do turbos kill mileage? bypass for mpg?
 

I've never seen a turbocharged car which gets even comparable mileage to a nonturbocharged car under the same specifications YET. Even if the total package may be a little better (like the Chevy Cruze Eco using it to pull higher gears) I would assume if you deactivated the turbocharger that even though you couldn't drive under normal habits, it might actually improve mileage assuming you didn't normally see hills and such anyways.


On that subject I was wondering, why not have a turbo bypass built in? Then it's there when you felt you needed it, and could be cut out when you were in Eco mode. I can't think of any downside not to except the expense of the turbo if it's added on and not stock.

Turbos help the fuel economy on diesels.

They do have a bypass for the exhaust, its called a waste gate.
The intake also has a bypass valve, called a blow off valve, its operated by intake vacuum, anything less than about 5''Hg and the blow off valve is open.

Diesels do not require blow off valves.
I intend to use a BOV on my diesel to allow air to by pass the compressor at low speed before the turbo lights off, I measured measured 1''Hg in the intake piping so far, driving it kind of easy.

Other wise turbos help diesels about 90% of the time. You wouldnt want to deactivate it.

Not to puttoo fine a. Point on it but there is a bypass of sorts.....your right foot. Having owned several turbo cars and also their normally aspirated brothers, the nonturbo versions in every case had more usable torque under 2000 rpm than the rurbo cars did. Above 2k, the turbo cars would run circles around their na brothers. My feeling is the turbo costs about 2 to 3% fuel economy for performance sake. Even if you could bypass the turbo, the lower static compression of the turbo engine would have a negative impact on fuel economy and bottom end torque. So while you can be gentle with the right foot and largely stay out of the boost except for long steep hills, there is a penalty paid for having that hardware.
Another sonsideration is cam timing. Most gas turbo cars use a milder camm grind than their higher compression na brothers. Bottom line is the laws of physics remain in force. The so called free energy driving the turbo is a mechanical load imposed on the engine as a pumping loss on the exhaust side. I guess there really is no such thing as a free lunch.

I have a diesel turbo, but when i drive just under the turbo spoolup rpm is use about 0.5L/100km less then when my turbo spools up. The turbo spoolup is just above 1800rmp and max torque is at 1900rpm. But my engine is from around the year 2000. One of the first electronicly controled turbo dieseld. My mate has a 2007 saab 93 1.9 turbodiesel 6speed. In gear 6 at 120kmh it uses more then 130kmh is 6. This is because the turbo spools up at 125. So i think you can say that newer diesels need their turbos, and the old could do without.

Guess i should. Have said that i have never owned a diesel and given the premium prices they seem to go for and the cost of diesel fuel, prob never will. But they do seem to have some real efficiency attributes. Wish icould afford one as a toy to play with....

The balance here i completly different. Diesel is about 20% cheaper at the pump. And whit better mpg it could save money. To bad thet we have to pay al lot of road taxes here in the netherlands. I pay around 1500+ dollar a year for my diesel. The same car whit a gas engine is 700 a year

Fascinating that it would be so different there. And the taxes....yikes,that is crazy. You'd haveto drive a lot of miles in that diesel to make the higher taxes worth it.

Some of the time turbo helps to lower fuel consumption like when accelerating etc. But when driving steady speeds you dont need it and it hurts the FE little. However you can remap your ECU programming to avoid that boost in steady throttle.

My lupo did have 0.5 bar boost when driving 100 km/h or 62 MPH. That boost if there so the EGR system could work. Now with custom ECU program there is no boost even at 120 km/h speeds. When you have done this modification turbon is always more fuel efficient.

Sounds like retuning the wrx ecu would buy me better economy....hnmmmmm

Usually the wastegate is activated when the pressure reaches a certain level. An electronic wastegate can be opened at all times to provide said bypassing effect at cruise to improve fuel economy. One might wonder, a turbo is supposed to recycle waste energy in the exhaust, so why does it even create backpressure? Well a turbo's main objective is to increase power, and then it needs to be driveable as well. For driveability (turbo lag/transient response) the turbine is extremely restrictive to be able to quickly produce boost.

Engines with continuously variable intake duration can benefit from the pressure produced by the turbo that would otherwise be wasted via blow off valve.

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.

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.

.

Just to name a few :

Turbodiesels.

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

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....

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.

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

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

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.

See points above and below.

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:

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.

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.

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.

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.

I think with a gas engine it hinders it via the lower compression. I was always disapointed in the mileage that friends got with their WRXs an STi's.

That D-serries thread looses a lot of creditability on about page 8.

Was outweighed.
The latest diesels have become very complex machines.
As a result they also have more issues.

Things have changed a bit since - especially with European diesels ;)

Dunno if you'll ever have the chance to drive say a 2011 or 2012 BMW 3L 6-in-line "330" diesel, but that's a good example of what modern diesels can be.

No, I can't...but I can show you two with improved FE. The Chevy Cruze & the Dodge Dart. It comes with the option of 2 different engines: a 1.8L naturally aspirated engine and a 1.4L turbocharged engine. Both have exactly the same horsepower rating according to Chevy (138 hp):
2012 Chevy Cruze | Powertrain & Chassis | Chevrolet

fueleconomy.gov rates the NA engine at 22 mpg city & 35 highway = 27 mpg combined. The turbocharged engine is rated at 26 mpg city & 38 highway = 30 mpg combined--a 10% improvement in FE.
Compare Side-by-Side

The Dodge Dart is very similar--two engines with the same exact 160 hp power rating: 1.4L turbo & 2.0 L NA (http://www.dodge.com/en/2013/dart/). The 1.4L turbo beats the 2.0L NA engine by 10% (http://www.fueleconomy.gov/feg/Find....32403&id=32404).


