Why does the engine wind out when in B mode?

Don't guess or do a 'sounds like', put a tach on it (OBDII engine monitor, such as ScanGauge-II or Torque, or any other). Mine doesn't go above about 4600 RPM, somewhat below Gen3's redline. Your Gen2 has a slightly lower redline.

The force is not extreme, it is significantly less than the acceleration torque when providing forward thrust.

 

I believe the redline of a Gen2 is 5k rpm, approx.

Having the engine make a lot of noise is perfectly normal for compression braking. It's the exact same noise when you downshift in a conventional car, where the engine is being spun by your wheels, as opposed to vice versa.

There is no regenerative braking in B mode. The motors are removed from the picture. It's purely the engine and friction brakes. It's thus inefficient and not recommended for normal braking.

 

Mine is closer to shifting down from 5th or 6th to 3rd. Yes, both of my household's other cars are still manual transmission versions.

I have inadvertently shifted lower and started letting the clutch out on prior MT cars, quickly re-applying the clutch when the error becomes blatantly obvious. Prius B mode is not like that.

Both MG1 and MG2 are always involved in B mode, usually to spin up the ICE to create drag. And also to dump charge into the HV battery too, if it is not yet full. If MG1 & 2 are disabled, then the car is in Neutral and both battery charging and engine compression braking become impossible, the car turns into a glider.

There is a low speed (<25 mph?), low drag (shallow hill descents) corner case where the ICE can remain stationary, and nearly all drag comes from MG1 charging the battery, slowly. MG2 spins as just the right speed to keep the ICE stationary. But when the battery gets full, it quits charging and MG2 changes speed to start driving the ICE at low RPM for some engine drag.

Friction braking comes from the driver's foot only. The car doesn't apply them, neither in B or CC modes nor any other mode.

 

No, there's nothing going on that's 'opposing' the engine force and direction; as described in #16, the kinetic energy from your descent is used to spin the engine in its normal direction of spin.

The only 'opposing' going on is that the engine, which is not having any fuel injected at that time, so is not producing any power of its own, is actually pretty hard to spin. You could say its friction, as well as the work of uselessly pumping air through the cylinders, is 'opposing' the spin being applied to it by the transmission.

And by attaching a tach, you will see that it is nowhere near 7,000, no matter what it sounds like to you. The engine is spec'd for around 5,000 in normal operation (that's its point of maximum power output, when it's producing power), and that sounds a mite whiny but it can do that all day without a sweat. The HV computer knows exactly how fast the engine is winding at every instant (and by every instant, I mean the computer is able, if there is a misfire, to tell you which cylinder, by tracking the rotational speed changes within a single revolution). It will never spin the engine to a speed you would need to be concerned about.

For exactly that reason, you never really need a tach, unless you just find it interesting, or to reassure yourself that the car really does know what it's doing.

 

The ECUs should be able to figure out how much current the battery can take. Battery charging is intentionally slowed if the battery is hot, and also tapered down as it approaches full. The ICE spins up before the battery fills, to absorb the difference.

I do suggest putting on an OBDII engine monitor and monitoring the RPM. If the engine noise is excessive, I suspect it is more from an old worn engine than from excessive RPM for a good young engine. The B mode control seems unlikely to apply a reduced RPM to compensate for engine age and wear.

 

5k is quoted in the Gen 2 New Car Features manual as the point of best power output. The actual redline is not mentioned in the manual (6 to 7k might be a reasonable guess), because the control system will never take it there anyway; power output tapers off beyond the peak at 5k so there'd be no point.

Watching a tach will be valuable for you.

 

The increase can seem to be sudden, especially if this B-mode spinup is new to you. It was only after watching many repeats that I noticed the battery tapering off near the top, and the RPM starting to ramp up.

We really want to know what RPM is displayed. Despite very many readers here commenting on the loud noise and believing the RPMs were getting too high, and some even switching back to D and applying friction brakes to 'save the engine', nobody here with an actual tach display has reported seeing excessive RPMs actually indicated. And very many of us do use tach displays.

 

MG1 is not involved in regenerative braking. It only charges the battery if needed when turned by the engine.

 

I definitely stand corrected. According to Hobbit's site, B may actually temporarily increase the regenerative braking current.

 

I think it sounds so much louder because of the lack of soundproofing in the Prius. In B mode MG2 acts as a generator, sending the current it produces to MG1. MG1 spins the engine using it as an engine brake. Just like when you downshift a regular car and take you foot off the gas going downhill.

The OP doesn’t state it in this thread but his battery has a P0A80 code. That might be why his engine speeds up so quickly. The car is trying to protect what little battery it has left, or not stress it so it explodes.

P0A80 and weird events (sped up on it&#039;s own.) | PriusChat