Quote:
Originally Posted by 2000mc
I would totally agree with oilpan4, I think 4x the heat for a 1/4 of the time would be far better.
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edit: so most ppl on the saturn forums seem to say 300w block heater. considering that, it doesnt sound as inefficient as i was thinking
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There's something to be said for both approaches...
A quick blast of high power avoids all those hours of wasteful cooling, but you need to closely control the heater both by time (to avoid wastefully leaving a powerful heater on too long) and by temperature (thermostat) to avoid a powerful heater overheating things or burning out. Also you MUST have a pump with a powerful heater, which adds further complexity, greater difficulty of installation, and an additional possible point of failure. (It took me three days in all to install my coolant heater and pump, what with the plumbing, the electrics and so on. The main dificulty was finding the exact spot and orientation to fit everything in so that no airlocks could develop and nothing was in danger of rubbing against anything else. Cars have so little space under the bonnet these days.)
A low power heater in the right place will heat the engine gradually over a longer period, but only to a relatively modest temperature. Because of the lower temperature, the heat wasted per minute is much less (Newton's Law of Cooling - see video below) although you need a lot more of those minutes to raise the temperature. A lower power heater requires no thermostat, no pump, and time control is much less important, so a low power heater should be much more reliable. (High power coolant heaters can and do burn out. All you need is a coolant leak, an air lock, a faulty thermostat or a faulty pump and your heater element will fry itself.)
My experience (diesel) is that the mpg hit from starting a cold engine is pretty much dealt with by pre-heating the engine coolant to 50 deg C (122F) Maybe even less. I've tried with 50 degrees C.
The greatest theoretical efficiency would be achieved by using the maximum amount of power for the shortest possible time, and having a thermostat which switches off at the lowest temperature the engine needs for reasonable efficiency. But that requires a relatively complex control system, and you can achieve the same temperature rise over a longer period with a much smaller heater, which is cheaper, easier to install and more reliable.
The big advantage of heating the engine coolant to full operating temperature is that you can use the cabin heater straight away - for de-frosting, de-misting and comfort purposes. This potentially contributes to fuel efficiency by obviating the need to idle the engine on the driveway to warm up the cabin or de-mist the windows, but using the cabin heater straight away cools the engine rapidly just when it's trying to get to full temperature.
If I was doing this again I would start with a simple, low power heater - an OEM block heater if your vehicle supports one (mine doesn't!) or an oil pan heater, or maybe both. That should improve mpg enormously over the first few miles, but won't help much to defrost your windows. Then I would run a mains cable inside the cabin, put a mains socket there, plug in a mains timer to the socket and put an ordinary household fan heater on its lowest power (say 1kw?) in the passenger foot well. That will deal with the frost on the windows and the -!! degrees interior very well indeed but without robbing the engine of heat.
You can run both these heaters on the same power outlet and on a single cable. Say you want to drive off at 8am, there's no reason why you can't have one timer indoors turning the power on at 5am for the low power engine heater, and set the 2nd timer in the car to 7:30am to turn the fan heater on. If you still want the mpg gains in the summer just unplug the fan heater and carry on using the engine heater. A low power heater is never going to overheat an engine, even on the hottest day.
(Obviously in the car you need to use an electronic mains timer with a battery - the kind that still keeps time when the mains power is disconnected! E.g.
Timeguard TG77 Day Compact Electronic Timeswitch: Amazon.co.uk: DIY & Tools Not one of those mechanical timers.)
Here in the UK with a 230v mains supply we can go up to 3kw for a single normal power outlet, so I can still add a 1kw fan heater to my 1.8kw coolant heater, and that is what I'm going to do, but if I was starting again I would do it the way I have described above with an oil pan heater.
(I went for the 1.8kw heater because I thought it was necessary to heat the engine to full operating temperature (85C) to get the mpg gains. Turns out that isn't the case. Also, I thought I'd be able to wire up the car's own heater blower to defrost the windows, but in practice, even on its lowest speed turning the blower on drags the coolant temperature down too low and consequently doesn't work as well as I expected. It would probably work better if the coolant heater was 3kw rather than 2kw, but still I think it's better to let the engine heater do its job undisturbed and use a separate fan heater for the cabin.)
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