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Old 22nd Aug 2009, 23:04   #91
Jonnycoupe
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Originally Posted by Overlag View Post
if you stop/slowdown, you have to start/speedup again, so yes it does effect fuel consumption.

smoothness is key.
and storing some of that kinetic energy

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Old 22nd Aug 2009, 23:04   #92
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Just returned from a 500 mile round trip. Mosty motorway but also about 2-3hrs standing still in traffic, grrr....

Trip computer reckons I averaged 34 mpg which I'm well impressed with.

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Old 23rd Aug 2009, 01:33   #93
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My 120d tells me to change down if I'm in too high a gear at 30mph, as it thinks the fuel is being burned inefficiently.
Does unburned fuel not get returned to the tank/fuel line anyway, so the efficiency with which fuel is burned is fairly pointless? Although I'm not sure how CO vs CO2 fits into this.


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Old 23rd Aug 2009, 01:39   #94
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Does unburned fuel not get returned to the tank/fuel line anyway, so the efficiency with which fuel is burned is fairly pointless? Although I'm not sure how CO vs CO2 fits into this.
I think it's the extra fuel in the cylinders during combustion so will end up going out with the exhaust gasses.

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Old 23rd Aug 2009, 10:23   #95
andrewdodd13
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Surely you'll save a miniscule amount of fuel if, when you're coming to a stop, you brake gently earlier rather than slamming on at the end, cause over the distance you started braking, you're using less fuel, because you'll be going slower?

Sort of a graph of fuel consumption like
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I don't really know, just curious
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Old 23rd Aug 2009, 13:59   #96
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I just took my car out and tried some of this 'aggressive braking'.
I drove as normal except that when I was slowing down I pulled angry faces, shouted and swore, stuck my fingers up at other drivers etc.
It's too early to say if there's any effect on mpg.
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Old 23rd Aug 2009, 15:23   #97
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i would say A but it depends on the car.

if we are talking about an 800hp skyline with a big turbo and you drive around in positive boost all day, then your going to use way more fuel than normal.
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Old 23rd Aug 2009, 15:36   #98
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Originally Posted by andrewdodd13 View Post
Surely you'll save a miniscule amount of fuel if, when you're coming to a stop, you brake gently earlier rather than slamming on at the end, cause over the distance you started braking, you're using less fuel, because you'll be going slower?

Sort of a graph of fuel consumption like
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I don't really know, just curious
as soon as you take your foot off the accelerator fuel consumption goes down massively, as the kinetic energy of the car rolling keeps the engine turning over, not the fuel. so rolling upto lights off gas is better than going upto lights, then stopping and letting the car idle. idling uses more fuel than turning the engine by rolling.

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Old 23rd Aug 2009, 15:38   #99
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Old 23rd Aug 2009, 15:45   #100
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All modern cars are far more fuel efficient than they were a few years ago; a few things to remember, When you lift you foot off the accelerator providing that the engine is turning fast enough (above 1500rpm approx) still in gear you don't use any fuel at all (none) In the old days people used to knock there car out of gear going downhill to reduce mpg.
2nd, 56mph is the most efficient speed the faster you go after this mpg plummets with air resistance.
Reading the road ahead to avoid unecessary braking will mean that you also avoid unecessary acceleration too.
I have an Octavia 1.8 vRS turbo and I have got 47mpg! Tameside to Chester Zoo!
It all went out of the window driving back though!!
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Old 23rd Aug 2009, 17:25   #101
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56mph is the most efficient speed the faster you go after this mpg plummets with air resistance
That depends on the car, as 56mph for some cars will be top gear at 3000 rpm and for others it'll be top gear at 2000rpm. Surely the most efficient speed to drive it is the speed at which the engine is idling, in top gear? That would actually be around 25mph in my car. However, that number goes up once you take into consideration hills and the like, because there is practically no torque at 1,100ish rpm, so to maintain speed and avoid stalling while going up a hill you would have to give the car considerably more throttle. So the most efficient speed for general driving would be the speed at which you can maintain top gear without having to give silly gas to, or change down a gear, to get up hills. This in my car would be around 40mph.
Clearly the faster you go the more air resistance becomes a factor, but as a general rule I would expect the fuel efficiency to go down the further open the throttle is, even regardless of air resistance (which will obviously increase as you open the throttle further).

In other words, the most efficient speed will depend on the car, but will be the point at which the engine produces sufficient torque that it can maintain speed up inclines without needing much more gas - in most cases considerably less than 56 mph.

I agree with your other points though.


