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Recently I got a bit bored with the daily grind so I decided to change things up. I normally operate in a pretty narrow band of SOC and speed. So now I've returned to my old route with a chuck of highway and high speed country roads. Well about half way through my tank, my sister up and bought a brand new Prius. She's asking all sorts of questions, and to most of them, my reply is "Get and EV-button and block heater". Well frustration got the best of her and she wanted to know what she could to without it. So I've limited my EBH and EV usage as well to try to find out what she can do with a stock Prius. This led me to B-gear.

The executive summary is basically, B-gear == DFCO. Decel Fuel Cut Off is old-hat to the Yaris and other drivers. Basically what DFCO does is cut off the fuel injectors when the car is in gear and no gas is being applied. To accomplish this, the transmission pulls the engine along keeping up engine RPM. Since the transmission is keeping the engine moving, the injectors can turn off giving you a decel without burning any fuel. This does come at the cost of rapid deceleration which translates to short glides requiring you to burn fuel to pulse back up to speed. But there are some definite situations where DFCO is better than decel-fuel-burn (DFB).

Now.. how does this translate to Prius Piloting? Well despite a pilots best efforts there are some situations where the engine just won't cut out. These usually have to do with the 5 stages of prius operation (http://www.techno-fandom.org/~hobbit/cars/five-stages.txt). These stages basically state that the car has to get warm before most good things happen, and that once warm, the user should try to force stage 5. Now, even when in stage 5, there is the infamous coolant pump 60 second cycle that will leave your engine running. So to summarize, here are the times when a Prius won't shut off even though you think it should:

When engine temp is below 165 F / 70c (in stage 1 or 2)
When entering your decel at speeds below 35 mph (in stage 3)
When engine temp first reaches operation range of 167f-175f / 75c-80c (coolant pump)


In all these cases B-gear will succeed where all else (except the EV-button) have failed. The one exception is that DFCO doesn't seem to come on until about 140 f / 60 c, but even when the engine is in these very cold ranges, b-gear seems to heat the engine much more than d-gear, so there is some advantage there.

Now, from what I've been able to find, B-gear will hold DFCO down to about 20 mph. In some cases, switching from B-gear to D-gear in the 20-30 mph range will get the engine to cut off, depending on when you entered your decel. The really wild thing about B-gear is that the Prius control systems seem to make their decision as to whether or not to cut the engine independent of what gear your in. So if you started a long decel in B-gear that would have triggered the engine to cut off, it will cut the engine as soon as you go from B-gear to D-gear it seems. One cool way to see this is to observe this in an EV-button enabled Prius. Enter EV mode at 30 mph and start a no-throttle decel. At 25 mph, switch to B-gear. The engine will start spinning, then at 20 mph it will shut back off.

So dust off the "LP" gauge on your SGII and look for "OPEN" which is your indication that the injectors are off. I've also verified this behavior with FCD in the Energy screen. Start an engine on zero-throttle decel in D-gear and recored the speed that you transition from 99.9 mpg to some number below 99.8. Now do the same thing with B-gear. If your reading "OPEN" on the "LP" gauge you should see 99.9 mpg hold all the way down to 20 mph in B-gear, where as in D-gear you will loose 99.9 mpg around 27 mph. What this really means is that you were holding about 100-200 mpg on the decel in D-gear, but you were holding 99,999.99 mpg in B-gear.

If anyone has a L/100km Prius it would be interesting to see what they record on there energy display in a D-gear decel versus a B-gear decel.

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OK... a bit more data... I've been testing things out a bit more and there is definitely a use case here for B-gear. What I've found is that there are three distinct readings that will signify DFCO in the Prius (two from the SG or CanView):
LP == "OPEN"
IGN == 5
FCD == 99.9 mpg
Poking around with this really did show that as soon as mpg dropped off of 99.9 in B-gear, IGN fell off of 5 and LP went to "CLSD". The interesting thing is that this only happens in B-gear. If I'm in DFCO and shift to D-gear, the injectors come back on, then if I switch back to B-gear they shut back off.

Another interesting thing I think you can hit DFCO in S1. So if you start your commute at the top of a big hill and can get up to 21 mph then drop to B-gear, you'll hold DFCO (I think) so long as your speed stays above 20ish mph. This is dependent on temp of course, I haven't quite ironed out what the low temp threshold for DFCO is.

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I was just thinking about how to attack a situation this winter.

4000' climb to mountain top, prius sits for 4-5 hours in below freezing temps.

How best to handle drive back down?

Starting traction battery temp below freezing on restart
SoC probably low after climb
Starting water temp ~20C?

Use heater to get battery temp up so EV is available asap?
Don't use heater to get water temp high?
Ride brakes to get SoC up then turn on B mode?
Use B mode immediately?

Sounds like B Mode might be helpful. If the variables you're researching aren't figured out by February I'll be able to add to the data.

What does B mode do when EV is engaged?

Well one thing I found out is that if the ride down is steep enough, you can do the whole thing in B. You won't burn a drop of fuel (most likely) and you will have the by-product of getting your block up to a manageable temp. I've seen 'B' trigger fuel cut as low as 30c so you may not need to get the block all that warm.

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I never much cared for the B gear. As soon as you come off the gas, your braking, even if you don't touch the brakes. In normal driving, you wouldn't be able to pull off a decent coast & might as well be one of those people who does nothing but gas/brake/gas/brake.

However, I don't have any large downhills. I was thinking of the last few times I drove through the mountains. In that case, I might see myself using "B", but being a front wheel drive, I wouldn't use it in bad weather. Nothing like braking with front wheels only to set you spinning out of control.