"Special Belt" on Engine

[NemesisUK]

Just curious about the only rubber belt visible on my RC300h engine. It runs round a large crank pulley up to a relatively small pulley on a 'thing' bolted to the nearside on the engine block.  Printed on the belt is "Special Belt" and just above on a pipe union is a label stating "Special Belt, see owner's handbook".

I've trawled through my handbook and can find nothing relating to this "Special Belt"

What does this one belt drive? I thought ancillaries on a hybrid were all electrically driven?

[Jayw13702]

It’s the water pump drive belt.

It’s a special belt In so far as it’s Lexus only, you can’t seem to find them aftermarket and it’s narrower that a “normal” serpentine belt


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[NemesisUK]

So is there a second electrically driven water circulation pump to keep the heater matrix fed when the engine is off?

[ColinBarber]

It's a 'special' belt because it has an amount of elasticity in it so there is no need for a tensioner/adjuster on the water pump. It's a sort of warning not to fit a standard one - if you could find one the correct size.

No water circulation occurs if the engine shuts down - if the heater matrix gets too cold compared to your cabin temperature/setting it will force the engine on - one reason why mpg isn't as good in winter.

There is an electric cooling pump that is used to circulate coolant around the hybrid inverter / dc-dc converter but that coolant doesn't flow into the cabin heater matrix (doesn't get hot enough).

[NemesisUK]

9 hours ago, ColinBarber said:

No water circulation occurs if the engine shuts down - if the heater matrix gets too cold compared to your cabin temperature/setting it will force the engine on - one reason why mpg isn't as good in winter.

This must obviously closely match engine coolant temperature as measured by the water temperature sensor that displays on the dash as I noticed the engine always starts when the display drops to approx 1/3rd full scale, irrespective of ambient temperature.

This is what prompted me to ask about a second, electric, pump cutting in to keep the flow going. Else wouldn't the engine radiator cool very much more quickly than the heater matrix?

All academic really as the system works really well, I just find it all fascinating!

[ColinBarber]

48 minutes ago, NemesisUK said:

This must obviously closely match engine coolant temperature as measured by the water temperature sensor that displays on the dash as I noticed the engine always starts when the display drops to approx 1/3rd full scale, irrespective of ambient temperature.

Another reason for the petrol engine to fire up is if it is getting out of a temperature threshold, i.e too cold for optimum efficiency.

If the engine isn't running then the coolant in the radiator isn't moving and so won't cool the engine unnecessarily, once the engine fires back up it will get mixed and the mechanical engine thermostat will regulate that.

 

On the IS, and I presume RC, there is a mechanical shutter on the lower part of radiator/grille which restricts air flow to stop excessive coolant cooling when the temperature is extremely low - I don't believe this is fitted to any UK vehicle as we don't have extreme winter temperatures.

[NemesisUK]

Just fired up Lexus-tech.eu and reading through the Air Con System description, there is a secondary, electrically driven, water pump to supply the heater radiator with engine coolant to prevent a decrease in the effectiveness of the heater when the engine shuts down.

[Britprius]

On all Toyota/Lexus hybrid cars there is an electric pump that runs to pump hot coolant from the engine to the heater matrix when the engine is not running, and is in ready mode. This is there to both improve heat output, and to save fuel by not requiring the engine to run just to pump coolant.

John.