Is a turbo more fuel efficient?

[xLibelle]

If someone asked me that question straight up w/o any other info, the answer is no. The compressor and turbine impedes/restricts airflow, thus harming fuel economy.
So then breathing ability is the most important thing about an engine, to obtain maximum fuel economy.

What about cars today? Many have turbos and average MPG's to match the NA cars. You see this from mercedes and bmw among other manufacturers.

So here's my question that i've had some trouble finding the answer to:

Lets say you have two of the same engine, exactly the same, except one is 6 cylinder and the other is 4. The 4 cylinder one has a turbo configuration and the system is designed to match the power output of the 6 cylinder NA engine.
Granted, this would be impossible in real life but lets say it was close, like 150hp/170tq on the 6 and 170/150 on the turbo. Now I cant say that those would reflect REAL comparisons between these two configurations - I'm just providing a for-instance.
The engines would be tested in various driving situations. Stop-and-go city driving and wide-open racing are not included because each engine would favor the opposite situations. This would be a general middle ground test comparison.

So what other important data is required to make a valued comparison? From this data alone, could you actually say that one configuration is more fuel efficient than the other? For example, lets say you asked which configuration is heavier? The answer is, they are the same weight. There should be no need for extra variables... unless i'm wrong.


Please share your thoughts.



So my take on it is, they should be equally efficient. Power output is directly proportional of the fuel provided, right?
I've GOT to be wrong because everywhere you look, its a car with smaller displacement and some form of boost involved.

[Julien]

I would venture to say that the I4/T would be more efficient because it's a more efficient/smaller motor when off boost. Realistically in a normal driving scenario you are doing stop/go style driving where you aren't using the full extent of the motor's potential. To me this is where the I6 will most likely be less efficient as they tend to be more efficient at constant revs (highway driving). Again this is a huge guess. My only supporting data is that BMW is turning to boost instead of displacement for more power and less pollution (but this could also have to do with tax restrictions in europe)..

Julien

[Collin]

I believe the turbo would be less fuel efficient as you have to inject more fuel into the combustion chamber since you're throwing the A/F mixture all kinds of out of wack with all that extra air.

But between a 4 and 6 cylinder? I don't know exactly, but I do know it would require more math than I'm willing to do on the internet.

[SamE30e]

Off boost it could be more efficient, I mean the engine doesn't have to suck in air as hard, the turbo still moves air.

Makes sense in my head.

[e30pwr]

well, on a personal note, fro road trips for my family the 02 volvo s60 T5, aka a 5cylinder turbo, gets better mileage than then NA 03 Acura 3.2tl

power is off a good bit between the two, but the Volvo seems to cruise at around 80 righta t the kick in of boost, I think this helps, however I'm no scientist.

[Pinepig]

My 2002 got better mileage after I put the turbo on, no other changes.

[fretburnr]

One point to keep in mind - with modern EFI systems and expertly calculated fuel curves, a smaller boosted engine can putt around at partial throttle quite economically. A boosted engine cruising along at a near zero vacuum/boost level has significantly lower pumping losses as well.

[nando]

I believe the turbo would be less fuel efficient as you have to inject more fuel into the combustion chamber since you're throwing the A/F mixture all kinds of out of wack with all that extra air.

But between a 4 and 6 cylinder? I don't know exactly, but I do know it would require more math than I'm willing to do on the internet.

A/F ratio doesn't change a whole lot actually. Most guys run a rich tune to hide their poor ignition map, restrictive turbo, poor combustion chamber design, small exhaust, etc. Or because they're running way, way more power than the stock engine and want a margin of safety.

A factory turbo probably runs close to the same ratio as an N/A car - fuel doesn't burn very well below ratios of 12:1, and N/A and turbo engines should both be targeting a ~13:1 ratio at full load regardless (depending on the above stated conditions). You aren't going to make more power by going richer than that, unless it's hiding some other problem.

older turbos were a lot less efficient and did pose a significant restriction to the engine, but things have changed quite a bit in the last 10 years thanks to all the turbo diesel development.

