different spark tuning
-Different valve timing
-Different valve duration
-Different valve overlap
-Different valve sizes
-Different valve train losses (fewer losses from smaller, lighter valves)
-Different parasitic losses (turbo engine has to pump extra oil and possibly coolant through the turbo)
-Different throttling losses
-Different intake charge temperatures

 

[Bignate603]

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".

Yes, there must be SOME extra back pressure from a turbo. We never denied that, we just have been saying that the effect is relatively small especially when compared to everything else going on in the engine. The effects of this backpressure on the power and efficiency of the engine can easily be overcome by the other effects of the turbo.

If a turbo was placed in the exhaust path without having the compressed air it produces routed to the engine's air inlet you'd see it harm efficiency, no question about that. However, when you dump that compressed air into the engine's inlet the engine now has to do significantly less work to compress the air up to the high pressure that gives it the best efficiency and power. Overall there is a net benefit to the engine cycle that is much larger than the parasitic loss from the relatively small increase in back pressure.

Here's something that is related, where a bit more work for part of the process is required but the effects on the power and efficiency of the engine is beneficial in the end. Think about increasing an engine's compression ratio. When you do that the piston during the compression stroke has to do more work because the cylinder pressure gets much higher. However, the higher compression ratio benefits the engine cycle, allowing more energy to be taken out of a given quantity of air and fuel. The extra work to compress the air is small in comparison to the increased amount of power you get from it. A turbocharger is similar, the parasitic energy losses from them exist but are much smaller than the effects it has on the thermodynamic efficiency of the engine.

 

[Zivic]

The problem here is people aren't distinguishing between static compression ratio (CR) and dynamic CR - and they are different between turbo engines and normally aspirated (NA) engines. For example - NA engine with a 10.5 static CR will have dynamic CR of ~10 when running, a turbo engine may well have a static CR of 8, but during operation (or more precisely on boost) the turbo can force in more air into the cylinder - so if forces in ~50% more air you have a dynamic CR of ~ 11.5.

now days we are seeing higher compression ratios on turbo engines.

the MS3 is 9.5:1
VW 2.0T in the GTI is 9.6:1
chevy cruze turbo motor is 9.5:1

we are seeing higher compression turbo motors for a few reasons. better design, better materials, better computers controlling them.... but we are seeing the available N/A motors increasing in compression as well...

 

[Demo24]

What about adding a turbo to an engine and not changing the compression ratio? Has anyone ever done that and not lost efficiency?

The problem with that is gasoline then becomes the weak link. Sure you can build an engine designed to cope with such pressures, but you run the risk of premature combustion from gasoline you buy at the pump.

 

[exdeath]

Also keep in mind mass and inertia of the piston+rod+crank vs mass of the turbine. When you shoot a bullet from a gun there is an equal and opposite force on the gun and bullet, but which one moves more? When you shoot a .22 in a big heavy rifle, does the rifle move much at all compared to the bullet? Technically it does move, we know it has to because of physics, but due to its mass compared to the bullet, its insignificant and hardly noticeable.

 

[EightySix Four]

now days we are seeing higher compression ratios on turbo engines.

the MS3 is 9.5:1
VW 2.0T in the GTI is 9.6:1
chevy cruze turbo motor is 9.5:1

we are seeing higher compression turbo motors for a few reasons. better design, better materials, better computers controlling them.... but we are seeing the available N/A motors increasing in compression as well...

9.5:1 is not high in comparison to the majority of N/A engines anymore. Little four cylinders are rocking at least 10:1 or 10.5:1.

As mentioned, there are multiple reasons why turbo motors generally get worse gas mileage. If you have two exactly equal motors, and one has a turbo and one does not, you will see a mild loss in MPG due to both back pressure and the inlet restriction (long intake tubes, turbo sitting in the way, intercooler, etc.). What everyone has been trying to say though is that there are very few situations where everything is equal.

Turbo motors have stronger internals, valves designed for higher performance, lower compression ratios, different cams, different tuning, and often different gear ratios.

