Drop in efficiency over the last week then?

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aminorjourney
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Drop in efficiency over the last week then?

Postby aminorjourney » Wed Jul 04, 2007 8:24 pm

So, I am interested to know if like us, other Prius owners are noticing quite a dramatic drop in mpg due to all this bad weather.

I know that surface water makes it harder for the car to move along, and so it uses more fuel, but a week or more of this really makes it obvious.

A week or two ago I was constantly getting 60 mpg. Now we're struggling to get 51!

Anyone else experience such a dramatic illustration of how road conditions impacts on mpg?
Nikki Gordon-Bloomfield

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qdos
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Postby qdos » Wed Jul 04, 2007 9:16 pm

Actually,no, my Zest seems to like it and it goes better in the damp. But then there's no windscreen to de-mist or wipers to wipe nor a rear one either. And the aircon is of the natural kind too. ;)

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Postby marktime » Thu Jul 05, 2007 11:13 am

Nikki you are not alone.
The weather does make a difference, anyone who constantly monitors fuel consumption (on any car) will see a Winter / Summer cycle, a major factor is air temprature, cold air is more dense and provides greater risistance. The drag coefficient of the Prius is very good (I cant find it on the web site but I know it's in the brochure at home) but anything that affects the Cd (like a roof rack) has a very significant adverse effect.
Pushing all that rain water about does not help either.
It's also much more noticable in the Prius because you know how good it can be!

I peaked at 68mpg in April when the weather was glorious, I am now back to 63mpg, but I'm still smiling :)

Mark
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Postby marktime » Thu Jul 05, 2007 7:43 pm

Found it, for what it's worth the Drag coefficient of the Prius is 0.26.

Isn't it funny how numbers stick in your head, I can't remember when or what model it was but I know that there was a big fanfare when an Audi was launched and it had a Cd of 0.39! Am I alone :o

MT
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Postby qdos » Thu Jul 05, 2007 11:10 pm

It was a drag factor of 0.30 not 0.39 and it was just marketing hype as there had actually been several cars with lower drag factors around many years previous. The drag factor I think is much more affected by the raindrops on the surface of the car. I fly hang gliders and the stall speed definitely goes up in the rain. Trust me I know from some dicey landings I've had ;), but it's due to the water droplets on the wing not air density.

I really don't believe it will affect your MPG by that much though and it's far more likely it's because of all the wipers, fans, lights, demisters etc. putting a load on the engine via the battery. It's simple to see this as next time you come to stop your car after a run at night in the rain just before you turn the engine off listen to the tone of the engine with all your electronics turned on then turn them off and hear the engine note change and the revs pick up by possible 200rpm. Why because the engine is having to work to put a charge into the battery system.

However the increase air density is kind of correct but in actual fact it helps the carburation more in that there is a better oxygen content in the fuel air mix and consequently you get a better burn i.e. better efficiency in the engine. Which is why my Zest gets more perky in the cool and damp. With only 20bhp on tap you notice every little increase in power / aerodynamic efficiency

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Postby marktime » Fri Jul 06, 2007 5:25 pm

Doh, unreliable memory, still I was right about it being an Audi!

I know what you mean about more dense air going in, I have driven cars that felt more lively when it was cold.

The trick of turning off the auxillaries wont work on the Prius, the engine will probably have already stopped!

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Postby qdos » Fri Jul 06, 2007 10:08 pm

marktime wrote:
The trick of turning off the auxillaries wont work on the Prius, the engine will probably have already stopped!

Mark


True! I hadn't thought about that :lol:


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