Latest Reader Question: Air Conditioning & Sunroofs

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Here’s the latest reader question, along with my reply!

Martin asks: I live in France. In Europe, the sun roof has disappeared also – because of air conditioning. Also, I believe that heating is via the air conditioning increases gas consumption since not using natural heat of engine.

My reply: Increased glass area definitely increases the load on the AC system by making it harder to cool the car’s interior down. A notorious example is the AMC Pacer of the’70s, which had so much glass area,  it created a greenhouse effect inside. May the Motor Gods have mercy on you if you didn’t order AC but did order the optional Levi jeans seat covers with authentic brass rivets. . .

On the rest of your question: Whether the car has AC or not, heat is just repurposed waste energy – excess engine heat carried away from the engine by the coolant, which takes it away from the engine to be radiated to the outside via airflow over the radiator and to the inside, via the heater core, which is  just another radiator; the difference being it radiates the heat of the coolant into the cabin (via an electric fan which pulls air through it and into the cabin).

This does not cost energy – except for the small electrical draw to run the fan motor.

However, if the car has climate control AC, the system may cycle the compressor on, depending upon the temperature set as well as setting chosen. For example, choosing the defroster will usually cycle on the AC, which will help clear the windshield by dehumidifying the air.

The main energy cost of AC is the mechanical load of the compressor – which (in most cars) is powered by an engine-driven pulley/belt and the added weight of all the AC components. These have gotten much lighter than they used to be (my ’76 Pontiac’s compressor weighs an obnoxious amount compared with modern compressors) and they don’t load the engine as much (in my car, you can actually feel the engine lose power when the AC is turned on) but there is still a cost.

The tragedy is that cars were once designed to be pretty livable without AC, via vent systems which they no longer have. Which is why AC has become not only de facto standard in all new cars but something you pretty much have to have!

. . .

Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!

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  1. Sunroofs which open at the rear provide venting so that you can choose to ride without the a/c. One PT Cruiser I owned had a sunroof that would slide open all the way or would raise open at the rear. I could open the windows partially and raise that sunroof at the rear side and the slipstream venting would keep my head cool without using the a/c.

    It also worked well with the windows closed and the system blowing outside air into the cabin at the dash and into the footwell.


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