Interesting thing

[Rovy]

Interesting thing happened to me. I was on a long trip in the Defender with trailer to pick-up a bike. As it was going to be a 10hr round trip I made sure all the fluids where topped up. When I stopped for a bite to eat 5hrs into the trip I checked the water again. It was down to almost the bottom of the reservoir, as I couldn’t find a leak anywhere except near the water pump seal I topped it up and went on my way. The next day after breakfast I checked again and it was down a bit, about a quarter or so. Topped it up again and on my way. Anyway rather than boring you with the details of continuing to stop and topping the water up that was leaking from the back seal on the water pump I will quickly get to the punch line.

So after using all my water I was using I drove at about 95 km/hr to the next rest stop. Filled up my water containers and went to top up the water. To my amazement I found the water level to be exactly where it was when I last topped up. That is where it stayed for the last 300km of my trip as I continued to drive at about 95 to 100 km/hr. I assume what was happening is that at the lower engine rpm there was lower water pressure from the water pump.

The other big surprise was at the pump, for the last 300km I averaged 9L/100km. At my usual highway speed I would be getting 13 to 14 L/100km. I don’t believe the difference was due to the higher wind resistance at the higher speeds alone, but also to the high rpm needed in fifth gear to maintain 110+ km/hr. I have heard that trucks can do Sydney to Brisbane on 10L/100km.

Two questions:

Does the new defender have better fuel economy with its sixth gear?
Is it possible to retrofit an overdrive into the five speed gearbox?

Not that I want to get into my gearbox after just spending $4k on it.

Cheers

[langy]

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

[aew849]

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

[Dougal]

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

[JDNSW]

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

[Chase]

Thats it:o......i'm taking the roof racks off the Rangie:lol2:

[dmdigital]

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

[justinc]

Thats it:o......i'm taking the roof racks off the Rangie:lol2:

...:o And I think I'll stop driving mine:p:p:p:p

JC

[Dougal]

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.

[pawl]

Ummm.....I would have to agree with JDNSW here. Cd is the drag Coefficent . Well, thats what I was taught in Helicopter Aerodynamics years ago.
It is a dimensionless number used to describe the drag or resistance of a shape through air or water.

[lokka]

...:o And I think I'll stop driving mine:p:p:p:p

JC


I need to put the 4BD1 in mine :cool::cool:

[JDNSW]

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.
1. A 93 defender 110, 95 discovery and 94 defender 90 all have a Cd of 0.55.

2. 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.

1. Drag coefficients published by manufacturers (and parrotted by everyone else) are in many cases very suspect (I don't know whether this is the case with Landrover but I would want to see proof it wasn't rather than the reverse). There are two reasons for this - in most cases they are measured on models in wind tunnels, and do not properly account for the roughness of the underbody. This is particularly important for vehicles like the Defender with a very high ground clearance. The second reason (probably less important for unstreamlined shapes like Landrovers) is that for most body shapes the Cd is very sensitive to small changes in body attitude, which means that the quoted figure is only achieved in practice when loaded exactly as the test model. It would be surprising if all body styles of Defender 90, 110 and the Discovery had the same Cd, regardless of the fact that the published figure is the same, although I would not be surprised at their all being similar, as they are all more or less boxes, and even more important the very similar underbody must represent a fair bit of the drag - but agreement to two decimal places does sound unlikely.

2. I am surprised to hear this - certainly in the US pickups are commonly operated with flowthrough tailgates or no tailgate, although this may well reflect marketing rather than anything else. In my own experience, tailgate or no tailgate on my big trailer is good for more than 10kph in maximum speed when empty, although I will admit that the tailgate on the trailer is a lot further back than on a ute.

John

[Dougal]

Ummm.....I would have to agree with JDNSW here. Cd is the drag Coefficent . Well, thats what I was taught in Helicopter Aerodynamics years ago.
It is a dimensionless number used to describe the drag or resistance of a shape through air or water.

A coefficient of any type is a correction factor, whether it's for fluid drag, friction or otherwise. It corrects the ideal situation to fit with a measured result.
They have to be dimensionless (unitless) otherwise they unbalance the equations they're correcting.

As for landrovers all having pretty similar Cd's, well they're all at the upper end of the scale, so getting it worse is difficult.:D
Even a hummer is listed as being better (0.5).
With a dyno plot of actual wheel power and a measured top speed you could work it out yourself.

Here's the most comprehensive list I've found.
The Mayfield Company Homepage - Coefficient of Drag Tables and Curves (http://www.mayfco.com/tbls.htm)
Landrover list the D3 Cd as 0.41 in their tech specs, but they also say it has 24 valves per cylinder.

[isuzurover]

Everyone is right. Cd = drag coefficient, and is basically a correction factor.

For values of Re < 1 (Stokes flow), Cd = 24/Re. (Re = reynolds number) - so Cd values can go up to around 1x10-5.

For Re > 1000 (where vehicles usually operate), Cd ~ 0.44 for spheres, and my texts say the ranges are from about 0.04 for (subsonic) planes, and up to almost 1 for vehicles such as "delivery vans".

Dougal is right on the ute tailgate issue. It was even on mythbusters! IRC, both having the tailgate up and having a tailgate replacement of wide plastic mesh were better than having it down or removing it.

[JDNSW]

.........
Even a hummer is listed as being better (0.5).

Higher ground clearance and cleaner flow underneath, plus a better aspect ratio.

With a dyno plot of actual wheel power and a measured top speed you could work it out yourself.


With appropriate adjustments for temperature, pressure and wind velocity - but probably give a more realistic figure than some of the manufacturers' ones.

John