Brake specific fuel consumption

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Guys
Was out West at the weekend and found diesel at a very good price so filled up both main and auxiliary fuel tank. I figured I'd saved a few quid but then realised that I'd have to carry the auxiliary 170Ltrs around for 400 miles before I could start burning it. Then I was wondering how much diesel I burnt carrying around the auxiliary tank. Now I think I didn't save anything - possibly the opposite.
Anyone ever done the calculations to see what the Brake specific fuel consumption is of a LC or come across it anywhere?
Brake specific fuel consumption is a measure of an engine's efficiency - the rate of fuel consumption divided by the rate of power production.
If we knew this we could possibly calculate the cost of carrying additional weight when driving.....
Perhaps there's a rule of thumb out there - something like every additional 100KG reduces fuel efficency by 2% or something.....
Niall
HDJ80 - heavier than it needs to be.
To calculate BSFC, use the formula BSFC =3D Fuel_rate / Power
Where:
Fuel_rate is the fuel consumption in grams per hour (g/hr)
Power is the power produced in Kilowatts where kW =3D w * Tq / 9549.27
w is the engine speed in rpm
Tq is the engine torque in newton meters (N=B7m)
Note: The Power in the BSFC calculation is not weather corrected.
The resulting units of BSFC are g/(kW=B7h)
The conversion between metric and U.S. units is:
BSFC_US(Lbs/(HP*Hr)) * 608.277 =3D BSFC_METRIC(g/(kW=B7h))
BSFC_METRIC(g/(kW=B7h)) * .001644 =3D BSFC_US(Lbs/(HP*Hr))
 
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Niall,
I see the thinking but the problem is how are you going to measure the extra
power output (and then integrate it) without doing the same 400 miles
without the fuel load.
I imagine if you driving is flat motor way the extra fuel won't be a big
issue, if you are doing start / stop driving it will could more of an issue
and if you are crossing the alps your in trouble!
Perhaps it's the vehicle brakes or more specifically the extra energy the
brake are dissipating that you need to measure rather than the brake
specific fuel consumption?
Our works transit will do over 35mpg at 60mph loaded or unloaded but loaded
on a hilly A road and it's about 10mpg worse than when empty.
Malcolm Bagley
Stafford UK
FJ45 '75 & FJ45 '76
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Malcolm
Its a bit of a brain teaser alright. - I sure don't have the
mathematical horse power to figure it. When you factor in different
stylkes of driving and all the other random influences it bercomes even
more complicated
Perhaps toyota should publish this type of data. You kknow the way they
publish figures for urban driving, combined motor way/urban
Would be interesting to see these figures for a landcruiser with a half
a tonne in the back!
Cheers
Niall
 
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Don't underestimate the effect of fuel price increases during the time
the extra fuel sits in the auxiliary tank. The last time I filled I
paid E 1.06 per litre, the time before it was only 98 cents which is
more than an 8% increase.
I figure you need an empirical solution to this problem: i.e. check
the gas consumption during mixed condition driving over a single tank
full and then with both tanks full.
Simple calculation:
Cost both tanks full to empty/ distance
verses
Cost main tank full to empty/ distance
Cheers,
--
Andy Haxby
1994 HDJ81
Den Haag NL
Macclesfield UK
 
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On a less practical track;
I was talking to an supplier of aftermarket ECU's, had was interested a
development they are currently working on. The are using a three axis
accelerometer with drag and wind resistance calibrated in to the ecu to give
power output from the engine, I know dedicated race systems use this and a
gps input to give very detailed information.
Alternatively if the vehicle has a OBDII connector you might be able to see
the rack position which is all you really need to determine rough fuel
consumption on a diesel, integrated over time and you have consumption (well
relative consumption).
Malcolm Bagley
Stafford UK
FJ45 '75 & FJ45 '76
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Andy
Its the only way Andy . Too many variables to factor in a
calculation....
Good food for thought though.
Niall
HDJ80
 
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