Size and aerodynamic efficiency

[hawk20]

Aerodynamic drag at the speeds most of us travel in the UK for most of the time, is pretty irrelevant.

Have a look at 'miles since reset' and the average speed and shock yourself and ask why any of us care about either grunt or drag.

Average speed in cities is about 11-17mph. Outside on most trips it is quite hard to average much above 45/50.

Even at the legal maximum of 70mph, aerodynamic drag is not a big part of total resistance.

 

[Dryce]

Even at the legal maximum of 70mph, aerodynamic drag is not a big part of total resistance.

ISTR seeing an average figure of around 40% of energy used to overcome aerodynamic drag on open roads/motorway driving.

That would imply that at 70mph it is a large part of total resistance.

As you say - it's not so significant in other driving phases.

 

[hawk20]

ISTR seeing an average figure of around 40% of energy used to overcome aerodynamic drag on open roads/motorway driving.

That would imply that at 70mph it is a large part of total resistance.

As you say - it's not so significant in other driving phases.

Let's hope someone expert on this can give some links. I read somewhere a BMW spokesman saying that the difference between a low drag design and a pretty ordinary design on total resistance at speeds below 70 mph was really quite small.

 

[Dryce]

Let's hope someone expert on this can give some links. I read somewhere a BMW spokesman saying that the difference between a low drag design and a pretty ordinary design on total resistance at speeds below 70 mph was really quite small.

Ahhhh.

This doesn't really say that drag is or isn't a major contribution to total resistance. What it does say is that design doesn't have much impact on whatever that contribution is.

So whether it's 40% or 10% at a given speed it is what it is and there's not so much they can do about it that has much impact below 70!

 

[Mactech]

Newton was pretty bright and his laws still apply at any speed below the speed of light.
Whilst I fully agree that within the city limits the weight and the engine will define most of the effiecncy, in steady state conditions aero drag is a very real factor.
In our race car simulations only about 4% of engine power is used up to overcome rolling resistance and transmission losses. The rest is used to overcome inertia (if accelerating) and areo drag. (Proportional to the square of the speed).
At 70mph a typical car will use about 35bhp to maintain its velocity. Well over 50% of this is to overcome aero drag. (Depending on Cd)
My average road speed is about 50mph and while the aero is not as important as on a race car, it is a very real factor.
Try driving a Luton van against the wind if you are still unsure!

 

[hawk20]

Newton was pretty bright and his laws still apply at any speed below the speed of light.
Whilst I fully agree that within the city limits the weight and the engine will define most of the effiecncy, in steady state conditions aero drag is a very real factor.
In our race car simulations only about 4% of engine power is used up to overcome rolling resistance and transmission losses. The rest is used to overcome inertia (if accelerating) and areo drag. (Proportional to the square of the speed).
At 70mph a typical car will use about 35bhp to maintain its velocity. Well over 50% of this is to overcome aero drag. (Depending on Cd)
My average road speed is about 50mph and while the aero is not as important as on a race car, it is a very real factor.
Try driving a Luton van against the wind if you are still unsure!

But we are not talking about either racing cars at racing speeds or about a Luton Van. If you take a normal car of normal shape the point someone from BMW was making was that spending vast amounts of time and effort on the aerodynamics has very little effect on economy at speeds below 70mph.

 

[Dieselman]

http://www.atmosphere.mpg.de/enid/Information_ss/Velocity___air_drag_507.html

More than 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed, according to the drag equation.

So reduce drag and save say 20% fuel at cruising speeds. Reduce weight and save the same at town speeds and accellerating.

 

[television]

http://www.atmosphere.mpg.de/enid/Information_ss/Velocity___air_drag_507.html

More than 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed, according to the drag equation.

So reduce drag and save say 20% fuel at cruising speeds. Reduce weight and save the same at town speeds and accelerating.

That fits in with my trial exactly in the cruising at 60 mph. I must shed some weight off the SL, and me

 

[hawk20]

http://www.atmosphere.mpg.de/enid/Information_ss/Velocity___air_drag_507.html

More than 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed, according to the drag equation.

So reduce drag and save say 20% fuel at cruising speeds. Reduce weight and save the same at town speeds and accellerating.

