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.

It's actually coming from both. The direct benefit may come from the gearing, but without the supercharger, they wouldn't be able to gear it like that, so they both help.

Not really. You'd still be able to do it with the NA engine, you just wouldn't have as much power in 5th gear.

That's not accurate.

There are other benefits than just being allowed to reduce engine size. Your intake charge needs to be compressed before ignition. This is typically done in the cylinder by the piston. If you compress it with a supercharger, cool it down, and then compress again with the piston, you can compress the intake charge more efficiently.

As mentioned earlier, a supercharger can allow you to use lower gear ratios than NA which can improve fuel efficiency as well.

Most cars have superchargers added for performance reasons, so looking at empirical data won't help you much. However, if the engineers design for efficiency only, it's entirely reasonable for a supercharger to increase mileage.

Mike

Its pretty accurate. If you compress too much air in the engine you have problems with preignition and pinging, thus the need for most super/turbocharged engines to use higher octane fuel. If you don't use this higher octane fuel you have to sacrifice optimal ignition timing and thus efficiency. Another remedy for this problem is to reduce the compression ratio which also lowers efficiency.

Yes, lowering the compression ratio of a NA engine lowers efficiency. The reason why is because your cylinder pressure is reduced. With forced induction, you have two compressors increasing the cylinder pressure (the supercharger and the piston) so lowering the compression ratio of the piston doesn't neccessarily mean your cylinder pressures have been reduced.

You're good at explaining the reasons why a typical supercharged engine has lower fuel efficiency, and I concede that. However, as I've already stated, superchargers are typically used for performance reasons, not fuel efficiency. I still stand by the statement that a properly designed engine can be made more efficient with a supercharger. Pistons are inefficient at compressing air and a lot of heat is added. An efficient supercharger running at low boost through an intercooler on an engine designed to utilize it should be more efficient than a NA engine.

I'll agree, it may be possible to design an engine to run more efficiently with a turbocharger, not sure about supercharger since it gets its power directly from the engine where as the turbo gets some of its power from waste heat. Anyway, what do you think you'd have to do to an engine to get it better mileage with a supercharger?

Well:
1) Needs to be an efficient supercharger. Probably not a straight lobe, roots blower here... Probably Lysholm screw or a centrifugal.
2) Needs to be intercooled.
3) Moderate levels of boost. Two compressors running a low pressure ratios will be more efficient than one at a higher ratio.
4) Higher gear ratios to make use of the higher torque at low RPMs.

On a related note, I was reading through the latest SAE magazine last night and it had a long article about supercharging/turbocharging to improve power and efficiency. While most are going to turbos, both VW/Audi and Mercedes are going with twincharged engines (both engine driven supercharger and a turbocharger). The VW 1.4 TSI is already available (except in the US) and is a 1.4L 4 cylinder that makes 177ft-lbs of torque at 1500 rpm. That makes it pretty easy to keep the revs way down at highway speeds.

bump for recent interest in the subject

Wouldn't just upping the compression ratio and leaving it normally aspirated be way more efficient than adding more moving parts and over-compressing the air on the way into the engine? I think this super charger thing might be lust, not love ;)

...NA engines do not produce torque at LOW rpms anywhere as well as SC or TC engines do!

...NA engines have to SUCK air in, ie: 'work to breath.'

...SC engines BLOW air in, with a power penalty, but no "lag."

...TC engines BLOW air in, using wasted power of exhaust, but have "lag."

supercharged engines have horrible bsfc
Computing BSFC for Supercharged Engines - Moderated Discussion Areas

I don't like seeing them "greenwashed".

...how many TURBO-charged WWII fighter aircraft were there compared to SUPER-charged WWII fighter aircraft? Uh, one, the P-38 Lightning!

...(with all due respect to the Hallmark Cards™ slogan):

• "...when you absolutely NEED maximum HP, use SUPER-charging."
• "...when you absolutely NEED maximum FE, use TURBO-charging."

...is brown-washing better than green-washing?

For some reason turbos seem to help efficiency on diesels, but make it worse on gassers as compared to normally aspirated. I haven't quite thought that one through yet.

Even the Rutan Voyager that flew nonstop around the world was normally aspirated. It also was essentially true multi displacement, with a 200 in back running for the whole flight and a 240 up front to help w/takeoff and initial flight. I haven't seen any cars try the multi-IC approach yet.

Uh, the B17 had exhaust driven trubosuperchargers. The P51 had two stage supercharging in the Packard built Merlin engine copy that ws originally on the Spitfire and many other Brit planes.

