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As said, use the appropriate gear to keep at 2000rpm. I've no idea of the Audi gearing ratios.
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I find it hard to believe that keeping the engine at 2000 rpm (or whatever represents peak torque), no matter what the road speed, produces the best economy.
That would mean, for example, using a low gear for speeds around 20 - 40 mph, when wind resistance is minimal and even if the road is flat or downhill. I just don't see why that uses less fuel than using a higher gear.
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Basic physics. Ever tried riding a 12 speed road bike in 12 gear at low speed? Your legs soon let you know how much energy is required to keep the wheels turning. That’s why pro cyclists concentrate on cadence to maintain speed whilst minimising energy use age.
A car is no different. Too high a gear for the speed and you use more fuel.
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Basic physics. Ever tried riding a 12 speed road bike in 12 gear at low speed? Your legs soon let you know how much energy is required to keep the wheels turning. That’s why pro cyclists concentrate on cadence to maintain speed whilst minimising energy use age. A car is no different. Too high a gear for the speed and you use more fuel.
Yep, ride a push bike and you will soon understand from an engines point of view how much load is on it.
At low speeds, you might be using higher revs but you are using less throttle maintaining that speed, because the load on the engine is reduced by using a lower gear.
Edited by corax on 09/11/2019 at 11:18
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My very basic understanding is that the lambda sensor seeks to maintain the air/fuel ratio close to the optimum - for petrol engines around 14.7 air to fuel by weight.
When the lambda sensor identifies a variation to permissable limits, the ECU changes the fuel injected to bring it back to "normal"
So as a generality I have always assumed that using as high a gear and low revs as possible should give the best fuel consumption. This can obviously be taken to extremes:
- no sense in stressing enginet/drive train using too a high gear at very low revs
- using a high gear obviously reduces flexibility of power delivery.
At (say) 30mph a mid size, mid range 5 speed car in:
- 5th gear at around 1000-1200 rpm will be ok on the level but unresponsive.
- 4th will be a comfortable drive at 1500-1700 rpm with some capacity to accelerate
- 3rd gear would give prompt acceleration.
My conclusion is that the faster the engine rotates the more fuel it will use - or can someone explain why not.
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My conclusion is that the faster the engine rotates the more fuel it will use - or can someone explain why not.
You might equally argue that whatever gear you use, it requires the same amount of energy (and hence fuel, assuming correct combustion) to drive a given vehicle along a given stretch of road. Experiment shows that neither hypothesis is true: all engines have a power output curve against engine speed, with a peak somewhere near the middle. And why the gearbox is to help the driver keep the engine rotating near the optimum.
And this thread started with a diesel engine - does it have a lambda sensor?
Edited by Andrew-T on 09/11/2019 at 17:46
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That would mean, for example, using a low gear for speeds around 20 - 40 mph, when wind resistance is minimal and even if the road is flat or downhill. I just don't see why that uses less fuel than using a higher gear.
I think, to keep the discussion simple, so far we have all been assuming a level road. Obviously if you reach a downhill gradient you get help from gravity and can ease off the accelerator and maybe use a higher gear. But on the flat, peak torque is the place to be. Then at higher speeds wind and rolling resistance are more noticeable and your engine will need more fuel.
Scientists have been measuring fuel consumption under all kinds of conditions for many years. You may have difficulty seeing why it happens, but that doesn't change the science.
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It is power that does the work - calculated by multiplying RPM by torque.
Maximum power is usually developed towards the upper end of the RPM range as the benefit of rotational speed outweigh the gradual drop off in torque as speed increases.
For diesels peak torque is usually in the 1500-2000 range, pehaps 500 rpm lower than petrol.
Taken to its logical, but absurd extreme, optimum economy arises where the engine is generating only the power to overcome frictional, aerodynamic and gradient losses.
Using maximum torque as a point at which to drive and adjusting gear selection accordingly is probably a good compromise between economy and flexibility but I don't think it is necessarily the optimum speed for economy.
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Using maximum torque as a point at which to drive and adjusting gear selection accordingly is probably a good compromise between economy and flexibility but I don't think it is necessarily the optimum speed for economy.
In responding initially I was thinking about what feels right and gives appropriate flexibility when driving. The OP spoke of engine labouring rather than economy - that was introduced later.
In early days of diesel ownership having moved from a petrol BX to its non turbo diesel equivalent I found the diesel's pull from around 2000 revs a revelation. I therefore adopted a technique of using the gears to keep revs in 1800-3000 range. I didn't meed to change that moving to later PSA diesels with direct injection and turbo charging nor when I acquired a Skoda with VAG EA189 engine. The only oddity is that the later PSA engine can drop out of its torque band without labouring and leave you flatfooted when power is needed - most noticeable on Alpine or similar passes. Remedy is to watch the tachometer as there are no aural or vibration cues as were present on IDI versions.
That technique has brought me reasonable economy. Never given thought to whether refining technique might give better economy, obviusly possible it would.
