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The Electric Supercharger |
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Written by Devin Durham
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Saturday, 14 April 2007 |
 We all have seen ads for them on E-bay: the electric supercharger. A device that you add to your intake, that is guaranteed to make 25+ horsepower and increase your miles per gallon. But does it all add up? Does the math work? Is this just a sham? Could you really add 25 horses for 100 bucks? In short, do these things work at all?
The short answer is that they don't work. In physics there is no free ride. The long answer is that they DO work, if you can put enough energy into them.
There is a difference between moving air and compressing air and that's one of the reasons why you can't just stick a simple fan onto your intake and go to town. If you want a graphic example of this, find a fan that moves a lot of air, a funnel and a balloon. Place the balloon over the small end of the funnel and see if you can blow that balloon up with the fan. It won't work because although air is moving around, it is not being compressed above one atmosphere.
So your first hurdle with an electric supercharger is going to be finding a fan that is capable of compressing air as well as moving air. The second hurdle is finding a fan that won't restrict the airflow, causing more harm than good. There are some electric axial-flow fans that seem to fit this bill.
But you are still going to have to flow enough air to make a difference. Here's where we are going to get into some math, and excuse me while I crib a little bit from another website, but what we are looking for is how much airflow (cfm) would be required by such a fan to provide enough airflow and pressure to make it viable:
vfr = (rpm * 0.5 * cid * ve) / 1728
Where:
vfr is the volumetric flow rate of the engine
rpm is the engine RPM where expected maximum power is developed
cid is the cubic inch displacement of the engine
0.5 is a factor that takes into account that an gasoline engine only moves air for two of its four cycles
1728 is the conversion factor from cubic inches to cubic feet
This is assuming a 2.5 liter V6 engine, and of course there is no correction for real world efficiency drop. So here's the formula:
vfr = (5850 * 0.5 * 152.4 * 0.82) / 1728 = 211.5 cfm
Meaning that the engine in question can, on its own, suck in 211.5 cubic feet per minute of air. This figure was chosen deliberately because to move that much air with a fan, you are going to have to expend about 746 watts, or roughly 1 horsepower. Since car voltages are at about 13v coming off the generator then 746*13=57.4 amps of required power to, in theory, deliver 1 pound of pressure.
 e-RAM Electric Supercharger
It should probably be noted that this type of draw on most generators is probably detrimental to the life of that generator, and for the cost of the system I am not sure if the power gains are worthwhile.
On E-bay you can find many many people selling electrical superchargers with outstanding claims, but math is math, and they just don't stand up. You can't hook up a blower motor with 15amps of draw and expect power gains from it. It simply will not happen.
There is also a very real electrical supercharging system that does 100 percent work, and produces about 9 pounds of boost. It has three electric motors working in concert driving a roots style supercharger, and requires a literal trunk full of car batteries that it will discharge in about 30 seconds. This system will draw from 600 to 1200 amps of power (wow!) and even the manufacturer only recommends it for engines under 2.5L. What it comes down to is the reason everyone uses an engine driven supercharger or an exhaust driven turbocharger is because those systems work, while a car's electrical system is by far the weakest of its power systems.
If you look around on YouTube you can find a video of some guys attaching a leaf blower to a Honda Civic and getting a 20+ horsepower gain. That will work, assuming you can carry around an A/C powerplant (or power inverter) capable of delivering about 1320 watts of power and the leafblower is running all the time, otherwise it is a major exhaust restriction.
If you want to see someone who actually did this in his truck, here is a link.
Here is a link to a review of the eram supercharger, complete with some discussion of dyno pulls done by Two Guys Garage
And finally here is the "real" electric supercharger.
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Last Updated ( Saturday, 14 April 2007 )
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