On-Line Toyota "Fail-safe" Ad--Claiming more than they prove?

The brakes are NOT drive by wire. The brake will ALWAYS override the gas in every auto. The electronics are beside the point.

-Brad

 

Yes, the Prius CVT(PSD) works by always having a mechanical connection between the ICE, MG1, MG2 and the drive axles.

 

Sorry, but for the Toyota HSD system, the brakes ARE DBW.

The very instant you apply the brakes, at least lightly or moderately, the brake fluid pressure is blocked from reaching the caliper pistons. That remains true for just a few microseconds until the skid control computer and the HSD system control computer can decide which will supply braking forces and how much for each.

The skid control computer has EXCLUSIVE control of frictional braking while the HSD system control computer has control of regen braking.

 

If you shut off the power the engine is NOT connected to the wheels. It spins the gearing, yes, but without power to the MGs that power cannot get to the wheels. The MGs have to provide resistance.

As for the brakes, except for the ABS (just like any modern car) the brakes don't need the electronics. In fact in a panic application of the brake pedal the brakes go directly to hydraulic brakes, but again, through the ABS system. I suspect that if the ABS fails the brakes will then just be non-ABS hydraulic brakes. There is no electronic switching required for panic braking to go direct. It's mechanical.

I would interpret this as fail safe. If we think the electronics is more failure prone than the mechanical parts, which is just not true. The electronics is the most reliable part of the Prius.

At any rate, most of this is just FUD. The brakes do work a little differently than other old technology cars. Adapt to it. They still work just fine.

 

These MGs have permanent magnet rotors, so they will NOT COAST without generating back EMF. Lots of back EMF, enough to blow all of the inverter drive electronics if you are rear ended at low speed.

 

Try flooring the brake pedal. It goes directly to brake hydraulics overriding all electronics.

Also try shutting down the car completely. Brakes still work, you just need to press the brake pedal harder as there will be no electric braking and no power assisted braking, but braking will be, which means that brake pedal is connected with brake hydraulics. :cheer2:

 

"so they will NOT COAST without generating back EMF. Lots of back EMF, enough to blow all of the inverter drive electronics if you are rear ended at low speed. "

Is there documentation of this you can point to for our review? Far as I know, "coasting" means the MG's are not doing anything, as they are awaiting instructions to either regen or accelerate. Or did you mean to say that in regen (not while coasting), getting rear ending causes this? Do EMF's really matter the instant you get rear ended? Your car's going to the shop anyways.

 

I doubt very much being rear-ended will blow the inverter. You have to get the car up to a pretty good speed. I read somewhere that even 15 MPH is safe. To get the car above that speed it would probably be totalled in a rear end collision. At any rate, most rear-end collisions are "on the road", meaning the car is on. I'd bet the electronics could take the back EMF with the power on better than with it off. What happens if you release the throttle at 60 MPH, for example?

This discussion is about "fail safe", meaning if the electronics fails it does so into a safe condition. Dead inverters are pretty safe.

Note that it's common to have Prius pushed around shops in "N" with the power off. So at that speed nothing "bad" happens.

 

Check out page 10: http://john1701a.com/prius/documents/Prius_Power-Split-Device_details.pdf

The Prius PSD CAN coast without frying any inverter electronics.

We often DO coast, when pulse-and-gliding at speeds below 41MPH. And I haven't heard of ONE single instance of anybody having an inverter failure from pulse-and-gliding a Prius yet. Nor any instances of any Prii having an inverter failure from a low-speed bump from behind.

 

I think there may be varying definitions for "coasting" in play in this thread.

Some mean by "coasting" using the pulse and glide method where the gas pedal is set to where there is no energy flowing anywhere, and the car is being carried forward by its momentum. I think of that as gliding, but some may think it the same as "coasting."

Others mean by "coasting" the act of taking one's foot completely off the gas pedal without stepping on the brake -- and the car will slow down, much like a conventional car does from engine braking.

Are some of the posts in this thread using different definitions?

 

"Coasting" as we Prius drivers know it would be as you say-- Setting the gas pedal to where there is no energy flowing anywhere when you are pulsing and gliding.

Taking your foot completely off the gas pedal doesn't coast, because there is MG2 regenerative braking (battery recharging) occurring when you do that above 6mph.

 

It separates the flow of power from the 3 power sources to the wheels, but yes, even in N, if you turn the wheels, gears go around in the Transaxle. (In all cars, some gears still go around in the transmission)

 

And just to keep the statements in this thread close to reality, even when in "N", off or not, the Prius MGs will turn when the car is moving. They WILL generate back EMF (a generator works just as well with no load as with a load at generating a voltage). The back EMF may not drive a current, but the diodes/transistors in the inverter still see the voltage. There are no relays to disconnect the MGs from the inverter. So IF the voltage was high enough it -could- stress the electronics.

But as I said above, the electronics sees the back EMF at all times, for example when gliding or coasting (same thing). If it could blow the inverter it would do so when you go 95 MPH and let off the pedal. It doesn't, so I don't think this is an issue.

Again, the biggest danger to the car and the passengers is the nut behind the wheel.

Drive carefully!

 

I think you really need to read this ...
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_H*