Supercharging for economy?

[comptiger5000]

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.

[Daox]

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.

[darcane]

Originally Posted by Daox

If you don't downsize the engine the best you can hope for is the same mileage which is highly unlikely.

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

[Daox]

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.

[darcane]

Originally Posted by Daox

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.

[Daox]

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?

[darcane]

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.

[ShadeTreeMech]

bump for recent interest in the subject

[dcb]

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

[gone-ot]

...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."