Aircon keep vs. letting car heat up and cool down

[doggy1]

What do you mean by worked on?

Diagnosed & repaired aircons, replaced compressors, replaced accumulators, replaced evaporators, regassed, repaired air movement motors, replaced sensors, repaired flaps and so on

 

[JonDarian]

Some one worked it out, from memory there was a thread about it somewhere, If you set the temperature to a certain temperature that is the temperature that the air is at the vents, the only way to stop the heaters from coming on is to set the temperature to Lo

It is difficult to engage with this without seeing that original thread. I would be very surprised to find that the air is being cooled to 4C using an energy-efficient heat pump, then heated to the desired temperature using an inefficient resistive heater. The thermodynamics of that are pretty terrible.

Assuming you are right, and the air is heated to the thermostat temperature, that would also slow cooling the cabin immensely. For example, a 28C cabin and 22C output would take ages to bring the cabin temp down to 22C. Instead, if the air entering the AC system at 28C is cooled significantly (e.g by 15-20 degrees) and is then expelled into the cabin, the cabin will cool much more quickly.

It is in most cars. It is controlled by the fan speed at lower temperatures 16~19 but as you dial it up it also uses the heater core to warm up the air because slowing down the fan to the lowest doesn’t cut it.

I gather you are assuming a situation in which the ambient temperature is below the thermostat temperature. I have been assuming we were discussing trying to cool the cabin with AC.

 

[Rosier]

Diagnosed & repaired aircons, replaced compressors, replaced accumulators, replaced evaporators, regassed, repaired air movement motors, replaced sensors, repaired flaps and so on

Then I believe you already know the climate control uses a combination of aircon and the heater core. You must’ve seen the flappers.

 

[Rosier]

I gather you are assuming a situation in which the ambient temperature is below the thermostat temperature. I have been assuming we were discussing trying to cool the cabin with AC.

Well, the original experiment was to see the net energy use difference between leaving the climate on to maintain a cabin temp of 21c vs leaving it off.

My idea was to run the same experiment with a car that has a heat pump system, see how the increased COP affects the original results.

On one hand; it will use less energy if left on. On the other; it could possibly cool down the car quicker and more efficiently.

Remember in the ‘leave on’ scenario The cabin is already cooled down to 21 ambient. So it will try to maintain that. Not necessarily cool down. Perhaps the inside-outside gradient is enough.

The only way to know for sure is by rerunning the OPs experiment.

 

[bay74]

I've worked on the aircon in my last 8 ICE cars (all European). Although I acknowledge that one COULD use the heater in this way and maybe early Teslas did this, it sounds rather inefficient. None of mine has used the heater in this way. To maintain temperatures less than ambient, they have always cycled the compressor on and off as required. Thermal mass in the system (incl Evaporator) ensured that you did not experience wild fluctuations in temperature

Cycle the compressor? That sounds positively last-century. These days I'd have thought the "intelligent" inverter-controlled compressor would just slow down such that the outgoing airstream was at the required temperature.

Now what that required temperature is, is another question. If it's trying to as rapidly as possible cool the cabin, sure, just go full compressor power, but there are other circumstances where another approach would be warranted. E.g. if the cabin is near the temperature set-point, and the dewpoint (or relative humidity) is sufficiently low inside the cabin (such that there's no risk of condensation on the windscreen) then it would be more efficient to not cool (much) below the temperature set-point. No?

Dunno whether there's a humidity sensor in the car somewhere. Does it ever attempt to "defog" the windscreen automatically?

 

[cafz]

Dunno whether there's a humidity sensor in the car somewhere. Does it ever attempt to "defog" the windscreen automatically?

There is definitely a humidity sensor in the cabin because I had to get it replaced under warranty!

 

[ZeeDoktor]

I think think there is a humidity sensor indeed - but I don't have it on my current CAN signals decoding list.

Would indeed be interesting to figure out the difference in energy use to the heat pump models. Does anyone else in this thread with a heat pump model have the ability to extract CAN bus data? And knows which signals to decode for heater/AC compressor energy consumption?

 

[Rosier]

I think think there is a humidity sensor indeed - but I don't have it on my current CAN signals decoding list.

Would indeed be interesting to figure out the difference in energy use to the heat pump models. Does anyone else in this thread with a heat pump model have the ability to extract CAN bus data? And knows which signals to decode for heater/AC compressor energy consumption?

Will try to run a similar test, if not the same when I get my hands on the new car. I will keep you posted/reach out to you when I have some free time and suitable weather.

 

[ZeeDoktor]

I left the car parked in the sun yesterday for a few hours with the ipad thermally secured. When I came back the inside temperature was around 40C, so definitely the overheat protection must have worked. Alas, the app crashed by the time I came back to the car. So, still no news on the energy consumption of overheat protection. I'll keep trying...

Meanwhile, another question that surfaced in this thread dealt with what temperature is blown out of the vents. It won't come as a surprise to most that even with the cabin temperature near the set point, the air temperature coming out of the vents is cold, less than 10C. The car just lowers the air flow when near the set point, and uses less power to drive the AC compressor.

In the attached screenshot you can see the left and right floor vent temps are around 11C, while the passenger facing vents are around 9C. The CAN bus reports cabin heater power in %, and THC compressor power used in kW. I don't remember the number exactly, but I think the heater has a max power draw of around 8kW, so for the integrated power ("Cabin heat used") I'm doing a conversion from %. The gauge to the bottom right is the total power consumed at the HV stage - includes drive power etc. This screenshot obviously was taken while standing still. The car just sitting there without climate running consumes about 600W when it's on. I did some measurements of other components like lights, autopilot, MCU use, fan settings etc. in another thread here: Demystifying the power consumers in my Model S