Hill Start Assist - how does it work

[Thackeray]

I've often wondered how the Hill Start Assist feature works and mentioned it in a thread a couple of years ago. I initially thought it must work by Battery power being fed to the MG2 electric motor to prevent the road wheels turning. But then a couple of years ago I drove a diesel Citroën which also had the feature. So I realised that the Citroën must be working by holding the brakes until the car started moving. This made me think that this must be the way the IS300h worked too.

But recently I thought it was time to do some experimenting. And the answer seems to be that the IS300h (and maybe other models) uses both methods.

This is what I found. I parked on a hill facing uphill and held the car in Drive using the footbrake. Then I released the footbrake and waited. The car didn't roll backwards. Checking on the energy monitor there was initially no power flow from the Battery to the motor. So the car was being held by the brakes and this was confirmed by Hybrid Assistant. But after a couple of seconds the brakes were released and simultaneously power began to flow to the motor to keep the car held on the hill. There was a slight movement of the car, which I've often noticed, as the brakes were released and the motor took up the strain of holding the car. This slight movement (just a couple of millimetres probably) has always made me slightly nervous and I've always hurriedly pressed the accelerator to get the car moving.

I then did a second trial, this time on a much steeper part of the hill. Same procedure as before. But this time on the steep hill the electricity flow comes on immediately even while holding the brake. This means that when the brakes are released there's no perceptible movement of the car at all because the MG2 motor has been holding the car stationary, along with the brakes, right from the moment the car stopped. I didn't time it but this stationary position, held by power to MG2 and to the road wheels, seemed to continue indefinitely, though I probably didn't try it for more than about 10 seconds.

So the answer is that both the brakes and MG2 are used to keep the car from rolling backwards. And the slight movement backwards you get on less steep hills is just the transfer of the load from the brakes to the motor and does not mean the Hill Start Assist feature has stopped working.

I hope this helps anyone else who's interested in how this feature works.

[PaulWhitt20]

I thought the Hill Start Assist just kept the brakes on for a few seconds to give you time to move your foot from the brake to accelerator when setting off. After a few seconds the brakes are released and the Hill Start Assist is no longer active. In a non automatic car you would then start rolling back. However in an auto you don’t seem to be able to roll back on gentle slopes so the transmission is then keeping the car from rolling back. Probably best not to rely on this though. 

[Herbie]

23 minutes ago, PaulWhitt20 said:

I thought the Hill Start Assist just kept the brakes on for a few seconds to give you time to move your foot from the brake to accelerator when setting off. After a few seconds the brakes are released and the Hill Start Assist is no longer active.

Yep, that's correct.

Hill Start Hold may work differently but Hill Start Assist does exactly that.

[ColinBarber]

Hill-start assist controls the brakes by maintaining the master cylinder fluid pressure produced by the driver depressing the brake pedal. After 2 seconds that pressure is released (or before if the accelerator is pressed). Whether the vehicle maintains it's stationary position, rolls back or creeps forward is to do with the hybrid system, independently of hill-start assist.

[Thackeray]

18 hours ago, ColinBarber said:

Whether the vehicle maintains it's stationary position, rolls back or creeps forward is to do with the hybrid system, independently of hill-start assist.

Exactly so. This is what I was interested in finding out about. I was wondering what MG2 does to back up the brakes. In fact, it seems to take over from the brakes and hold the car on a hill indefinitely after the system has released the brakes.

Below are a couple of charts created by Hybrid Assistant which show what is happening. The first shows the progress of the car up the hill. The blue line shows when MG2 is turning and therefore when the road wheels are turning. At all other times the car is stationary. There are four short advances up the hill along with longish stationary periods in between.

The second chart shows in blue the torque MG2 is outputting over the same period. This shows that even when the car is stationary, MG2 is outputting around 30 ft-lb of torque to prevent the car from rolling back on the hill. At the steepest point of the hill, the torque goes up to around 40 ft-lb but the car is still stationary.

The temperature of MG2 rose from around 26C to 30C over this period, even though it was scarcely turning. I didn't leave the car stationary on the hill for longer than 15 seconds, as I imagine too much heat without any rotation might not be good for the motor.

It was also interesting that at the steepest point of the hill, the car's energy monitor showed that the Battery was feeding power to MG2 from the moment the car was shifted into Drive, even though both the parking brake and footbrake were both applied. I expect this is aimed at making the start as smooth as possible. There must be a sensor monitoring how steep the hill is for the car to know when to apply more power to MG2 even though the parking and footbrake are both holding the car stationary. Perhaps this explains the difference between the 30ft-lb most of the time but around 40 ft-lb at the steepest point.