Progress at any Cost?

Discussion on Advanced and Defensive Driving. IAM, RoSPA/RoADA, High Performance Course. All associated training. Car training.

Postby Kevin » Fri Apr 20, 2012 11:33 am


YorkshireJumbo wrote:
Kevin wrote:I see someone has suggested that brisk acceleration is part of a hypermiling technique, which makes me think that someone has worked out whether or not there is some benefit in it. I dunno, just putting it out there to those that might have a better knowledge or greater access to resources than I have.

But they still cut the engine once they've reached their set speed, so that the engine only runs for a small %age of the time. That technique IS proven to work better for mpg, but it only really works if you are not sharing the road with other users, as their speed varies widely. Unless you plan to drive like that, I'm not sure there are significant benefits.

We're talking about driving with the engine running to maintain a set speed.


Whatever speed you reach and whether or not the engine is cut once the desired speed is reached isn't the area of fuel saving that I'm exploring. The area of fuel saving I'm focusing on (and I accept that it is probably only a small area and that there are many other ways of saving fuel that probably have greater effect) is just the acceleration phase. My gut feeling, is that longer spent in lower gears means the engine uses more fuel for a given distance. So early changing up gears is probably beneficial.

Now, that could mean selecting higher gears very early, almost labouring the engine. But, lets say you want to make reasonable, every-day normal driving sort of progress.

You could gently accelerate, changing gear, lets just say purely for the sake of example, at about 2,500rpm, until you reach whatever speed you want to be travelling at. Or, you could accelerate briskly, still changing gear at about 2,500rpm, until you reach the speed you want to be travelling at. Before this is discected in minute detail (Gareth! luv 'im to bits, but he does like to over-analyse sometimes) :D I accept various engines, drivers etc may, do, or want to change gear at different engine rpms and that journey time may affect the way people may or may not want to drive at any given moment etc. Just concentrate on whether brisk acceleration or gentle acceleration is better for fuel economy if all other variables remain constant. Otherwise I'll get a headache. :P I can feel one coming on already.
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Postby ScoobyChris » Fri Apr 20, 2012 11:59 am


I think you're never going to know with maths alone as there are too many variables and you'd need to do a measured experiment on a car to be scientific but...

If you assume that there is a 1:1 correlation between the amount of fuel an injector is able to flow and the throttle position, and find the flow rates of an injector:

eg a Bosch injector spec I found on t'interweb
FWIW, All flow is measured in cc/ml using using ISO fluids temperature regulated to 40C using the following test methods:
BACK FLOW - 19.8 - 45.2 cc/min
PRE-INJECTION - 1.5 - 4.9 cc/min
LOW-SPEED - 2.0 - 6.0 cc/min
MEDIUM - 19.9 - 28.1 cc/min
MAXIMUM - 45.8 - 58.2 cc/min

you might suggest that spending twice as long at a medium throttle would be roughly equivalent fuel consumption to full throttle.

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Postby YorkshireJumbo » Fri Apr 20, 2012 4:50 pm


The problem is that an engine does not deliver a linear response: double the fuel is significantly different to double the power as an engine can only work with the air it is breathing in. Then for all sorts of reasons, such as pumping losses and variations in the mass of mixture actually compressed, the power produced by that fuel is non-linear. Then you get into how the ECU decides the air:fuel mixture: should it be set lean for maximum economy or set for maximum power, or somewhere in between to make the engine respond properly (ie no "flat spots")?

As I mentioned earlier, I've seen articles over the years claiming both ways are better. For instance, when the 525e came out many years ago, BMW released some research saying the best way to maximise economy was to accelerate swiftly. Similarly in this article, they recommend wide throttle openings and changing up at low revs (2-2.5k) to minimise losses due to internal friction and pumping.

I think you're right that are just too many variables to make a general statement, and I also think that any difference will be tiny: this guy got a 3% improvement. You could make a much bigger difference by checking your tyres, removing the roofrack or unnecessary spoilers and emptying junk out of the car...
You may have speed, but I have momentum
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Postby Silk » Fri Apr 20, 2012 9:41 pm


ROG wrote:
Silk wrote: If you're turning left at a T-Juntion in a 30 limit and you have a blind bend on the road approaching from the right, it may be a better idea to wait until a car is approaching, so you can judge its speed and pull in front safely. If you pull into the empty road, you risk a collision someone approaching too fast who hasn't seen you.

EH!!

Best advice is to turn left when clear and then briskly get up to safe speed

If an empty road with good vision each way and nowt there then use eco driving to save fuel which will be slower than the former


I think this thread has moved on a bit, so getting back to the original subject, I wonder how many times we've all said, "I could have made that" after the event? The difference being that, after the event, we've had more time to take in information.

I agree that good planning on the approach to a junction is the key but, where there is limited time due to poor visibility, a little hesitation can go a long way.

Even where visibility is good, it's not always a good idea to thrash the car to within an inch of its life in order to get one space in front. More often than not, if I see a car approaching a roundabout from the left and there are no others behind, I'll make the decision to let it go first, especially if I can't be sure I can pull in front smoothly and without alarming the other driver.
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Postby MGF » Fri Apr 20, 2012 10:36 pm


I prefer to be driven by someone who hesitates in order to gain sufficient information to make a decision than by someone who makes decisions a bit prematurely for the sake of progress. On the other hand it is a real joy to be driven by someone who can process the information sufficiently quickly (or early) to not miss the opportunities. :)
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Postby Silk » Sat Apr 21, 2012 9:05 am


MGF wrote:I prefer to be driven by someone who hesitates in order to gain sufficient information to make a decision than by someone who makes decisions a bit prematurely for the sake of progress. On the other hand it is a real joy to be driven by someone who can process the information sufficiently quickly (or early) to not miss the opportunities. :)


A good answer. My worry is people will be encouraged to imitate drivers in the latter group, when they don't have the skill and confidence to do so safely.

