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Fuel and fuel systems Petrol vs Diesel, LPG Conversions, premium fuels, and all that gas

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Old 29-01-10
Set0 Set0 is offline
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Join Date: Apr 2009
Posts: 19
Thumbs up Diesel Fuel Economy, I got almost 100km more for some reason...


Strange thing happened to me today, i drove back to Adelaide from Innes National Park on the tip of Yorke Peninsula. My trip out and back and totaled 680kms, as i got back to Adelaide the fuel light came on.

Now, i drive a 1991 NH Pajero, 5sp 2.5TD LWB. Last year i did a trip around the south eastern coast, great ocean rd and so on. Generally i got around 580 to about 600km ish to one tank full tank of diesel.

The strange thing i got a substantial distance more, WHY???

Ok, potential factors that may have played a part:

Got a wheel alignment
Cleaned Air filter
Carried less weight
Kept the speed down, minimized going 110km/h, tried to keep it to 80~90km/h

Could these factors have contributed this much ??

I didn't use and ultimate or premium diesel fuels.

Also what do other 2.5 TD generally get ? in terms of fuel economy?

I'm pretty happy now.

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Old 29-01-10
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ziggy ziggy is offline
Join Date: Feb 2008
Posts: 205

Im sure wind resistance plays a huge part. As soon as I try to maintain 100+ the eco goes way down.

After all they have the aerodynamics of a brick
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Old 31-01-10
speewa158 speewa158 is offline
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Join Date: Jan 2010
Posts: 41

in the NH you will find on the drivers sidea big pedle in the right corner try to appily preasure to it gentle for as little time as possiblr through out the trip this can improve fuel consumption greatly
i have achived 58-6kml yes 58-6kml yes i was rolling down a hill at the time with the motor running the air,con stearing & under gravity speed that gave me a projected range of over 4500 km . when the road flattened out back to 7-5kml .was good while it lasted
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Old 12-04-10
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Mitzified Mitzified is offline
Join Date: Jan 2009
Location: Elizabeth Sth Aus
Posts: 49

It is the slower speed that makes the biggest difference. It is the wind resistance that is the big enemy.
As a rule of thumb travelling at 80 klms half of your fuel consumption is used fighting the wind and the faster you go above that the ratio increases.
I have experienced the same on a few trips now.Even towing shows the same result.
I can use more fuel at 110 klms while not towing than travelling at 90 klm towing a 14 ft caravan.
As mentioned earlier it is all in the right foot action.
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Old 12-04-10
Rocketsurgery Rocketsurgery is offline
Join Date: Dec 2008
Posts: 109

Got it in one. Speed. To double your speed you require 8 times the power - more power means substantially more fuel.
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Old 16-04-10
RovingTech RovingTech is offline
Join Date: Jan 2008
Location: Darwin
Posts: 109

I nearly ran out of diesel towing a trailer from Threeways to Mount Isa. Must have had a 20 knot headwind at least. Fuel light came on about 30km out of town. I never realized a headwind could make such a big difference to fuel consumption.
2005 NP Exceed DiD .TJM Alloy Bull Bar. Dual Battery
TX3440 UHF. Airtec Snorkel and false floor. Intercooler and sump guard.
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Old 18-04-10
FamilyMan FamilyMan is offline
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Join Date: Oct 2009
Posts: 273

A bit of high-school maths.

Work = force times distance
Force = mass times acceleration

Distance is fixed
speed is constant (mostly) so acceleration is zero, so force required to keep mass moving is zero.
Force is wind resistance, as moving a constant mass at constant speed requires no force, so no work.
Wind resistance rises (as a complex function) with speed, so going faster will use more fuel to cover the same distance.
Now an idling engine has some losses (it still uses fuel even though producing no output). So say idling an engine at 2000rpm would use say 3.5 litres an hour.
The best I have achieved is about 6.5l/100km around town at say 60km.h. Now guess 100km would take 1.5 hours at this speed, so equivalent idling consumption around 5.3 l/100km ish. So we are using about say 1.5l/100km in overcoming air resistance.

Now skip to say 90km.h. I get around 8.5l/100km at that speed. About 4l/100km idling losses, so air losses have increased to 4.5l/100km.

Then at 100km/h, I get about 9.0 l/100. 3.5l/100km idle losses, so 6.5 l/100km wind losses.

Not that I do it, but I'd then project, based on the above numbers losses at 120km/h to be up around 10l/100km, to give around 13l/100km.

Now there are some gross assumptions in there, but it gives you an idea of how the numbers are working.

Look at it another way, in case you are better with pictures than numbers.

Stand beside a suburban road at 50km/h at 2m away and you don't feel a car go past.
Stand beside a highway in a layby at 5m away and feel the rush of wind when something goes past at 110km/h. So in effect the vehicle is towing around all of that wash. It is pushing the air out of the way much faster and so it goes much further. More work to do, more fuel used.

Other complexities. If you travel slower, you use your A/C for longer to cover the same distance, so that costs more, negating some of the difference.
Intercooler efficiency is very dependent on airflow. Cooling rises rapidly with increasing airflow. At 40km/h your intercooler is probably doing very little, but at 120km/h probably working well. I'm not sure of the relationship between intake air temp and efficiency. Anecdotally my diesel seems more efficient in the cool of the evening, but that might be an incorrect conclusion based on A/C use and lighter traffic. The intercooler is there to increase available power, but I don't know what the impact on efficiency is (either in direction or amount). A hotter engine and transmission with have less viscous oil - on the face of it fewer losses. However, engine temp is regulated by the thermostat so making it work harder won't increase its temp much, but the transmission I don't believe is so regulated, so there may be a slight gain there.

Hills vs flat also makes a difference. Obviously the assumption of no force required to maintain speed is no longer valid as you need to put in work to raise the mass up the hill. But do you get all of that benefit back when coasting down the other side? Also, rising may be done slower and descending more quickly, or perhaps not. Again more variables to make it more complicated.

Lots of variables to make it complicated, but I'm sure for such a brick-shaped vehicle, the wind resistance is the major one. Speed is evil in this regard.

Last edited by FamilyMan; 18-04-10 at 09:44 AM.
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Old 18-04-10
Rocketsurgery Rocketsurgery is offline
Join Date: Dec 2008
Posts: 109

Familyman, the wife's VW has an instantaneous fuel consumption readout on the rip computer. When stopped, the display changes from L/100km to L/hr. The 2 litre 4 cylinder diesel uses .7l per hour idling, and 1 lire per hour with the A/c on. I'd doubt your vehicle uses 4.5 litres per hour idling unless you are talking something WAY bigger.

"speed is constant (mostly) so acceleration is zero, so force required to keep mass moving is zero." No, rolling resistance is another issue quite apart from wind resistance. There are also mechanical losses to overcome, each set of bearings/gears etc steals some energy from the system.
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