do turbos kill mileage? bypass for mpg?

[oil pan 4]

My diesel suburban got 22mpg in factory configuration, unloaded.
Gas suburbans got closer to 14mpg

With my modifications I hit 20 to 22 mpg towing now.

If diesel is $4/gal that costs 18 cents per mile.
If gas is $3.50/gal thats 25 cents a mile.
And the gas to diesel price difference is almost never more than 50 cents difference, you would have to shop around for the lowest gas and highest diesel at different stations to find that price difference.
So how is diesel fuel more expensive?
Dont tell my you only looked at cost per gallon and stopped there.

[slowmover]

So how is diesel fuel more expensive?
Dont tell my you only looked at cost per gallon and stopped there.
s

It's the usual stopping point. There is no understanding of cpm and operational costs, much less ownership costs.

On my truck so long as diesel is no more than 50-cents higher than gasoline the " fuel economy" is a wash . . but as the diesel engine is designed with a B50 life of 350k miles, it pulls away from the gas version at around 150k miles of service (given initial expense differences). That it can also do more work at any point along those miles is left from this comparison.

.

[euromodder]

Originally Posted by stillsearching

I've never seen a turbocharged car which gets even comparable mileage to a nonturbocharged car under the same specifications YET.

Just to name a few :

Turbodiesels.

The BMW ?20i (184 HP) and ?28i (245 HP) models with downsized, turbocharged 2.0L engines.

[thomason2wheels]

Thanks to both of you for pointing out my errant comments. Chalk it up to the hour of the day it was posted. Insomnia is an ugly thing.... I hope via this forum I learn enough about diesels and the way they work to make an intelligent buying decision the next time I am faced with a four wheel vehicle purchase. Seems as though from what you folks are saying that the higher initial cost is outweighed several times over by the mean time between failure. My opinion about automotive diesels, as opposed to trucks, has unfortunately been skewed by the early GM diesels and their rather sketchy reliability. I must admit a certain predjudice against diesels because of all of the bad PR GM received because of those early engines.

On another note, when towing with my subaru wagon my fuel economy is pretty dreadful. 18-22 is about what I can expect and that is very terrain dependent and speed dependent. Flat land and low speeds without too many stoplights and low 20s are possible, if it is hilly or I am trying to run 70 all bets are off....

[jamesqf]

I think your problem is with "same specifications". A turbo allows an engine of a given displacement to produce more power, meaning you could a) have the same displacement engine (same specifications) and drive it harder; or b) use a smaller engine to provide the same performance.

Now if you think about it, it seems pretty obvious that a) is going to lower mpg, while b) can increase mpg.

[Diesel_Dave]

I'm more familliar with the diesel world, but I can't recall the last time I heard about a turbo hurting fuel economy (assuming the turbo match has been done properly).

The turbine side of the turbo captures some of the exhasut energy and turns it into pressure on the intake side (which decreasing pumping loses). In fact, with some particularly good turbo matches, the intake pressure can actually exceed exhaust pressure to the point where pumping losses are completely eliminated--or even reversed (positive pumping work).

Turbine efficiencies in the 60-70% range are pretty common and compressor efficiencies of 70-80% aren't uncommon either--that makes the combined turbo efficiency over 50%, which is better than the base engine. As has been noted before, a "turbo bypass" is called a wastegate. I've tuned them before on diesels. When they start opening, you start losing efficiency pretty quickly. That's actually the whole point of a wastegate--it reduces turbo efficiency to keep the turbo from overspeeding.

Also, as has already been mentioned, turbocharging allows for downsizing and/or downspeeding of engines, which increases fuel economy by allowing the engine to run at lower speeds and higher loads (lower friction, pumping losses, etc.)


Just to make my point further, I Googled "turbocharging and fuel economy an came up with several stories:

Auto industry boosts turbocharger sales to save gas
Top 5 Turbocharger Tech Innovations: The Truth about Fuel-Sipping Turbos - Popular Mechanics
Honeywell Turbochargers Enable 20 To 40 Percent Better Fuel Economy Helping Automakers Reach CAFE Targets
Design News - News - Turbocharging Technology Reduces Fuel Consumption
Today's turbochargers focused on fuel economy over performance - SFGate

[stillsearching]

I will clarify - everything i'm referring to refers to gas engines in this instance.

Originally Posted by oil pan 4

They do have a bypass for the exhaust, its called a waste gate.

I haven't seen ones you can set to 0psi yet though. :-/ Maybe they're out there though. Just if it's in the exhaust path, it's acting as a restriction.

Originally Posted by thomason2wheels

Not to puttoo fine a. Point on it but there is a bypass of sorts.....your right foot.

See points above and below.

Originally Posted by Diesel_Dave

I'm more familliar with the diesel world, but I can't recall the last time I heard about a turbo hurting fuel economy (assuming the turbo match has been done properly).

Can you find me even one vehicle on fueleconomy.gov that shows the same MPG figure for a turbo engine vs a nonturbo engine if gas powered? Those are being driven about as easily as possible in the mileage loop. The mileage drop is often about the same as a bigger engine producing the same power without the turbo.


Out of order response:

Originally Posted by Vekke

Some of the time turbo helps to lower fuel consumption like when accelerating etc.

I don't know that it lowers it, it might let you accelerate in a higher gear though, or give you power without the drawbacks of larger displacement carried around all the rest of the time. But that seems to be the case for "normal" drivers. What i'm wondering with all the ecomodders already lowering axle ratios, lowering aerodynamic load, driving slower and swapping in smaller engines if there's even a point to the turbo... wondering whether a total bypass for 0psi and no exhaust restriction would work better under many conditions of cruise, short of pulling a hill that would normally require you to downshift. (and maybe you could throw the bypass off/turbo back in with a little switch on the shifter or something then)

All I know is that i've never seen a turbocharger added to a gas engine which didn't take away 1-2mpg under steady state low load conditions.

[oil pan 4]

Originally Posted by stillsearching

I haven't seen ones you can set to 0psi yet though. :-/ Maybe they're out there though. Just if it's in the exhaust path, it's acting as a restriction.

Any vacuum accuated waste gate can be opened at 0 PSI.
Most waste gate accuators are pressure operated
Vacuum accuated waste gate contol seems to be harder to tune for.

Or rig up a pressure activated waste gate to run off stored compressed air. Thats what I'm doing.

[02ws6]

Just keep reading...

I did some FE testing 64.8mpg - D-series.org


Alot of people who go through the trouble of boosting are looking for a happy medium of power and economy.. But if just going for economy, with the right know how alot can be done.

[oil pan 4]

If you are going to lean burn I think you will want to avoid 15.7 to 16:1 air fuel ratio.
I believe that to be the exhaust valve burning a/f ratio, at least if you use it under heavy load.
18:1 is safe.