Hybrid engine braking or mechanical brakes?

[johnatg]

As a relative newcomer to hybrid systems (GS300h), I have a question. Approaching my home village I descend a long (a mile or so) gentle hill, but one on which I need to take action to keep within the speed limit. I can either use the paddles to change to a lower effective gear ratio or apply the brakes gently but fairly continuously. Which is better?, for driving mechanically sympathetically and/or for optimum traction Battery charging - from the bottom of the hill I can usually drive most of the way home in EV mode - it doesn't seem to make much difference which technique I use.

I am not one for changing down whenever possible to minimise use of the brakes in old school fashion - if you are going to stress part of a car, brakes are cheaper than engines and pretty effective these days.

So what do you all think?

[Sherra]

gentle braking will be all regenerative so no detriment to mechanical brakes at all.

[NemesisUK]

"Gears to go, brakes to slow" was drummed into me in my early driving days..

In the hybrid gentle braking will be regenerative as already said but one can brake quite firmly and still only use regen, it all depends on road speed and pedal application speed. Using the paddles to slow brings in the engine to slow the car with no benefit from regen.

I too have a steep long downhill to my home and always "ride the brakes" to keep within the speed limit. By the time I reach the bottom the traction Battery is fully recharged and ready for a silent getaway in the morning!

The attached docs from Lexus-Tech.EU (for my RC300h) may be of interest?

Brake Control System.pdf

HYBRID TRANSMISSION SYSTEM CONTROL SEQUENTIAL SHIFT.pdf

[ColinBarber]

1 hour ago, johnatg said:

I can either use the paddles to change to a lower effective gear ratio or apply the brakes gently but fairly continuously. Which is better?

Provided your power meter stays within the regen section using the 'brakes' you won't be using the mechanical ones - get to the end of the scale and the mechanical brakes will also start to come into play. Using the gears, depending on speed etc, will be achieved by the same thing - the system will apply electrical regen to simulate engine load in a lower gear. So there really isn't a difference.

[Britprius]

If you can drive the rest of your journey in EV mode it would be best to avoid doing this if possible. When you start from home cold the Hybrid system will try to use as much EV or electric assist as possible.This is to reduce emissions from the cold engine by reducing the load on it while it is effectively in "choke mode" running rich by injecting more fuel.
Given that you also have to climb that hill it would be more fuel efficient to have Battery power available to assist than to use it on a flat section of road with a warm engine.

John. 

[Shahpor]

17 hours ago, NemesisUK said:

"Gears to go, brakes to slow" was drummed into me in my early driving days..

In the hybrid gentle braking will be regenerative as already said but one can brake quite firmly and still only use regen, it all depends on road speed and pedal application speed. Using the paddles to slow brings in the engine to slow the car with no benefit from regen.

I too have a steep long downhill to my home and always "ride the brakes" to keep within the speed limit. By the time I reach the bottom the traction battery is fully recharged and ready for a silent getaway in the morning!

The attached docs from Lexus-Tech.EU (for my RC300h) may be of interest?

Brake Control System.pdf 263.75 kB · 13 downloads

HYBRID TRANSMISSION SYSTEM CONTROL SEQUENTIAL SHIFT.pdf 151.97 kB · 10 downloads

Thanks for sharing.

Wow, this is impressive stuff.  I had no idea that the brake force I apply to the pedal doesn't directly translate to the fluid pressure in the brake calipers.  I suppose, in hindsight, I should have expected this though since, if it did, it wouldn't allow for regen assistance.

Something interesting is the picture on page 5 of the Brake Control System;  it looks like the regen braking isn't linear and greater mechanical braking is applied when the pedal is first pressed, or am I reading this wrong?

[NemesisUK]

I see it as initially all braking effort is regenerative but as pedal pressure increases mechanical braking force increases. If one can see far enough ahead and gently apply the brakes one can come almost come to a stop just under regenerative braking. As speed drops very low (~10mph?) regen effort is inefficient and braking becomes totally mechanical and one gets that characteristic 'groan' or as my good lady describes it, goose-like honk as one come to a complete stop.

Braking has to be "brake by wire" as the internal combustion engine isn't always running to supply vacuum assistance. One can hear the electric pump pressurise the brake fluid reservoir, especially when manoeuvring to park/slow speed crawl in traffic