Discussion
Can someone please check my maths.
I am trying to calculate how much it costs me to sit at traffic lights in my 4L Chimaera.
Given a 4 stroke engine pumps an air/fuel mixture for only 2 of it strokes, I have halved the 800rpm
idle speed to 400rpm to keep things simple.
4000cc/rev * 400rev/min = 1600000cc/min = 1.6 m3/min
To get the amount of fuel used I am converting the volume of air/fuel to mass of air/fuel by the conversion 1.2041 kg/m3? I am assuming optimum efficiency and that for every 14.7 parts of air we need 1 part of fuel.
If mass of air/fuel = 1.6m3/min * 1.2041kg/m3 => 1.92656 kg/min
then mass of fuel = 1.92656 kg/min / 14.7 => 0.131kg fuel/min
Given the density of petrol is = 702.5kg/m3
converting to volume 0.131kg/min / 702kg/m3 = 1.86767x10^-4m3
converting to cc = 186.767cc fuel/min
converting to litres = 186.767cc/ 1000 = 0.187 litres per minute.
If average stop time at traffic light is 1.5 minutes = 0.285 litres @ 120pence per litre => 33.66p!
I appreciate at idle the fuel mixture may be a lot richer, but does the maths look OK?
I am trying to calculate how much it costs me to sit at traffic lights in my 4L Chimaera.
Given a 4 stroke engine pumps an air/fuel mixture for only 2 of it strokes, I have halved the 800rpm
idle speed to 400rpm to keep things simple.
4000cc/rev * 400rev/min = 1600000cc/min = 1.6 m3/min
To get the amount of fuel used I am converting the volume of air/fuel to mass of air/fuel by the conversion 1.2041 kg/m3? I am assuming optimum efficiency and that for every 14.7 parts of air we need 1 part of fuel.
If mass of air/fuel = 1.6m3/min * 1.2041kg/m3 => 1.92656 kg/min
then mass of fuel = 1.92656 kg/min / 14.7 => 0.131kg fuel/min
Given the density of petrol is = 702.5kg/m3
converting to volume 0.131kg/min / 702kg/m3 = 1.86767x10^-4m3
converting to cc = 186.767cc fuel/min
converting to litres = 186.767cc/ 1000 = 0.187 litres per minute.
If average stop time at traffic light is 1.5 minutes = 0.285 litres @ 120pence per litre => 33.66p!
I appreciate at idle the fuel mixture may be a lot richer, but does the maths look OK?
Chequred Demon said:
Can someone please check my maths.
I am trying to calculate how much it costs me to sit at traffic lights in my 4L Chimaera.
Given a 4 stroke engine pumps an air/fuel mixture for only 2 of it strokes, I have halved the 800rpm
idle speed to 400rpm to keep things simple.
4000cc/rev * 400rev/min = 1600000cc/min = 1.6 m3/min
To get the amount of fuel used I am converting the volume of air/fuel to mass of air/fuel by the conversion 1.2041 kg/m3? I am assuming optimum efficiency and that for every 14.7 parts of air we need 1 part of fuel.
If mass of air/fuel = 1.6m3/min * 1.2041kg/m3 => 1.92656 kg/min
then mass of fuel = 1.92656 kg/min / 14.7 => 0.131kg fuel/min
Given the density of petrol is = 702.5kg/m3
converting to volume 0.131kg/min / 702kg/m3 = 1.86767x10^-4m3
converting to cc = 186.767cc fuel/min
converting to litres = 186.767cc/ 1000 = 0.187 litres per minute.
If average stop time at traffic light is 1.5 minutes = 0.285 litres @ 120pence per litre => 33.66p!
I appreciate at idle the fuel mixture may be a lot richer, but does the maths look OK?
I think it all goes wrong when you're converting to volume.I am trying to calculate how much it costs me to sit at traffic lights in my 4L Chimaera.
Given a 4 stroke engine pumps an air/fuel mixture for only 2 of it strokes, I have halved the 800rpm
idle speed to 400rpm to keep things simple.
4000cc/rev * 400rev/min = 1600000cc/min = 1.6 m3/min
To get the amount of fuel used I am converting the volume of air/fuel to mass of air/fuel by the conversion 1.2041 kg/m3? I am assuming optimum efficiency and that for every 14.7 parts of air we need 1 part of fuel.
If mass of air/fuel = 1.6m3/min * 1.2041kg/m3 => 1.92656 kg/min
then mass of fuel = 1.92656 kg/min / 14.7 => 0.131kg fuel/min
Given the density of petrol is = 702.5kg/m3
converting to volume 0.131kg/min / 702kg/m3 = 1.86767x10^-4m3
converting to cc = 186.767cc fuel/min
converting to litres = 186.767cc/ 1000 = 0.187 litres per minute.
If average stop time at traffic light is 1.5 minutes = 0.285 litres @ 120pence per litre => 33.66p!
I appreciate at idle the fuel mixture may be a lot richer, but does the maths look OK?
