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Old 10-06-2008, 08:00 AM   #1
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EV Conversion - Power needed on Highway

I am converting a 94 Saturn SC2 to an electric vehicle. I am doing this on the cheap and I want to get the smallest motor I can that will handle the job. Electric motors are rated at continuous power. They can provide over 4 times that much power for short periods, the shorter the period the more power you can push through it.

Therefore I want to buy a motor rated at a continuous power rating for highway driving. My commute is 45 miles on a 55 MPH two-late highway most of the way with few hills. I want to know how powerful of a motor I need to cruise at or a little under the continuous rating of the motor. I want to go at least 50 or other drivers get very anoyed. 55 would be best. If they want to go faster than the speed limit they can pass me.

I will be doing a lot of aeromods on this car. I will remove the side mirrors and replace with cameras. I can put a very clean underbody tray on it since with an electric there is no exhaust and no oil or gas leaks to worry about. I will add some rear fender skirts as well. The SC2 has a low drag coeficient to begin with. I will also run LRR tires.

So, what kind of horsepower should I need to cruise at 55? I will be running through a manual transmission, so there are losses there to consider.

Also, what about the rear spoiler; will drag be better with or without it? And the car has pop-up headlights as well. I will be doing a lot of driving at night and I want to convert to HID anyway. Should I mount some new lights up front and leave the pop-ups down all of the time? I don't need to worry about blocking the grill since I don't need as much cooling as an ICE does.

Thanks for your expert advice.

-- Paul
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Old 10-06-2008, 08:19 AM   #2
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One question,

Why are you running it through the manual transmission? Electric motors make gobs of torque down low so you shouldn't need anything more than a single gear.

In fact, depending on the electric motor you end up choosing, a 1:1 straight to the wheels (no final drive only a diff) might be your best, most efficient setup.
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Old 10-06-2008, 08:30 AM   #3
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The transmission could be useful for reverse. I think you'll find that the only gears you'll need are reverse and 4th, if 4th is a 1:1, and 5th is overdrive. You will definately never need 1st or 2nd anymore.
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Old 10-06-2008, 08:51 AM   #4
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Reverse is one reason for the transmission. Another is the differential. Lastly most the the motors that are used for EVs are most efficient at higher RPMs, around 8K. Most of the running with be in 2nd gear. 1st will be used if a lot of tourque is desired from a stop but usually I will leave it in 2nd. I won't have a clutch, I will couple the motor right to the transmission (with a lovejoy coupler, most likely).
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Old 10-06-2008, 10:30 AM   #5
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Reverse is easy on an electric motor, even an AC one like you'll most likely be using.

One thing you will need to take into consideration. Is the transmission gear reduction in 2nd going to cause more of a loss than the motor running at a lower RPM.

Do you have any efficiency charts for the motors you are thinking about using?
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Old 10-06-2008, 10:47 AM   #6
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Are you driving in a hilly area? It may take twice as many hp to keep you at 50 mph depending on how hilly the highway is.

EV's would be best for great plains drivers...
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Old 10-06-2008, 12:16 PM   #7
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I was just thinking... is the donor car OBD II compatible? Use a scangauge to determine how many HP are required @ highway cruising speed. If the donor car isn't (94 is questionable - some cars were, some weren't) then do the SG experiment with a similar Saturn of 96 or newer vintage just to find out how much HP is required.

-Jay
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Old 10-06-2008, 12:44 PM   #8
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According to our Mustang Dyno information it's 9.6 HP at 50 mph.

So the 250cc engine in my motorcycle can get your car faster than 50.
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Old 10-06-2008, 12:51 PM   #9
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The car is not ODB II and the engine is in pieces anyway. That's why I got it cheap as a donor core.

The engine will be DC. AC controllers are still far too expensive.

The differential is still a huge reason for the transmission.

Most EV conversions are done like this, a DC motor dropped in a manual transmission. It is simple and works well.

The trip is mostly flat with a few rolling hills. Remember that when an EV is coasting down a hill it is using (nearly) Zero energy. There is no "idle" like with an ICE. I can recover most of the energy from small hills just in momentum.

I'm just looking for a ballpark figure. They estimate that most conversion EVs use 200-400 wh/mile, which is 15-30hp. I figure the SC2 is on the low end of that scale, say 15hp. If it is 15hp for a car that averages say 38 mpg then if I did enough careful driving to get the equivilent of 57 mpg would I be only using about 10hp? Is a 50% increase in mileage possible?
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Old 10-06-2008, 12:59 PM   #10
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In that car with some good aero mods (grille block, full belly pan) yes. Also, convert as many bulbs to LED as you can.

-Jay
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