Topic: Turbocharging: theory versus reality

[Mitlov]

With an increasing emphasis on fuel economy, some manufacturers are treating turbocharging as the holy grail.  Ford, for example, has dumped a lot of R&D into its upcoming "ecoboost" engines.  Leaving aside the fun factor of turbocharged engines, is this a good idea?

IN THEORY:

Turbocharged engines should be more efficient because they recapture lost energy in the exhaust.  They should also be more efficient because they let the engine effectively run in two states of tune--on boost when accelerating hard, and off-boost when idling, cruising, braking, etc.  This should make it more efficient than a naturally-aspirated engine with similar peak horsepower.

IN PRACTICE:

It doesn't seem to be a huge advantage.  It's certainly not a consistent advantage.  Let's do some comparisons with ratings from the US EPA website:

Jetta 2.0T (200 hp): 22 mpg city, 29 mpg highway.
Accord i4 (190 hp):  21 mpg city, 30 mpg highway.
Accord v6 (271 hp): 19 mpg city, 29 mpg highway.

Mazda CX-7 AWD: 16 mpg city, 22 mpg highway.
Mazda CX-9 AWD: 15 mpg city, 21 mpg highway.
Toyota RAV4 v6 AWD: 19 mpg city, 26 mpg highway.

BMW 335i: 17 mpg city, 26 mpg highway.
Cadillac CTS (3.6L DI): 17 mpg city, 26 mpg highway.

So what's the problem here?

[Railton]

Welcome back Mitlov.
Railton

[tpl]

As horsepower is just a number derived from Torque,  ( hp=torque*rpm/5252) the torque curves of the various engines would be more interesting if the discussion is about turbocharging only.    if the discussion is about the relative efficiencies of the cars mentioned as a package then that really needs a detailed look at how the EPA allows for total drag  as well as the specific amount of fuel burnt.

With the cars mentioned I suspect that each company has equally good engineers with equal facilities and those engineers are under equal pressures to produce powerful efficient cars.I have confidence that the EPA's testing methods are scrupulously fair between manufacturers.

Interesting thread coming up...


Did you have a good vacation form CTC ? Its good that you are back.

[Mitlov]

As horsepower is just a number derived from Torque,  ( hp=torque*rpm/5252) the torque curves of the various engines would be more interesting if the discussion is about turbocharging only.    if the discussion is about the relative efficiencies of the cars mentioned as a package then that really needs a detailed look at how the EPA allows for total drag  as well as the specific amount of fuel burnt.

First, I don't agree with the premise that "horsepower is meaningless and only torque matters."  If that was true, MkIV diesel Jettas would not be as painfully slow as they are.

Regardless, here are the torque measurements for the above-mentioned cars.

Jetta 2.0T: 207 lb-ft
Accord i4: 162 lb-ft
Accord v6: 254 lb-ft

The Jetta is slightly less fuel-efficient than the bigger-but-less-torquey Accord i4, and far less torquey than the bigger-and-only-slightly-less-fuel-efficient Accord v6.

CX-7: 258 lb-ft
CX-9: 270 lb-ft
RAV4: 246 lb-ft

The CX-7 is about as fuel-efficient as the far-larger, slightly torquier CX-9, and far less fuel efficient than the similarly-sized, barely-less-torquey RAV4.

BMW 335i: 300 lb-ft
Cadillac CTS: 273 lb-ft

The Bimmer is torquier, but the Caddy is far larger--arguably closer in size to a 5-Series.  Given the huge weight difference in the cars (500 lbs, I think?), I would have expected the twin-turbo Bimmer to be far more fuel-efficient.  But it's identical.

Did you have a good vacation form CTC ? Its good that you are back.

Thanks, this time I'm going to try to participate without getting drawn into the same nonsensical circular arguments.

[G35X]

Interesting subject… several questions come to mind:
 
1) Does the process of converting thermal energy > mechanical energy (expanding hot gas and force-feeding of air/fuel mixture) > thermal energy really improve the efficiency?  Judging from the high running temperature of the turbocharged engine in general, a good amount of the thermal energy created by burning more fuel is lost in the air.
   
