Cruise Control not available when drive selector is in B

Sure ... while you're thinking about it, remember the mechanical disk brakes are rather primitive devices; they are making fierce heat by clamping down pads on a gnarly rotor covered with road grit, cooled only by whatever airflow is going over them. The engine is moving precision-polished surfaces in a bath of filtered oil with an engineered cooling system taking away heat, and doing that under less stress than it is under when pulling the car, which is probably what you really should avoid if that stress is concerning.

Lastly I found this:

That's almost right but not quite ... whatever the direction of power flow, the power-split device has a fixed ratio of how much torque follows the mechanical path and how much the electrical path—the split of power can vary, being the product of torque ✕ rpm, according to the relative speeds of wheels, MG1, and engine, but there has to be power taking the electrical path. When there isn't, none passes mechanically either; MG1 just freewheels.

-Chap

 

But only use "B" if the battery is absolutely full - it's not using the friction brakes, but the regeneration braking. I've only had it once when the battery was full, (top bar) going down a mountain range, and I could hear the Engine revving the same as in "B" mode while I was just using the foot brake. Not entirely sure what it was doing.

 

No. Use it from the top of any descent that will 'fill' the battery, that way both the friction brakes and the battery stay cooler longer, both a very good thing!

 

OK - I wrongly assumed that when the Manual says:

"If the vehicle is driven continuously in the B position, fuel efficiency will
become low. Usually, select the D position."

that it wasn't charging the battery - but it's only giving poor fuel economy. I think I'll stick with "D".

 

It does charge the battery and still uses Friction brakes if needed, it just adds engine braking to the mix.

If the hill is not steep enough to make you not worry for gas mileage as much as living through the day, use D. Here is an example where useing B would be a good plan: https://www.oregon.gov/ODOT/MCT/docs/emigranthill.pdf

Chinook Pass down to Yakima is another example.

 

I don't know if you've noticed but the car will automatically use engine braking even when it in D if you are braking and the battery fills up. So it is only under somewhat extreme conditions that it is useful to manually put the car in B.

Yesterday I was coming down a local hill from 1800' down to ~400' using DRCC and after the battery filled the system used engine braking with it going up to about 4000rpm. The same thing occurs if using manual braking.

kevin

 

Kevin the only difference is that if you manually choose B at the top, the battery and the friction brakes stay cooler from the top, waiting for the automatic engine braking after 600 foot vertical drop just heats them more. You are right in that there is an automatic engine braking function, it just can't guess that this is a big hill and start from the top. It reacts, not plans.

There is some hint that high heat is bad for the traction battery, high heat is definitely bad for friction brakes.
Brake fade - Wikipedia

 

Not quite true - another way for there to be no power transferred electrically is if MG1 is providing torque to balance the engine but is rotating at zero RPM. Toyota has designed the gearing for this to occur at cruising speeds on a level road. This is because for best efficiency it is desirable to minimize the power going through the electrical path as it is only about 80% efficient whereas the mechanical paths about 97% efficient.

kevin

 

Well, ok, that is a corner case of power being torque ✕ rpm. And also the case where MG1's efficiency as a motor drops to exactly zero (because "providing torque to balance" requires current in the windings, becoming heat without any rotor motion produced). I suppose the engineers could have found a sweet spot where the resulting overall powertrain efficiency benefits nevertheless.

Is this a Gen 4 innovation? Did you read about it in the New Car Features manual or an engineering paper?

-Chap

 

This sweet spot has been a fundamental aspect of the split-hybrid approach since power-split device was devised in the 70's.

I don't think I read about it in any specific place although here is a paper that describes the modeling of such a system including the quote: " For highway driving, similar to a parallel hybrid, power flow is mostly through the mechanical path to improve fuel economy."

http://www-personal.umich.edu/~hpeng/Liu TCST split paper.pdf

You are correct that the power in MG1 is not zero at the balance point and in fact MG1 will be operating close to maximum torque much of the time so the resistance losses will be similar to those at full output except when being allowed to spin free.

The operating line of the ICE is such that when it is running it is at 80% or so of maximum torque - the engine speed is changed to control its power. As a result MG1 has to operate at close to maximum torque as well. This is an example from a 2004 paper showing the operating line (i.e. gen 2 but the principles haven't changed)

https://www.dropbox.com/s/5easclkh88tl66j/BSFC_Prius_SAE_2004.gif?raw=1

I used to monitor MG1 RPM on my Gen2 Prius and it very rarely stayed at any one speed. Even when I tried to keep it in the sweet spot MG1 would dance back and forth from +1000RPM to -1000RPM. I haven't monitored it in my gen 4 yet - it's probably even more erratic as it can go anywhere in the range +/-17,000 RPM now!

kevin

 

That's been my experience too. So I think I'll stick to saying that except for isolated moments when MG1 rpm happens to be zero, there is always power on the electrical path when there is power on the mechanical path. I've never seen anything that looked like the Prius control laws were trying to hold MG1 stationary for any length of time, and I don't think I've read of them introducing such a strategy.

-Chap

 

The control laws do tend to result in a low speed of MG1 as that is when the minimum loss occurs in the transaxle although it is a fairly shallow minimum - e.g. if 10% of the system output power goes through the electrical path then only 2% will be lost in that path (assuming 80% efficiency) compared to 3-5% through the mechanical path. During low speed acceleration or when hill climbing most of the power could go through the electrical path and 20% of the power could be lost that way.

The other thing that forces the speed of MG1 though is keeping the ICE at its optimum speed for the particular power demand and road speed. The choice of the gear ratios selected at design time will be to attempt to keep the engine RPM and the percentage of power through the mechanical path at the optimum for a large percentage of the operating cycle while still meeting all the other requirements. For the Prius this is done with a single gear setting but in the the higher performance cars such as the Lexus hybrids Toyota had to resort to selectable gear ratios as a single one was not adequate.

kevin