Thursday February 6 2014

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. 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.

Paul: We need to reconsider how we load, how do you take off, how do you brake?

Mustafa: Paul, you have been at the lights and you look at the guy next to you, seriously you want to race away.

Jude: But if I have a sports car, I need to put it to the test. If I have an AMG say 63 or 55, I need not worry about fuel consumption.

Mustafa: The only thing you waste at the lights with your AMG is time. They are delaying you. Jude, what three things off head would you advise people to do in order to have better fuel consumption for people with a car like yours?

Jude: Improving their driving skills, by braking evenly and not suddenly, using the AC sparingly, know when to have your windows down or up and maybe the load capacity. Sometimes we go upcountry and come back with sacks of food and charcoal and forget all about the impact this has on fuel and trickles down also to the tyres.

Paul: The other thing is if you are driving a manual car, shift as quickly as possible, some people take long to shift from the lower gears to the higher ones, driving style is very important.

Jude: These tips cut across whether one is driving a diesel or petrol car, some people think they are a bit safer with fuel if driving a diesel car but forget all about other tips, so whether it is a small or midsize engine, the tips are the same, they cut across.

Mustafa: So Paul for the kind of car you drive, how is it?

Jude: Is it a 4.0litre or 4.2litre?

Mustafa: It is a big SUV in the line of the Mercedes MLs, Prados, so what tips would you give for people with such cars?

Paul: Driving styles, is what I will emphasise , but if you have a car that has cruise control, I have found it useful, when I am on the highway and select the appropriate gear at the appropriate speed. When I use cruise control, my fuel consumption is better, not so fast between 80 and 100kph and I will ease off the gas pedal. The car actually selects the appropriate gears without revving, driving in the highest gear also helps, when you take off, don’t accelerate aggressively, do it gently, because the more aggression you use, the higher the revolutions, the higher the fuel consumption. It is also good to select the right source of fuel. Do not go to unheard of fuel stations.

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