The word "sparkle" is often used to describe good driving and is often considered an essential element in order to pass an advanced test. In my view, "sparkle" shouldn't be used as a synonym for speed and aggression. "Sparkle" can also mean confident, smooth and uneventful, qualities I always admire in a driver.

Of course, if a driver is unable to demonstrate they can drive at the maximum safe speed and take all reasonable opportunities, including overtaking, then they shouldn't be passing an advanced test, but they shouldn't feel bullied into driving in a certain way that doesn't feel confortable.
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Postby hir » Sat Apr 21, 2012 9:49 am


Silk wrote:... but they shouldn't feel bullied into driving in a certain way that doesn't feel confortable.


I'm intrigued by this comment. Was this your experience... and was it this that led to your original posting?

If it was, then that would be a completely different discussion to the one we are having on this thread; and I would wholeheartedly support your assertion that I have quoted above. Unfortunately, there are some observers/tutors out there who are insensitive to their associate's feelings. While it is sometimes necessary to push associates just a little way out of their comfort zone it must never be at the expense of safety nor must it unsettle the driver.
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Postby TripleS » Sat Apr 21, 2012 10:43 am


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Postby Silk » Sat Apr 21, 2012 1:58 pm


hir wrote:
Silk wrote:... but they shouldn't feel bullied into driving in a certain way that doesn't feel confortable.


I'm intrigued by this comment. Was this your experience... and was it this that led to your original posting?

If it was, then that would be a completely different discussion to the one we are having on this thread; and I would wholeheartedly support your assertion that I have quoted above. Unfortunately, there are some observers/tutors out there who are insensitive to their associate's feelings. While it is sometimes necessary to push associates just a little way out of their comfort zone it must never be at the expense of safety nor must it unsettle the driver.


Having to make good progress has always been a part of Advanced Driving as far as I'm aware and I'm sure it comes as a bit of a shock to drivers new to Advanced Driving, especially given the poor image of Advanced Driving organisations as being for ladies and gentlemen of a certain age and type.

However, I worry that in order to appeal to a broader audience including the young and fans of Jeremy Clarkson, the IAM is in danger of throwing away its principles in favour of popularity.
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Postby Silk » Sat Apr 21, 2012 4:03 pm


TripleS wrote:
hir wrote:Unfortunately, there are some observers/tutors out there who are insensitive to their associate's feelings. While it is sometimes necessary to push associates just a little way out of their comfort zone it must never be at the expense of safety nor must it unsettle the driver.


Agreed on both points. :wink:

Best wishes all,
Dave.


Good afternoon Dave. We're missing you in "the other place".
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Postby Horse » Sat Apr 21, 2012 5:59 pm


Silk wrote: [ I wonder how many times we've all said, "I could have made that" after the event? The difference being that, after the event, we've still been alive :) to take in information


Corrected your typo ;)
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Postby Horse » Sat Apr 21, 2012 6:01 pm


Silk wrote: [ The word "sparkle" is often used to describe good driving and is often considered an essential element in order to pass an advanced test. In my view, "sparkle" shouldn't be used as a synonym for speed and aggression. "Sparkle" can also mean confident, smooth and uneventful,


A few years ago there was a video on-line of the Met's SEG taking a convoy through into Central London, recorded from inside the lead car.

It was extremely boring . . . which was the whole point, really.
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Postby WS » Sat Apr 21, 2012 9:02 pm


YorkshireJumbo wrote:I think you're right that are just too many variables to make a general statement, and I also think that any difference will be tiny: this guy got a 3% improvement. You could make a much bigger difference by checking your tyres, removing the roofrack or unnecessary spoilers and emptying junk out of the car...

As you said, the engine's efficiency is not always the same. It depends on the level of engine revs and on engine load, generally being the best in low revs and high load. There is a lot of research on this topic, although it is not easily available on the internet. The conclusion is that in test lab conditions it is the most economical to accelerate decisively, using at least 50% of gas pedal position, but changing gears very early.

However, in real world conditions it does not matter that much whether you accelerate briskly or slowly. In the advanced driving school I work for we use telemetric equipment plugged directly into the ECU to measure the driver's technique and various aspects of fuel economy during the drive. There is hardly any difference in fuel economy between accelerating techniques, even in urban traffic where the driver needs to change their speed relatively often. On a motorway or a country road, the difference between accelerating techniques will be negligible. Under one condition: the driver changes gears early. 2000 rpm in diesel engines and 2500 rpm in non-turbpocharged petrol engines is the level that is usually recommended and it works well, at least from the point of view of fuel economy.

But I agree with the others who said that other factors have a much stronger effect on fuel economy than accelerating technique. Primarily, anticipation and planning. Accelerating technique is actually one of the least important aspects of ecodriving.
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Postby Silk » Sat Apr 21, 2012 9:48 pm


Horse wrote:
Silk wrote: [ I wonder how many times we've all said, "I could have made that" after the event? The difference being that, after the event, we've still been alive :) to take in information


Corrected your typo ;)


:wink:
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Postby TripleS » Sat Apr 21, 2012 10:42 pm


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