Some cars (like the Toyota Prius) have a system where hot engine coolant is pumped into an insulated storage bottle. This can be used to heat the interior when the ICE is not running.

I seem to remember Saab experimenting with this for overnight storage, where the hot coolant would help heat up the engine when you start the car the next morning. Has this ever resulted in a usable product? And could this be retrofitted to other cars?

I'm looking into ways to improve my winter commute. A coolant water heater (electric or petrol) is the obvious solution (electric is difficult since I can't park my car on private property), but I'll consider alternatives.

  • I'm just wondering if there would be enough energy to actually accomplish anything? Secondarily, there are such things as "engine heaters" ... we used to say we needed to "plug the car in" which would get the coolant warm and keep the engine well above freezing in the winter. Plugging a car in has a whole different connotation today! Dec 6, 2016 at 13:56
  • The VW Touareg had a system where an electric pump would circulate the coolant to keep the heat functioning for a while after the engine was turned off.
    – cory
    Dec 6, 2016 at 15:31
  • @cory - Wasn't that actually to continue to allow the engine to cool after shutoff? I mean, it would continue to allow heat as well, but that would be a secondary effect, not a primary one. Dec 6, 2016 at 22:52
  • 2
    The primary function for this system on the Prius is to preheat the engine not the passenger compartment. The reason is that Toyota wants the engine to be ready for closed loop fuel control before engine start. It is also why the engine will not start for at least 7 seconds after switch on to allow time for the coolant pump to run and the O2 sensors to heat up to operating temperature. Dec 7, 2016 at 6:21
  • @Pᴀᴜʟsᴛᴇʀ2 It was for heat. You had to activate it using a button on the dashboard. The radiator fans will continue to run after turning off the engine to cool the engine without having to push any buttons.
    – cory
    Dec 7, 2016 at 14:00

1 Answer 1


I don't believe this can be retrofitted into old vehicles, because it takes some amount of time to circulate the coolant around the engine. Typically, Toyota hybrids have a <10 second delay when starting the engine (on my 2016 RAV4 hybrid, the delay varies depending on the outside air temperature; there's no such coolant storage system on this car).

Now, when starting a normal non-hybrid car, you will definitely not want to wait for few seconds before the engine can be started. On a hybrid car, about 20-25% of the total system power is available from the battery, and you don't need to drive at wide open throttle when backing up from the parking spot, so the engine start delay is not a problem.

Also, your reason for the existence of this system is incorrect. It is to help reduce emissions, not to heat up the interior of the car when the engine is not running. Seriously, providing heat to the interior will very quickly drop the temperature of the coolant. This effect is so quick that the storage system would accomplish nothing. On my RAV4 hybrid, when it's cold the electric drive won't work for more than a minute at a time because the coolant temperature has by then dropped so much that extra heat is required.

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