Turbocharger MPG(Km/L)

Okay, so I have been looking all around online to see whether or not a turbo can increase your MPG(Km/L). I have seen people saying that they not only got more power but their tanks lasted longer, but that's not very reliable seeing as my weekly MPG changes all the time even just going the same route. I've also seen people saying that it decreases your MPG due to the fact that it's forcing more air in, meaning the engine still needs to use more fuel to meet the mixture ratio. I have also seen people saying that technically your vehicle becomes more fuel efficient because it's reusing the exhaust to generate more power, but that just relates to your emissions and relating to more power for less emissions. I have also read that there are "turbos" that wont neccisarily give you a performance boost but actually are built to increase you MPG, but I haven't been able to find anything to support that or what the difference is between the turbos.

What do you guys think? Have any of you found an actual article relating to testing the vehicle with and without a turbo and showing the increase in power, decrease in emissions, and MPG change? I've been searching for quite some time.

[Killrob]

Thing is: this very much depends on how you use the turbo. For normal every-day driving (i.e. cruising), a turbo will not make much difference to MPG because it's not spooled 99% of the time. It only ups the maximum power output of the engine.

On the other hand, having a turbo on means that you can build a smaller engine with still the same power output as a larger NA engine.
For cruising you need on the order of ~10kW of power, which for most engines in modern cars is a very small amount. That means that the throttle needs to be pretty much closed all the time while driving, making the big air-pump (your engine) horrible inefficient. The smaller, turboed engine has a smaller displacement and thus is closer to "open throttle" while cruising, which helps its efficiency and your MPG (smaller engine = less absolute amount of friction)! As you don't sacrifice any maximum power you're happy... one downside is that a small turboed engine is not as responsive as a bigger NA engine.

If you'd build engines which just allow you to cruise (i.e. max power ~10kW), one NA and one with a turbo, the even smaller turboed engine would probably (that is my guess... haven't thought that one through properly, it melts my brain ) be slightly more efficient due to the additional feedback loop... not sure though.

Interesting. That makes a lot of sense. What about when it comes to the turbo's range? If i were to have the same small engine with a low range turbo, 1k-3k RPM, will that mean that my car stays just as fuel efficient but gains more power accelerating from a stop just driving down the street, or will that only increase the power from a stop if I were racing?

the efficiency of a turbocharged engine varies greatly with load, rpm and A/R ratio.
when the turbo is not in operation (low rpm) the engine is very inefficient as it struggles to pass air and exhaust through the turbocharger.
at mid-range cruise (3500 rpm on highway) the engine becomes very efficient as the turbo eliminates much of the pumping losses caused by the throttle.
the moment you put your foot down the extra air needs to be complemented by more fuel or the pistons will develop "exhaust ports'', this basically throws fuel economy out the window.
overall a turbocharged engine will have worse fuel economy than a NA engine (except for diesels , which become increasingly more efficient with higher intake pressures)

EX: averages
- 2.0 l NA engines use 7 to 8.5 l/ 100km of fuel
- 2.0 turbo engines use 9 or more depending on manufacturer
BMW 3.0 l NA 7.5-8 l/100km average
3.0 l Turbo 9.5-10.5 l/100km average

smaller engines are worse, for example:
- 1.6 turbo engine of a mini cooper S (170 hp) uses 9-13 l of fuel in the city. you are better off with a V6
- 1.4 tsi engines use 8 l/100km in low power tunes and about 9 for the 170 hp versions. these are comparable to 2.0 l units.
- 0.9 TCe (90 hp) engines from renault manage 8-8.5 l/100 in the city which is more than 1.4 NA engines of similar power, besides being very unresponsive and rough.

the efficiency of a turbocharged engine varies greatly with load, rpm and A/R ratio.
when the turbo is not in operation (low rpm) the engine is very inefficient as it struggles to pass air and exhaust through the turbocharger.
at mid-range cruise (3500 rpm on highway) the engine becomes very efficient as the turbo eliminates much of the pumping losses caused by the throttle.
the moment you put your foot down the extra air needs to be complemented by more fuel or the pistons will develop "exhaust ports'', this basically throws fuel economy out the window.
overall a turbocharged engine will have worse fuel economy than a NA engine (except for diesels , which become increasingly more efficient with higher intake pressures)