2) Force-feeding means effective increase of compression ratio requiring premium grade gas in most cases.  Combustion chamber and piston stroke (and possibly intake valve timing) are designed with this high compression ratio in mind, which is not necessarily working efficiently when cruising with little boost.

3) An NA 2000cc engine becomes a 3000cc engine in effect when the boost is 7psi (500hPa).  Can automakers imply this engine has the power of 3000cc with the fuel economy of 2000cc?  They never say turbocharger is a device to burn MORE fuel.
   
4) Exhaust pipes of turbocharged automobiles such as 335i, CX-7 and RDX are often covered with soot.  Does this mean automakers are optimistic in programming fuel feed while the turbo is spooling up?
   
5) Re the point 2 above, modern turbocharged engines such as those in TSI, 335i, RDX and CX-7 have rather high geometrical compression ratio (close to 10:1 or even higher).  With a rather low boost of 7psi the effective compression ratio will become as much as 15:1 or even higher, certainly making it in the knocking zone.  How do automakers keep the engines from knocking at full boost?  Retard spark timing way back, delay intake valve closure effectively making the engine a Miller cycle design or ?

[Ice]

It'll be interesting to see how Fords engines turn out.  Hopefully they have done their home work...

I don't know allot about Turbo charging except that there is more than one way to do it and that how you do it greatly affects the performance of the engine and in what circumstances.  I wonder if there is a way to do a turbo so it provides a small power boost but a greater efficiency boost?  I don't know.

What about supercharging?  Any hope for that?

[Snowman]

Turbo charged cars are lighter than larger displacement normally aspirated engines with similar power ratings and only have marginal fuel mileage advantages at the tallest gear traveling at the posted speed limits.

Considering premium fuel is required economics are hardly a consideration and I would be very surprised if Ford brings something to market that proves otherwise. I suspect that there is some production cost advantage at play here.

[tpl]

It used to be that there was a formula in F1 ( several  decades ago)  that   forced induction engines could only be 1/2 the capacity of  atmospheric engines.  this was to achieve equality in performance.
So i wonder why the BMW for example is not generating ist 300 bhp with 1/2 the capacity that the Cadillac has.


G35x
1) It is still a heat engine so ideally all that heat would be used for something as you want the exhaust gas to be at ambient having given up all its energy to providing power.   Trouble is this requires considerable complexity... people spent 100 years doing these things for steam engines.

2) I think you are correct here. AND importantly  there is the NoX problem with high combustion temps is there not?

3) A 2000 cc engine does become a 3000 cc engine but of course it will burn the same amount of fuel as a 3000 cc engine.  You can't get more power without more fuel and air.

4) I have wondered about that. I think that you may be right.... a rich mixture is "safer"   for the engine... assuming that you have to run on pump gas

5)  I don't know.

[The Mighty Duck]

So i wonder why the BMW for example is not generating ist 300 bhp with 1/2 the capacity that the Cadillac has.

  Why should they?  It's not a challenge to get the most power from the smallest number of cubic inches...  and we all know it's technically possible - Mitsubishi has 2.0L turbos that spit out 400 horsepower.  BMW's stated reasoning for going with a twin turbo setup in the 335 was that this engine would be more economical than a similarly powerful naturally aspirated engine.

As for Mitlov's argument...  it's fundamentally flawed because the cars being compared are not the same.  As a VW driver, I'm surprised the rather obvious comparison of the same car with different engines was lost on him, the Jetta 2.5 and the Jetta GLI.

Both cars get a combined rating of 24 MPG - this in spite of the GLI having an extra 30 hp on tap.  What's more, the GLI gets better mileage than the 2007 Jetta 2.5, which had only 150 hp - a deficit of 50 hp.

I don't think an unreasonable extrapolation is that, if VW detuned the GLI's engine to match the HP of the 2.5, it would get better mileage...

[mmret]

You can't get more power without more fuel and air.

This is true.

However with a turbo you can pump out far more torque at lower RPM for the same displacement vs. a NA motor, and therefore save yourself a whole lot of mechanical losses.

The question becomes what is the thermodynamic efficiency at any given crank hp for this particular motor? I imagine you would find quite a lot of differences in the lab, perhaps not so much on the road given the vagaries of gearing, drag coefficient, etc....

