Thread: Interesting thing

Welcome to the 2500rpm club.

Your fuel economy surprise occurs to most LR diesel owners sooner or later, when they realize how to use a diesel properly. It took me quite a while to figure it out, after a v8.

From my point of view, after 2500rpm, and about 10psi boost, you've reached the top of the fuel/power curve - and it seems that 95 is the magic number when it comes to wind resistance for big vehicles.

I'll leave it to a Puma owner to discuss 6 speeds, but from a 300tdi viewpoint, there isn't sufficient power to justify an overdrive. Besides, you need to keep rpm up to keep coolant circulating when the engine is working hard.

Remember to fix your water pump, flush and use proper coolant - aluminum does not like ordinary water

Am not surprised that pushing from a cruisy 100kmh to 110kmh results in a marked increase in fuel.

Airflow is a big factor, given the aerodynamic qualities of the Deefer. In the helicopter world, drag increases proportional to square of airspeed.

From the books:

Drag = Coefficient of drag x 1/2 rho (air density) x velocity (squared) x area. So if the C drag, air density and area all remain the same, drag = V squared.

Increasing your speed means a huge increase in drag, which is what the engine power has to overcome. The power required to beat this drag increases as a cube of the velocity. I can't remember the mathematics to resolve all the formula but I can the answer!

So the 90 odd kw of the TDi and TD5 struggles to get the Deefer to decent speeds, and because it's flogging it's rings out above the economic engine seeds, the fuel economy really slides.

Smoozing around the metro area I would easily pull 600 kms out of the tank. On the 1700 trip to Karratha after New Years saw 450-500 km's per tank if I was lucky, and that was sitting on 110kms....windows, up, aircon off, air vents closed, mirrors folded back, aerial down.........

I think you've solved the water leak - the lower rpm reduces water pump pressure.....if it starts making water, then be worried!!

aew849
04 130 HCPU

Originally Posted by aew849

The power required to beat this drag increases as a cube of the velocity. I can't remember the mathematics to resolve all the formula but I can the answer!

You were doing so well, here's the final hurdle.

You've already got F = 1/2density*Cd*A*v^2
Power is simply force * velocity, so change the F to a P and the V^2 to a V^3 and you're there.
P = 1/2density*Cd*A*V^3

P = watts
density = kg/m^3 (1.2kg/m^3 is pretty good).
Cd is just a correction factor, maybe 0.6 for a landrover.
A is the area of the hole your landrover punches in the air (square meters)
V is velocity in metres per second. 100km/h = 27.7 m/s

Originally Posted by Dougal

You were doing so well, here's the final hurdle.

You've already got F = 1/2density*Cd*A*v^2
Power is simply force * velocity, so change the F to a P and the V^2 to a V^3 and you're there.
P = 1/2density*Cd*A*V^3

P = watts
density = kg/m^3 (1.2kg/m^3 is pretty good).
Cd is just a correction factor, maybe 0.6 for a landrover.
A is the area of the hole your landrover punches in the air (square meters)
V is velocity in metres per second. 100km/h = 27.7 m/s

Cd is not exactly a correction factor - it is the Coefficient of drag and is the factor that relates the measured drag to that of a flat plate normal to the airflow - it is the ratio of the areas. And I think 0.6 for a Landrover is optimistic, although it will depend on the body style - probably the wagon is the best except for a soft top minus the top and with the windscreen down, where the reduced area is a major advantage. A trayback is bad unless the tailgate is open or removed. Add a roofrack and things really start to go downhill!

The formula is actually fairly easy to derive from basic physics. The key part of course, is that drag increases as the square of the velocity. There are a few minor effects working in the opposite direction, such as the reduction in rolling resistance due to the centrifugal force on the tyres as speed increases, but nothing can overcome the fact that over about 60kph aerodynamic drag is by far the major part of resistance. In petrol engines, the changes in energy efficiency with rpm and throttle opening can have an overriding influence, particularly when the Cd is very low - a good example is my Citroen DS, which, with a Cd of under 0.3, and a rather oldfashioned carburetter engine, the most economical speed is 110kph.

With diesels, including all the ones fitted to Landrovers, the fuel efficiency varies much less with power and rpm, and although there will be a peak, it is not a very pronounced one, so that basically, fuel economy keeps dropping the faster you go from about 50kph where aerodynamic drag starts getting significant.

I do not think that the fact that going over 95 takes you past the most economical engine speed is the problem - I think it is just that the fuel usage curve implied by that V^2 (the change back from V^3 is when you go from fuel/unit time to fuel/unit distance) starts to get pretty steep about there.

John

Thats it......i'm taking the roof racks off the Rangie

Puma will do better at higher speed but in general the economy is actually a little worse than the Td5.

As for the gearbox question there's an interesting thread on Defender2.net about Td5 and G6 box

Originally Posted by Chase

Thats it......i'm taking the roof racks off the Rangie

... And I think I'll stop driving mine

JC

Originally Posted by JDNSW

Cd is not exactly a correction factor - it is the Coefficient of drag and is the factor that relates the measured drag to that of a flat plate normal to the airflow - it is the ratio of the areas. And I think 0.6 for a Landrover is optimistic, although it will depend on the body style - probably the wagon is the best except for a soft top minus the top and with the windscreen down, where the reduced area is a major advantage. A trayback is bad unless the tailgate is open or removed. Add a roofrack and things really start to go downhill!

The Cd is a correction factor, it corrects the actual projected area to an equivalent projected area. Your description is correct, but your interpretation isn't.
It is measured and and applied to correct the formula.
A 93 defender 110, 95 discovery and 94 defender 90 all have a Cd of 0.55.

There have been many studies done on US pickups aerodynamics, they do better with the tailgate up than down. They do even better of course with a streamlined canopy fitted.