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Try Not To Use Air-Conditioning

No Air Con

Having air-conditioning is a nice luxury, and when it is really hot in the summer air-con seems like the greatest invention ever! However air-con does increase your petrol consumption by as much as 10%, so when its warm just have your fans on cool instead of having the air con on, or just have the window down slightly. If this fails to cool you down then try to just have the air-con on in short bursts.

If you are traveling at over 60mph and it is too hot then it is recommended you actually have the air-con on instead of putting the window down, this is because having the window down increases drag which in turn reduces your fuel consumption.

Why does air-con use fuel? (the long explanation!)
The air-con in most cars consists of 5 basic components and an assortment of refrigeration lines and sensors. The primary components are a Condenser, a Compressor, an Evaporator, a Thermal Expansion Valve, and a Drier. Air conditioning cools your vehicle's interior similarly to how a refrigerator keeps its interior cool. In most air-con systems a gas refrigerant such as Freon or the refrigerant R-134a (which is better for the environment) is pressurized by the compressor and circulated in a closed system. The gas absorbs heat from the air around it and dissipates that heat as it travels through the condenser and a series of refrigeration lines. As the gas is depressurized it becomes a liquid and passes into the evaporator. Air forced through the evaporator cools down as it touches the cold refrigerant lines and blows into the cabin of the vehicle. The thermal expansion valve controls the amount of liquid refrigerant entering the evaporator and by extension how much cooling the air entering the vehicle cabin undergoes. Refrigerant leaving the evaporator turns back into gas as it warms up again, and passes through the drier before reentering the compressor. The drier ensures that only gas and no liquid enters the compressor. This process requires energy from the engine to compress the gas and pass it through the system.

The reason air conditioning can affect your fuel consumption is that the compressor is driven by an engine belt. The air-con requires power from the engine in order to compress the gas into the refrigerant lines and puts additional strain on the engine. At low speeds (under 30mph or so) the air conditioner requires the most power from the engine to run, so if you are driving slowly in town it is generally best to open your windows. At higher speeds the air conditioner's effect on fuel efficiency is negligible. The heat that the air conditioner dissipates into the engine compartment may also adversely affect engine performance.

Problems
If it is really hot then don't leave the air-con off just to save a couple of pounds! So sometimes you can't get round it, but most the time just have the window down slightly.

Image credit: Some rights reserved by exfordy