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Automotive Temperature

At Homershams our bedrock parameters are Temperature, Pressure, Level & Flow and it’s interesting and resonant when these show up in day-to-day life. This month, one of our reps had an epiphany while on a sales trip, and in this article he shares his thoughts.

On a recent sales trip to Northland, my rental car was a Holden Cruze. One unusual thing about it was that it had a digital temperature display of the cooling water. (Commonly the temperature gauge simply shows COLD and HOT). And it was interesting to watch the water temperature vary between 84° C and 107° C.
Cruze-dashboard-200 Std Temp 200
The Cruze Dashboard A Typical car Temperature Gauge

I could clearly see when the thermostat opened and the temperature would drop. And of course, it was interesting to watch the effect of the Gas Laws; allowing water to rise above 100° C without boiling because it’s under pressure. (In fact it can reach 120° C without boiling when under 7-bar of pressure).
cooling-system Radiator-Cap
Automotive Cooling System Thermostatic radiator cap

The cooling system in a vehicle uses a pressurised system; the radiator cap is an integral part that opens and closes in response to temperature. The cap is in fact, a thermostat.

Later, while kneeling beside a car to hook up a trailer, I felt the heat from the exhaust burn my knee, and my reverie drifted to the nature of heat in the engine. (Being a Homershams rep I of course had a thermometer on board and determined the exhaust temperature was 48° C)

Like most, the car motor is a Heat Engine. The power it produces is directly proportional to the difference in temperature of the air entering the intake divided by the temperature at the exhaust manifold. All other things being equal (e.g. Piston displacement, size etc.) the way to increase power output is to increase combustion temperature. For example if the hardware could take it, running our cars on nitro-methane would raise combustion temperature and hence output, but would melt the valves!
A/C and Passenger Comfort
With 20/20 hindsight it’s curious that it took until 1933 for Ford to think of using all the waste heat of the engine to warm the cabin of the car. 

In days past, I remember the automotive tip, that if the car was overheating (a venerable HJ Kingswood in my case), that turning on the cabin heater (even on a hot day) and winding down the windows let the heater act as a second radiator. (Ask a ‘millennial’ why the verb for lowering a window is ‘wind’! They will have no idea!)
cold glass
In-car Air Conditioning came much later. As you probably know, as well as heating and cooling, air-conditioning lowers humidity by condensing water-suspended-in-air, just like water condensing on a cold glass of beverage.

Because the interior of a vehicle is a very small, controlled area, it’s also interesting to view the effect of the Air Conditioning on humidity levels.

Again at Homershams, we measure and calibrate humidity, so I was able to measure the results. The following photos were taken one minute apart. The 1st shows the humidity with the AC off and the second 1-minute later with AC on. The difference is vast!
RH 68 RH 48

The controlled interior of a closed vehicle as compared to an open one, is one of the reasons Homershams don’t do on-site/mobile pressure calibration. The Gas Laws (see a future article) say that temperature and pressure are linked and a change in one will grossly effect the other. If we were to mount a calibration rig in a vehicle that was open to the air, our ‘Total Uncertainty’ would be much worse.
Car Temps
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