EX: averages
- 2.0 l NA engines use 7 to 8.5 l/ 100km of fuel
- 2.0 turbo engines use 9 or more depending on manufacturer
BMW 3.0 l NA 7.5-8 l/100km average
3.0 l Turbo 9.5-10.5 l/100km average

smaller engines are worse, for example:
- 1.6 turbo engine of a mini cooper S (170 hp) uses 9-13 l of fuel in the city. you are better off with a V6
- 1.4 tsi engines use 8 l/100km in low power tunes and about 9 for the 170 hp versions. these are comparable to 2.0 l units.
- 0.9 TCe (90 hp) engines from renault manage 8-8.5 l/100 in the city which is more than 1.4 NA engines of similar power, besides being very unresponsive and rough.

I would think that they would have a bypass for the turbo when it's not pumping. I didn't think about the passage through the turbo when it was off though, I automatically figured that there might be some lag in the passage but not a reduction.

I also recently saw a separate discussion board that posted this: http://www.dyersblowers.com/223/268.html

5. Will there be a severe loss in fuel economy with a Dyer's Blower Kit?
Under Normal driving conditions, you should PICK UP around 3 mpg over a normally aspirated engine. This is possible by the efficient mixing and vaporization of the fuel due to the supercharger. Also, the additional low end torque allows for a taller rear end ratio.

Now I understand that the supercharger is a bit different from the turbo, but from running of the belt I would think that the supercharger automatically takes some MPG just to run it off the belt where the turbo is just running off exhaust.

there is no bypass as this will add a great deal of complexity in the piping. diesels have variable vane turbos which close when the turbo cannot spool allowing exhaust gases to escape freely. only in recent years they started appearing in petrol engines because of the high temperatures in a petrol engines exhaust.

roots superchargers use a large amount of engine power just to run itself. this means it will increase fuel consumption by a lot.

Hmm, It seems that this is having just as much difficulty as the rest of the Websites. I have a friend who works for BMW and says that the reason they are now doing twin turbos on almost all their cars is for MPG reasons.

My main reason for asking this was that recently my girlfriends 1990 Volvo 740 had a leak of oil into the turbo causing it to blow out smoke, I removed it and a couple weeks later I went to good old Pick and Pull, removed a new one for her and just installed it last week. Amazingly, her gas mileage has improved exponentially from when the turbo was removed. Her car is a hunk of junk so she has just been using to go to work and back. Her solid work schedule has made it to where while it was out we were having to fill up the tank once a week, now we are at a week of having it in and have only used a half a tank. I'm wondering if maybe the part that we aren't thinking about is whether or not the companies design the car for a turbo. It seems to me that the Volvo has already shown an extreme of how much the turbo matters to her car.

what your friend at bmw told you is true in part. modern direct injection systems allow ultra lean burns and improve mileage in certain conditions, compared with a larger NA engine, but overall they will use more fuel. it depend on how you use the car.

[Daffyflyer]

The way I understand it, An appropriately turbocharged engine, of smaller capacity, as used for example in VW Golfs, where they have a forced induction 1.4, in place of an N/A 2.0, will be more efficient for the following reasons.

The Turbo uses otherwise wasted exhaust energy to pump air into the engine, this reduces the work it needs to do to suck in air, thus meaning less wasted energy & fuel.

You can downsize the capacity of your engine, and make up for the lost power using turbocharging. The smaller engine will usually be lighter overall, have lighter weight components and spend more time at higher throttle settings, reducing pumping losses again. This also all saves fuel.

Because you can make good power at low RPM using modern turbo tech, you need to spend a lot less time at high RPM, and can stay in higher gears more of the time. The frictional losses in engines are GREATLY reduced by running at lower RPM, so this is another big fuel economy gain.


This all depends on the ability to run it no richer than an N/A motor at most medium loads, only richening up at full power, but modern direct injection + premium fuels usually takes care of that.

but the 1.4 TSI unit uses more fuel than 2.0 FSI. the advantages are lower weight, lower emissions and lower taxes and a cheaper engine for the manufacturer.
turbocharging sounds good on paper, but in practice things are not so bright.