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Old 23rd Aug 2009, 17:52   #102
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How can 25mph be the most efficient speed? You aren't travelling fast enough to get anywhere!! 56mph is to the best of my knowledge the most economical speed for a road going vehicle. You only need a very modest amount of bhp to propel a car at that speed. Any slower and you are not covering enough distance to give good economy. You have only 2 things to overcome on a level road, wind resistance and rolling resistance. So It would stand to reason that these laws of physics would apply to all road vehicles. Try it anyway (I suggest 60mph as 56mph results in irate HGV drivers!!)
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Old 23rd Aug 2009, 18:54   #103
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No. Stopping inertia doesn't effect fuel consumption.
umm... I dont think so lol, think about it.
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Old 23rd Aug 2009, 19:04   #104
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Surely the most efficient speed to drive it is the speed at which the engine is idling, in top gear? .
no, not at all. Labouring the engine will make it alot worse on mpg.

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Old 23rd Aug 2009, 19:32   #105
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In other words, the most efficient speed will depend on the car, but will be the point at which the engine produces sufficient torque that it can maintain speed up inclines without needing much more gas - in most cases considerably less than 56 mph.
The easiest way to explain (without sounding condescending.. I hope) would be to imagine what Miles per Gallon means. To get a high mpg figure, you'd have to cover the most distance using a gallon of fuel. Simple enough.

However, if you're poodling along at 25mph, you might not be using much fuel, but you're not going anywhere either.

If you travelled at 50mph, the increase in the fuel consumption in 99.9% of cases would be a lot less than the increase in speed (distance covered using that fuel).

I think the efficiency of the engine at certain speeds may be a factor? Not sure. It's a balance thing anyway, there's a point when more fuel is used up than the proportionate increase in speed due to air resistance etc. (if that doesn't make any sense, I'm tired!) A lot of car manufacturers seem to give the guideline of 56mph, maybe they try gearing the cars for this?

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Old 24th Aug 2009, 06:54   #106
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How can 25mph be the most efficient speed? You aren't travelling fast enough to get anywhere!! 56mph is to the best of my knowledge the most economical speed for a road going vehicle. You only need a very modest amount of bhp to propel a car at that speed. Any slower and you are not covering enough distance to give good economy. You have only 2 things to overcome on a level road, wind resistance and rolling resistance. So It would stand to reason that these laws of physics would apply to all road vehicles. Try it anyway (I suggest 60mph as 56mph results in irate HGV drivers!!)
60mph on motorways for 200 miles (occasional queue) got me 66mpg the other day - considerably higher than normal. It's far harder to test out slower speeds though, as on any roads where you're not stopping and starting, you'd be going faster than 25mph.
Anyway, I'm not saying it's the most efficient speed - because, as mentioned below (and indirectly in my last post), that would be labouring the engine/needing silly throttle to maintain speed on inclines.

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no, not at all. Labouring the engine will make it alot worse on mpg.
But at what point are you no longer labouring the engine? Is it (as I presume) the
Quote:
point at which the engine produces sufficient torque that it can maintain speed up inclines without needing much more gas
?

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Originally Posted by LostPhil View Post
The easiest way to explain (without sounding condescending.. I hope) would be to imagine what Miles per Gallon means. To get a high mpg figure, you'd have to cover the most distance using a gallon of fuel. Simple enough.

However, if you're poodling along at 25mph, you might not be using much fuel, but you're not going anywhere either.

If you travelled at 50mph, the increase in the fuel consumption in 99.9% of cases would be a lot less than the increase in speed (distance covered using that fuel).
How do you know that though? I'm not saying you're wrong, just that what is there to say that you use less than twice as much fuel going at 50 rather than 25?

Quote:
I think the efficiency of the engine at certain speeds may be a factor? Not sure. It's a balance thing anyway, there's a point when more fuel is used up than the proportionate increase in speed due to air resistance etc. (if that doesn't make any sense, I'm tired!) A lot of car manufacturers seem to give the guideline of 56mph, maybe they try gearing the cars for this?
They may give that guideline because people might drive along the motorway at silly low speeds aiming for low fuel consumption? I note that 56mph is the speed limiter on most HGVs (albeit they have more accurate speedos, showing as c.60mph on most car speedos).
Remember of course there's not only air resistance and rolling resistance to overcome but internal resistance related to the engine rpm, and all those resistances are lower at slower speeds (assuming same gear). There is indeed a point when there's more fuel used up than the proportionate increase in speed, it's just hard to measure - and I suspect it changes depending on the gradient being driven up.

Perhaps someone with a real-time display of mpg could test this out? (i.e. when driving in a 30, go to 5th gear, adjust to maintain constant speed, give it 30 seconds, note the reading; do the same at 40, 50, 60)
Not that I've got a great deal of faith in them, the one in my parent's car seems to record about 10mpg higher than they're actually getting but they should be alright for relative comparisons.


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Old 24th Aug 2009, 10:56   #107
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How do you know that though? I'm not saying you're wrong, just that what is there to say that you use less than twice as much fuel going at 50 rather than 25?
I don't know that for sure, but it's from years of trying to work it out.

I shouldn't have posted it as fact, more of my understanding of it.

