How much power does a supercharger really take to run?
 

I can't really find a definitive answer on the Google so I thought I would bring it up here, as I suppose it's an efficiency question anyway.

Ahem, anyway, let's discuss. I'll assume there is certain variables like, how much the supercharger is being overdriven/underdriven, how big the charger is (in terms of CC/REV, the efficiency of the charger itself.

The discussion here is for a roots type. I'm asking for... Reasons.

More than a turbocharger! :)

If I recall from Thomas Knight, to get about 7 psi on a 2.4 liter engine, they needed about 15 horsepower from their electric motor set-up.

Which may not seem like much... but that's a piddling amount of boost for a piddling amount of engine (errh... relative to most supercharged American iron).

When you start talking about big displacement American iron and big boost numbers, that drain can rise fast. It takes a lot of power to compress air.

Here's one scientific test... right on the first page of Google search:

http://www.dragzine.com/news/lose-po...-loss-testing/

Up to 353 hp at 32 pounds of boost.

Yowzah.

It's not actually that much, if you can get your hands on an Eaton TVS model.

The geometry of the housing and rotors is designed to take advantage of acoustic effects, and the rotor sealing is not great so generally you see high efficiency at high speed.

A centrifugal supercharger loses efficiency at lower than design pressure ratio, and a turbocharger loses efficiency because they rely partially on backpressure to generate power. A Roots supercharger equipped engine maybe loses 5-10% more power than a turbo pushing similar boost, but who cares because you're not always on boost.

If you actually want to calculate it, you can approximate it (since you don't know how the intercooler will affect temperature just yet). Compressor flow is 1x rotor displacement per revolution * volumetric efficiency (which you can look up on the chart), pressure ratio you assume adiabatic compression, then use PV=nRT or the formula for work in adiabatic compression to estimate the temperature increase, which you can derive work per unit mass airflow from assuming ideal efficiency. Then divide by the efficiency written on the compressor map and you'll get your number.

On the supercharged Mustangs and Corvettes, they say it's around 40hp for 9psi boost (for a 6L engine at 6000rpm ish). If you only want 120hp from a Honda Insight, I imagine it can't be more than 10hp even with an older blower.

The only unknown variable here is the VE of the ECA1 engine. I can't find it anywhere, but I can imagine Honda has it close to 80-85%. Problem is, if I'm off by even 5% it can make a huge difference in how much boost the engine will get.

Its an air compressor.
Run a blower specific air compressor calculation.
The only thing that makes it blower specific is the efficiency, other wise its always the same.

Once you start boosting the engine more than 3 or 4 psi V.E. goes above 100%.
At 7psi you may have V.E. of 130% or more.

This isn't very sciencey, but only thing I noticed in my search: Horsepower Consumed by Centrifugal Supercharger

I remember hearing a number on the first GT (44), but can't find it online. Must have been Top Gear commentary.

We have a roots blower at work for inducing air flow in ducts that we make.

The blower is a 240v 3 phase unit with a 10 hp industrial motor, and the blower puts out 500 cfm wide open and 3 psi dead ended.

Hope this helps.

Jim.

That's the best you can do. I believe 80-90% to be a reasonable range for that Honda engine. The tricky part is that when you have cam overlap, the blower might be blowing air straight out the exhaust port.

Honestly, the way you have to look at a Roots supercharger setup is how much CFM you want to push through (taking into account how much boost it will generate) and thus specific torque/specific power. You cannot really be sure how much power the supercharger will consume because it is way too dependent on temperature and VE. I'd shoot for something safe like "supercharger will displace 1.4L per 2 revolutions", pick a blower based on that goal, and then work with it once you get the blower. There is not much of a choice in blowers anyhow.

It's 500cc/REV.

Thanks for the info guys. It does help a lot.