The problem here is people aren't distinguishing between static compression ratio (CR) and dynamic CR - and they are different between turbo engines and normally aspirated (NA) engines. For example - NA engine with a 10.5 static CR will have dynamic CR of ~10 when running, a turbo engine may well have a static CR of 8, but during operation (or more precisely on boost) the turbo can force in more air into the cylinder - so if forces in ~50% more air you have a dynamic CR of ~ 11.5.

That is why I repeatedly said at cruise. If you're cruising (off boost), their dynamic ratio would not be anywhere near the same.

What about adding a turbo to an engine and not changing the compression ratio? Has anyone ever done that and not lost efficiency?

As previously stated, there will still be a loss in efficiency due to the added back pressure and intake restriction created by the turbo. Problem is, now you have a high compression motor getting huge amounts of air force fed into it, the heat is going to go up, the engine is going to start knocking/pinging, and you're going to find the weak link in that motor really quickly.

 

[Zivic]

9.5:1 is not high in comparison to the majority of N/A engines anymore. Little four cylinders are rocking at least 10:1 or 10.5:1.

But now, you are getting turbo motors with compression ratios that 5 yrs ago were only found in N/A setups.

As mentioned, there are multiple reasons why turbo motors generally get worse gas mileage. If you have two exactly equal motors, and one has a turbo and one does not, you will see a mild loss in MPG due to both back pressure and the inlet restriction (long intake tubes, turbo sitting in the way, intercooler, etc.). What everyone has been trying to say though is that there are very few situations where everything is equal.

For what it's worth, my Z gets comparable milage now (on the highway -> mid to upper 20's) with with a twin turbo setup and 9:1 compression as it did with the stock 10.1:1 CR and N/A.... the loss in MPG comes from the boosted situations. but apples to apples comparison is hard to make. My Z makes 500+ with turbos and 250 N/A. Same displacement.

I agree with you, that given equals, the inlet/outlet restrictions will cause a slight loss in MPG. That said, the boosted app will likely make more power - like my Z. A fair trade off IMHO


Turbo motors have stronger internals, valves designed for higher performance, lower compression ratios, different cams, different tuning, and often different gear ratios.

I have seen some boost builds in the CR over 10:1. We are seeing the CR climbing on both N/A and boosted applications because of the stronger internals

 

[Demo24]

Well, the CR can increase with intercoolers but mostly these days to DFI which conveniently also somewhat cools the mixture as it sprays in. Cooler temps on entry mean less likely to combust before the right time.

 

[Bignate603]

I have seen some boost builds in the CR over 10:1. We are seeing the CR climbing on both N/A and boosted applications because of the stronger internals

Stronger internals don't prevent knock.

 

[Zenmervolt]

But at cruising speed, with no acceleration, a turbo and a NA engine are putting out the exact same horsepower. Why would one be less efficient than the other, except for exhaust restriction?

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

 

[Throckmorton]

Yes, there must be SOME extra back pressure from a turbo. We never denied that, we just have been saying that the effect is relatively small especially when compared to everything else going on in the engine. The effects of this backpressure on the power and efficiency of the engine can easily be overcome by the other effects of the turbo.

If a turbo was placed in the exhaust path without having the compressed air it produces routed to the engine's air inlet you'd see it harm efficiency, no question about that. However, when you dump that compressed air into the engine's inlet the engine now has to do significantly less work to compress the air up to the high pressure that gives it the best efficiency and power. Overall there is a net benefit to the engine cycle that is much larger than the parasitic loss from the relatively small increase in back pressure.

Here's something that is related, where a bit more work for part of the process is required but the effects on the power and efficiency of the engine is beneficial in the end. Think about increasing an engine's compression ratio. When you do that the piston during the compression stroke has to do more work because the cylinder pressure gets much higher. However, the higher compression ratio benefits the engine cycle, allowing more energy to be taken out of a given quantity of air and fuel. The extra work to compress the air is small in comparison to the increased amount of power you get from it. A turbocharger is similar, the parasitic energy losses from them exist but are much smaller than the effects it has on the thermodynamic efficiency of the engine.