The difference between an SUV at say 0.375 and a saloon at say 0.325 (average figures from your excellent link) is under 15%. And 15% of 60% is only a 9% reduction in fuel. Or say 32.7mpg versus 30mpg. Not a whole lot.

But the BMW statement was for under 70mph average speed as most of us do. At 50mph aerodynamic drag is under 50% of total resistance. And then 15% off the CD only saves under 7.5%. Or say 32.25mpg versus 30mpg. Not enough to sway many owners choices IMO.

 

[Dieselman]

You forget the rolling drag is significantly higher with a 4 wheel drive vehicle, which also usually have a larger frontal area as well as drag factor and increased weight.

Had I made my post a political post against 4x4.? I didn't thing so but I'll have a review.

reviewed.

Nope..!!

 

[hawk20]

You forget the rolling drag is significantly higher with a 4 wheel drive vehicle, which also usually have a larger frontal area as well as drag factor and increased weight.

Had I made my post a political post against 4x4.? I didn't thing so but I'll have a review.

reviewed.

Nope..!!

No, indeed not, Nor was I suggesting that. Merely using 4x4s as an example of not very good drag coefficients.

But forget all the theory for a second, the truth is that a state of the art saloon car like the Mercedes S class 320cdi does 35mpg combined govt figure and an ML320cdi with the same engine does 30mpg combined. Not that much difference and not enough to dramatically affect the choice of someone who wants the not very aerodynamic car rather than a more aerodynamic one.

And even in the example I give a part of the difference is due to 4x4s having extra rolling resistance. So I think the BMW point that aerodynamic drag is not such a big deal at average speeds below 70 is a pretty fair one?

 

[st4]

I assume 4x4 people mean an SUV. Technically an Audi A6 quattro is a 4x4 as its AWD.

 

[wallingd]

I've not read this thread yet, but want to post an idea.

Given the rising fuel prices how feasible + wot mpg gains could be achieved by doing very simple aero tweaks to the front of a car e.g, primarily, taping over the bonnet shutlines + maybe covering some of the air intakes? Maybe even with Sellotape?

A very topical and interesting thread.

 

[stats007]

Really not worth doing. Changing your driving style would benefit you more.

 

[DieselE]

Just a thought but I wonder what improvement in the fuel efficiency of 4x4s and SUVs would be achieved by deleting the standard roof bars.

 

[Brian WH]

Just a thought but I wonder what improvement in the fuel efficiency of 4x4s and SUVs would be achieved by deleting the standard roof bars.

Good point, but it would make no difference, as most are only used on the school runs.

Ducks.

 

[wallingd]

Ducks.

 

[BTB 500]

But forget all the theory for a second, the truth is that a state of the art saloon car like the Mercedes S class 320cdi does 35mpg combined govt figure and an ML320cdi with the same engine does 30mpg combined. Not that much difference and not enough to dramatically affect the choice of someone who wants the not very aerodynamic car rather than a more aerodynamic one.

However the 'extra-urban' figures are 36.2 mpg for the ML320 CDI and 44.1 mpg for the S320 CDI. The average speed for the extra-urban cycle is only 39 mph, so that 8 mpg difference is pretty significant ... at a steady 70 mph the better aeodynamics of the S class would really be felt.

 

[hawk20]

However the 'extra-urban' figures are 36.2 mpg for the ML320 CDI and 44.1 mpg for the S320 CDI. The average speed for the extra-urban cycle is only 39 mph, so that 8 mpg difference is pretty significant ... at a steady 70 mph the better aeodynamics of the S class would really be felt.

Yes but the point is we don't live at a steady 70 in the UK. It is quite hard to average 50 over mixed driving, week in week out.

And believe it or not the S class 320cdi at 70ish does about 37mpg (drove mine to South of France) tankful to tankful. ML320cdi does a tad over 30 in similar conditions. Similar difference to your figures.

 

[Dieselman]

Even using Hawks combined figures there is a 20% difference. That's quite significant over a years motoring.

Say 10,000 miles at 35mpg = 285 gallons/1300 litres

10,000 miles at 30mpg = 333 gallons/1516 litres.

At £1.27 that's a difference of £275.