600 miles from a tank???

MikeMarsUK said:
Well, that's to do with feeding the engine, but I was referring to aerodynamic drag (which is linearly related to air density). That's why aircraft go high...
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But drag is proportional to the square of the speed, so aircraft have no choice if they fly fast and can go high. John is travelling at an average 26km/h! At that speed altitude won't be the most important factor affecting fuel economy!

RAB
 
RAB said:
... John is travelling at an average 26km/h! ...
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True, at that speed aero drag is irrelevant. It starts becoming detectable > 60kp/h, but once past 90kp/h it accounts for over half the vehicle's fuel usage (increasing at the square of speed, as you noted).
 
Yes. Aerodynamics shift massively once you get to high speeds. Air density and altitude are fairly insignificant on the performance of a car.
I thing that in my case steep hills and forced use of low gears due to incredibly low speed limits do not help with efficiency.
Certainly when driving longer distances along the coastal road (which is relatively flat) makes a huge difference.
Interestingly an A2 at 80-90km/h seems markedly happier and burns much less than at 30km/h. Also I find that noise increases until about 40km/h and then decreases before becoming noisy again around 100km/h.
There seems to be a fuel burn curve and a noise curve that both have a sweet spot somewhere around the 50-60mph zone.

John
 
John L said:
Yes. Aerodynamics shift massively once you get to high speeds. Air density and altitude are fairly insignificant on the performance of a car....
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Air density and altitude are absolutely significant to performance of the internal combustion engines! It affects the amount of oxygen it takes in each intake stroke! Hill climbing cars lose a lot of power as it goes high up the mountain, which I have done so with NA car. It was so obvious the performance was going away as the car travels up.

And also at speed, the thinner the air, the less resistance the car goes up against. (This is the performance impact of the 'car' rather than just the engine) Things travel a little easier in the air at high altitudes. But not that I have experienced this effect with an NA car, the engine was working very hard there was hardly any speed left to be significantly benefited from thinner air and lower resistance!
 
The ultimate of this was Concorde -she flew with a cruise-climb profile so the longer the flight went on, as fuel burned off, she was allowed to climb higher and so during its flight she became more fuel-efficient. Also, somewhat bizarrely, the faster she went, the more efficient fuel-burn rate became due to the increasing efficiency of the engines design in supersonic flight. At her designed cruise speed of Mach 2.02, the air intakes alone accounted for some 50% of the total thrust of the aircraft - that 10ft of air-intake was one of the major design successes of Concorde.

In service, she flew up to a ceiling of 60,000ft, but in development she flew up to 68,000ft - up there it was even more efficient and could cruise even faster (Mach 2.3!) due to the colder, thinner air, but the service-ceiling was kept to 60,000ft due to the time to return to a low flight ceiling in the event of losing a window and the cabin depressurising and everyone blacking out due to hypoxia!



RAB said:
But drag is proportional to the square of the speed, so aircraft have no choice if they fly fast and can go high. John is travelling at an average 26km/h! At that speed altitude won't be the most important factor affecting fuel economy!

RAB
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Re: the effect of altitude - NA engines
The fuel injection system's maximum fuel delivery setting assumes the air density for ~ sea level. As humps commented, for naturally aspirated engines there is a performance drop-off due to the reduction of air density at altitude. This lack of air means that the fuel injected will not burn completely - which is one of the reasons that the trucks in the Himalayas seem very smoky. It is true that some systems offer an altitude compensation device that reduces the max fuel to allow for air density - but often the trucks originated in Europe where it is not required.
It should be less of a problem with turbocharged engines as long as the turbocharger can maintain the boost pressure.

Mark
 
My S2 had some sort of altitude sensor and would adjust fuelling and boost to try and maintain the same engine performance regardless of altitude
 
A modern vacuum or electronically controlled turbocharger should largely automatically compensate for any change in altitude. An NA engine will lose nearly 50% of power at 5000m.

RAB
 
The S2 was based on late 80s engine and ECU tech so relatively unsophisticated compared to modern engines but boy did the 5cyl motor have character!
 
I best I've ever had from my A2 (114K on the clock) - was after a decent full service last year,

I can't figure the MPG - but it cost £18 to Bristol and Back (from Leeds/Bradford). I didnt even bother removing the rear seats - I'm well impressed - It would have cost £100 by train

I managed this by driving right up the arse of HGV's all the way on a sunny summers day - not an nice view to the front - but the view to the sides was ok.
 
OldBanger said:
...I managed this by driving right up the arse of HGV's all the way on a sunny summers day - not an nice view to the front - but the view to the sides was ok.
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I wouldn't recommend that as a hypermiling technique ... there are two main problems, firstly if the hgv drives over debris such as a shed tyre you don't get time to react, and secondly if it does an emergency stop, and the thing is empty, it will stop pretty quickly.

You can get similar mpg by using other techniques (although it is more work).
 
617 miles on 40 litres from my last tank! DIS said 10miles to go but the gauge was very low and I didn't want to risk it any further! That's 70mpg I think...
 
Hi,

I've recently achieved my personal best mpg.

Travelled from home Hartlepool to Harry pottery studio in London which is a straight run down for me, Sat nav was saying a return trip of 500miles. I thought this could be my chance to achieve my best mpg.

Filled up on death con deep, 34.07 litres travelling at a averaging speed of 56-60, at times 70 to over take etc..

Total miles travelled 619 miles to the beep.

82.49 MPG..

I have mapped tdi 75 with extended 5th gear 0.659.

I'm a very proud A2 owner. Never though this figure would be achievable.

Ross
 
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Good effort! By contrast, I did 51MPG off my last tank, my worst in a LONG time. New commuting schedule is screwing my MPG!
 
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