Research the EPA test procedure--there's a whole lot more to it than just driving around easily in a loop.

AWD and tuning for power (read: rich) will do that. If the turbos are more realistically sized, perhaps fuel economy would benefit.

For a turbo that does better, fe-wise, than the N/A version: the '88 Saab 9000 turbo matches it's N/A version EPA combined, and betters it by 2MPG on the highway. Same car, same engine....one had a turbo bolted; one didn't.

It was also extremely laggy, which shows the path to success: a really big, heavy turbine wheel.

Diesel dave: you mentioned compressor/turbine combos at 50% efficiency...better tgan piston power. Then wny not delete the piston portion and go with a pure turbine engine? I was told in my flying days that a piston would handily beat a turbine in "power-specific fuel consumption": is that no longer true?

The wrx is pretty 'laggy' too. Turbo starts working around 2k, cruising rpm at 55 is about 2200, so its in boost range at normal highway speeds. Next set of tires will be taller to force slightly lower rpms in top gear. I think getting it down to 2k if possible would give me 10% improvement at eco speeds but when needed i would stillhave boost available above 55.

Some of the more economical turbos I've driven have very little lag. Lag sometimes indicates that the turbo was not optimized for the rpm range you're driving in.

For terrible lag, nothing beats the previous STI automatic. Absolutely nothing in first or second gear below 5k rpm. Sucks like a hoover to drive around town.

Gas turbines have a very narrow high effeciency margin.
Usually you want the piston engine at its most efficient speed and load while the turbo is usually running at 10% or 20% efficiency.
You could size both the pistion and turbine engine to be most efficient at cruising speed but that would be all the vehicle would be good for. Every other driving condition would be less drivable. You would have to have a tiny engine turning about 2500rpms with a properly sized turbo boosting it to between 10 and 30 psi.

Hello meanjo

What Dave was referring to is mechanical efficiency, not thermodynamic. The best low power turbines can return about 20% thermodynamic efficiency. And conversely the mechanical efficiency of a piston as compressor and expansion engine is better than 95%.
-mort

Thet 3.0 td in the M550D does 385hp. And then you can even gain 25 to 30% by chiptuning.

Well thats why i'm assuming the saab LPT tech is worth emulating elsewhere... i'm just wondering if 0psi wastegate options or total bypasses were worth pursuing too.

For EPA the city is more of a struggle, but isnt the highway a steady state 65mph or 70mph now?

You could get an over sized ricer blow off valve and use it to bypass some of the intake restriction. This is what I am doing to reduce turbo lag.
Another option might be to do an external WG in addition to the built in one.

It's a transient test and the average speed is 48.3 mph. Here are the details:
Emission Test Cycles: EPA Highway Fuel Economy Cycle

http://www.epa.gov/nvfel/methods/hwfetdds.gif

Turbos have little effect on diesels because diesels operate at an "excess air" condition. The combustion event occurs on the surface of the fuel droplets where in a gas engine the fuel/air mixture is (more or less) uniform through the whole combustion chamber.

Otto-cyle engines are married to an air:fuel ratio. Stuff more air in and you have to add more fuel. If you don't have enough load, you'll have to throttle down and that reduces thermodynamic efficiency.

This is why direct-injection is exciting. It generates a local sufficient mixture in the combustion chamber without the thermal losses of a pre-combustion chamber. The "rich" area around the centerline of the injection pattern "blowtorches" the leaner outlying areas into combustion.

Unless you have direct injection or a pre-combustion chamber (Honda CVCC) increased vol eff means increased fuel flow.

Sorry for resurrecting this old thread. But there is so much misinformation.

Cars don't come with blow off valves. They come with bypass valves to recirculated compressed inlet air to intake to prevent compressor surge. Diesels don't have throttles so this is very difficult to do.

Turbos raise engine efficiency. The manufacturer needs to do gearing and other changes to take advantage of this. This is why all BMW's are turbo'd now.

The wastegate is to keep turbine speeds and boost in check.

On cars like the wrx. They lower compression to increase engine longevity which lowers mpg. They could keep high compression but there would be no boost or need race gas.

Or they could build a diesel engine.

This is pretty iffy. The 335 and the 328 are the same engine pretty much but the 335 has lower compression and twin turbo. The 335 has poorer fuel economy. Fuel economy goes up if you decrease the engine size and then turbo to make up for the power loss, because it decreases the friction you have to deal with.

BMW engines are all Valvetronic equipped except the high performance Sxx variants. This means off load boost actually can help thermodynamic efficiency.

As people have already figured out with the best BSFC at 80% load rule, increasing power production stops increasing efficiency before the engine hits full load. A usual throttle plate controlled engine might see a slight improvement in thermodynamic efficiency from a turbo at an operating point where the throttle is nearly fully open, but the valve timing is set up so that the engine is not at maximum torque output.

Sema show mpg award went to sts turbo for having highest percentage mpg increase

Cams,timing, comp ratio,Afr all play into it.and all need to be adjusted accordingly to a high performance turbo motor.

BMW also switched over to a turbo 2.0l over the na n54 3.0 i6. And said the new M's will be turbocharged cause it raises efficiency which means lower emissions and higher mpg

As for the diesel ... not everyone wants a heavy diesel drive train. I love diesels, had a 750rwhp cummins. That averages 21-25 mpg (25 when stock 21 modded)

Also, iirc, turbos help with exhaust scavenging and help with reducing pumping loss from intake stroke. https://sites.google.com/site/shooti...ine-efficiency

Also have to look at the technology behind it.ball bearing takes less friction than journal. Comp wheel and turbine shape. Exhaust housing size etc. Just slapping one on isnt going to give you mpg.

Forgive my android writting