[Iso Octane]

"MPG" isn't really a measure of efficiency.

"Power Per Gallon (per unit of time?)" is more appropriate, and in terms of PPG I bet they're all about the same.  Isn't it the general consensus that reciprocating gasoline engines are only about 30% efficient?   

More power being generated = more fuel consumed, period. 

If my 3.5L car uses 100hp getting to 60kph, while my 1.8L car uses just 50hp and takes a longer time doing it, than the 3.5L car is going to use more gas.  These turbo charged SUVs are producing and using a lot of power to get going, and therefore, a lot of gas as well.

[Iso Octane]

The question becomes what is the thermodynamic efficiency at any given crank hp for this particular motor? I imagine you would find quite a lot of differences in the lab, perhaps not so much on the road given the vagaries of gearing, drag coefficient, etc....

Given the maturity of piston engine tech, I really really really doubt that there would be much difference in thermodynamic efficiency between them.

[Mitlov]

"MPG" isn't really a measure of efficiency.

Maybe it isn't from an engineer's standpoint, from a theoretical standpoint, but it (or L/100km) is really the only measure of efficiency that matters for real-world drivers.

[Mitlov]

As for Mitlov's argument...  it's fundamentally flawed because the cars being compared are not the same.  As a VW driver, I'm surprised the rather obvious comparison of the same car with different engines was lost on him, the Jetta 2.5 and the Jetta GLI.

The comparison you want doesn't exist in reality, because nobody sells the same car with two different engines (a turbocharged engine and a naturally-aspirated engine) making the same horsepower.  So once again, we're talking about "what turbos SHOULD be capable of" when we start talking about retuning the 2.0T to 2.5 horsepower levels.

As for eliminating variables, I think the CX-7 and CX-9 is about as close as you are going to get.  It's remarkable that the two vehicles get almost the same fuel economy.  Acura's got the same problem.  The RDX gets 17 mpg city, 22 mpg highway.  The more powerful and far larger MDX gets 15 mpg city, 20 mpg highway.

And finally, so what if not every single variable is the same?  Is it so unreasonable to think that the Jetta 2.0T should get better highway fuel economy than the absolutely immense, 270-hp Accord?  The variables that are not the same (namely, size) should have helped the VeeDub.

[rrocket]

Oh man...there are soooo many reasons why this is...

But one of the biggest is often (not always) the turbo engines have lower compression ratios.  So when you are off the turbo, lower compression engines are less efficient than a well done high compression engine.  The design of the combustion chamber as well as pistons in turbo cars is also sometimes optimized to prevent detonation while on boost...again, not the best design while off boost.  Sometime even the timing of the opening/closing of the valves is optimized to give better pulses for faster turbo spool-up.  Cylinder head design and also cam selection plays a crucial role in the efficiency of a turbo engine. All of these things combined can make a turbo engine less efficient off boost than a well designed NA engine...


But what rules for me for turbo engines, is when you mod them.  Toss on a bigger turbo as I have and you end up with 600+ HP..while still getting the exact same fuel mileage as before (or very slightly better).  Feeling bored?  Add in a meth injection kit, which is akin to running race fuel, and you add in another 50-100HP..and again...no loss in fuel mileage.  Try getting HP like that from a NA engine and you'll see fuel efficiency go out the window.

It's not correct to assume that a turbo car should have better efficiency just because it...well...has a turbo...

[Spheric]

I'm not fully aware of the physics and engineering aspects of the turbo debate, but I rank the ability to put a smile on my face as quite high.... 

Right now, I think I would find it emotionally hard to switch back to a non-turbo vehicle...

[ktm525]

I have become addicted to the low RPM, flat broad torque delivery of a turbo engine. My chipped VW 1.8T was great aon gas and had reasonable power. The 2.5T in the VR is just awesome. I am not a complete turbo snob, wouldn't  turn my nose up at a NA, large cubic inch V8.

[wing]

OMG I love twin turbos!

[ktm525]

OMG I love twin turbos!

Double the fun.

[ovr50]

OMG I love twin turbos!

Yep....