Let's chat: Fuel consumption and the air conditioner

This is a continuation of last week’s discussion about small engines some times not reflecting “good” fuel consumption and the causes of this.

Thursday February 6 2014

This is a continuation of last week’s discussion about small engines some times not reflecting “good” fuel consumption and the causes of this. This time though, we delve more on the use of the air conditioner and its impact on the engine and subsquently, fuel consumption figures.

Mustafa: What is your experience with air conditioners and fuel consumption?

Jude: I have travelled in people’s cars with the AC on all the time. Well, in Uganda we have issues such as dust. To feel safe and have a clean interior, people prefer switching on the AC.

Mustafa: Is it dust or heat or both?

Jude: Both, now it has just rained. If it is not raining, then you have dust, some people will switch on their ACs.

Paul: AC is like any other consumer to the car. AC is one of those things that add a huge load on fuel consumption. You are driving a mortar called the AC compressor. When you switch it on there is an impact on the engine. The amount of fuel your car guzzles will be determined by your speed. If you want to floor the pedal and also have the AC on, your car will really guzzle, the two loads will be competing. But also when you drive fast with your windows down, the amount of air sucked into the car will cause a drag and this affects fuel consumption.

Jude: The way I do it, all my windows are always up save for the driver’s, that’s how I drive. Isn’t that some kind of balance?

Paul: You could say so but if you drive very fast, that wind will come into the car and slow you down. It still boils down to how fast you are driving.

Mustafa: I have a friend who drives cars from Mombasa with friends. He often argues with his friends on whether to have the AC on or not. He believes the AC should always be on and his friends say no. I gave him the analogy of an umbrella. Try running with an umbrella up; it is the same thing when your windows are down. However, when does it become drag? If I am driving around town with all my windows down at the speeds I am doing, it won’t make a difference, but if I am driving from Kampala to Jinja with all my windows down, there will be a problem. If my windows are down, the aerodynamics are different. If you are on a highway with windows down, there is that drag and many other variables.

Jude: So how can we achieve a balance?

Mustafa: That is a difficult question.
Paul: Do you have wind deflectors to deflect the wind when your windows are up to some level? I think it boils down to one thing, driving tips. But generally speaking when you use the AC, expect your fuel consumption to go up.

Mustafa: That is it, because you are adding a load to your engine. So for your engine to operate ordinarily, it will require more fuel because you have added a load.

Paul: It is at that point that we give solutions. How do you load your car? If you load your car in such a way that you affect the aerodynamics,.(the fluid motion of air over your car), there is a problem. These days people are into load boxes, people have bought cars with these boxes it is like they are going to ski.

Mustafa: They look cool!

Jude: Ugandans like “fancy” things.

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