Do some cars have A/C which works when it's below freezing? I thought most had a cut off at 4degC. The light still came on on my Mondeo the other day but I don't think the AC was actually running.
Maybe not, but of course the air intake temp can still be above 4C when it's snowing ;-)
Chris
Hence why any competent manufacturer puts the temperature sensor in the condensor air flow
Speelung corrected..
It's my understanding that Ford just use the air intake temperature, but I'm happy to be proven wrong.
Of course, you might not consider Ford to be a competent manufacturer...
Chris
The ignition/injection systems takes an appreciable amount of current.
Yes but the efficiency of conversion isn't the main thing that effects the heating. Given similar amounts of fuel necessary to maintain idling the spark ignition engine will have a fixed air:fuel ratio and high adiabatic flame temperature, so the average temperature in the cylinder will be high with a low mass flow. The compression ignition engine will have high volumetric efficiency at idle so the air fuel ratio is much higher, hence the fuel heats up a much greater massflow, so the average temperature is lower.
This average temperature is what controls how much heat is rejected to the coolant.
AJH
Which isn't the case
That doesn't help, but also reduces the amount rejected to the exhaust.
No but it actually makes the heat rejection to the compression ignition coolant circuit less because it's conversion of fuel to motion is better than the spark ignition engine. The original postulation was that the diesel runs cooler at idle, which we both seem to agree on, we differ in that I believe the major difference in this lower heat transfer to coolant is to do with the higher massflow rather than conversion efficiency.
I don't think so, I'd guess the relative proportions rejected to exhaust and coolant remain about constant for a given engine speed. The heat lost to coolant is proportional to average delta T across the cylinder walls and this is related to the combustion temperature less the rejection temperature.
AJH
Yes
Apart from anything else they're not independent.
No they don't.
It'll be proportional, but it won't be the same proportion for different engine designs.
As you wish, goodnight.
AJH
Tony Houghton presented the following explanation :
Whilst sat waiting in the car at A&E for 3/4 of an hour, in the last freeze up a couple of weeks ago - I tried it, in my CDTi at -8 deg.
Initially, with the heating / a/c set on auto (its a full auto climate control system), which runs the cooling fan on slow. The thermostat was still closed, so the only effect the fan was having was to cool the engine block, rather than the rad. Yet the engine temperature went steadily down and the amount of heat produced by the heater decreased.
I then shut the auto a/c off, which also cut the fan. It was just barely able to hold the engine temperature and put some heat into the cab. Manually decreasing the heater fan speed helped a bit, to boost the input temperature. So I would suggest any half way efficient diesel might be struggling to keep you warm in cold temperature at a tickover and you may need some manual intervention to keep warm with a climate control type system.
Hence why vehicles for really cold climates, and many a truck , has an auxiliary diesel heater.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Harry Bloomfield saying something like:
I keep hearing this about diesels been crap at heating. My Tranny idles over and supplies copious quantities of heat to the cab (I used to go down the coast in all sorts of winter weathers and we'd sit in the cab lovely and warm when it was wild outside), likewise my ShiteOldCarina and there's a generation between them. Just this morning, I started up and left the SOC for ten/fiften minutes, came back out and the entire glass area was utterly clear of the fairly thick film of ice on it. Nothing special about the heater controls, just used them to best effect. Actually, the Toyota has a lower hose t'stat, which doesn't open until the heater circuit is fully satisfied and the block temp starts to climb.
Harry Bloomfield gurgled happily, sounding much like they were saying:
It's why a fair few common-rail diesels have auxiliary fuel-burning heaters.
And some have electrical heaters in the cooling circuit, often utilising glow plugs.
Chris
Perhaps these old designs are far less efficient than modern ones? And that poorer efficiency must result in more waste heat?
But as a generalization, it's accurate.
Never seen one of those, how do they glow if they're in the cooling circuit?
Mk1 Focus diesels have them, there is a heating chamber on the bulkhead that has three glowplugs in. The ECU limits operation when insufficient battery voltage, presumably using the same strategy as for the HFS.
As they are immersed in coolant, I'm pretty certain that they don't glow as in emit visible light.
Chris
Yep, because of indirect injection vs direct injection. I don't know how old his Tranny is (I think they were using DI for some years before most cars) but the SOC is most likely indirect injection. They lose more heat into the cylinder head (and therefore ultimately the cooling system) than DI.
If it's quite a new and/or top of the range Tranny it might have an auxiliary heater.
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