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Old 07-22-2010, 05:36 PM   #21 (permalink)
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Arragonis,
I think the plan would be for it to just assist, so the engine would still be turning with out this, it would just be putting out less toque than with the whole system.

Brucey,
Which motor & controller are you getting?

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Old 07-22-2010, 09:22 PM   #22 (permalink)
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Quote:
Originally Posted by brucey View Post
Shadetree: Watching the scangauge, at low speeds (<40 mph) it is very hard to get my automatic to go into fuel cut. It's also downright impossible to get it to fuel cut with the A/C on, but that's rare that I use it anyway. But when the engine does go into fuel cut, the electric motor is going to have to move the weight of the car AND turn over the (not running) engine it might be a bit much power to ask of it, which is why guessing from the scangauge, the motor will still spin up to accelerate but be using very little fuel. Even with the the throttle closed I still don't always see fuel cut unless I'm decelerating. I can go down a mountain and will gain speed and hover around 100 mpg. This is true from about 35 mph all the way up to 75 mph. Even though I'm gaining speed, the motor has to use more fuel to spin the motor up to X RPM. It might take some getting used to, but I do plan to have 2 independent throttles. This also has the nice ability that anyone can drive the car, I'll just have a switch that I can turn the E-drive off.
On the engine not going into fuel cut, it could simply be a matter of adjusting your throttle position sensor so it reads 0 when the throttle is closed.

I realize it'd have to move the vehicle + turn the engine, but even with just extending your coasts, it may be worth it.

Assuming your Subie wouldn't get too pissed about being turned into a 2wd, I like the idea mentioned of using your rear (or front, depending on ease of application) as the drive axle for just the electric motor. But your AWD transfer case would likely get pissed off about it. Where is your tranny located, under the hood, or under the floor? Ie, is it a front wheel drive car with auxillary prop shaft to the rear, or is it a rear wheel drive with transfer case sending power to the front? Only Subie I ever saw was an older Legacy, and it had a transverse mounted engine like a front wheel drive.
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Quote:
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 07-22-2010, 11:07 PM   #23 (permalink)
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regarding option 3 (turn the engine over with an electric motor and drive the car that way). Most cars already have a motor that turns the engine over, the starter, so it is easy to get an approximate idea how much power/efficiency that will cost.

If your starter takes 200 amps, at 12ish volts, thats a 3.2 hp penalty for turning over the engine at sub-idle speeds, say 500 rpm. If you need 3000 rpm then you need at least 6 times 3.2hp to keep it running, or 20 hp, probably a lot more.

This is totally wasted power from the battery to wheels perspective for option 3.

Compound with extra weight/cost of the electrical setup.

The other aspect is if you "boost" the stock engine during cruise you will move the engine farther from bsfc and reduce its efficiency.

Just my $0.02, maybe I'm wrong.
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Old 07-23-2010, 02:38 AM   #24 (permalink)
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Arragonis: Oops. Didn't see your question. Yeah, it would have to force the motor to turn over too. I think anyway. I'm yet to see anyone actually do it this way so it's all pretty much theories right now.

Jerry: AC-12/ whatever controller comes with it.

Shadetree: Yup! It's possible, although I don't know how much my trans would like it. They're notoriously picky. I guess I could swap with a FWD trans but they are hard to find.

Here's a picture of the subaru AWD Set up:



The engine and transmission are located as if they were in a RWD car. The front differential, transmission, and rear transfer case are all a single unit. Then a regular drive shaft and rear differential (R160) to the back.
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Old 07-23-2010, 06:54 AM   #25 (permalink)
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So why does the clutch pack burn up if you coast in neutral? Doesn't it run in an oil bath?
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Old 07-23-2010, 09:49 AM   #26 (permalink)
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Quote:
Originally Posted by Patrick View Post
So why does the clutch pack burn up if you coast in neutral? Doesn't it run in an oil bath?
I think its the centre diff or something which lunches itself if only one set of wheels (front or rear) is turning. Similar advice is never to tow a Subaru AWD with wheels on the ground - i.e. use a flatbed.

There is a 2WD version of the Imprezza so the gearbox could be swapped leaving the rear shafts in place and then drive that assembly seperately as shadetree tapped.

Starting to get expensive though.
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Old 07-23-2010, 10:24 AM   #27 (permalink)
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Quote:
Originally Posted by dcb View Post
If your starter takes 200 amps, at 12ish volts, thats a 3.2 hp penalty for turning over the engine at sub-idle speeds, say 500 rpm. If you need 3000 rpm then you need at least 6 times 3.2hp to keep it running, or 20 hp, probably a lot more.
But it's 200A to START a cold engine, not to keep a hot engine spinning at 500RPM. Maybe a better place to start with this math would be to measure engine braking on flat land in low gear, or to consider your gph in neutral at 2000 RPM.

Car-part.com tells me you can get a tranny for $150, but then you need the shift linkage, pedal set, maybe some wiring... not an easy job.

Iirc, I've read about people converting from AWD to FWD by welding shut the center differential. But this is probably all off-topic, right?
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Old 07-23-2010, 12:38 PM   #28 (permalink)
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Motor, controller, instruments ordered!

Yeah, I don't want to get into a lot of fabricating if I don't have to. And I have no idea what kind of increase this is going to show. My best guess is the car is going to drive like it's always going downhill. Which shows me around 100 mpg. As the speed gets higher, the rpms in the motor get higher too and it cancels each other out. But I'll have proper gearing at least!

Robert: I've heard the opposite, people weld their center diff and get a RWD drift car. The EA (80's) Subies might be possible to do something like this, since they have a real transfer case and selectable FWD/4WD/LO
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Old 07-23-2010, 12:47 PM   #29 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
But it's 200A to START a cold engine, not to keep a hot engine spinning at 500RPM. Maybe a better place to start with this math would be to measure engine braking on flat land in low gear, or to consider your gph in neutral at 2000 RPM.
Don't forget that a starter is a high-torque low-efficiency device as well; most of that 200 amps goes to heat, which is why you burn up the starter if you run it for more than a couple of minutes at a time.
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Old 07-23-2010, 01:37 PM   #30 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
...consider your gph in neutral at 2000 RPM...
It was offered as a swag, but I offer my gph readings from my 1.9 liter saturn warmed up in neutral if you would like to try to make sense out of them:
RPM GPH
1000 0.35
2000 0.65
3000 0.80
4000 1.02

Here's the bsfc chart if a correction is in order. Dunno what to use for load or even if it is constant. Maybe just go with 15.6Nm, though that doesn't explain the non-linearity of the readings (they should be more evenly spaced if the load is 15.6 looking at the chart at these rpm) Image:Saturn 99 1.9l dohc bsfc.jpg - EcoModder

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