Mini experiments: 300w vs. 800w block heater warm up time
 

I thought I had done this already with my 300w external bolt-on block heater, but I couldn't find the data anywhere on my computer.

Moot point. I've since added an 800w inline coolant heater, and haven't collected any info from it yet.

So, what I'm going to do is plug the car in & check the temperature every 15 minutes for an hour or so.

The results will be skewed somewhat by the fact that the coolant is not heated uniformly, and I need to run the engine briefly to get an accurate temperature reading.

I know the coolant temp isn't uniform because when I read the temp on the ScanGauge before I start the engine, it gives me a value that's significantly lower than the value I see within a few seconds of starting the engine.

So the water pump has to circulate things briefly to mix it up and get an accurate (or "average") reading from the coolant temp sensor.

I can say with some confidence it's not the heat of combustion responsible for the temp jump, because it's quite large (say 50 degrees F) and shows up on the SG within 10 seconds.

The best way to run the test might be to take off the belt & spin the water pump manually/electrically, but I can't think of anything offhand that I can use to do that easily.

Anyway, I'll report back later with some DATA! ...

Oh, another fly in the ointment:

By circulating the coolant every 15 minutes to get a temperature reading, I'm probably distributing the heat differently than the convective currents would on their own.

Meaning, not counting the small amount of additional heat I'm adding from combustion, the final temp by this method will probably be different than if I just plug it in and leave it for the same amount of time.

Still, I'll carry on.

Looking forward to the results Darin. Could you please show a picture or a diagram of the placement of the heater as well for visual reference?

Results!

Darin,
Do you think there would be any hit in FE if you used the heater for 45 minutes instead of the 2 hours? I think the Prius holds the temp at 125 in the coolant reservoir and in 45 minutes you're sitting at 136. You might need to do a test on that:D Nice results by the way.

Yeah, talk about diminishing returns, eh?

Fact is, I never plug in for more than an hour anyway; I just did the 1:45 today for the purposes of the graph. 30-45 minutes is more typical.

Now that I've charted it, I can see that anything more than an hour and 15 minutes (#6) is probably not worthwhile in terms of return on time & energy invested.


Note that this probably isn't directly applicable to other cars. The 993 cc motor in the Firefly is a smaller heat sink. I'd guess that bigger motors may "level out" at a higher temperature (though they'll take longer to get there @ the same 800w).

I may do some other tests - like compare this to the performance of the 300w external heater (it's still bolted to the motor).

Also, I have plugged them both in at once a few times on really cold days.

It would be interesting to see the comparison of time compared to temperature.

How long would it take to get to 135 when it's -20 outside compare to 30?

I've read that block heaters are best when on consistently at a lower setting in terms of engine wear. This seems supported by intuition, in that the same amount of heat over a longer time will result in a better distribution of heat through the engine. The thermal conductivity of Fe also drops as the temperature increases, which could exacerbate this. Since automatics tend to rev higher when cold, presumably to heat up the rings and avoid excessive wear and oil contamination, I wonder how effective a block heater would be at reducing wear given the ECU would think the engine is warmer than it actually is and the warmup portion would be somewhat retarded. Manuals of course are in the hands of the driver at anything other than idle.

Also, I think a significant portion of fuel consumption at startup involves a rich mixture lighting off the catalytic converter/s, and I wonder how much fuel block heaters save compared to cold. I imagine a car with a single pre-cat oxygen sensor would likely realize a greater decrease in consumption compared to a car with a pre and post-cat oxygen sensor, so testing in that case would likely be fruitless, but testing the second system could provide useful info as to how much fuel consumption is associated with the higher idle, and how much is associated with lighting off the emissions system.

In any event, it seems that less over more is better than more over less for engine wear, but I imagine both are better than nothing in the cold.

I suspect the plot would just be shifted up or down relative to ambient temperature. IE - if it's 30 F degrees colder, things will level out @ about 131 F instead of 161.