[xLibelle]

well, i can see that there are a number of factors that many are missing.

pinepig, you probably saw better fuel economy because the 2002's engine was underpowered. driving style can also compliment the power to weight ratio. So, if you are the average lead foot, your engine was under less stress and gulping less fuel once you added the extra HP.

e30pwr, just guessing here as a suggestion to why you experience what you do, that the tubo config is pretty well configured. BUT while on the highway, turbos are not producing boost. (Look it up if you dont understand why)
Unfortunately, the two cars are completely different and cant be considered for comparison.

sam, off boost is the worst efficiency. turbos are efficient for a specific part of an engine's rpm range, depending on the configuration's design. For example, a 6-cyl NA is more efficient at cruising speed than the same 6-cyl with a turbo strapped on. The 4-cyl with a turbo is running 2 less cylinders so pushing less fuel.

julian is right on the money. ...its just that this seems such a complicated concept to explain both sides of the coin, as if its going to land on one side or the other.

good ideas guys, please keep em coming. only way to get to the right answer, it seems, it to eliminate all the wrong ones ;)

[xLibelle]

One point to keep in mind - with modern EFI systems and expertly calculated fuel curves, a smaller boosted engine can putt around at partial throttle quite economically. A boosted engine cruising along at a near zero vacuum/boost level has significantly lower pumping losses as well.

from what i understand, its about 5% loss at cruising speeds


and FYI, i'm researching to build a common rail diesel to put in the new family car (weighing in at around 3500lbs)... just that i need to understand which engine to start my investigations on.

I brought this topic up in an automotive engineering forum and... i was ousted without a single hint at the answer or how i should have approached the question. It was pretty disheartening.

[Liam]

The baseline answer is no, because turbocharging is not inherently more efficient than natural aspiration.

A direct comparison is hard to make though because a properly integrated power-plant for turbocharging is not efficient NA and visa versa.

[nando]

The baseline answer is no, because turbocharging is not inherently more efficient than natural aspiration.

A direct comparison is hard to make though because a properly integrated power-plant for turbocharging is not efficient NA and visa versa.

how are you defining efficient? a turbo engine all things equal will make twice the horsepower of an equivalent N/A engine with ~1 bar of boost, so it's more efficient as far as HP/displacement. It's using the turbo to increase the thermal efficiency of the engine by recapturing some of the energy lost in the exhaust.

a supercharger on the otherhand, is much less efficient since it's using some of the generated power to create boost.

[dsobering47]

I always thought that N/A would be more efficient than F/I. My reasoning was that with any F/I, a restriction is placed in the intake path and even at zero boost/vacuum, the engine still has that restriction.

So basically if you want to have a N/A 6cyl vs F/I 4cyl battle, then the N/A 6cyl would win....assuming you de-tuned it for efficiency.

Right?

[Liam]

how are you defining efficient? a turbo engine all things equal will make twice the horsepower of an equivalent N/A engine with ~1 bar of boost, so it's more efficient as far as HP/displacement. It's using the turbo to increase the thermal efficiency of the engine by recapturing some of the energy lost in the exhaust.

a supercharger on the otherhand, is much less efficient since it's using some of the generated power to create boost.

I'm no major guru in this subject and can't explain thoroughly in detail but

All things are not equal in this equation even though your logic is sound about thermal efficiency.

Stoichiometric is the same at one bar as it is at two and while it is true you gain output with any compressor but it is not inherently more efficient.

[rThor432]

As far as the turbo 4 versus a normally aspirated 6 I'd have to go with the i4 turbo for the same reasons as Julien stated above.

I believe the turbo would be less fuel efficient as you have to inject more fuel into the combustion chamber since you're throwing the A/F mixture all kinds of out of wack with all that extra air.

Not necessarily. I hate to bring up ancient technology, because it doesnt apply to this thread very well, but Motronic 1.3 can adjust the addition of fuel in standard form up to 7 psi or so. Off boost it behaves like a stock B25i motor, for the most part. Toss in a taller rear end and you have better than stock efficiency when cruising with more power than a standard motor when you are in the boost. May sound crazy, but I've got a friend with a 2.5i turbo doing 6 psi that dynoed 248 whp and with the 3.25 rear end he gets upper 20's for fuel mileage with 50/50 driving.

Sorry to take it off the subject at hand, but I had to mention my experience with that particular car as it relates to the turbo vs. efficiency subject.