I've had my car for four years now. I opened the sunroof the day I bought it just for a play, and it has remained shut ever since. I'm a window's man (no aircon).
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>>So here is the question: which leads to less fuel consumption between opening windows or sunroof?
I stand by the part of my answer in one of the threads linked to by jbif.
At low speeds, in town, the extra aerodynamic drag caused by lowering the windows is minimal, and it's probably more efficient to leave the A/C off. However, if you're in a jam, there won't be any cooling air, and you'll soon be too hot.
At high speed, the drag caused by opening the windows makes turning on th A/C the sensible option.
In one of the other threads linked to by jbif, the ever shrewd Cyd suggested that the important parameters are the sizing of the air conditioning system, the solar loads borne by the car (the rear windows of old Ford Anglias would be good in this regard), and how the system is used and controlled. These parameters define the total power consumed by the A/C, and so, they define the fuel used - it's nothing to do with the engine size.
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Depends on the car. In mine an open roof doesn't make a lot of buffeting at medium-to-high cruising speeds, and I assume that means it isn't making much extra drag. Open windows are noisier and make more buffeting, so they obviously make more drag.
Of course I am from the days of moveable quarterlights and even opening windscreens, so what do I know?
I still occasionally see those plastic aftermarket driver's-window deflectors on cars.
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Well I have finished a 6 week experiment running our Yaris diesel with the aircon switched off.. and only switched on when needed to cool the car. for warm days, open the sunroof.
Previously I had the aircon on all the time. And averaged 57mpg - mainly around town.
I still leav ethe heater blower motor on to increase airflow. Tested results show mpg - same driving conditions and style - 59mpg..
Over 12,000 miles that's 7.2 gallons of diesel or £40 tops.
Is it worth it?
Well the car does have more power starting off when the aircon is not working. And there is no noticeable impact on comfort as we still use aircon to cool on hot days.
The only downside is the 1 second to push a switch.
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Skoda offer these after market wind deflectors - mine have them, they look a bit naff, but I think they are effective.
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I'm a window's man (no aircon).
I prefer to have both aircon and sunroof fitted, and sometimes use both together. Tilting the sunroof increases the airflow through the heater. Without the fan in operation the airflow through the heater is virtually nil ~ a result of modern body shapes, I suspect. And I'm more concerned with my comfort than any piffling increase in petrol consumption that might result from using the aircon or sunroof.
Edited by L'escargot on 07/06/2008 at 09:36
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Just back from a quick break in Cyprus. Didn't hire a car or bike this trip and just used buses and taxis for a week. I think this is the longest period of time I have gone without driving anything in years ! Served to remind me how much I would miss it if I couldn't for some reason.
Anyway to return to topic, there was a grizzled old local taxi driver we came to a regularish arrangement with and we got to chatting about things automotive. He used an oldish diesel E class saloon in ubiquitous mediteranean white. He had those deflector things fitted and just left the windows down. When I asked him if he used the aircon, his droll response was along the lines of "only eef it get hot". ( It was a steady 32 deg. all week ) Have to say we were never uncomfortable in his car. Maybe it was due to the lack of humidity but it did rather make a mockery of my habit of using that facility in my car at anything over 18 deg or even lower if assisted demisting is required.
(Seem to think diesel was 1.18 Euro / Lt there BTW)
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"What's the size of the engine got to do with this?"
Well, I'm no expert, but I have seen others argue, and many users report, that A/C is in effect an overhead on power consumption. Let's say that's 10 bhp. If your engine delivers 100 bhp, that's 10% power consumed by A/C; if it's 200 bhp, that's 5% -- with commensurate impacts on fuel consumption. When I have reported no difference in fuel consumption with 3 litre engines, others have responded in those terms.
Always happy to be corrected, I await further views.
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Remember, with sunroof open, you are going to need extra lashings of suntan lotion.
Oh, and more use of cupholders to secure mineral water.
NO! This is NOT a facetious statement - lots of folk who use sunroofs/ragtops in sunny weather dehydrate and get sunstroke.
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The biggest problem I had with the tilting sunroof on my car arose from the very long hair I had at the time (1990s).
With the vents open, I got a wonderfully cooling through-draft, but my hair was sucked out the sunroof.
Edited by NowWheels on 07/06/2008 at 15:20
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The biggest problem I had with the tilting sunroof on my car arose from the very long hair I had at the time (1990s).
Likewise. But I had the misfortune of a very large bird (If they could fly, I'd have suspected an Ostrich) in a leafy tree above my car one day when i was waiting for the light to change.
Hair, suit, shirt front, particles strewn all over the place as the mess spalettered off my bonce.
Needless to say, I did NOT make the meeting...
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Let's say that's 10 bhp.
That's 7.5 kW, the equivalent of 2.5 x 3kW fires. That's an awful lot of power/heat needing to be dissipated somewhere!
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Surley the size of the engine IS a factor in determining the extra fuel used.
E.g. Lets say the AC uses 20BHP on maximum cool, if you have a small, 60BHP, engine that does 45 mpg, then you will be using a third as much again in extra fuel to run it (34mpg). If you have a large, 200HP that does 25mpg, with the same AC load, then you will only be using 10% more fuel (23mpg).
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In partial answer to kith and Roger Jones;
When travelling along a road at a given speed, the amount of power required is defined by the vehicle, namely its rolling resistance and its aerodynamic drag. This power is usually (for legal speeds!) much less than the rated power of the engine, so, for most cars an extra 10BHP load would be a significant change in the engine's operating condition.
In other words, the peak power of the engine isn't the one to consider, it's the road load power.
If you have an engine that doesn't require more fuel to deliver more power, then you've at least got part way to a perpetual motion macine on your hands!
As a proportion of the fuel used, I would say that for inefficient high drag vehicles, for example a Range Rover, the air conditioning load will be a small proportion of the road load, and so, proportionally, it will make less difference to the high speed fuel consumption figures, than it would to a more efficient vehicle where the road load power at speed is much lower than the Range Rover's.
Of course, very inefficient vehicles NEED large engines, and so this is where a large engine is more likely to be found.
The basic point I was making still stands though - the extra fuel quantity [not proportion!] needed is not dependant upon engine size, it's all to do with the spec and efficiency of the air con system, and the solar load on the vehicle.
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