The "Infamous" Breaking Gear

1) Why would you be wasting energy in B mode on your driveway? Why not use the brake pedal and get all Regenerative braking instead of some engine braking?

2) I spend most of my time on my driveway when my engine is cold, how sure are you that you are not watching the Prius try to warm up?

 

I would recommend B when there was over 600 feet of drop, but it is not straight down!
And if it is dangerous, safety wins over MPG, don't you think? Be safe and get good gas mileage later.

 

I always use B on any dirt road. It's like using a lower gear. There is only so much regen the battery will take and then the car will pretty much do B any way. Personally I'd rather know I am in control.

 

Don't use B in this situation unless you are starting with a full charge (all green). B just wastes gas.

As to your question, it depends on whether the engine is running. The Prius often needs to run the ICE to make heat, even when it is unnecessary for propulsion. At low speed you don't cover much ground, so the MPG is low when the engine is making heat.

Tom

 

I am pretending that Northern Michigan is closer than Mississippi. (I will actually be in Bellingham WA June 10 to 15th, but that is only close compared to Michigan)

 

I have a similar situation here with my driveway. When I start down the driveway, the engine is in warm up mode, so it runs regardless until it gets up to temperature. Also, since I am going less than 8 mph, there is no regen braking possible, so friction brakes take over. Hence the mpg is not good, but then it is a small distance so not a concern.

It's getting UP the driveway that makes for some serious low mpg readings. But much to my pleasure, it can make it up, which was a real question on the day I drove it home.

 

Hi CyperPrius, that's a 1 in 5 gradient, so yeah pretty steep.

Since you're only getting 12 MPG at idle then I assume you're only doing around 10 MPH. In this case the distinction between using "D" or "B" is pretty much moot. Apart from needing to push the brake pedal a little harder in "D" there is no actual difference at such low speed. At higher speed however (can't remember the minimum off hand) "B" mode will cause the engine to always turn. So even though there may be no fuel injection once the warm-up phase has completed, "B" mode will regenerate slightly less efficiently due to energy lost in turning the engine.

BTW. If you want to you can leave the Prius in "P" and let it warm up for about 50 seconds. It doesn't really waste the gasoline, as it uses it to gently pre-charge the battery. It probably wont change the overall fuel efficiency either way, but at least you can pose getting 100MPG down your driveway.

 

Would the regen-energy recovered picture change significantly if cyberprius had an
EV mode switch?

The sequence of events would be:
Put the car in Ready.
Switch on EV mode to surpress ICE On. (Note: Temps are neither too hot or too cold.)
Negotiate the driveway -- preferrably at more than 8 MPH where it is said that
regen kicks out.

 

Huh? It is not much like using a lower gear. Going up hill, for example B is exactly like D. Going down hill, it uses engine braking instead of regen braking, but you only want to do that when you might 'run out' of regen braking. (that is, fill the battery) I can't imagine what difference the road surface makes. Nor how B is more 'in control' than D.

 

Tom, I thought that while in braking (B) mode on a downhill without the gas pedal being pressed the engine merely spun and the pistons compress air without any gas being injected to the cylinders . When the gas pedal is pressed, either uphill or downhill, while in B mode the engine preforms exactly as in D mode. If that is true then how can the engine waste gas as you say while in B mode? What am I missing?

 

You aren't wasting gas per se, but wasting energy which could otherwise be going into your battery. Thus you will need to use gas instead of battery at some later point.

 

OK that makes sense - but given the choice of potentially going out of control on a steep downgrade , experiencing brake fade from constant use, or saving a few ounces of gas.... well .....

 

No one has been able to explain where is 'loss of control' comes from. Brake fade would only occur if you are using your mechanical brakes, as opposed to either your regen brakes or your engine brakes. If your batteries aren't full, 'D' is NOT more likely to result in any loss of control than 'B'. If your batteries ARE full, that is exactly the time when you DO want to be using 'B', as has been mentioned.

 

I'd expand this to say that on hills where the battery is going to get filled, there is no reason to wait until it actually is filled. I.e. on those 7 mile, 7% downgrades, I always enter 'B' mode at the top to slow the filling of the battery and minimize the time the ICE howls like a jet engine.

On the moderate 400 foot drop in one direction from my house, the battery usually doesn't fill (it depends on the vagaries of starting SOC / outside temperature / engine warmup cycle), so I never use 'B' there. For slightly taller hills, optimizing this matter becomes a juggling act between B and D and some footwork on the 'go' pedal that won't be worth the effort for many drivers, and missing the optimum won't lose enough energy or heat up the brakes enough to be of any concern. But somewhere between 600 and 1000 vertical feet, steep hills become tall enough to always use B from the very top.

 

I can't imagine what difference the road surface makes. Nor how B is more 'in control' than D.[/quote]

Even ABS can lock up on loose dirt, muddy or icy surfaces. Using B to decelerate and avoiding unnecessary touches on the brake means that you have just a little more control on these very uncertain surfaces.

 

This is most definitely due to being in Stage I.