This is the first explanation that makes sense :thumbsup:

 

[Zivic]

Stronger internals don't prevent knock.

no but more effecient intercoolers, direct injection, better computers and fuel mapping does... the stronger internals do protect the motor in case of knock to some degree. You aren't as likely to bend a rod, crack a piston, or ringland if knock does occur

 

[Arkaign]

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.

I'm not disagreeing explicitly, and I know there are a lot more variables in play, but while love turbo and SC setups, I haven't seen much real-world examples of them being more fuel efficient even for the same power. Maybe it's the fault of the manufacturers and how the cars are tuned. I do know about the turbodiesels, those are pretty much the standard these days, and they must be really efficient.

Anyhow, examples (and yes I do know that AWD = more powertrain losses)

2011 WRX STI, 6-speed manual, 2.5L Turbo, AWD, ~3300lbs, 305hp/295tq, 17/23mpg (premium required)

2011 MazdaSpeed3, 6-speed manual, 2.3L Turbo, FWD, ~3250lbs, 263hp/280tq, 18/25mpg (premium required)

2011 Mustang 3.7L, 6-speed manual, 3.7L NA, RWD, ~3400lbs, 305hp/280tq, 19/29mpg (regular)

2011 Mustang 5.0L, 6-speed manual, 5.0L NA, RWD, ~3600lbs, 412hp/390tq, 17/26mpg (premium recommended)

2011 Vette, 6-speed manual, 6.2L NA, RWD, ~3200lbs, 430hp/424tq, 16/26mpg (premium recommended)

^^ Common examples, but it stays the same even if you throw in stuff like GTI, Cobalt SS, etc. Maybe it's because a lot of my focus is in the more sport-tuned world. How does the ecoboost V6 shape up against say the 6.2L V8 in terms of power/efficiency?

edit : added mfg fuel type spec

 

[thedarkwolf]

90 dodge daytona 2.5l 5 speed 100hp 21/29mpg
90 dodge daytona 3.0l 5 speed 142hp 17/25mpg
90 dodge daytona 2.5l turbo 5 speed 150hp 18/26mpg
90 dodge daytona 2.2l turbo 5 speed 175hp 18/26mpg (this was the performance version)

The old turbo dodges are the best examples I can think of since they offered turbo engines in everything along with v6s and regular 4 bangers.

 

[Bignate603]

This is the first explanation that makes sense :thumbsup:

Yeah, the thermodynamics in engines is pretty complex and sometimes its hard to see how one change can affect things. There's hundreds of details that need to be worked out for a modern engine to function well, and many of them aren't completely obvious.

 

[Throckmorton]

Isn't t he reason for using turbos on diesel engines that they make low horsepower per unit of displacement without the turbos? So a full size pickup would need like a 20L engine if it wasn't turbocharged.

 

[Arkaign]

90 dodge daytona 2.5l 5 speed 100hp 21/29mpg
90 dodge daytona 3.0l 5 speed 142hp 17/25mpg
90 dodge daytona 2.5l turbo 5 speed 150hp 18/26mpg
90 dodge daytona 2.2l turbo 5 speed 175hp 18/26mpg (this was the performance version)

The old turbo dodges are the best examples I can think of since they offered turbo engines in everything along with v6s and regular 4 bangers.

Wow, pretty amazing how far we've come in 20 years. Huge, huge, epic increases in performance, while getting superior fuel economy now. Also of note is that the old fuel economy standards were a ton looser, the new standard is a lot tougher, and includes a lot of slowing down/speeding up, a/c use, etc.

 

[PlasmaBomb]

That is why I repeatedly said at cruise. If you're cruising (off boost), their dynamic ratio would not be anywhere near the same.

Which is why the it wasn't directed specifically at you...

I was just pointing out that there is a valid reason for static CR to be lower in turbo engines - you have to design them with boost in mind in case people hadn't realised.

Another factor in implementing turbo engines is likely quality assurance/testing - see the ecoboost testing by Ford. I would imagine that turbo units require more testing, which in turn drives up cost or lowers profitability in price sensitive markets... C3PO might be able to comment...