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I am currently sitting far up in nothern Norway and I am early for an appointment.

So I figured I'd wait in the car for the time to pass. But since it's currently -15°C outside: I need to have heating on. My car (a Volvo V40 D2 - 2013) is fitted with a heating unit that warms the engine and cabin using diesel without having the actual engine running.

The engine consumes 0.5 l/h when warm (0.9 l/h cold). The numbers for the pre-heater I got no idea about.

So what is more efficient and economically best to use while waiting? The pre-heater or the engine itself?

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    The pre-heater heats the cabin while the engine foremost heats the motor compartment which should be less efficient. Anyway less than 1l/h doesn't sound bad really (unless you are days early). The best would be to not be too early for the appointment or to wait inside somewhere if possible. :) – Trilarion Feb 16 '18 at 13:24
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Let's do a little back-of-the-envelope calculation: Assuming that the heating unit has an efficiency close to 1.0 and that automotive diesel has an energy density of ~39 MJ/l which translates to ~10kWh/l.

With no knowledge of your pre-heater, let's assume 5kWh. Thus, 0.5l/h, i.e. comparable with a warmed-up engine.

Now, the diesel engine has a lower efficiency (if nothing else, due to mechanical resistance.) Also, wear and tear favors the pre-heater-based warming.

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    I disagree that mechanical resistance is a problem here. If the goal is to warm up the engine, mechanical resistance is eventually turned to heat, i.e. it is towards achieving the goal of warming up the engine. – juhist Feb 16 '18 at 12:55
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    @juhist You are mistaken about the goal. If he is early for an appointment, then it is unlikely the goal is a warm engine, but rather a warm human. In that case, the pre-heater is the clear choice. – Eric Hauenstein Feb 16 '18 at 13:22
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Either way you are converting fuel to heat. However, the engine is built for transferring heat away from the engine in order to keep it from overheating. It has radiators and a lot of heat leaves via the exhaust pipes. If this could be turned down to zero, there would not be a case for the pre-heater to be built in the first place.

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Just look at the possible efficiency : you leave the main engine on idle just for the heater - it is not the most efficient at that speed : better under load at say 2000rpm.

The pre-heater is doing exactly what it was designed to do so it will be running at or very close to its design point ie most efficient.

Ergo, use the pre-heater, or use neither and go and sit in reception... You may be lucky and get a coffee!

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    I have to say I partially disagree with the claim that efficiency is relevant here. In fact, engines are designed to create mechanical power, not heat. The more efficient the engine is, the more mechanical power and the less heat power. If the goal is to warm up the engine, inefficiency rules! – juhist Feb 16 '18 at 12:54
  • @juhist You are mistaken about the goal. If he is early for an appointment, then it is unlikely the goal is a warm engine, but rather a warm human. In that case, the pre-heater is the clear choice. – Eric Hauenstein Feb 16 '18 at 13:22
  • @juhist my engine left to run on idle cools down ie produces less heat so your “inefficiency rules” may not be true... – Solar Mike Feb 16 '18 at 13:45
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In the world of larger diesels, the answer is pre-heater because the engine probably isn't going to start if left at ambient temperature. :)

And as others pointed out, a properly working pre-heater is more efficient than a cold idling engine any day.

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