0.131kg/min/ 702kg/m3 = 0.0001866
Converting to cc = 01.866 fuel/min
Converting to litres = 1.8666/100 =.01866 litres per minute
(Only 100 cc in a litre)
So 1.5 x .01866 = 0.0285 @120 p = 03.42pence
Edited by V8 GRF on Monday 7th July 16:03
Keeping it simple (as Im no maths genius). My 3.0 Litre V6 omega on tick over in traffic uses (according to the computer) 0.4gal/hr. Now thats with Aircon on and in Drive (its an auto). Now that works out at 3.6p per minute. Now assume the TVR is 3 x as thirsty (who knows maybe it is!) thats still only 10.8p per minute.
Works for me....
Works for me....
Edited by bigdods on Monday 7th July 15:58
Edited by bigdods on Monday 7th July 15:58
bigdods said:
Keeping it simple (as Im no maths genius). My 3.0 Litre V6 omega on tick over in traffic uses (according to the computer) 0.4gal/hr. Now thats with Aircon on and in Drive (its an auto). Now that works out at 3.6p per minute. Now assume the TVR is 3 x as thirsty (who knows maybe it is!) thats still only 10.8p per minute.
Works for me....
Totally inaccurate fuel computor on those..... better off wetting your finger and sticking it out the window,better idea of fuel consumption that way lolWorks for me....
Edited by bigdods on Monday 7th July 15:58
Edited by bigdods on Monday 7th July 15:58
Used to be a vaux tech for many years and often wondered how vauxhall could fit one of these and consider it to be worthwhile to the driver.....
You have of course forgotten those of us who want to look after the planet
and thus turn our engines off at lights... well now then, I wonder what that means in terms of a stop and restart. Some of this would i guess be exponential in terms of the injected pressure of the fuel and the expansion of the explosion against the volume available. This does not of course take into account the potential loss of volume due to average wear meaning that there would of course be an increasing volume available for the gas to expand in and thus changing the amount of fuel used.
As an Explosives and Weapons specialist, I could probably work all of this out including the relative bending moments of the rods and material characteristics that will have an effect, but damb, I was just too busy to blipping the throttle and enjoying the sounds...
and thus turn our engines off at lights... well now then, I wonder what that means in terms of a stop and restart. Some of this would i guess be exponential in terms of the injected pressure of the fuel and the expansion of the explosion against the volume available. This does not of course take into account the potential loss of volume due to average wear meaning that there would of course be an increasing volume available for the gas to expand in and thus changing the amount of fuel used.As an Explosives and Weapons specialist, I could probably work all of this out including the relative bending moments of the rods and material characteristics that will have an effect, but damb, I was just too busy to blipping the throttle and enjoying the sounds...
Cost of switching off at lights, You would also have to calculate the life term cost of the premature replacement of the starter motor and ring gear, plus the fuel issue of increased alternator load from the re-start and any related increased wear issues on the alternator, drive belt etc.
Getting back on topic!
V8 GRF
Thinking about it there can't be 100cc in a litre? A litre motorcycle is 1000cc and all the reference material I can find says 1cc = 1ml and there are 1000 ml in a litre?
In addition the reference says there are 1000000 cubic centimetres (= 1000000 ml) in a a cubic metre, which sounds about right?
So now I wonder what is wrong with the original calculation?
0.131kg/min / 702kg/m3 = 0.0001866m3/min
Converting to cc = 0.0001866m3/min * 1000000 = 186.6 cc/min
Converting to litres = 186.6/1000 = 0.1866 litres per minute
cost per minute = 0.1866 @ 120ppl = 22.39p per minute
So 1.5 minutes * 0.1866 = 0.2799 @ 120ppl = 33.48p per traffic light stop?
Assuming 20mpg @ 60mph using this rough method I calculate:
60 mph = 1 mile per minute
20 mpg @ 1 mile per minute = 1/20 gal per minute = 0.05 gal per minute.
gallons to litres = 0.05 * 4.54 = 0.227 litres a minute.
cost per minute = 0.227 * 120ppl = 27.24ppm ?
Or 4.54 litres costing 544p at 60mph/20mpg will last 20 minutes.
544/20 = 27.2ppm ?
I will be the first to admit my maths is rubbish but the above looks to be in the right ball park to me?
Help!
V8 GRF
Thinking about it there can't be 100cc in a litre? A litre motorcycle is 1000cc and all the reference material I can find says 1cc = 1ml and there are 1000 ml in a litre?
In addition the reference says there are 1000000 cubic centimetres (= 1000000 ml) in a a cubic metre, which sounds about right?
So now I wonder what is wrong with the original calculation?
0.131kg/min / 702kg/m3 = 0.0001866m3/min
Converting to cc = 0.0001866m3/min * 1000000 = 186.6 cc/min
Converting to litres = 186.6/1000 = 0.1866 litres per minute
cost per minute = 0.1866 @ 120ppl = 22.39p per minute
So 1.5 minutes * 0.1866 = 0.2799 @ 120ppl = 33.48p per traffic light stop?
