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Old 11th June 2007, 11:05   #1
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Default Does Weather Affect Fuel Economy

Hi,

Till a few days back, when delhi was around 40-42 Degree Celcius, The FE in my XETA with AC was around 16.65 per Kg, But Yesterday on my last fill- I noticed the FE is gone done to 14.37 per KG, Also I have noticed that the AC works with More Labour and Ride is Rough during the day but smooth during the Night when its slightly cooler.

The Temp Needle although remains constant at the Half way mark, Could it be the Weather that influences the FE and Driveability.
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Old 11th June 2007, 11:14   #2
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A very good question. I don't see any great relation between the temperature & FE.

I too tanked up twice during the current heatwave, but mu truck returned the usual mileage that I have been constantly getting all along. While at home it is parked in the open, at office it is in a shade. Maybe some quantitiy of fuel is lost due to evaporation.

Well there are quite a few benchmarks for optimum efficiency that included the fact that your AC is serviced thoroughly thus not putting much strain while cooling.

I have yet to understand the relation ship between driveability and temperature except that in case of a vehicle smooch it leads to road rage...

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Originally Posted by wolfinstein View Post
Hi,

Till a few days back, when delhi was around 40-42 Degree Celcius, The FE in my XETA with AC was around 16.65 per Kg, But Yesterday on my last fill- I noticed the FE is gone done to 14.37 per KG, Also I have noticed that the AC works with More Labour and Ride is Rough during the day but smooth during the Night when its slightly cooler.

The Temp Needle although remains constant at the Half way mark, Could it be the Weather that influences the FE and Driveability.
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Old 11th June 2007, 11:18   #3
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Well, I've been driving the same in the last 3 days, Running on the same CNG- I have noticed that ever since the Temp went upto 44-45 Degree in delhi in the last 3-4 days, The car is riding rough and does not like moving about in the day..!
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Old 11th June 2007, 11:34   #4
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I would think there would be a difference simply because the AC has to work overtime as it were; efficiency of the AC does vary with the ambient temperature AFAIK.

I can see it over the past week even at home so I'm guessing the car ACs would be no different
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Old 11th June 2007, 13:10   #5
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My obserbation is also in the similar lines; FE drops marginally (may 1kmpl); I think folloing might be the reasons:
high temp, high volume and thus lesser qty of fuel
more load on AC due to increase in amb temp
high sp vol of air and thus lesser qty of air for comb. (Not so sure)
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Old 11th June 2007, 13:59   #6
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Ummm... I think the car struggles in a hot weather and hence the FE figures do come down. You see the engine response with cold air is always better than hot air (the reason they add an Intercooler). I believe this causes a difference in FE.

However in your case the drop seems to be pretty big (of almost 2kms). I can think of the following two reasons for your CNG vehicle,
* Due to the intense heat the gas volume increases. So actually you are getting lesser gas than a kg, since the pumps are calibrated based on the volume dispensed.
* Also, due to the high temperature the gas that goes in for combustion is also much rarefied. This probably equates to a leaner mix and hence the harsh engine response as well.
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Old 11th June 2007, 14:05   #7
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Quote:
Originally Posted by Zappo View Post
Due to the intense heat the gas volume increases. So actually you are getting lesser gas than a kg, since the pumps are calibrated based on the volume dispensed.
.
Wolf, it may be a good experiment to fill up early in the morning and see if there is any difference.
Also, does the filling not depend on the pressure at the CNG pump? If the pressure is low, it would not fill completely, and if you are using the Full tank to full tank method, it would return you a smaller figure. However, the next fill should compensate and return higher than average fugures.
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Old 11th June 2007, 14:35   #8
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Quote:
Originally Posted by Subodh Kumar View Post
My obserbation is also in the similar lines; FE drops marginally (may 1kmpl); I think folloing might be the reasons:
high temp, high volume and thus lesser qty of fuel
more load on AC due to increase in amb temp
high sp vol of air and thus lesser qty of air for comb. (Not so sure)
Right if some-one drives indica, i am sure he would have experienced the same. In hot summer afternoon, pickup becomes pathetically poor, not so smooth engine & drop in fuel economy too. By the way i drive a diesel one.

In the night the things become normal.

I think, it may be due to, hot air=less denser=lnefficent combustion

A/C has to overwork to beat the heat, thankfully my indica's a/c is chilling, needed no service in last 3.5 years.

something awkward with air intake design/fuel injection system design, as i have never heard, drop in pick-up & FE with other car owners, if air becomes thin, it is for everybody else too.

anyhow i am converted long ago being an indica owner & loves it.
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Old 11th June 2007, 14:57   #9
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I have the same observation. 40 litres and only 424 kms .Last month I got somewhere close to 475kms for the same quantity of fuel. In both the measurements AC was on all the time. May be inefficient combustion due to hot less dense air is the reason.
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Old 11th June 2007, 14:57   #10
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IC engines are based on Carnot cycle where efficiency is proportional to (T2-T1)/T2.
T2 is the engine temperature and T1 is the ambient temperature.
There are a lot of other factors also, but this is a very important factor.
So if its hot outside, i.e. the air intake is taking in 50 degree C air, its going to be much worse than the winters where the air intake takes in 10-15 degree air.
This is the whole point of cold air intakes.

Now comes the pressure part, in hot summer the air pressure does not become that low as to effect efficiency, and most diesel cars with Electronic fuel pump can cope with this. However above 2500mts you may face a problem in non turbo cars.
At 3500rpm or whereabouts, the diesels become extremely smoky, esp the non turbo ones.
However all these effects will not be seen at Delhi Altitudes.
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Old 11th June 2007, 17:11   #11
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Quote:
Originally Posted by tsk1979 View Post
IC engines are based on Carnot cycle where efficiency is proportional to (T2-T1)/T2.
T2 is the engine temperature and T1 is the ambient temperature.
There are a lot of other factors also, but this is a very important factor.
So if its hot outside, i.e. the air intake is taking in 50 degree C air, its going to be much worse than the winters where the air intake takes in 10-15 degree air.
This is the whole point of cold air intakes.

Now comes the pressure part, in hot summer the air pressure does not become that low as to effect efficiency, and most diesel cars with Electronic fuel pump can cope with this. However above 2500mts you may face a problem in non turbo cars.
At 3500rpm or whereabouts, the diesels become extremely smoky, esp the non turbo ones.
However all these effects will not be seen at Delhi Altitudes.
You still remember carnot cycle . Good explaination Tanveer. But T2 is very high so the difference is the temperature gradient is minimal so shouldnt make that much of a difference right?
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Old 11th June 2007, 18:03   #12
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at higher temperatures the ac would take longer to reach the cut off temperature.lowering the fuel efficiency
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Old 11th June 2007, 22:35   #13
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Quote:
Originally Posted by rishibravo View Post
You still remember carnot cycle . Good explaination Tanveer. But T2 is very high so the difference is the temperature gradient is minimal so shouldnt make that much of a difference right?
Imagine a temparature difference of 282K vs 322K
So you can have around 3-5% efficiency drop.
Add to that fuel density etc., things the difference can be upto 10%
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Old 11th June 2007, 23:56   #14
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But is the validation correct for Gas.
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Old 12th June 2007, 09:28   #15
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No problem about gas, since its filled by weight. Gas will be 10Kg whether its 10 degrees or 40 degrees, however volume may change.
But due to over use of AC and the lowering of engine efficiency, you will still see a drop in FE.
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