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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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#41
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Graph of car fuel consumption versus speed
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember The Natural Philosopher saying something like: WE suspect that, at full power, its less efficient than part power. So the curve of efficiency is definitely sort of parabolic. Where IS the most efficient part? Useter be peak of the torque curve and probably still is. I agree that slowcoach acceleration is pointless and wasteful of everbody's time, as is boot to the floor (but godammit, it's fun), so when I'm driving economically it's always a compromise, usually about half boot to get up to speed and a whiff of throttle to stay there. On all sort of vehicles and engine types I've found this pays off without holding anybody up. I digress... Years ago I had to accompany a rider on a clapped-out 175cc bike on a motorway trip of around 120miles. I was riding one of my GS850s and of necessity had to accelerate gently and limit my top speed to ~60mph. At the end of the trip I was astounded to find I'd done at least 60mpg on a bike that normally returned 38mpg. -- Dave GS850x2 XS650 SE6a "It's a moron working with power tools. How much more suspenseful can you get?" - House |
#42
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Graph of car fuel consumption versus speed
AJH wrote:
On Wed, 08 Oct 2008 15:06:50 +0100, The Natural Philosopher wrote: It makes a fuel efficiency case for allowing long distance trucks to couple up! AJH It's called a railway... Railways may be fuel efficient but being able to choose when to leave the train could also be resource efficient. OK it's a long way off but intelligently controlled traffic management may be on its way, in which case the chief benefit of a train, in fuel effectiveness due to low frontal area to payload, may come to roads. AJH In an ideal word, railways make the most sense for medium-long haul. typically miles and up..to the limit of the continent! What holds it back chiefly is the lack of efficient container loading/unloading. And no interest in providing it. |
#43
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Graph of car fuel consumption versus speed
Roger wrote:
The message from The Natural Philosopher contains these words: We know, that at idle, producing no actual acceleration, the powertrain is necessarily 0% efficient. WE suspect that, at full power, its less efficient than part power. So the curve of efficiency is definitely sort of parabolic. Where IS the most efficient part? Intelligent guesswork suggests its not close to idle at all. There frictional loses in the engine will be nearly all the losses. It is more than 40 years since I did any thermodynamics and we didn't do much on IC engines anyway but through the mists of time ISTR that an IC engine is most efficient at max bmep (brake mean effective pressure IIRC) and max bmep equates very closely to max torque. One of the reasons I remember this at all is because it seems to conflict with the age old advice that the way to save fuel is to get into as high a gear as possible as early as possible. Yup. We've been playing with 'sport' mode on the auto, and it seems to use less..goes higher in revs on the deezil.. The optimal seems to be aroudn 2000-3000 RPM. Turbochargers make the torque peak a lot lower in most engines. |
#44
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Graph of car fuel consumption versus speed
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember The Natural Philosopher saying something like: You might even find that periods of acceleration and periods of coasting is in fact a better way to drive. Christ, no. I was in a car once being driven like that and I couldn't wait to get out of it - the prat was doing it all along the motorway. I've also been behind prats doing it - total ****wits. -- Dave GS850x2 XS650 SE6a "It's a moron working with power tools. How much more suspenseful can you get?" - House |
#45
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Graph of car fuel consumption versus speed
In article ,
AJH writes: On Wed, 08 Oct 2008 15:06:50 +0100, The Natural Philosopher wrote: It makes a fuel efficiency case for allowing long distance trucks to couple up! AJH It's called a railway... Railways may be fuel efficient but being able to choose when to leave They aren't fuel efficient. Whilst efficiency of many modes of transport (such as cars) have been a consideration for many years and resulted in many improvements over those years, efficiency of trains (at least in the UK) was completely ignored until very recently and is miles behind. -- Andrew Gabriel [email address is not usable -- followup in the newsgroup] |
#46
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Graph of car fuel consumption versus speed
Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like: WE suspect that, at full power, its less efficient than part power. So the curve of efficiency is definitely sort of parabolic. Where IS the most efficient part? Useter be peak of the torque curve and probably still is. I agree that slowcoach acceleration is pointless and wasteful of everbody's time, as is boot to the floor (but godammit, it's fun), so when I'm driving economically it's always a compromise, usually about half boot to get up to speed and a whiff of throttle to stay there. On all sort of vehicles and engine types I've found this pays off without holding anybody up. I digress... Years ago I had to accompany a rider on a clapped-out 175cc bike on a motorway trip of around 120miles. I was riding one of my GS850s and of necessity had to accelerate gently and limit my top speed to ~60mph. At the end of the trip I was astounded to find I'd done at least 60mpg on a bike that normally returned 38mpg. same as me i my old Triumph spitfire. Surrey to Cambridge used to be around 38mpg..till I had to do it in 6" of snow. With wheelspin at anything more than a gentle prod, and a top speed of more than 50mph impossible, I recorded over 50mpg on that run.. |
#47
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Graph of car fuel consumption versus speed
Grimly Curmudgeon wrote:
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like: You might even find that periods of acceleration and periods of coasting is in fact a better way to drive. Christ, no. I was in a car once being driven like that and I couldn't wait to get out of it - the prat was doing it all along the motorway. I've also been behind prats doing it - total ****wits. Oh, its a nightmare to sit in, but it might be fuel efficient, if bloody dangerous. |
#48
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Graph of car fuel consumption versus speed
Andrew Gabriel wrote:
In article , AJH writes: On Wed, 08 Oct 2008 15:06:50 +0100, The Natural Philosopher wrote: It makes a fuel efficiency case for allowing long distance trucks to couple up! AJH It's called a railway... Railways may be fuel efficient but being able to choose when to leave They aren't fuel efficient. Whilst efficiency of many modes of transport (such as cars) have been a consideration for many years and resulted in many improvements over those years, efficiency of trains (at least in the UK) was completely ignored until very recently and is miles behind. I think you will find its always been miles ahead, and is slowly having its advantage eroded actually. I am pretty sure a 4500 bhp loco can pull 50 35 ton containers. At very sensible speeds. I don't think a 90 bhp truck could do the same for one 35 ton container. with any adequacy. |
#49
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Graph of car fuel consumption versus speed
Nige Danton laid this down on his screen :
I'm a cyclist and aerodynamic friction plays a huge role in determining speed and above ~30kph the benefits of drafting behind another cyclist are considerable. There's an energy saving of ~20% for the first cyclist in a pace line and that rises to maximum of ~30% for the fourth cyclist. I really would be surprised in cars are so slippery that aerodynamic friction does not play a significant role at speeds slower than 60mph. I fancy it will be a trade off in a car between the gear in use and aerodynamic friction. Going just fast enough to be able to stay in top gear will be the most economic speed in most cars, which will be slightly below 60mph. -- Regards, Harry (M1BYT) (L) http://www.ukradioamateur.co.uk |
#50
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Graph of car fuel consumption versus speed
In message
, Nige Danton writes I'm a cyclist and aerodynamic friction plays a huge role in determining speed and above ~30kph the benefits of drafting behind another cyclist are considerable. What sort of advantage does wearing lycra so tight that we can tell what religion you are play or is that, as I have long suspected, just a fetish thing? BTW, if you tell me you shave your legs for aerodynamic advantage like my neighbour does, nothing in the world will persuade me the lycra isn't a fetish thing. -- Nige Danton -- Clint Sharp |
#51
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Graph of car fuel consumption versus speed
In message , The Natural
Philosopher writes I m not convinced that slow acceleration is as effective as its made out to be. Under heavy acceleration the engine management system will go open loop and injects more fuel than is strictly necessary for very good reasons, the excess fuel is burned in the cat and wasted so slow acceleration will use less fuel even if the eventual speed is the same. the energy needed to get a car up to speed is the same. Of course, but unfortunately real life internal combustion engines don't run best with theoretical energy inputs so even though the amount of energy required is the same you'll find that a heavy foot will still use more fuel. -- Clint Sharp |
#52
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Graph of car fuel consumption versus speed
Roger wrote:
It is more than 40 years since I did any thermodynamics and we didn't do much on IC engines anyway but through the mists of time ISTR that an IC engine is most efficient at max bmep (brake mean effective pressure IIRC) and max bmep equates very closely to max torque. Me too, that's why it should be better to get to your target speed by keeping the revs in the max torque band as you change gears, which implies full throttle but I suspect the fuel map is such that its best to back off from this a bit. AJH |