Assuming 20mpg @ 60mph using this rough method I calculate:
60 mph = 1 mile per minute
20 mpg @ 1 mile per minute = 1/20 gal per minute = 0.05 gal per minute.
gallons to litres = 0.05 * 4.54 = 0.227 litres a minute.
cost per minute = 0.227 * 120ppl = 27.24ppm ?
Or 4.54 litres costing 544p at 60mph/20mpg will last 20 minutes.
544/20 = 27.2ppm ?
I will be the first to admit my maths is rubbish but the above looks to be in the right ball park to me?
Help!
Chequred Demon said:
Getting back on topic!
V8 GRF
Thinking about it there can't be 100cc in a litre? A litre motorcycle is 1000cc and all the reference material I can find says 1cc = 1ml and there are 1000 ml in a litre?
In addition the reference says there are 1000000 cubic centimetres (= 1000000 ml) in a a cubic metre, which sounds about right?
So now I wonder what is wrong with the original calculation?
0.131kg/min / 702kg/m3 = 0.0001866m3/min
Converting to cc = 0.0001866m3/min * 1000000 = 186.6 cc/min
Converting to litres = 186.6/1000 = 0.1866 litres per minute
cost per minute = 0.1866 @ 120ppl = 22.39p per minute
So 1.5 minutes * 0.1866 = 0.2799 @ 120ppl = 33.48p per traffic light stop?
Assuming 20mpg @ 60mph using this rough method I calculate:
60 mph = 1 mile per minute
20 mpg @ 1 mile per minute = 1/20 gal per minute = 0.05 gal per minute.
gallons to litres = 0.05 * 4.54 = 0.227 litres a minute.
cost per minute = 0.227 * 120ppl = 27.24ppm ?
Or 4.54 litres costing 544p at 60mph/20mpg will last 20 minutes.
544/20 = 27.2ppm ?
I will be the first to admit my maths is rubbish but the above looks to be in the right ball park to me?
Help!
On the CC thing I thought that there were 1000cc in a litre but saw a website that I either read wrong or had the wrong answer but it gave me an answer that would put the decimal point in the right place in your initial calculation so I took that... V8 GRF
Thinking about it there can't be 100cc in a litre? A litre motorcycle is 1000cc and all the reference material I can find says 1cc = 1ml and there are 1000 ml in a litre?
In addition the reference says there are 1000000 cubic centimetres (= 1000000 ml) in a a cubic metre, which sounds about right?
So now I wonder what is wrong with the original calculation?
0.131kg/min / 702kg/m3 = 0.0001866m3/min
Converting to cc = 0.0001866m3/min * 1000000 = 186.6 cc/min
Converting to litres = 186.6/1000 = 0.1866 litres per minute
cost per minute = 0.1866 @ 120ppl = 22.39p per minute
So 1.5 minutes * 0.1866 = 0.2799 @ 120ppl = 33.48p per traffic light stop?
Assuming 20mpg @ 60mph using this rough method I calculate:
60 mph = 1 mile per minute
20 mpg @ 1 mile per minute = 1/20 gal per minute = 0.05 gal per minute.
gallons to litres = 0.05 * 4.54 = 0.227 litres a minute.
cost per minute = 0.227 * 120ppl = 27.24ppm ?
Or 4.54 litres costing 544p at 60mph/20mpg will last 20 minutes.
544/20 = 27.2ppm ?
I will be the first to admit my maths is rubbish but the above looks to be in the right ball park to me?
Help!
That said I think there is something wrong with your calculations, I don't think that you use 1/4 of a litre sitting at the traffic lights.
Doing a reverse calculation that would mean if you sat at the lights for 4.4 minutes you'd use 1 litre which means you'd use a gallon in 20 minutes? Or on a 12 gallon tank that means you'd run out just idling after 4 hours. If you drove for 4 hours at 60mph you'd cover about 240 miles.
I think you've worked out the cost per mile while driving but not idling.
I think there must be some adjustments to be made for weight, load and windresistance that would make your idle consumption much lower.
Sorry to double post but using actual data...
Going to ROAR 08 took 4 hours on A roads and I used 36.6 litres of fuel to cover 160 miles.
this roughly equates to 8 gallons, so averaged 20mpg / 40mph - sad but true!.
Therefore 36.6 litres / 240 minutes = 0.1525 litres a minute, which again looks to be in the right ball park of our .187 litres per minute at idle?
So it looks like the decimal point is in the right place?
Going to ROAR 08 took 4 hours on A roads and I used 36.6 litres of fuel to cover 160 miles.
this roughly equates to 8 gallons, so averaged 20mpg / 40mph - sad but true!.
Therefore 36.6 litres / 240 minutes = 0.1525 litres a minute, which again looks to be in the right ball park of our .187 litres per minute at idle?
So it looks like the decimal point is in the right place?
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