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Old 02-22-2021, 11:42 PM   #1 (permalink)
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Question Insight Regen Braking vs Coast

2000 Honda Insight 5MT. I've got the auto-stop switch so I can EOC whenever.

Let's say I have a long coast to a light, a quarter mile or more.
Let's say there's no one in front of me and I'm in EOC going 40 mph or so.

I can do one of two things:
1 -- Coast until braking is needed and then apply brakes and stop the car.
2 -- Blip the throttle and shift into gear (this restarts the ICE) and then let go of the gas so the car regeneratively brakes. In this situation I usually get less than 10 seconds of regen. Depending on the situation and is there a grade or not I might get 15 to 20 amps of regen for those 5 to 10 seconds.

Note:
The Insight restarts the ICE with the IMA motor, but I HAVE to hit the gas pedal as well or else I'll wreck my transmission. I usually choose third gear. I might need to get up to 3000 RPM or so to match the engine so I can safely release the clutch.

So here's my question:
During that rev-up of the engine am I using more gasoline energy than I am going to get back by regenerating while coming to a stop?

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Old 02-23-2021, 12:44 AM   #2 (permalink)
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I tried to do some napkin math, but realized I'm technically missing some details. Does that regen of 15-20A have 12v or pack voltage? I'd assume it would be pack voltage, which I don't know. Google says 144V.

20A*144V = 2,880W of power re-generated.

Gallon of gas = 33.7 kW's of power. 33.7 x .4 (efficiency of ICE) = 13,480

So the amount regenerated converted to a gasoline equivalent would be .21 gal

If it is based on the 12v system you can just reduce by a factor of ten and be close. So ~.02 gallons equivalent recovered. So as long as you use less than .02 gallons then I think option 2 would be more efficient.

Does my thought process make sense? That's pretty much how I would consider the problem, so if anyone could double check the math and thought process I'd appreciate it.
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Old 02-23-2021, 03:20 AM   #3 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
20A*144V = 2,880W of power re-generated.

Gallon of gas = 33.7 kW's of power. 33.7 x .4 (efficiency of ICE) = 13,480
Need to equate time in to your calculations. The gallon of gas is already in kWh.

10s = 0.002777778 of an hour, so 0.00277778*2880 = 8Wh = 0.008kWh.

8Wh isn't insignificant, about the power of a single 18650. It's not a lot either.

OP: The only gas you would be using is the initial blip to rev up the engine, after that it would be in FCO until you get down to 1500rpm. To maximize the time between and the power generated, you could go for a lower gear, or shift again. Regen seems to stop about the same time as FCO cuts out.

You could continue the math...my Geo Metro used about 0.15 GPH to idle. Buuuuut....since its just the blip and the last moments when you get down below 1500rpm, I'd say it's negligible. Negligible vs "not insignificant".

When I got my IMA system working, at last, my gas mileage got worse at first because of all the times it wanted to go in to regen when I'd normally not-quite-coast in gear (the engine drag is very minor in higher gears)...I had to retrain myself to hold the throttle ever-so-slightly open(Keeping it >150mpg) unless I wanted to shift in to neutral (at which point I would pick up too much speed). Was rather annoying.

Point of that story is that regen does not make up enough for the losses along the way, so I generally only let it regen in place of braking, and since it doesn't get a lot of that, I try to make sure I use it every reasonable opportunity, so the engine doesn't have to "refill" the battery pack while driving. So far so good...it's almost always nearly full; I've only once had it get down to half where it forcibly starts charging while driving.

So, yes...I'd say it's worth it any time you're going to forcibly slow down for more than a couple of seconds.

Last edited by Stubby79; 02-23-2021 at 04:11 AM..
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Old 02-23-2021, 04:35 PM   #4 (permalink)
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Quote:
Originally Posted by Natalya View Post
Note:
The Insight restarts the ICE with the IMA motor, but I HAVE to hit the gas pedal as well or else I'll wreck my transmission. I usually choose third gear. I might need to get up to 3000 RPM or so to match the engine so I can safely release the clutch.
One other option .. You could also combine FAS with IMA motor (MIMA or IMA-C&C) to rev up the ICE , without using any gasoline for the power/energy to do the ICE reving .. keeping zero gasoline flow the entire rev-up time .. once at desired RPM and in desired gear enter regen as desired.
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Old 02-23-2021, 07:46 PM   #5 (permalink)
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Quote:
Originally Posted by Natalya View Post
Let's say there's no one in front of me...
My question is if there's anyone behind you. The slower the speed you end up at from pure coasting the better.

But my guess is that any needed blip would cause a negligible effect in fuel mileage.
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Old 05-09-2021, 01:43 PM   #6 (permalink)
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Quote:
Originally Posted by Stubby79 View Post
When I got my IMA system working, at last, my gas mileage got worse at first because of all the times it wanted to go in to regen when I'd normally not-quite-coast in gear (the engine drag is very minor in higher gears)...I had to retrain myself to hold the throttle ever-so-slightly open(Keeping it >150mpg) unless I wanted to shift in to neutral (at which point I would pick up too much speed). Was rather annoying.
You can avoid this problem entirely by installing a clutch switch. I often use mine on the highway to avoid Regen+DFCO drag and just get DFCO drag. It also prevents hidden background charging of the IMA battery.

An OBDIIC&C gauge will let you see background charge.

EFAS would let you easily engine-off-coast at highway speeds if you really wanted to turn your car into the Space Shuttle, but the clutch switch and OBDIIC&C will help a lot with the issue you're mentioning here.


Quote:
Originally Posted by IamIan View Post
One other option .. You could also combine FAS with IMA motor (MIMA or IMA-C&C) to rev up the ICE , without using any gasoline for the power/energy to do the ICE reving .. keeping zero gasoline flow the entire rev-up time .. once at desired RPM and in desired gear enter regen as desired.
I didn't realize this could be done!! I guess I finally have a compelling reason to install MIMA. I don't have a plug-and-play version, I've got like a box full of extras to make a kit for it that Mike sold me at a discount. I never got around to installing it because it looked like a big job.

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