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In 2011 we got a new Vauxhall Meriva 1.7 CDTI, and while it was great on the open road, at slower speeds it was hard work. The problem, as with most modern cars, is that the gearing is simply too high. This is to enhance the figures gained in the economy tests, which is in no way shape or form, relevant to real life driving situations. Since then, fortunately, i managed to persuade SWMBO to try going auto (which would have made the Meriva so much nicer to drive in stop start traffic) and she is now, like i was decades ago, an auto fan!.
For my work, i drive a 2010 VW Caddy 2.0SDI (non turbo). It (helped by short gearing to mask the lack of power) will happily pull third gear from walking speed and fifth would be no problem at 30mph assuming the road was more or less flat.
Also feel it worth pointing out that one of the reasons i bought the 2.0SDI was due to the reputation for unreliability of the 1.6TDI..........
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We bought a 1.9 TDi Golf in the mid 90's. On the test drive it seemed great especially considering what we were driving but once we got it our opinions changed. On the open road it was fine, excellent mpg and an easy cruise but once in town what a nightmare. You would never seem to be in the correct gear, 2nd too low, 3rd too high, b***** hard work wife hated it on her commute. We kept it 7 trouble free years but after that were not tempted by another VW. We drove a Mondeo TDCi and what a revelation. On the open road fine and in town fine, a doddle to drive, wife loved it, rubbish mpg though.
If Ford could do that why not VW?
Whilst the Golf may have been reliable and economical I would not want to wish one on my worst enemy from a driving perspective.
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... what a revelation. On the open road fine and in town fine, a doddle to drive, wife loved it, rubbish mpg though.
If Ford could do that why not VW?
As said above, poor mpg figures don't help to sell cars ?
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Another characteristic of the 1.6 Tdi engine is the ease which it stalls when pulling away from rest. It comes as a shock to drivers who traditionally remember diesels as difficult to stall , which fitted the bill for driving school cars.
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Trawling the internet I can’t find any reference to maximum torque in advice about driving for economy. The following is a fair sample of what I found:
“Probably the biggest secret to achieving high mpg is driving in the highest possible gear for your vehicle while keeping within the speed limit. The best advice in urban areas is to change up through the gears as quickly as you can with the lowest revs possible, probably at around 2000rpm. Remember: the faster an engine spins, the more fuel it uses.” www.rac.co.uk/drive/advice/how-to/fuel-saving-tips/
“To maximise your fuel efficiency, the Energy Saving Trust recommends trying to change up a gear before you reach 2,500 revs in a petrol car, and 2,000 in a diesel car. Changing gears like this can help you achieve a 15% fuel saving, according to UK Road Safety, the Bristol-based instructors who offer "eco driving" courses.” www.theguardian.com/environment/green-living-blog/...s
“As a general guide, change up before 2000rpm in a diesel and 2500rpm in a petrol, without letting the engine struggle at low revs.” www.whatcar.com/truempg/fuel-tips
“Change to a higher gear as soon as its possible and safe to do so. The Department for Transport recommends diesel drivers try changing up a gear before the rev-counter reaches 2000 revolutions per minute (rpm), and petrol drivers do so before 2500rpm.” www.ageas.co.uk/solved/your-car/how-to-maximise-yo.../
“It’s more efficient to move up a gear at 2,500 (petrol) and 2,000 (diesel). Use high gears, such as fifth and sixth gears, sooner than later.” www.moneyadviceservice.org.uk/en/articles/how-to-s...g
But maybe none of the above knows what they’re talking about.
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<< As a general guide, change up before 2000rpm in a diesel >>
That's what we are trying to tell you, FP - 2000rpm is about the point of maximum torque in a car diesel engine. If you rev higher you are going past the peak, so change to a higher gear. These authorities have not mentioned torque, possibly to avoid technicality?
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All the articles FP refers to say change up before 2000 rpm (2500 petrol), not that 2000 is the target.
It seems obvious to me that each rev is a combustion, and uses fuel each time. There is a lot of internal friction in a car, due in part to the tight piston rings. Have you tried pushing a car in gear, it's not easy for this very reason. If the car is doing 2000 rpm rather than 1200 or 1500 then this friction will require more energy to be overcome.
On one level you are right, a car needs a minimal amount of fuel at idle, just to keep going. This fuel does no useful work, and at higher revs 2000 Vs 1200-1500 the baseline energy as a percentage of total energy used is lower. Some hypermiling website recommend accelerating to just above peek torque then coasting (with or without engine on) until the cars engine speed is just above idle, then repeating. I'm not sure this is practical in the busy UK road network.
I still think that traveling at a constant speed will be more fuel efficient at lower revs. This is the reason the OP car can run nicely at 30 in 3rd not 4th or even 5th. If 5thwas low enough to ruin at 30then it would be too low to run at the 1500 rpm range at higher speeds.
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There is a lot of internal friction in a car, due in part to the tight piston rings. Have you tried pushing a car in gear, it's not easy for this very reason.
I'm not quite sure what you are saying here. Many of us will have tried pushing a car in gear, it's difficult because of trying to compress the gas in the cylinders, not because of friction or tight piston rings etc. But I don't think that is what you meant? (it's easiest if the car is in top gear)
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They don't know what they are talking about.
Roadsmart have it right.
Use the highest safe gear to save fuel. However explaining what that means will take more than two lines the other 'experts' have used.
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