The effect of FOG on mpg!

John H

John H

Member
I have just driven home from Newcastle, a 165 mile journey I have done four times now in my new A2 TDi under different conditions. In a strong headwind I got 48 mpg, in neutral conditions about 55 mpg, with a strong tailwind I have managed 62 mpg.

Tonight I have just done the journey from hell in thick pea soup fog, mostly crawling along at barely 50 mph, with only a few blats up to 80 where it cleared.

Result ... 70 mpg!

:D

Though I'll be happy to pay for a bit of extra diesel if I can avoid that damned fog next time! ;)
 
That's because you were travelling at a steady(ish) speed that is almost bob on with the most efficient and the air entering the engine was very dense (close to dew point), hence had a lot of oxygen, hence was burning the fuel efficiently (the air meter measures flow, volume and temperature, not density).

Result = RESULT!

Cheers,

Mike
 
Actually, result = OK in fog, but I'd die of boredom if I had to drive at that pace that on a regular basis! :(

However, by being occasionally virtuous in my fuel usage, I don't feel so bad about the odd profligate blast in the Porsche!

:D
 
Yeah (i think) 58mph is the ideal speed to to travel at for saving fuel, I guess that is why most large lorries travel at this speed.
 
Lorries at 58mph

All large lorries in the UK have to have a speed limiter set at 90kmh (56mph). A small tolerance is allowed. Without the limiter, I think you would find that most would travel a bit faster as most people would rather get to their destination a little earlier (or make up lost time stuck in traffic jams).
 
I used to get equally better performance on my moped back in the days, these 50cc's would fly in moisty foggy evenings.;)
ah yeah, the same joys still apply, we'll never grow up.
 
Hmm, I'm not sure that the intake telemetry doesn't measure air density. I'd always thought it did because it measures the air mass.

Most air mass meters (to give it what I think is the proper title) are a metal filament or, on most VW/Audis, a metal film that is electrically heated to a set temperature. As the air flows over it, the air tries to cool it and the electronics sense this initial small drop in temperature by lower resistance of the film. It then increases the voltage across the film which increases the current flowing through it and therefore it's temperature increases back to preset. The electronics seek always to keep it at the constant preset temperature. The voltage applied to the film is a function of the mass of air (velocity, density and temperature). The temperature and atmospheric pressure (from weather systems and/or altitude) should have no effect on the consumption of fuel as far as the engine management system is concerned.

There are a few other effects that need to be considered, discounting variables like climate control on, low tyre pressures, roofracks and windows open: (1) On short journeys especially, the increased time to warm up the engine will result in increased consumption. (2) Lower temperatures will result in colder and therefore lower pressure tyres yielding lower consumption (slightly offset by the tyre compound being harder and therefore decreasing roll-resistance), (3) Having to push the car through denser air (fog will have an effect) will increase consumption, as will (4) wet roads with even a small amount of standing water.

Gee, this is boring!

If you are still with me, a lot of people think that 56mph is the most fuel efficient speed for driving. It isn't. I think this probably grew as myth because before the advent of consumption figures given as combined cycle, it used to be quoted at three different speeds, one of which was 56mph (90kph) - the other was 75mph and I can't remember what the third was. The 56mph figure was usually the lowest of the three and therefore appeared to suggest it was the best speed for eco-warriors. My guess would be that you'd find doing 900rpm in fifth gear (~28mph?) would give the lowest consumption. The amount of energy that is used just in moving air increases hugely with speed (think speed squared and you won't be too far wrong).

You can see why my kids, when they had homework problems, always pleaded with me for the briefest answer possible!
 
How about drive absolutely flat out then you'll get there before the fuel runs out ;)

I once heard a traffic police officer report that a driver of a certain gender (ahem) used this as an excuse for speeding.
 
ULP said:
I think the last suggestion of circa 900rpm is incorrect - there are many more factors than just air resistance, not least of which is the most efficient point at which to run an engine. In the real world, it is most-likely the point of peak torque as this is where you will vary the engine speed least when coming up against inclines, varying speed increases consumption. Accordingly, the A2 TDI 75 is in theory most efficient at about 60mph, 2200rpm.
Click to expand...

(Trying to restart thread on a bit more serious vein as it interests me)

Granted the speed of peak efficiency may not be 900rpm - although it's not likely to be too far off - it will certainly be at the lower end of the rev range. It doesn't have anything to do with either peak torque or peak power as these could even coincide with peak inefficency (ever seen racing diesel saloon cars belching black smoke to get the torque and power??). You also need to remember that oil drag in engines is significant and also increases with engine speed.

I don't have DIS on my A2 but anyone with DIS and Cruise Control could tell you the best speed! On my 1.9TDI Passat, which has both, I know that fuel consumption keeps dropping with speed although it's difficult to do long journeys too slowly without being antisocial and a bit dangerous.

See the following for a bit of a horror story in this respect: http://www.mirror.co.uk/news/topstories/2007/12/22/10mph-on-m32-89520-20262578/

The following are recent journeys from my Passat in still conditions:
165 miles at 85mph - 52.1mpg, return at 65mph - 67.7mpg (2 passengers)
330 miles at 60mph - 70.9mpg (solo)
625 miles at 50mph - 75.1mpg (solo)

I think if the speed keeps dropping below 50 then consumption will still decrease, though with diminishing effect. Where the consupmtion/speed graph would bottom out would be interesting to find out.

