Better MPG with Turbo? What?

[BlasterMaster555]

So, I make an economy car that meets all the modern day safeties, and weighs 2250 lbs roughly. A 1.6L 16V engine powers it, making 109 HP, and with a 5 speed standard I get 43 MPG running on 80 octane.

I make a new 1600cc engine with the same base but VVL and Turbo, 194 HP on 91 Octane, weighs 100 lbs more. Same car, now gets 51.5 MPG. How??

Remember, only changes are engine and gearing (optimized for engine)

[nialloftara]

To push almost 200hp from a 1.6 I'm guessing you've got plenty of boost in there, that turbo lag actually helps your MPG for very technical reasons relating to how open your throttle is, plus as the turbo makes twice the horsepower your top speed was probably set higher and the taller gearing also helps. Do a few searches here in the forums, I know killrob has explained it in greater detail at least a few times.

[VicVictory]

Yes, properly tuned (as in not massive performance tuning) turbos will give better efficiency for the displacement.

Real world example: the newer Chevy Sonic is made with a 1.8L naturally aspirated or 1.4L turbo engine. Both put out 138 HP. Both get the same EPA rating in the Sonic. So they're squeezing out the same power and efficiency from a turbo motor with more than 20% less displacement than its NA counterpart.

When most people think about turbos and their negative effect on fuel economy, they think of things like the WRX or Evo, which are tuned for performance, not economy. They're also generally driven by rather... aggressive... drivers, which doesn't help economy either.

[BlasterMaster555]

The Variable Valve Lift and a very finely calibrated turbine minimize turbo lag. When I get home I will fire it up and hope the save is still there, so I can post things.

Usually I think of turbos helping economy when the previous fuel and intake system can not deliver the fuel air mix well enough (see Jay Leno's Blastocene Special, which went from carbs to Fuel Injection with twin turbos to double both HP and MPG). Both engines are DI.

[TrackpadUser]

What are the mixture, ignition timing, compression and cam setting for both engines?

[NormanVauxhall]

Having a 91 RON instead of 80 indeed help a lot the economy. You have 11 RON points more to put in compression and ignition timing that you can spend saving fuel

[USDMFTW]

The mpg/turbo calculations in this game are a bit unrealistic.

http://www.greencarreports.com/news/108 ... as-mileage

Like other people have said, turbo engines really don't increase economy all that much, but they do increase horsepower and torque. For example, fords 3.5 ecoboost vs there 5.0 in thier f150, both get around the same mpg, but the ecoboost makes much more power.

Plus your NA engine has 11 octane points less than the turbo engine which aids to the big gap.

[BlasterMaster555]

Both engines are running 14.7 fuel air mix. If the car makes double the horsepower, it should halve the fuel economy regardless. The only time you MPG goes up with a turbo is on a diesel, where fuel air mix is not a constant.

Also, I took the turbo engine and made a nonturbo variant with higher compression, same 91 octane. I got a good boost in power over the economy engine, but the fuel economy was 38 MPG instead.

[USDMFTW]

Both engines are running 14.7 fuel air mix. If the car makes double the horsepower, it should halve the fuel economy regardless. The only time you MPG goes up with a turbo is on a diesel, where fuel air mix is not a constant.

Also, I took the turbo engine and made a nonturbo variant with higher compression, same 91 octane. I got a good boost in power over the economy engine, but the fuel economy was 38 MPG instead.

No, it should not halve the fuel economy regardless, that not the properties of an engine. As an example, 1960s GM L35 396 V8 made 325 hp, but gets much worse economy than an ls1 making around the same horsepower and torque. Mpg generally stays the same while power increases with modern turbocharger/engine technology. Another example is the 2.0 ltg and the early 5.3 vortecs, both make near the same hp and torque, the ltg gets much better fuel economy. Do note the ltg needs premium gas.

When you took the turbo off did you optimize the other settings for the cars new powerband?

[TrackpadUser]

Both engines are running 14.7 fuel air mix. If the car makes double the horsepower, it should halve the fuel economy regardless. The only time you MPG goes up with a turbo is on a diesel, where fuel air mix is not a constant.

Fuel consumption works in the following way.

An engine has what is called a brake specific fuel consumption, represented by the fuel economy stat and graph in the engine creator.

The graph is, IIRC, 1:1 the amount of fuel used per HP at each RPM at full load.

The fuel/hp is the important part.

If you have two engines with the following power curves, which are completely unrealistic, but should be fine for my explanation:



The Y axis represents the power in kW, the X axis represents the RPM

Now, let say both have a fuel economy of 100g/kW*h at all RPMs, and that due to drag and mechanical losses you need 50hp to cruise at 100km/h.

Since IIRC fuel economy is maxed out at maximum engine load for piston engines, the optimal gearing would be to have the engine cruising at under 2500 RPM.

Therefore, since both engines are producing the exact same amount of horsepower at the optimal gear for cruising, and that they have the exact same fuel economy, they will use exactly the same amount of fuel, even though engine 2 produces 3 times the horsepower.

OFC this example is completely unrealistic but it should be good enough to explain that an engine making twice the peak horsepower can burn just as much fuel while cruising. It will probably burn twice the fuel in racing though, as you will be going more in the high RPMs.



As for your other problem, did you change anything other than compression when making the engine 91 octane? Gearing might need to be adjusted for maximum fuel economy.

[BlasterMaster555]

Yes I optimised gearing and so on for every engine variant.

What I would like to see is a City/Highway fuel economy rating,which separates cruise economy from start/stop under load economy. While the eco engine and super engine can get the same mpg on the highway, the super engine goes through a lot more fuel in city driving, because of the accelerations.

[nialloftara]

The mpg is a combinition rating. Killrob explained it a while back. It's calculated across several speeds.