HSD implementation of regenerative braking

That's a simple question to answer. The HSD in the Prius mimics an ordinary transmission. With your foot off of the accelerator pedal, the system applies a small amount of regenerative braking to simulate engine drag in a normal car. This is done for safety and comfort, making the Prius drive like any normal car. It is not a requirement of the HSD.

This is also the reason the Prius creeps when you lift your foot off of the brake in D. It is simulating the creep of a normal automatic transmission.

Obviously Toyota felt it was better to make the controls behave in a normal and recognized manner than to go for all out efficiency.

Tom

 

No, it's not that. The battery is completely protected by the charging algorithms in the controller. It doesn't need to play games with the accelerator response. My original answer is all there is to it; nothing more, nothing less.

Tom

 

Hi Tim Walsh,

I find it interesting that you consider the zero pedal regenaration inthe Prius to be slowing at a rate much greater than a standard car. Because that "feature" was programmed into the Prius to make driviers of previous cars confortable with the Prius. Rather than have it coast along without and drive-train drag.

You can read all about it in "Prius the Car that Shook the World" novella of the Prius developement. Google that name and you will find it online somewhere.

 

I believe the NHTSA are involved as well. They're the reason behind pressing the brake pedal to both make the car READY, and to put it into gear. They wanted to keep EVs and hybrids similar to how automatic transmissions work, and they want the driver to signal he's in control before the transmission engages.

This makes the car a bit unfamiliar for many UK drivers, who mostly have come from manual cars - the automatic transmission hasn't taken off to any great extent, the manual being more efficient, more reliable, less costly, and giving a greater feeling of control. Here there is no requirement to press the brakes before starting or engaging a gear from neutral, and manual transmissions do not creep. If the clutch is engaged, slowing down below a certain level will cause the engine to stall; with the clutch fully disengaged the plates do not touch and the wheels get no power from the engine - indeed it will roll back on a hill. A common complaint among new Prius owners is that the creep is too powerful and makes manoeuvring at low speed quite difficult, because it's always pulling against the friction brakes (which stick, making the problem worse).

The US and Canada appear to have a self-certification process to meet NHTSA regulations; the regulations are only checked if a complaint is raised or a vehicle is involved in a higher-than-expected number of incidents. This allows a little more creativity than countries operating under UN ECE regulations, where cars must receive a type approval before going on sale. I suspect this is why the fuel bladder and the coolant thermos are only present in North American cars; NHTSA hasn't specifically ruled either way, but they might well not have passed type approvals in Japan (now a member of the World Forum for Harmonization of Vehicle Regulations) or Europe.

 

Hobbit has a link to the FMVSS that covers the creep force.

FR Doc 03-12051

It covers the reasoning and requirement for the creep force, but doesn't spell out the need for regenerative "drag". There is a requirement for a transmission braking force that has a lower final ratio in at least one forward gear than the top gear ratio; this is manifested by "B" mode in the Prius.

 

Being sued for causing accidents is not something Toyota wants. Look at the lockouts on the MFD for navigation and phone use while driving.

As for the regenerative braking to simulate engine drag, I got that straight from the Prius chief engineer when I was talking to him about the 2010 model. I can't understand why you are so reluctant to believe it.

Tom