[brpeterson]

Hi. I have found myself down-shifiting when I come into a stop-light so that I get more regenerative breaking. A little more info: When I press the brake, but not the clutch, when decelerating, I get a surge of regen. But then the regen goes to zero and I am still breaking and moving. If I down shift into second gear, re-engage the clutch, and then break again, I will get another surge of regen. I can even go 5-3-2, when decelerating from highway speed to put a bunch of juice in the battery. Does anybody else do this? Why don't I get regen until I come to a complete stop? Is it bad for the engine to do what seems to be a whole bunch of compression breaking? Please fill me in.

 

[Jim]

down shift regen

Hi br,

I don't downshift to get regen, I downshift because that's how I was taught to drive a stick shift. The car is going to regen when the engine is slowing the car down, whether going down hill in a low enough gear, or downshifting. The regen cuts out at the same point the auto stop kicks in, but when I downshift into first, I get regen again. I usually don't do that, though. I'd rather take the auto stop.

 

[Foxpaw]

I've considered this, but I'm not sure that it will improve mileage.. I only use regen to the bottom of the current gear, then coast to the light or whatever the reason for stopping may be. I believe that shifting like that will take the engine out of fuel cut mode, and thus although you get more regen, you end up burning fuel.

 

[Delta Flyer]

Good question, which is better, downshifting or light breaking?

 

[Holicow]

brpeterson:

I do it, but not all the time. Usually if it is a prolonged slow-down, like getting off the freeway. It doesn't hurt anything, but it is good to try to match the revs before you disengage the clutch, to minimize extra wear.

There is no regen braking below about, what 1500 RPM?, in any gear, so that's probably why you see the charge indication stop at some point.

 

[dan]

I think you have to evaluate the value of the extra electricity verses the value of the extra clutch wear.

Traditionally one downshifted during deceleration because 1) It allowed maximum control of the car (the stopping force was divided between the engine and the breaks) 2) It reduced break wear 3) It reduced break overheating. The effects are much more pronounced in vehicles of high mass (trucks).

The equation is much more complicated in the Insight because: 1) Light breaking gives you regen with little or no break wear. In other words more of the force is automatically shifted to the motor until you get down to the low RPMs 2 )IMHO it seems harder to match the engine/motor RPMs to the wheels, in this vehicle therefore causing slightly rougher downshifts therefore increasing clutch wear. 3) I suspect that the little goose of gas that one gives the engine to match RPMs could eat up the incremental increase in electricity generated. 4) If your battery is “topped off” already I am not sure how much more regen you really get.

Since part of the conventional wisdom of Insight driving is to avoid hard stops whenever possible, radical downshifting is generally unnecessary. I have wondered about how much more regen one gets from choosing lower gears. I suspect it is not much. My gear choice is usually based on what gear I want to wind up in. When coming to a complete stop it seldom seems worth it to shift more than twice.

I think it comes down to: new clutches are expensive compared to the price of gas and how far a gallon will take you in an Insight. How smooth are your downshifts?

 

[Tim Maddux]

Short answer: don't downshift.

http://www.insightcentral.net/KB/faq-ef ... html#regen

"Usually I leave the car in whatever gear I was cruising at, using only regenerative braking until the engine gets down to 1000 rpm. At that point I shift into neutral and only using the mechanical friction brakes to bring me the rest of the way to the stop."

If you downshift, you must take your foot off the brake before shifting into neutral and thence to the new gear. This is because entering the new gear with the brake pedal depressed will defeat regen.

I can usually regen brake from highway speeds in 5th gear all the way down. When regen brake stops, I depress the brake pedal a little harder to make up for the stop of regen, and I simultaneously drop into neutral. This almost always is at autostop / idle stop speeds. In this way, I consume no gas during the entire deceleration and I get regen for almost all of it.

 

[Aaron Cake]

I always downshift and use regen until about 18MPH, at which point I switch to the friction brakes. Battery always nearly full, and 70 MPG+ in the city. Can't argue with that, even though it specifically goes against the advice in the Encyclopedia.

 

[Armin]

more regen than you think

I'd also like to remind you that the recuperation continues, even after the green charge lights on the dash go off. So, as others said, just tap the brake ever so slightly to get max. recuperation and then push the clutch just before the engine stalls.

One caveat: once rpm drop below idle speed, there will be gas injected into the engine again. So maybe push the clutch a little before that...

 

[kapps]

When you downshift you're hurting your syncros (the things that allow you to shift without grinding gears). I've found that when slowing down from the highway, I just let it recharge in 5th gear and then bring it to autostop. When driving off the highway, I usually do a downshift from 5-3 and then autostop from there. My downshifts are always double-clutched and rev matched so that my transmission and clutch have very little wear. The same thing goes when going into corners - downshift 4-2 and you don't even have to touch the brakes due to the fact that regeneative braking in 2nd really slows you down. I just urge you to try double clutching so you will save your transmissions. There is nothing better than the feeling of a perfectly rev matched downshift anyway.

 

[dan]

Hi folks-

I would like to clarify for my own knowledge: As I understand it;

“Double clutching” is a term left over from the time before the invention and popularization of the synchromesh transmission. Early manual transmissions needed to be brought up to speed to match the high revs of the drive shaft before they would allow downshifting. The procedure was:

Clutch depressed > shift to N
Clutch released > Tap gas peddle to bring inner workings of transmission up to speed
Clutch depressed > downshift to desired gear
Clutch released > tool on down the road.

A true art. Very difficult to do well, failure to execute precisely led to gear grinding and possible failure to attain lower gear.

What I think we are all referring to here is “matching the revs” to avoid clutch wear.

Clutch depressed > tap the gas as you downshift
Clutch released > tool on down the road

Much easier than true double clutching...Right?

 

[Holicow]

Right.

 

[Insightful Trekker]

Hi,

There seems to be some small confusion as to clutch and synchro wear, fuel cutoff and efficient regenerative braking (recharging IMA pack) so I thought I'd throw in my 2 cents.

1. Most clutch wear happens on upshifting when more "power" is being routed to the transmission. VERY rapid clutch wear occurs if the surface of the friction disk is heated above a specific point. As you approach this point wear also increases. A lighter touch on the gas pedal during upshifting can make a significant difference in long term clutch life.

2. Synchro wear. The synchro gears act as mini clutches on each gear set to bring them up to sync speed before the locking ring engages the gear. You can usually hear a gear set "whine" in complaint when your forcing a sync at too high a speed difference. Downshifting too early for the lower gear selected does increase wear on these parts.

Only a long slowdown from a relatively high speed will it be beneficial for regenerative braking to downshift. The relative short term surge of current vs. a blip out of fuel or engine cutoff in a normal stop can be close to a break even energy savings. Your MPG indicator will be your best trainer.

3. Fuel cutoff is maintained with low throttle angle, high manifold vacuum and the engine above 1100 RPMish. Slip outside of these parameters while downshifting and you'll spend some of the liquid gold.

HTH!