Thread: Aerodynamics.............

IME every 10km/h over 100km/h costs around a litre per hundred.

When I travel with a mate with a dual cab Hilux he gets better open road economy than me, but a soon as we hit the dirt I get better - the Deefer's push a lot of air...

Yep. You are about right. 120 kmh say for 300 km. that's six litres. I drove 570km till the lights came on. Usually comes on at 620km in the 130. That's 50km shy of usual

After the bas remap in the old 110 puma the efficiency in that thing was good. 650 to the tank before the lights came on. Highways were similar too. The torque gain must negate or overcome the aerodynamics to a certain degree. Also the useable rev range widens giving reducing need to change gears.

I might book a remap in next week.

I'm not surprised.

Let's suppose that your around town running includes mostly freeways and you can sit on 65km/h most of the time.

How much more power do you think it takes to push something through the air at 120km/h compared with 65?

Twice as much? Two and a half times as much?

I think the answer is about three and a half times as much. Even if we allow for the stop start driving in town, the power needed to push through the air at 120 is a heck of a lot more, so naturally you will use quite a bit more fuel.

I haven't tried driving at 120 to see how bad my fuel consumption would be, but I have observed the opposite effect. Whenever I go to Tasmania, I get better fuel consumption even with the camper on than I do on long trips around here. The roads, the distances and the scenery mean that I drive slightly slower and the fuel consumption improves.

Driving south from the Alice and east from Kalgoorlie I always seemed to hit headwinds,it's not unusual for me to get less than 500 from a tank,600+ in town. Pat

The mention of headwinds has reminded me of the worst fuel consumption I have had on the open road.

Driving west from Dubbo with the camper on, I encountered hundreds of kilometres of headwinds that I suspect were over 50km/h. Even when I slowed down about 15km/h slower than I would normally drive, I used 2 or 3 litres/100km more than usual.

With that headwind, I calculate that I was trying to push the Defender through the air at about 130km/h.

All said and done. Would not trade it for anything else. If we wanted aerodynamic we would have bought something else I guess. These buggers are made to plot along... Smell the roses (or dust).

That said. She is good for building work. Site to site. Pick ups and drop offs. Way better fuel consumption than my mates 78 cruiser.

Remember the important stuff.

Aerodynamics don't count in low range.

There are two major aerodynamic problems with the Defender - the first is the large cross section area compared to most light vehicles, but the second is the high ground clearance (and rough underbody including wide wheel arches to accommodate long travel suspension) mean that a large part of the aerodynamic drag is from turbulence under the vehicle. There is little that can be done about either of these and retain the offroad ability and interior space.

A future Defender replacement is likely to get some improvement by reducing the 'waste' cross section area represented by the chassis, but at the expense of flat floors.

The square rear of the wagon/hardtop is almost certainly a lot more aerodynamic than you think, as the sharp corners allow the airflow to break cleanly from the sides and roof. The front is more problematic, and drag would almost certainly be improved by a row of vortex generators across the front edge of the bonnet and mudguards - this would also help reduce the number of insects on the windscreen.

Aerodynamic drag is proportional to the square of the speed, which, when you think about it, represents an increase in energy consumption per kilometre to overcome aerodynamic drag in direct proportion to the speed. Other energy losses, primarily rolling resistance of tyres and frictional losses within the engine and drive train are generally independent of speed or may actually decrease with speed, for example, centrifugal force will decrease rolling resistance of tyres as speed increases, but these effects are usually small. For most practical vehicles, aerodynamic drag reaches 50% of total drag at around 60kph, so that speed reductions below this give little advantage in fuel economy.

Variations in engine efficiency with rpm and throttle opening also come into it, and as a general rule you can say an engine is most efficient at the maximum torque rpm. The variation in efficiency is quite small for diesels, but can be very large for petrol engines, especially carburettor engines - this means, for example, that my Citroen, with exceptional aerodynamics, has its most economical speed at 110kph.

All these losses discussed above are in addition to the energy actually needed to move the vehicle over the road, and this depends mainly on the road surface and how it is driven - every time you apply brakes you waste energy, so the most economical driving technique is to drive as if you knew the brakes did not work - anticipate the need to slow and take your foot off so you rarely have to touch the brake pedal. Similarly, slow on crests of hills so you don't have to use brakes on the down slope to stay within the speed limit etc.

And if you drive too slowly, remember there are parasitic loads on the engine, such as the airconditioning and alternator, that depend mainly on how long the engine is running, so that on a per kilometre basis these increase as you travel slower.

As a last comment - some Series Landrovers were fitted optionally with an economy device consisting of a strong spring that came into effect on the accelerator pedal at about half throttle, so that unless a determined effort was made, you never got more than half throttle. I have never owned one of these, but they were said to be quite effective!

John