11 Tips to save fuel

Not sure about point 11. That's what an A2 1.2Tdi and a Lupo 3L do in Eco mode when you take your foot off the throttle.

RAB
 
I don't think point 11 has enough warning on the 'switch off' quote! In case someone silly enough to try that, you'll have no power steering and brake which makes it extremely dangerous. Further, if you're stupid enough to take the key out, the steering lock engages and you may end up like http://www.themercury.com.au/article/2009/08/25/93061_scalesofjustice.html this Darwin Award candidate in the name of saving fuel!

Point 11 is kind of valid, my friend who was an engineer (engine testing) at ford told me just leave it in gear.
 
As usual, it is not as simple as that. On over-run, a diesel uses no fuel but there is an engine braking effect. The A2 1.2Tdi is just about the most fuel efficient four-door yet made and I'm sure VAG would not have used automatic coasting in neutral (in Eco mode) if it had no advantage. The proof of the pudding is in the eating.

RAB
 
I'm no engineer, but surely if you have the engine turning at 2000 RPM and the gearbox, turning creating resistance, it must be less economical than the engine turning at 1000 rpm and the gearbox in neutral.

I would also question the point raised on wind resistance. 50mph before resistance becomes a factor. In cycling the wind becomes a factor at 15mph.

Cessna aircraft can take off at 50mph!
 
I would also question the point raised on wind resistance. 50mph before resistance becomes a factor. In cycling the wind becomes a factor at 15mph.
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In aerodynamics drag increases with the square of velocity so a small increase in speed equals a big increase in drag.

You see a lot of drag coefficient Cd values quoted in magazines which show how slippery a car is (small Cd = more aerodynamic) but actual drag force is measured by Cd x frontal area (cross sectional area). That's why the most aerodynamic versions of cars are usually the ones with no spoilers and narrow wheels - same Cd but less area.

So a small cyclist (by area) with a big Cd could in theory create more drag force than a large car with a small Cd.

That's not the whole story though. A human (12st = 0.18tons) can only generate just less than half a horsepower for a short time but even small cars (1.05 tons) can manage 50hp. If you use the old power to weight ratio that means human = 2.8hp/ton but car = 47hp/ton. The poor cyclist has so little power that he'll feel the drag at the much lower speed. I think my sums are right:)

Aeroplanes and formula one cars do create a lot of drag as a by-product of lift/downforce but they have a lot more power to overcome it. That's why aeroplanes fly so slowly with flaps down when generating lots of lift and why F1 cars run with less 'wing' on straighter faster circuits.

Brian
 
Tincan, i'm an engineer and i can shed some light into your thoughts, but it's not about that but about common sense.:)
Driving means that you will at one time or another need to brake, because of cars in front of youand/or the bends in the road are not all attackable at the speed you're aproaching them ( the need to brake again). Pedal braking is quite inefficient because is 100% loss through heat and friction generated by the pads and rotors. Add to this the 0.6l/h or whatever fuel your engine consumes at idle when you're braking and in neutral.
Now imagine that with all that friction generated by the gearbox (and the engine if you don't press the accelerator pedal) is nothing compared to the one that is generated by a good foot on the brake pedal, but is somewhat constant. Where the true fuel saving comes into place is when you use engine braking for a forseeen need to brake ( eg: seeing a red light/traffic jam/bend early on and slowly decelerating in engine-braking insted of pedal-braking, in order to stop at it ), because while doing this, all modern ECUs cut the petrol/diesel from being injected into the engine. That means 0l/h fuel consumption while engine braking, while still traveling at 50-70-100 km/h. Also, this also keeps the engine at good revs if the need to accelerate back up arises (instead of working up from idling if you were in neutral while pedal-braking).
All of the above can translate into a substantial fuel saving if coupled with a light foot, traffic awareness and route planing.
Now that's hypermiling for you.:D
 
I would also question the point raised on wind resistance. 50mph before resistance becomes a factor. In cycling the wind becomes a factor at 15mph.
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Tincan - I really should have read the full thread before replying so my last answer made sense:eek:

Opening a window in a car can break up the smooth airflow causing turbulence which increases drag. 50mph is usually accepted as the speed where aerodynamic drag goes up sharply in a car so that's when it starts to affect fuel economy as the car engine has to work harder to overcome it. Being a nice slippery shape with very flush side glass the A2 illustrates this quite well because it gets the buffeting effect from the open drivers window at around 40-50mph. This is why some folks fit the wind deflectors or open a rear window which seems to reduce the turbulence.

An even better way to feel the drag increase is to stick your hand out of the window at 30mph then try the same at 60mph. Big difference. Leaving a roof rack on the car does exactly the same which is why this is bad for fuel economy.

Regarding the cyclist again, he isn't the most aerodynamic shape anyway but combined with the lack of 'power' that's why 15mph starts to hurt.:)

Brian
 
Well, I may be dreaming but my average went from 380km per tank to 420km when I started neutralling my A2 1.4 petrol instead of letting it roll on engine braking in 5th. And yes, I keep my eyes upwards to see the traffic lights from afar.

Are you SURE that the ECU strategy of the A2 favours engine braking?
 
If you know that you still have to do 2 more kms till you need to stop and you don't mind slownig down in neutral (and road conditions are optimum) then you'll get a better mpg figure because engine braking is still braking. But real-life conditions or most roads don't offer this option. Think of it this way:

Better MPG: idling>engine braking>normal braking
Faster Braking: normal braking>engine braking>idling

So idling is the most efficient but means you need perfect conditions, and normal braking is the least efficient but is the easiest to use because is instant and needs almost no road "awareness". In the middle of these two is engine braking that has some of both worlds.;)
 
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