When to use the B drive mode.

[preperationlaunch]

12 hours ago, TonyHSD said:

It is not a problem to let the car rolling in N up to the max allowed speed and then switch back to D without even notice anything, no shift shocks, no sound, no jerky action, nothing, it’s smooth and damage less experience., the whole point of having a reliable drive train is to have all possible scenarios thought by the engineers and solved mechanically and electronically before the car went into production.

There is no jerkiness or banging, because there is no resistance from the motors. Quite simply, all that happens in an ideal world is the car stops pushing. There is nothing to do so until it goes bang. You can design for mechanical limitations, you can design around user error, but no system is idiot proof.

The biggest problem comes if you're coasting and increasing speed above the EV threshold.

If you're driving with the EV light on, MG1 is spinning freely with little resistance, which stops the engine from being spun. To start the engine at speed, a current is applied to MG1 to slow it down, therefore, the engine speeds up and starts.

Toyota peg the safe limit for EV running (green light on the dash, NOT the button) on the Gen3 Yaris as around 45mph or so. The Auris is the same I believe.

So, let's do some maths, based on the drivetrain. These are using figures for a Prius, but they're broadly similar, as the cut-out speeds are very similar. The exact figures don't really matter, because, we're only using them to establish a limit.

The speed of MG2 is a function of the wheel speed. They're directly linked. 59.1*MPH is the equation I've seen for the HSD gearing. So at 45mph, MG2 is spinning at 2659rpm, whether or not the engine is running

We can work out the speed of MG1 by working backwards from that and the speed of the ICE. MG1 = 3.6 * ICE - 2.6 * MG2. 3.6 and 2.6 are figures for the ratios in the HSD.

So, at 45mph, with the ICE off, MG1 is spinning at -7000rpm (backwards). With the ICE on, at idle, that falls to about +2500rpm (being turned forwards by the ICE). We can safely work backwards from that to infer that around 7000rpm is the safe limit of the motor per Toyota.

Now, with that maths, you can figure out very quickly that if MPH continues to increase, the speed of MG1 will continue to increase. The car can do nothing to prevent this if the ICE is not running and the car is in neutral.

The maths will vary slightly, depending on iterations of the transaxle, but we can safely ascertain that the safe limit of MG1 is that at which EV mode stops working even when coasting. That magic 45ish mph figure. 

However, if you're doing 60mph, and the ICE isn't running, MG will end up spinning at -9200rpm. Well over the limit Toyota have deemed safe. Will it go bang immediately? Maybe not. Is it worth finding out what happens when a motor like that has a bearing failure from overspeed though? Almost certainly not.

Interestingly, as your speed and thus the speed of MG2 climbs, the minimum speed of the ICE has to increase, again to prevent damage to MG1 by spinning it too fast. 1200rpm is about the minimum safe speed for the ICE at 70. That rises to ~2500rpm at 100mph. You can see this in what speed the engine will coast at at various speeds in Hybrid Assistant.

12 hours ago, TonyHSD said:

when the car detects issue with the hybrid system either been from the electric motor, (clutch damper) or the engine problems the car will set itself into N at any speeds and will display hybrid system malfunction. Once you stop and restart the car if nothing mechanical has been broken you may be able to continue your journey. 
Thanks for all your experience shared. 👍

Limp mode is a common feature of any modern car, it's about picking lesser evils. If a hybrid fault is detected, Toyota have decided the absolute safest thing to do is to stop applying power to the motors. In exceptional circumstances, the risk of damage from an engine about to blow up is greater than that of overspeeding a motor. In an ideal world, the engine is likely still running if you're over the threshold speed, which reduces the impact.

The question I'd be interested to know the answer to, is what the car does at say, 100mph when shifted to neutral. Does it hold the revs at that 2500rpm for motor safety, or does it fall down to idle and overspeed the motor? I would not be surprised if it does the latter, especially after the saga Toyota had about a decade ago with 'unintended acceleration' incidents.