How did we get here from fog? :confused:
 
Most economical speed

The engine works most efficiently at around the peak torque speed. So, the most efficient engine speed for a TDI75 is around 2200 rpm. However at this engine speed, the car will be travelling at around 60+ mph. Aerodynamic drag will be so great at this speed that the fuel consumption will not ne at it optimum.
The most economical speed is the speed that the engine will pull comfortably in top gear. So, for the TDI engine I would suggest around
1250 rpm, or around 35 mph. Much slower, and the engine becomes too inefficient, much faster and aerodynamic drag outways any engine efficency improvements.
As an aside, wet (fog etc) air a great for getting a bit more power out of an engine, as it has the same effect as an intercooler on an engine. As moist air is drawn into the engine, the suction causes a lowering of atmospheric pressure which evaporates the water content causing a reduction in temperature, and makes the air more dense. This means that more fuel can be burnt, giving more power.
The oposite is also true, in that during a heatwave, engines (especially turbos with very small or no intercooler fitted) produce less power.
High powered piston aircraft and early jet aeroplane engines used to inject water into the engines when high power was required inorder to optain a few extra horsepower. Once in a while, too much water was injected and this would 'put the fire out' resulting in an unwelcome silence!!!
 
As you say, water injection is a useful tool to keep post-turbo temperatures down and acts like a better intercooler. This is only really effective if the water is in droplet form as it depends on the phase change from liquid to gaseous with the corresponding heat required to do this. The air charge is cooler and more dense. More dense = more air per stroke, allowing more fuel and bigger bang.

(Strange thought: anyone yet tried injecting Barry Scott's Cillit Bang :D)

I doubt this is as noticeable in fog. I don't know the detail of what happens when water droplets in air (fog) goes through an air filter but would be surprised if they got through. What will get through is air at 100% Relative Humitidty (at point of intake) which, at 20 Celsius, is around 0.014 Kg water in 1 Kg of air (roughly equivalent to 1 cubic metre of air at sea level and room temperature) - not much! However the heat capacity of water is much greater than that of air so this will reduce the post-turbo temperatures and therefore make more power available. Certainly not the 30% extra that water injection gives (with or without ethanol as used in aero engines).

Of course all of this is only talking about increasing the power of the engine in absolute terms, not the power delivered per unit of fuel (efficency). I don't know whether water-injected engines or those running in high humidity are any more efficient, even if they are more powerful. I take it from the original post that there may be an efficiency gain.

Still insist you're barking up the wrong tree talking about peak torque. This is a red herring. In any case, peak torque is a "foot to the floor" engine running condition. Even if you think this may be the best point of efficiency (which it is unlikely to be) this would only be true of full throttle operation and no guarantee that it would be most efficient for very light throttle running.

Warning: anyone following the thread to this point is in serious need of getting a life. If you hear of a life going spare, I obviously need one too!
 
dilbert said:
Warning: anyone following the thread to this point is in serious need of getting a life.
Click to expand...

Au contraire ... this has turned into quite a fascinating debate, albeit at a tangent from what I expected when I started it.

My initial point was that the fog had forced me to reduce speed drastically, thereby increasing economy. I had not really considered that there might be climatic implications too.

I am an inherently impatient driver, and much more used to driving faster and more powerful cars. It is also fair to say that until recently I have never given fuel economy much thought, and it has never been a factor in my choice of car. Not until now anyway.

I bought the A2 for a very specific purpose, namely to do a commute from Fife to Newcastle every week, and to slash my fuel bills for the journey. This it has acheived at a stroke, but a by-product of this has been to give me an interest in hi-economy/low emissions driving as an alternative way of thinking to how I drive otherwise.

Anyway ... back to the debate!

:)
 
Economical Speed Cont.

The drag for a car increases by the power of three for the speed. Taking into account the A2's Co-efficient of Drag (cd) which is 0.25 - I think, and Frontal Area (in ft²) which is 23.4653 or something, the following equation (the attached thumbnail) will show how many horses are needed to pull the tank through the wind. Obviously, this result doesn't show how much power is 'wasted?' by air con or climate control or cd player, lights, Opensky roof, the list can be endless. . . .
 
ULP said:
A fatal error ;)

Peak torque output does not only appear with "foot to the floor", it is the mechanical created when the engine turns and, ergo, the difficulty of stopping the engine from turning. Peak torque demonstrates itself in the turning force of the crankshaft at a constant speed as well as during acceleration - otherwise a slight incline would see the need to accelerate to get up it, which is not the case.

However, try going up any incline in 5th at 28mph and see how far up it you get!
Click to expand...

Tee hee! This is fun if somewhat sad.

I'm not suggesting that peak torque or anything like it will be available at tickover - I guess everyone knows it doesn't. Peak torque that is always quoted for an engine is at one engine speed (e.g. 250Nm at 2900rpm) and is ALWAYS at full throttle. You do sometimes see a range of speeds quoted to show how broad the torque band is (e.g. 200Nm between 2200rpm and 3500rpm) and hence tractable the engine is.

But to repeat, peak torque has nothing to do with efficiency.

I think your confusion over "foot to the floor" is that, if you floor it and the engine produces more torque than is needed to get up that particular gradient it will accelerate the car. It is rare to use the engine in real life at peak torque for any length of time - usually the hill is too steep or not steep enough, making the car either decelerate or accelerate.

Peak torque is a dynamometer measurement where the retarding force on an engine under maximum optimal fueling (and wide open throttle in the case of petrol engines) is matched to the torque produced by the engine to give a non-accelerating situation (i.e. constant "speed" - or more accurately, angular velocity).

One thing I haven't worked out yet is why, when you reverse, doesn't the fuel flow back into the tank? :confused:
 
Ding, ding!

End of round one!!

John H, you certainly know how to start something - I'd suggest a career in diplomacy would not be a good choice - you might inadvertantly start World War 3 at this rate!!!

Cheers,

Mike
 
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