Cruise control and fuel economy

Sounds like cruise is employing B mode some. That could be good, especially on extremely long down hill runs.

 

battery management is more important than gas mileage

 

That's evil, when the grade is not steep enough to require braking. That behavior wastes time, wastes fuel, and wastes wear 'n tear. I don't think cruise control has any business taking over braking (including aggressive regenerative braking, as well as "B" braking). If a downhill is so steep, long, and dangerous that braking is actually needed, the driver should wake up and take control.

 

I've used cruise heading downhill, but I did feel two little bumps growing on my forehead.

 

I love the DRCC and use it when on the open road.

 

I only use cruise control when the road is flat, because I like to reduce the amount of power going uphill and then increase going downhill while cruise control would try to maintain a relatively steady speed.

 

Always

 

The Prius does not have an "automatic tranmission" in the sense any other car does. It is a single speed car and does not shift. So no need to be automatic. Power is redirected to differing tasks via the power split unit. While the car may feel different there is no loss of control. If anything, there is maximum control, as the car takes care of itself to deliver the right amount of power and efficiency.

 

Yeah, regardless of vagaries of what's in the transaxle, it behaves like a Continuously Variable Transmission, ie: an automatic without distinct shift points.

 

Actually, one of the only times I used cruise control on my Gen 2 was to keep the car under 75 mph on the long straight downgrade north of Mojave, CA on State Route 14. The road is straight, boring, and uncrowded with a mild downgrade (maybe 3% or so). I got tired hitting the brake to control the speed, so I just let it sit in Cruise Control for several miles.

 

Here is the observation I have seen via the instantaneous MPG gauge. (didn't have external scan gauge tool attached)

Going down hill when in CC the engine will spin to act in braking mode, and not fuel is consumed (instantaneous MPGs are pegged at the top, 100). This appears to be done to keep the car at the set speed. This is very similar to the B mode on the shifter. It is also done when the battery indicator reaches full or nearly full.

So I say it is save and economical to leave it in CC when going down hill. (You may encounter grades too steep and despite the engine spinning VERY fast you will still gain speed, then you will have to take over and brake.)

 

I don't want cruise engaging friction brakes while going downhill, but a couple of my cars won't downshift to hold speed going downhill and it irritates the crap out of me. All cars should downshift or use regenerative braking or the b mode when required to maintain speed. If it can prevent you from having to get on your friction brakes it is much safer.

 

The same as needlessly braking down hills to the same extent with conventional friction brakes (aside from non-fuel-economy implications, which clearly are not the same).

 

When in the mountains I've taken to cancelling cruise when I know the next uphill is getting relatively close and there's enough room in traffic to do it. I then let the car build up speed as it coasts up to whatever I feel comfortable with so the momentum carries me farther up the next grade before the ICE has to fire up to make it the rest of the way. I have nothing to say this improves gas mileage, but it kind of has to, right? I don't really do this in say Wyoming on I-25 because the car doesn't really pick up any speed freewheeling on the hills there, but I do it a lot in the mountains of Colorado.

It seems to me that B-mode just free spins the ICE as fast as it can but does not feed it any fuel. The oil pump is providing pressure at that point, I'm assuming that the coolant is circulating, just no gasoline being fed into the cylinders. This might cause extra wear, but I expect it to be minimal, if even measurable.