Supercharging for economy?
 

I was reading a review for the new version of the car I have, the Nissan Micra. The new car is rated at 56.5mpg, which is fairly impressive in itself, however next year a SUPERCHARGED version of the same engine in the same car is set to come in with 20% more power AND more economy (71mpg) any one have any ideas on how this could be working? It's against my understanding of how superchargers work.. i.e. trading more fuel for more power in a smaller space.

Link to review: Nissan Micra Hatchback Car Review - Auto Trader UK

A supercharger is belt driven off an engines crank pulley, such that its rotation depends on engine rpm. There are some that have a clutch operated pulley system that allows the supercharger to spin up to a desired speed and hold there throughout the rpm range; max boost instantly, or whenever you set it up to be.

"Forced induction" essentially increases the volumetric efficiency of an engine to that over the limits of natural aspiration. In a NA application, the VE of an engine varies on throttle position and rpm. In a FI application, you can have a VE of 100% (that is, 0psi of boost) at any throttle position and any rpm, if the turbo/supercharger is setup up to do so. This is possible because more air is being forced into your motor than the motor alone could suck in alone.

I used to have a 2000 Dodge Stratus Coupe with a mitsubishi 3.0L V6 engine. I extensively modified the car, adding a vortech v-1 s-trim supercharger, standalone computer, larger injectors, completely rebuilt motor, and much more. To make a long story short, having the supercharger on the car increased my cruising gas mileage about 10-15% compared to its stock NA form.

I sure do miss her. 550hp to the front wheels was very interesting:
http://i422.photobucket.com/albums/p...n/HPIM0362.jpg

A friend of mine has talked a little about replacing the throttle assembly with a rotary compressor driven by an inverter motor. The idea is that boost pressure can be controlled electronically to get the most out of the fuel under heavy load. At light load, the compressor acts as a turbine to recover pumping losses. Combine that with a turboalternator (exhaust gas turbine driving a high speed alternator) and it should work better than a conventional turbocharger.

@theycallmeebryan

How did the enormous white radome on top of the Stratos affect performance? :)

...probably no help, but the radar inside of it probably DID help detect approaching cop cars and speed traps (ha,ha)!

VW and AUDI us a supercharger as part of the TwinCharger engines.

A few comparisons of supercharged vs NA reveal some figures:

Mini cooper (NA) 42mpg
Mini cooper S (SC) 32mpg (similar 1.6 Brazilian built 4-pot) Source: Parkers.co.uk

The mercedes range is the only one I can find with lots of supercharged cars, and provides lots of interesting info:

Mercedes C180 (NA) 32MPG
Mercedes C180 Kompressor (SC) 37mpg

Mercedes C230 (NA: 163bhp) 30mpg
Mercedes C230 Kompressor (SC: 192bhp) 34mpg
Mercedes C230 (NA: 201bhp) 30mpg

All I've done here is confused myself!!

This has been talked about before, but more so with turbocharging since turbos actually use more of the waste heat. Superchargers steal power directly from the engine to make more power. Anyway, the conclusion has always been the main benefit of supercharging/turbocharging is the ability to now downsize your engine since it is producing more power. If you don't downsize the engine the best you can hope for is the same mileage which is highly unlikely.

That's what I'd always thought, although in new car's it's going to be more complex. A different tourque curve may lead to them fitting taller gears.

They mentioned on Top Gear that the supercharger on a McLaren Mercedes SLR "costs" 120+bhp just to run. That can't be good for FE.

That may may be true, but the benefit is coming from the gearing, not the supercharger at that point.