#53
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Graph of car fuel consumption versus speed
On 8 Oct, 13:26, Martin Bonner wrote:
I *think* that the force exerted by drag goes as the /square/ of the speed. * Only for a constant drag coefficient. Cd itself, however, depends on the flow pattern around the car and that in turn depends on the speed - it's often expressed in term of the Reynolds number. As speed increases the flow becomes more turbulent and drag increases enormously. Conversely, when the speed is low the flow is (largely) laminar and drag is low. Ian |
#54
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Graph of car fuel consumption versus speed
On 8 Oct, 11:20, The Natural Philosopher wrote:
Andy Burns wrote: yes they are, I've seen several artics with stickers on the back saying "this vehicle is restricted to X mph" where X is lower than the 56 mph limit (and I don't mean the 40 mph limit on single carriageways). 50mph on single carriageways.... 40mph. Highway Code, section 123. http://www.direct.gov.uk/en/TravelAn...code/DG_070304 Ian |
#55
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Graph of car fuel consumption versus speed
Clint Sharp wrote:
In message , The Natural Philosopher writes I m not convinced that slow acceleration is as effective as its made out to be. Under heavy acceleration the engine management system will go open loop and injects more fuel than is strictly necessary for very good reasons, the excess fuel is burned in the cat and wasted so slow acceleration will use less fuel even if the eventual speed is the same. Yes, I know that, but the real point is where is the optimal throttle setting for efficient acceleration. It ain't idle, and as you have pointed out, it ain't full welly either. the energy needed to get a car up to speed is the same. Of course, but unfortunately real life internal combustion engines don't run best with theoretical energy inputs so even though the amount of energy required is the same you'll find that a heavy foot will still use more fuel. But a mouse foot may be just as bad. That's the point. If what the engine is optimised for is top gear cruise at 56mph, thats medium revs and maybe 1/3rd throttle. Possibly that's where we should be accelerating? I've been dong research. Its fascinating. some facts seem to be well agreed. 1/. Full throttle is not efficient.It tends to cause over rich mixture. 2/. Max RPM is inefficient. It wastes power in engine friction. 3/. On other than racing engines, peak torque is arrived at at lower than max revs. This is generally true even with a turbo, as the sort of breathing or boosting that will make an engine develop peak torque at peak RPM is totally intractable in a road car. So that definitely means we want to not use full throttle or max RPM when accelerating.. Down at the bottom end, it seems to be the case that wide throttle on a very low RPM is not efficient either. Surprisingly, this area of car engine performance seems ill understood by almost all the sources I can find. Or its simply ignored. Interestingly, one poster on a US group said that sub 50mh, he needed to drop a gear to get his mpg back. Thats just cruising along. I can't be sure, but I get the distinct impression that medium light throttle and medium revs works best on my petrol machine, and in fact the turbodiesel as well. |
#56
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Graph of car fuel consumption versus speed
andrew wrote:
Roger wrote: It is more than 40 years since I did any thermodynamics and we didn't do much on IC engines anyway but through the mists of time ISTR that an IC engine is most efficient at max bmep (brake mean effective pressure IIRC) and max bmep equates very closely to max torque. Me too, that's why it should be better to get to your target speed by keeping the revs in the max torque band as you change gears, which implies full throttle but I suspect the fuel map is such that its best to back off from this a bit. I think so too. Certainly maximum welly at ultra low RPM is not ideal. AJH |
#57
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Graph of car fuel consumption versus speed
The Real Doctor wrote:
On 8 Oct, 11:20, The Natural Philosopher wrote: Andy Burns wrote: yes they are, I've seen several artics with stickers on the back saying "this vehicle is restricted to X mph" where X is lower than the 56 mph limit (and I don't mean the 40 mph limit on single carriageways). 50mph on single carriageways.... 40mph. Highway Code, section 123. http://www.direct.gov.uk/en/TravelAn...code/DG_070304 Ian Ah, Thats for over 7.5 tonnes tho, and Im not licensed for them, so I never learnt that ;-) Mind you, none of them keep to it. I drive the A11 frequently in a 3 ton camper, and I keep pace with the artics.;-) If they slow to 40mph, it creates a tailback from Thetford to the start of the dualled section..all ten miles of it :-) |
#58
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Graph of car fuel consumption versus speed
On Wed, 08 Oct 2008 13:22:05 -0700, The Real Doctor wrote:
On 8 Oct, 13:26, Martin Bonner wrote: I *think* that the force exerted by drag goes as the /square/ of the speed. Only for a constant drag coefficient. Cd itself, however, depends on the flow pattern around the car and that in turn depends on the speed - it's often expressed in term of the Reynolds number. As speed increases the flow becomes more turbulent and drag increases enormously. Conversely, when the speed is low the flow is (largely) laminar and drag is low. Have to diasagree the Reynolds numbers for object of size car, in air, mean that even a few mph and you are into unambigously turbulent flow, where the drag is proportional to speed squared. The Cd can be affected by things like a half closed door/hatch. But is largely unaffected by speed for Reynolds numbers of 10K+. -- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at http://www.diyfaq.org.uk Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html |
#59
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Graph of car fuel consumption versus speed
On Wed, 08 Oct 2008 03:42:58 -0700, Nige Danton wrote:
On Oct 8, 3:35Â*pm, Adrian wrote: Nige Danton gurgled happily, sounding much like they were saying: within any given class are likely to be fairly similar. As a rough rule of thumb, increasing drag starts to come seriously into play from about 60mph upwards. Drag cubes with velocity and so it may become important at speeds lower than 60 mph. Trust me on this... I've got plenty of experience with low-powered, unaerodynamic vehicles. It starts to come into play at about 60. I'm a cyclist and aerodynamic friction plays a huge role in determining speed and above ~30kph the benefits of drafting behind another cyclist are considerable. There's an energy saving of ~20% for the first cyclist in a pace line and that rises to maximum of ~30% for the fourth cyclist. I really would be surprised in cars are so slippery that aerodynamic friction does not play a significant role at speeds slower than 60mph. What sort of vehicles are you referring to? IIRC about 20 years ago, 30 mph, was about evens between the air drag and other resistances (mainly tyres). These days the shapes are more slippery so 40mph might be the balance point. It all depends on what you mean by 'significant' and 'dominant' If 40 mph was the balance between aero drag and other friction then At 60mph aero drag is about 70% of total which I call significant. -- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at http://www.diyfaq.org.uk Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html |
#60
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Graph of car fuel consumption versus speed
It happens that Ed Sirett formulated :
IIRC about 20 years ago, 30 mph, was about evens between the air drag and other resistances (mainly tyres). These days the shapes are more slippery so 40mph might be the balance point. It all depends on what you mean by 'significant' and 'dominant' If 40 mph was the balance between aero drag and other friction then At 60mph aero drag is about 70% of total which I call significant. I've done a bit of caravan towing rough research using the fuelling computer and given still conditions and fast roads, my most economical speed is around the 50 to 55mph mark - surprisingly, because you would think the poor aerodynamics would make it very uneconomical. When in no particular hurry, I can manage economy figures only slightly worse than when not towing. -- Regards, Harry (M1BYT) (L) http://www.ukradioamateur.co.uk |
#61
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Graph of car fuel consumption versus speed
dennis@home wrote:
You get better MPG in higher gears which gives less acceleration. True You also get better mpg by using just enough gas to give the acceleration needed, not the maximum acceleration you can get. You need to be able to drive to know what acceleration is needed rather than just putting your foot down and this is where most come unstuck. Bol... err.. that turns out not to be the case. Keep the revs near the torque peak, and the throttle butterfly wide open so it isn't b***ing up the airflow and wasting lots of power. Remember of course that a lot of engines will enrich the mixture at full throttle to get max. power, and so you want to avoid actually flooring it. Diesels BTW with no butterfly are far happier at lower "throttle" openings. I seldom need more than a quarter throttle to match motorway speeds on any of the slip roads I use. I have had to use the hard shoulder when some prat has decided to stop because he can't get into a gap. Using the hard shoulder is the correct way of doing it of course. Usually you can spot the prat, and stay well clear. I've never actually had to resort to the hard shoulder. Andy |
#62
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Graph of car fuel consumption versus speed
On Oct 8, 12:18*pm, The Natural Philosopher wrote:
Dave Baker wrote: "Chris" ] wrote in message ]... I would like to make a graph of car fuel consumption versus speed. My driving is not smooth enough to gather the raw data myself. Do you know of any reliable figures, or graphs? I'm interested in relative values, rather than those for any particular car. The equations and sample data are on my website. http://www.pumaracing.co.uk/TOPSPEED.htm Excellent site Dave. And it shows that for 100mph about 70bhp/ton is fairly close. and no more than 35 needed for 70mph. However it does seem to be a bit conservative - many cars exceed 60mpg at 56mph, and also the fact of the matter is that so long as the big heavy cars aero losses are well below the frictional losses it doesn't matter a damn what speed it goes at. I.e. my old Jaguar would never ever turn in better than 27mpg no matter how slow it was driven. Typically it did 19-20mg. To get 27mpgh required it to be trundled at - yas - 56mph constantly. One stop would ruin it! It does show that there is plenty of room fr improvement though. Low friction small turbo charged engine, and lighter more aerodynamic cars and skinnier tyres could still have the speed. *motorcycle has the speed but not the fuel consumption, after all. Hmm! 10 cubed = 1000 20 cubed = 8000 30 cubed = 27000 40 cubed = 64000 50 cubed = 125000* 60 cubed = 216000* Maybe the fact that it almost doubles between 50 and 60. And: 63 cubed = 250047 (Hey that's double the number at 50 so anyone going 'just a bit over' not plus ten per cent or anything! Just a thought anyway. PS. Seem to remember from wind loading on radio antenna on tall towers; something about Pressure = 0.003 times the wind velocity squared???? For example at 30 mph; p = 0.003 x 900 = 2.7 lbs per square foot. (Roughly say 3) Whereas at 100 mph p = 0.003 x 10,000 = 30 lbs per sq.ft. So that's at variance with the cubed theory? |
#63
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Graph of car fuel consumption versus speed
Ed Sirett wrote:
On Wed, 08 Oct 2008 13:22:05 -0700, The Real Doctor wrote: On 8 Oct, 13:26, Martin Bonner wrote: I *think* that the force exerted by drag goes as the /square/ of the speed. Only for a constant drag coefficient. Cd itself, however, depends on the flow pattern around the car and that in turn depends on the speed - it's often expressed in term of the Reynolds number. As speed increases the flow becomes more turbulent and drag increases enormously. Conversely, when the speed is low the flow is (largely) laminar and drag is low. Have to diasagree the Reynolds numbers for object of size car, in air, mean that even a few mph and you are into unambigously turbulent flow, where the drag is proportional to speed squared. The Cd can be affected by things like a half closed door/hatch. But is largely unaffected by speed for Reynolds numbers of 10K+. That is consistent with my understanding as well. Airflow behind a car is fully stalled from a few mph onwards. |
#64
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Graph of car fuel consumption versus speed
terry wrote:
On Oct 8, 12:18 pm, The Natural Philosopher wrote: Dave Baker wrote: "Chris" ] wrote in message ]... I would like to make a graph of car fuel consumption versus speed. My driving is not smooth enough to gather the raw data myself. Do you know of any reliable figures, or graphs? I'm interested in relative values, rather than those for any particular car. The equations and sample data are on my website. http://www.pumaracing.co.uk/TOPSPEED.htm Excellent site Dave. And it shows that for 100mph about 70bhp/ton is fairly close. and no more than 35 needed for 70mph. However it does seem to be a bit conservative - many cars exceed 60mpg at 56mph, and also the fact of the matter is that so long as the big heavy cars aero losses are well below the frictional losses it doesn't matter a damn what speed it goes at. I.e. my old Jaguar would never ever turn in better than 27mpg no matter how slow it was driven. Typically it did 19-20mg. To get 27mpgh required it to be trundled at - yas - 56mph constantly. One stop would ruin it! It does show that there is plenty of room fr improvement though. Low friction small turbo charged engine, and lighter more aerodynamic cars and skinnier tyres could still have the speed. motorcycle has the speed but not the fuel consumption, after all. Hmm! 10 cubed = 1000 20 cubed = 8000 30 cubed = 27000 40 cubed = 64000 50 cubed = 125000* 60 cubed = 216000* Maybe the fact that it almost doubles between 50 and 60. And: 63 cubed = 250047 (Hey that's double the number at 50 so anyone going 'just a bit over' not plus ten per cent or anything! Just a thought anyway. PS. Seem to remember from wind loading on radio antenna on tall towers; something about Pressure = 0.003 times the wind velocity squared???? For example at 30 mph; p = 0.003 x 900 = 2.7 lbs per square foot. (Roughly say 3) Whereas at 100 mph p = 0.003 x 10,000 = 30 lbs per sq.ft. So that's at variance with the cubed theory? No, pressure or force is not power. The pressure is squared, the power is cubed |
#65
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Graph of car fuel consumption versus speed
In an earlier contribution to this discussion,
The Natural Philosopher wrote: No, pressure or force is not power. The pressure is squared, the power is cubed You're obviously a quick learner! g -- Cheers, Roger ______ Email address maintained for newsgroup use only, and not regularly monitored.. Messages sent to it may not be read for several weeks. PLEASE REPLY TO NEWSGROUP! |
#66
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Graph of car fuel consumption versus speed
Roger Mills wrote:
In an earlier contribution to this discussion, The Natural Philosopher wrote: No, pressure or force is not power. The pressure is squared, the power is cubed You're obviously a quick learner! g sure. Once reminded it all comes back. :-) |
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