# Myths: Electric turbos and ram air.

Discussion in 'Forced Induction' started by Loco Honkey, Jul 18, 2004.

1. ### Loco HonkeyBanned

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ELECTRIC TURBO

Fans are not designed for creating positive pressure! They can only put out 5 to 6 in H20 (.18-.22psi). They are designed to move air from one location to another not to pressurize it.

OK, lets go with a 2001 celica gts for comparison because I'm tired of talking about Honda this and Honda that...

specs:
Displacement: 1.8 liters (1,796 cc)
Bore x Stroke: 82.0 mm x 85.0 mm
Compression ratio: 11.5:1
Horsepower: 180 hp @ 7,600 rpm
Torque: 130 lb.-ft. @ 6,800 rpm

Now that we have the specs of the motor, lets see how much air it consumes.

v = (pi/4)*(bore)^2*(stroke)*(Ncylce)*(# Cylinders)
v = 3.14/4*(3.228in)^2*(3.346in)*(7600/2)*(4)
v = 416369in^3/min(1ft^3/1728ft^3)
v = 241.0 cfm

Mind you this is 100% VE, so say at a more reasonable 90% that leaves us with:

v = 241.0cfm * 0.9
v = 216.9 cfm actual

Now I am sure you are thinking that the fan is forcing the extra CFM into the motor right? Wrong. The air will actually just pass back between the blades and the housing, providing no benefit to power. Here is the thermodynamic method of finding the power a fan needs to move and slightly compress air (between 0.18-.22psi), but lets say it will magically create 5 psi boost for a "30hp" gain.

Fan air power = w * Ht
Fan air power = (216.9cfm)*(5psi)*(144in^3/1ft^2)(1hp/33000ftlb/min)
Fan air power = 4.7 Hp

4.7hp = 3.5kw of power.
Divide that by 13 volts (average operating battery voltage) and that leaves you with 270 amps of power. It leaves you to wonder if you would need a 270 amp inline fuse for this monster fan. I dunno about you, but I've yet to see anything with a 270 amp fuse in a car. Keep in mind, this is with 100% efficient motor as well, so it would actually have to consume more power than that. But wait....the factory rating on most alternators are around 80-90 amps! Hm, something seems wrong... seems this electric turbo is consuming 3 times as much as the alternator can put out. hmm... odd.

Long story short an electric turbo just won't create more power than what they consume to run them.

RAM AIR

Operating Conditions
Temperature = Tair = 20C = 293K
Atm. pressure = Pair = 14.7PSIA
Cpair = 1005J/KgK
K = 1.4
The Cpair and K are constants for air.

Case 1 @ 100Kmh (62Mph) = 27.78m/s

Calculating temperature of the ram air

Tramair = ((Vcar^2/2gc)/cp) + Tair
Tramair = (((27.78m/s)^2/2(1kgm/Ns^2)/1005J/kg) + 293K
Tramair = 293.4K

The temperature increased by 0.4K or 0.4C.

Pram = Pair (Tram/Tair)^(k/(k-1))
Pram = 14.7PSIA (293.4K/293K)^(1.4/(1.4-1))
Pram = 14.75PSIA - 14.7PSIA
Pram = 0.05PSIG (gauge pressure)

So as you can see driving 100kmh will only have a gain of 0.05 psi! now lets try for 200kmh.

Case 2 @ 200Kmh (124Mph) = 55.5m/s

Calculating temperature of the ram air

Tramair = ((Vcar^2/2gc)/cp) + Tair
Tramair = (((55.5m/s)^2/2(1kgm/Ns^2)/1005J/kg) + 293K
Tramair = 294.5K

The temperature increased by 1.5K or 1.5C.

Pram = Pair (Tram/Tair)^(k/(k-1))
Pram = 14.7PSIA (294.5K/293K)^(1.4/(1.4-1))
Pram = 14.97PSIA - 14.7PSIA
Pram = 0.27PSIG (gauge pressure)

By seeing how the velocity of the car increases the ram air effect...it is barely anything! I'll conclude by saying that in racing circles where a 1/100th of a second counts, it's worth it, but don't let Pontiac fool you into thinking it works on the street.

2. ### allbottledupSenior Member

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good info...ill be sure to pass this on to all those hot rodders who brag about ram air!

3. ### Iron 1Senior Member

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man, you need to come back to Oregon, so I can learn all this. hahahahhha

4. ### buzz1167Member

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Don't mean to be nitpicky, but I've taken a few physics classes and I'd just like to make some of the equations more correct (In case you want this to be a sticky or something.)
If yould just look over some of your units better it'd be a cleaner article...

Since you seem pretty educated I bet you can find your own errors by inspection.
Very good article though.
I think manily you would use a fan system not to create boost, but to eliminate vaccum. The equations seem correct and in all reality they should work, except that car conditions are not necessarily the best. Here are my only disputes:

For the Ram air: You assume ambient pressure at the ram air entrance, but car's run vaccum everywhere. Being that you will be running a little bit of vaccum under full load, it's much easier to eliminate this vaccum with the "ram air" effect, although as you've shown, it doesn't do much for the compression of air after ambient. People use cold air intakes and adjust the air flow over them with good results (as much as you can expect for free).

For the 'electric turbo': While I don't think that it is even a remotely good idea to try to get 5psi out of an electric fan, I don't think that lower numbers are so unachieveable. Also, I highly doubt that the efficiency of a 'straight blade' fan and the normal turbo type compressor fan can be compared in their own rights, they were made for different things. You would get better results with a turbo compressor on an electric fan. And realistically you could pull 50-70 or so amps from your car for just a few seconds (quarter mile) without alot of trouble, as long as you let your battery recouperate afterwords for minute. If you were really carried away about it you could add a capaciter and run off of it. Also I wouldn't scuff off all these new electric motors, they have plenty of very efficient brushless motors that put out lots of power.

With that said, could you increase power with one? yes! But only if it was really well made and designed. Even with that it's no turbo, and not a replacement for one. The people who tell you that you will increase your HP 15% with their 50\$ 'electric superchargers' are full of %&\$^. All this 'well design-ed-ness' would come at a price, probably about 600\$ for the parts (compressor housing and blade 300\$?, brushless electric 200-300\$, electronics to control motor 50-100\$). I know I wouldn't get one, just not worth it.

5. ### K2e2vinSenior Member

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do the ram air systems work better on motorcycles or something, maybe smaller motors? ive read that the new yamaha r1 makes 180hp with it, and like 150 without it. pretty good for a 1 liter motor.

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I know this is an old post...however that was taken from my site...

Welcome to ACE Engineering

With a couple clever tweaks, just thought I'd clear that up.

7. ### phyregod!!YTINASNI

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Ram air really does work, but it has to be set up correctly. There has to be a chamber for pressure to build and regulate.. The intake / motor / chamber / blah blah blah all have to be tuned for eachother.

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I totally agree, ducting and tapping into high pressure areas of bodywork does a lot more than a pipe out the front of the car and trying to pressurize a chamber to feed an engine. Some people think of ram air as "Hey I can stick my hand outside the window and feel how much pressure is on the front of my hand", when little do they realize it's the drag that contributes to the majority of the "pressure". A ram air setup has no drag force, so that quick calc. proves how much pressure is actually there.

Just would have been nice to be cited is all. Good thing a friend of mine wrote an app that reads sections of sites and then can google it and find the same string on other sites, that's how I came across this thread. Hopefully Ji Simmons (Loco Honkey) can come up with his own material next time or at least cite something if he is going to copy it. I just wanted to clear things up.

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