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They may give that guideline because people might drive along the motorway at silly low speeds aiming for low fuel consumption? I note that 56mph is the speed limiter on most HGVs (albeit they have more accurate speedos, showing as c.60mph on most car speedos).
Alternatively, you could argue that they don't want to condone speeding by quoting a higher figure than 60mph

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Old 25th Aug 2009, 02:00   #108
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as soon as you take your foot off the accelerator fuel consumption goes down massively, as the kinetic energy of the car rolling keeps the engine turning over, not the fuel. so rolling upto lights off gas is better than going upto lights, then stopping and letting the car idle. idling uses more fuel than turning the engine by rolling.
It goes to zero as the injectors stop firing.

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Old 25th Aug 2009, 02:03   #109
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That depends on the car, as 56mph for some cars will be top gear at 3000 rpm and for others it'll be top gear at 2000rpm. Surely the most efficient speed to drive it is the speed at which the engine is idling, in top gear?.
The most efficient speed is where the engine BSFC is best.

That is the Brake Specific Fuel Consumption, ie the mass of fuel to produce bhp. Low speed lugging is generally pretty bad for a engine that designed to run between 1000-6500 rpm, the air is moving to slow.

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Old 26th Aug 2009, 08:04   #110
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All cars demonstrated decreasing fuel economy beyond 65 mph (105 km/h), with wind resistance the dominant factor, and may save up to 25% by slowing from 70 mph (110 km/h) to 55 mph (89 km/h).[12]
Here's a graph I found on wikipedia showing fuel economy for a small variety of cars. There seems to be a trend with several of the cars having a peak in fuel efficiency around 20-30mph and then again at 45-60mph.



Also:

Quote:
a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak.
You mentioned the air intake being too slow as reducing efficiency. How does it become 'unthrottled'? Just by a heavier right foot? That would seem somewhat counterintuitive.


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Old 26th Aug 2009, 08:34   #111
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You mentioned the air intake being too slow as reducing efficiency. How does it become 'unthrottled'? Just by a heavier right foot? That would seem somewhat counterintuitive.
The more air that enters the engine the more efficiently it runs. This is because the pumping losses are minimised (the engine doesn't have to work against the vacuum in the inlet manifold) and the effective compression ratio increases (because it is compressing more air). Thermal efficiency is directly related to compression ratio, the higher the better, but the octane rating of the fuel limits how high you can go.

At very low engine speeds the air speed in the ports drops off, this is bad for cylinder filling and reduces the 'swirl' or 'tumble' that is designed into the cylinder head to ensure complete mixing of fuel and air, and rapid and complete combustion.

Engines using the Atkinson cycle (Fiat MultiAir and most hybrid cars) partially work around this problem by using the inlet valve to effectively throttle the engine, but rather than closing the valve early to reduce the air in the cylinder, it closes it very late so some air is pushed back out of the cylinder before the valve closes. This means that pumping losses are reduced, but it still suffers from low compression ratio at small throttle openings. Variable compression would be a great feature, but no manufacturers have yet made a system good enough for production.
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Old 26th Aug 2009, 14:54   #112
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Would perhaps a (complicated) system whereby, for example, two of the cylinders aren't used while idling, work (presumably only on cars with multi-point fuel injection)? The idea being to increase compression ratios somewhat, and therefore fuel efficiency, while using less fuel - because there are two cylinders unused.

I'm sure much cleverer people than myself would have thought this through and prototyped it if it were viable though :/


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Old 26th Aug 2009, 15:06   #113
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I certainly remember something which shuts of cylienders being demonstrated.. I'm not certain if it's in any production cars yet though.

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Old 26th Aug 2009, 20:13   #114
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Honda VTEC-E in the Civic IMA closes the valves when the electric motor is called on at very low engine demands.

The engine isnt fuelled at all, but by closing the valves you get a series of air springs that store and release the engnergy each stroke rather than having losses from typical engine aspiration.

The Honda Accord hybrid had VCM, which is Variable cylinder management, 3 of the the V6 cylinders are shut off at low load.

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Old 27th Aug 2009, 07:34   #115
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Good to know I'm not in a different ball park!


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Old 27th Aug 2009, 08:20   #116
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How can 25mph be the most efficient speed? You aren't travelling fast enough to get anywhere!! 56mph is to the best of my knowledge the most economical speed for a road going vehicle. You only need a very modest amount of bhp to propel a car at that speed. Any slower and you are not covering enough distance to give good economy. You have only 2 things to overcome on a level road, wind resistance and rolling resistance. So It would stand to reason that these laws of physics would apply to all road vehicles. Try it anyway (I suggest 60mph as 56mph results in irate HGV drivers!!)

tucked up under the back of a truck can result in some EPIC MPG figures


oh, and the cadillac northstar V8 shuts down one bank in traffic etc i think

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Old 27th Aug 2009, 09:24   #117
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tucked up under the back of a truck can result in some EPIC MPG figures
9 miles behind a big Homebase lorry = 116mpg

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