But it was not a fighter. I think some of the radial engine planes were turboed, but the mettalurgy was the weak point in turbocharging until after the war.

regards
Mech

And neither turbo or supercharging is done for efficiency is the point I am trying to make here, not even in planes where enabling higher altitude and power to weight could possibly be a contributing factor. Normally aspirated is still the aspiration of choice for economy.

At 30,000 feet with 40 below ambient temperatures turbocharging and supercharging were essential when you consider atmospheric pressure is 50% of sea level pressure at 18k feet. Even in cars when the elevation gets above 6k feet the power loss is significant.

Ford apparently does not agree with the assumption that there is no place for supercharging in an economy car.

Counterpoint:

The most efficient diesel engines at just over 50% do not use supercharging. Instead they utilize a 3 to 1 stroke to bore ratio, with a 3 foot bore and a 9 foot stroke, running at about 100 RPM to minimize the losses due to reciprocation of their massive pistons and rods. The sctoch yoke between the piston and rod eliminates side loads on the piston.

Personally I like the idea of electric supercharging especially when it is designed to allow air flow to not have to pass through the supercharger. The energy to run the supercharger can be supplied by regenerative braking.

Combined with start stop and DFCO in an engine of about 50% of the normal displacement and you can get better mileage, just like the Edison car in the X prize.

regards
Mech

the vlc is a curious example, I wonder if they added a turbo only to make the performance metrics. I'm guessing it has an efficiency penalty over a slower acceleration curve, dunno though.

The Story of Turboing a Hybrid Prius - Part 5

Anyone want to rebut Julian?

:)

The only way I could see forced induction helping is if it is coupled with downsizing the engine displacement, and/ or giving the transmission a very long legged overdrive. A mild boost designed to kick in at low rpms would allow lugging in lower gears and being able to stay in overdrive in more situations.

IIRC the new Corvettes are supercharged, but with a 1:0.50 overdrive gear ratio (it may be even higher, can't find the info) it could achieve ridiculously high fuel economy for a v-8 performance car. A friend told me his buddy could reliably get 30 mpg from his Corvette 6 speed when cruising.

One example is this quote from Ford:
I'd been reading about OEMs using turbos to increase economy while minimizing performance losses, and this is one example.

Another quote worthy of consideration is this one from the above article.

I think it was a 250 cc bike engine. Considering the weight of the vlc and the performance requirements, a turbo was the only solution. Of course the durability would probably be pitiful.

regards
Mech

That's a very impressive effort by Julian.

Uh, one comes to mind. The P-38 Lightning was turbocharged.

Yep, Pop was going to ferry one from England to Paris and had a turbo fire and aborted.

Forgot that one, but I believe most of the radial engine planes were turbo charged.

regards
Mech

...after WWII, both (SC & TC) were used together, one example being the turbocompound Curtis-Wright R3350-42 engines, which had a single-stage, two-speed, supercharger and three 'Power Recovery Turbines" (PRTs). The PRTs extracted 500 HP from the exhaust gases from the 18 cylinders and put it back into the engine crankshaft, boosting total output power from 2,900 HP up to 3,400 HP per engine...plus delivering lowest fuel consumption per HP at lean cruise, only about 100 gallons/hr per engine.

IIRC, it wasn't long before they realized they didn't need the ICE in the middle.

...LOL, "true" ie: SC-ICE-TC.

...however, just about any ICE aircraft engine gets BETTER fuel economy (but not SPEED) than any equivalent JET engine.

...example: with only 8,760 gallons of 115/145 AVGAS, we (Lockheed EC-121) could stay airborne for 18-20 hours...and still have reserve! Try that in a jet--NOT (ha,ha)!

...that's what "patrol" planes need--endurance, not speed--ie: "time (loiter) on station" to the maximum extent possible.

Essentially a turbocharger coupled to the crankshaft (other than the 2-speed compressor part).:)

...exactly. I still have my copy of the NATOPS manual for the EC-121, and there's a nice picture illustrating how the PRT's worked.

..the coupling between the turbine and the crankshaft wasn't "hard," there was a "fluid coupling" (torque converter) in between each PRT and the crankshaft.

This past June 26, I went for a tour of a C-121 Constellation. This particular aircraft was Douglas MacArthur's personal transportation during the Korean War. It's airframe and engines are in flyable condition. It resides at the "Planes of Fame" air museum at the Valle airport between Williams and the Grand Canyon.

...to meander back to the original posting, "yes," both Super Charging (SC) and Turbo Charging (TC) can and do increase fuel economy, but the combination of BOTH can do even better job (hence, my R3350-42 example above).

...SC both MAKES usable power and TAKES usable power, while TC MAKES usable power from the "waste" exhaust heat and gas flow, hence the "title" Power Recovery Turbine (PRT) on the R3350-42 engines.

...both SC and TC have their own pros & cons.