sagot
Zone Head
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posted October 30, 2005 11:23 AM
Super charger!
So Peugeot has a super charged 125 cc scooter that puts out 20 HP! That's a lot for a 125 cc four stroke but the thing I'm wondering about is the speed ratio between the crank and the super charger. The scooter is 1:1 and most car applications it is 2:1 but why not 1.5:1 or .75:1. Can you adjust the power of an engine by changing a couple of pulleys?
Also, in some aplications, can you run the super charger off of an electric motor so as to not rob engine power?
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trenace
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posted October 30, 2005 02:47 PM
A supercharger takes way too much power at boost to run off a battery. Theoretically it could be done, for exanple with a battery pack such as used in electric cars, but the weight of the batteries would make it not worth it. The problem is also equivalent to why not have an auxiliary electric motor add drive to the crank to add power -- that's the same as having the motor drive the supercharger. It can be an OK arrangement for example in hybrid cars but from the performance aspect it isn't good because it's not weight efficient.
One often indeed can adjust the power of a supercharged engine simply by changing pulleys and perhaps fueling.
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frEEk
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ummm... yeah
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posted October 30, 2005 04:14 PM
i'm nto sure that's quite true about a supercharger not being able to run an electric motor, as there is at least one company doing exactly that for cars. probably very low boost tho.
i also had the same initial thought about "what's the poitn since u have to turn over a bigger alternator?" but then it occurred to me that u can spread that additional work around such that you lose a little more power all the time, but gain a bunch at WFO. basically, make the supercharger electric to save the X horses it takes to drive it resulting in a peak HP of say 23 instead of 20, but then have a larger alternator and batter that could compensate based on a 25% duty cycle of WFO time. net result: lose 1HP across the band but gain 3hp at peak. of course that assumes the supercharger takes little power when yer not giving her. if it does, then my argument is broken. ideally u'd want a supercharger that u can turn on only when u need it, like NOS.
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trenace
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posted October 30, 2005 05:23 PM
 Edited By: trenace on 30 Oct 2005 17:41
Actually I said that it can be done rather than that it was impossible, but said what the general limitations are (high battery weight required for any real power to the supercharger), what it's equivalent to (an auxliary electric motor driving the crank or any other part of the powertrain), and how it's really not efficient in a performance application, any moreso than what it's equivalent to.
BTW, what production manufacturer (not the aftermarket outfits selling hair driers that don't work) is using an electric-motor driven supercharger? I hadn't known that.
Basically, say a battery puts out 200 amps. That battery might weigh 10 lb. 200 amps at 14 volts is 2800 watts. That is only 3.75 hp. So, not even counting the weight of the electric motor, your weight is 2.7 lb per crank hp.
So let's scale this up, let's say you want to get a 15 crank hp benefit out of your system.
Is adding 15 crank hp worth adding 40 lb of batteries to your scooter? (Not even counting weight of the motor?)
You could add those 15 crank hp with a much smaller increase in weight of the gasoline engine -- perhaps a reasonable weight/performance ratio there is close to 1 lb per crank hp. And its available all the time instead of for brief periods.
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frEEk
Administrator
ummm... yeah
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posted October 30, 2005 11:30 PM
i meant aftermarket of course, defintiely not aware of a engine manufacturer using them. and yeah, i wouldnt be surprised that they dont work real well.
not sure how u figure those HP/lb figures tho. u'd not driving the drivetrain itself with teh electric motor, but a blower. u shoudl gain significantly more power than the 3.75HP (or whatever) that you introduce via the electric motor. if that werent true, then a standard supercharger wouldnt make any power.
regardless, teh disadvantage of added weight (not to mention complexity) definitely does exist, no doubt.
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trenace
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posted October 30, 2005 11:35 PM
I wasn't talking about the power difference between the motor getting pressurized air vs normally aspirated, which is the case totally independent of how the supercharger is driven, but the issue of the hp involved in driving the supercharger.
The whole point of someone advocating driving it with an electrical motor is to save power being drawn from the crank.
The conversion factor I gave is simple physics at 100% efficiency: that many amps at that many volts is that many hp. In practice an electrical motor will deliver a little less but that's the best case.
And so if the idea of driving your supercharger from a battery is to save power being drawn from the crank, the problem is, it takes approximately 10 lb of batteries to yield, by that calculation, each 3.75 hp "saved" by the electrical motor delivering it rather than the crank doing so.
That was what I was saying -- not how many hp is produced from the intake charge being pressurized, which is an entirely different matter and doesn't require an electrical motor at all, and is better done without an electrical motor but rather directly off the crank.
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frEEk
Administrator
ummm... yeah
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posted October 30, 2005 11:38 PM
ah gotcha. agreed, and makes more sense than what iw as thinking too.
btw, do u live on this board? i can't say anything without u replying by teh time i finish checking the rest of the forums 
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trenace
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posted October 31, 2005 10:59 AM
I'm at the computer a lot in the work that I do, I work from home. I have to admit I also take a fair amount of breaks. Or when not working, I don't watch TV, so that saves time.
I also type 90 wpm so it doesn't necessarily take as long as you'd think
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Lorcan
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posted October 31, 2005 11:07 AM
Edited By: Lorcan on 31 Oct 2005 11:08
Turning mechanical energy into electrical energy and then back again is extremely inefficient. The TTS superchargers (which are basically belt driven turbos via a gearbox) run at a ratio of 9.68:1. As with a turbo, they don't make much boost until they are spinning hard, unlike a positive displacement blower which will boost from idle, and adjusting boost levels is not as easy as with a turbo. That said, Mark Fisher of Greenhill Racing has got his blown Busa in the 7s with a sniff of gas. It's not something I would consider as they will always lose the efficiency war against turbos, but if you want something different and don't want to rule the world then it's worth considering (just go direct to Mark, not TTS).
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trenace
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posted October 31, 2005 11:20 AM
There is an approach where I think one might get something out of it.
One of the car manufacturers -- VW I think but I'm not positive -- has just introduced or is just introducing a new dual supercharger/turbocharger set up. The concept is that turbos are more efficient at the high end but can suffer lag and not give much boost down low, while superchargers can be effective down low but won't be as good on top and also draw a lot of power on top. The combination has the turbo responsible for the top end and the supercharger filling in the bottom and filling in any turbo lag.
Well, if a turbo is going to do most of the work anyway, maybe it would work -- if also mechanically feasible -- to have an electrical motor that could drive that same impeller. In other words, it's driven by both the exhaust turbine and, if provided current, an electrical motor.
So, the fact that the system is not so efficient is OK at the very low end because, first, not that much power for compression is involved anyway in that regime, and second, relatively little total tim is spent at low rpm and needing boost. (During cruise, no current need be consumed since no boost is needed then.) And moments of filling in turbo lag would also be brief.
So the question is whether that would be lighter and cheaper, not requiring a separate blower, than having a separate mechanical supercharger.
So maybe there could be a "fill-in" role for electrical supercharging, even though not efficient as the principal source of forced induction.
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