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

I was looking at the LR bulbar on the front of a Defender at the weekend. It has sloping fronts on the two uprights either side of the grille. I was thinking a mesh screen might keep a lot of bugs out of the grille. I wonder if such a screen, sloping backwards at the top, might also deflect a lot of air over the front edge of the bonnet and improve the aerodynamics? Just a thought...

Originally Posted by DiscoMick

I was looking at the LR bulbar on the front of a Defender at the weekend. It has sloping fronts on the two uprights either side of the grille. I was thinking a mesh screen might keep a lot of bugs out of the grille. I wonder if such a screen, sloping backwards at the top, might also deflect a lot of air over the front edge of the bonnet and improve the aerodynamics? Just a thought...

Might do (the aerodynamics - certainly catch the bugs), but I suspect it will not go high enough. The problem is the smoothly rounded front edge of the bonnet and guards tends to keep the airflow following the top surface due to the Coanda effect - and then the airflow hits the near vertical windscreen and is deflected straight up. What is needed is either a row of turbulence generators along the top edge to break the airflow away, or a deflector fixed to the top of the bullbar that extends above the bonnet - often seen on trucks, but of doubtful legality.

One possibility occurs to me - the Citroen DS wagon I used to own had a fixed roof rack as standard. This had the front bar of the roof rack sleeved with a plastic tub that had spiral ridges on it, so that it acted as a turbulence generator. I wonder what the effect of wrapping the top bar of the bullbar with something similar would be? Perhaps a length of 10mm laid rope wound with spiral about 25mm between turns and maybe back to other way to give a diamond pattern. It could be temporarily taped in place, and if successful, held with a hose clamp at each end.

John

Originally Posted by JDNSW

Your first diesel? Petrol engines vary more in efficiency than do diesels, and are less efficient in stop/start driving than in continuous operation, where diesels are pretty much the same efficiency.

John

Yes, our first diesel.
Thanks for that, Pickles.

Originally Posted by DiscoMick

I was looking at the LR bulbar on the front of a Defender at the weekend. It has sloping fronts on the two uprights either side of the grille. I was thinking a mesh screen might keep a lot of bugs out of the grille. I wonder if such a screen, sloping backwards at the top, might also deflect a lot of air over the front edge of the bonnet and improve the aerodynamics? Just a thought...

And what else do you think it would do?

Like raise the load on your cooling system

Originally Posted by Tombie

And what else do you think it would do?

Like raise the load on your cooling system

I did wonder about that, but would an angled bug screen really make much difference? The electric fans on my Disco come on whenever the air con is on anyway.

Originally Posted by JDNSW

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,

hi john - i've seen you make this comment a few times before so thought i'd ask. i've got a skoda yeti with the 2.0 vw diesel & dsg. i've been driving around with the instantaneous fuel consumption showing on the dash. the 'simple' rule seems to be the lower the revs for a set speed the lower the fuel consumption.

the specs have the peak torque from approx 1700 rpm upto whatever. the car always drops into the lowest gear it can and will often be on 1100 to 1200 rpm. if i use cruise control to drive at various speed points there is never a reduction of the instantaneous fuel usage when in its peak torque range.

as a further test, if i use cruise control and drive at a set speed at 1800rpm in a gear then manually change up a gear the rpm drops well outside of the peak torque range yet the instantaneous fuel usage always drops.

it seems like there is a host a variables which are working here but the primary factor is the engine rpm?

Originally Posted by pibby

hi john - i've seen you make this comment a few times before so thought i'd ask. i've got a skoda yeti with the 2.0 vw diesel & dsg. i've been driving around with the instantaneous fuel consumption showing on the dash. the 'simple' rule seems to be the lower the revs for a set speed the lower the fuel consumption.

the specs have the peak torque from approx 1700 rpm upto whatever. the car always drops into the lowest gear it can and will often be on 1100 to 1200 rpm. if i use cruise control to drive at various speed points there is never a reduction of the instantaneous fuel usage when in its peak torque range.

as a further test, if i use cruise control and drive at a set speed at 1800rpm in a gear then manually change up a gear the rpm drops well outside of the peak torque range yet the instantaneous fuel usage always drops.

it seems like there is a host a variables which are working here but the primary factor is the engine rpm?

There are two concepts at work here. One is efficiency and the other economy.
They are often misused/interchanged, but they are quite different things.

Efficiency is how good the engine is at turning fuel into power. Economy is how good the vehicle is at turning litres of fuel (or $ spent) into distance travelled.

John is correct that the best overall efficiency point for a diesel engine is at rpm around max torque and high load (about 90%).
But this is only the most economical operating point if you require the maximum power the engine can produce at that point.

If you require less power then the best efficiency points (for the lower power) are found at lower rpm. Hence your gearbox changing up to lower rpm. The Engineers who programmed the box knew where to find the best economy.

I have a 6sp manual tdi scout and have the same thing. ~1200rpm is the lowest usable rpm resulting in the best economy if you can run smoothly at that speed.

Originally Posted by pibby

hi john - i've seen you make this comment a few times before so thought i'd ask. i've got a skoda yeti with the 2.0 vw diesel & dsg. i've been driving around with the instantaneous fuel consumption showing on the dash. the 'simple' rule seems to be the lower the revs for a set speed the lower the fuel consumption.

the specs have the peak torque from approx 1700 rpm upto whatever. the car always drops into the lowest gear it can and will often be on 1100 to 1200 rpm. if i use cruise control to drive at various speed points there is never a reduction of the instantaneous fuel usage when in its peak torque range.

as a further test, if i use cruise control and drive at a set speed at 1800rpm in a gear then manually change up a gear the rpm drops well outside of the peak torque range yet the instantaneous fuel usage always drops.

it seems like there is a host a variables which are working here but the primary factor is the engine rpm?

My remark primarily referred to petrol engines and is simply a rule of thumb anyway - as is peak torque; usually the shape of the torque curve is more important than the actual peak torque rpm.

The variation in efficiency for diesels (and in fact for some of the latest petrol engines) is quite small, although it should be noted that the equivalence of maximum efficiency and maximum torque really only applies at full throttle. But you are right, there are a host of variables. Just a few - combustion efficiency, which is the one that varies little for diesels, thermal efficiency, that is, what proportion of combustion heat goes into the cooling system and exhaust (turbocharger recovers some of this), pumping losses - this is where petrol engines lose when throttled compared to diesels, aerodynamic losses within the engine - proportional to rpm or more likely the square of the rpm, fortunately pretty small for most engines, frictional losses, usually pretty small, parasitic losses (alternator, aircon, fan, oil pump, power steering, water pump, all pretty much proportional to rpm, but can represent a large proportion of power use at cruising speeds).

There are two confusing factors affecting your observation that peak efficiency is dependent on engine rpm - the first is that for most cars normal driving is at very small throttle openings, using perhaps 10% of available power, so that actual engine efficiency variations are swamped by other losses, particularly parasitic losses. The second is that your observations include variations in gearbox efficiency - which will usually be less as you change up.

John