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.
Chris gurgled happily, sounding much like they were saying:
They don't exist.
They'll vary widely between classes of car - although individual cars 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.
Googling your subject gives a load of hits. Here's the first one:
Drag cubes with velocity and so it may become important at speeds lower than 60 mph.
-- Nige Danton
My response would have been better phrased as "drag cubes with velocity; are cars really so slippery that drag does not become significant until 60mph?"
-- Nige Danton
I think that frictional losses including the rolling resistance are pretty much linear with speed, but aerodynamic drag is the cube of velocity. Or it might be the square.
Hence economy driving broadly falls into these categories
To reduce frictional losses - otherwise approximately constant per mile
- lighten the car and pump the tyres up. And fit eco-tyres. You can probably get 3-4% this way
- keep speeds below 60 mph at which point aero losses start to mount sharply. This is significant. On cars with consumption meters 50-70 represents about 10% increase in fuel consumption, over that it goes up massively.
- strip all external junk like roof racks and the like. There is probably at 70mph a couple of percent to be had here.
- try and drive at a gear and speed where the engine is most efficient. For a diesel that is at the lowest throttle setting IIRC where the fuel-air ration is leanest. That possibly means use revs and less welly to get acceleration and power, not slogging in a low gear at higher throttle settings. For a petrol it may well be the other way around I am not sure. This can net you about 5% from typical driving styles.
- reduce acceleration and braking to a minimum by anticipating the road. Braking represents a net loss of energy that is never recoverable. This is as great a contributions as speed reduction. Especially in towns.
The Natural Philosopher gurgled happily, sounding much like they were saying:
Not always. You're ignoring the effect of gearing and the efficient rev range of an engine.
Nige Danton gurgled happily, sounding much like they were saying:
Trust me on this... I've got plenty of experience with low-powered, unaerodynamic vehicles. It starts to come into play at about 60.
So why do large commercials use lots of fuel at a constant 30, more at a constant 40 and even more at a constant 50? If drag isn't the cause what is?
There's been a lot about this on the radio in recent months, with people wanting to cut their fuel consumption because of the price of it (now that oil is back to $85 a barrel, why is petrol still £1.09 at the pumps?) and the consensus is that the greatest savings to be had are by using gentle acceleration. Fair enough. Unfortunately, a lot of people seem to have heard this, and have taken it to heart without any thought. When joining a motorway, or dual carriageway, your boot should be on the floor, especially where it is an uphill slip road. You need to get your vehicle up to at least the speed of traffic on the inside lane, so that you can make the judgement to slip in behind or in front of any vehicle near you in that lane, without causing any problem to them.
More and more people now seem to gently meander up the slip, without a thought for anyone other than themselves, and then expect to be able to just join the main carriageway, whilst everyone else takes care of letting them. It has reached the point where I have almost been brought to a stop on the slip by these thoughtless people, so many times in recent months, that I groan whenever I swing off a roundabout, and see another motor chugging up or down the slip, in front of me. Careful smooth driving ? Fine. But think of the implications to others around you, when carrying out this 'eco-driving'.
Arfa
"dennis@home" gurgled happily, sounding much like they were saying:
I'm not a truck driver, so I don't know if that's the case or not. If it was, I'm sure the haulage firms would be restricting their driver's speeds - yet they aren't.
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).
Either - it depends on what parameter you're talking about!
Aerodynamic *force* is proportional to the square of velocity. The *power* (force x speed) needed to overcome aerodynamic drag is proportional to the cube of velocity.
I m not convinced that slow acceleration is as effective as its made out to be.
the energy needed to get a car up to speed is the same. Its just whether the engine is operting more efficiently at high or low power outputs.
Now very high power outputs ARE inefficient, that's without doubt. But whether 'tickover plus one' is more efficient than - say - half throttle
- is a really moot point.
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.
My gut feel is that a petrol engine does best at about half RPM and half throttle. A diesel possibly at somewhat higher RPM and somewhat less throttle
50mph on single carriageways....
I've been playing with this, having recently got my first car which displays fuel consumption. It wasn't quite what I expected. Best fuel consumption is flat between 50MPH and 65MPH. Between
45MPH and 70MPH, efficiency doesn't drop off much. Below 45MPH, efficiency drops off significantly (even when you can comfortably stay in 5th gear, although I'm not sure it's making any difference if I stay in 4th instead at lower speeds). Urban driving (start/stop and never getting above 30MPH) is the worst of all.Before I actually measured it, I would have guessed highest efficiency would have been at lower speeds than it is. (The effect of start/stop driving is no surprise of course.) I suspect engine, gearbox, etc has probably been designed for good fuel consumption at higher speeds instead, and this is more than countering increased drag up to 65MPH.
The Natural Philosopher gurgled happily, sounding much like they were saying:
Is that for efficiency reasons or for PR to try to cut back on the "dualling artics"? I suspect the latter - because it would be FAR more widespread, especially amongst smaller/pikier outfits - whilst the ones who seem to do that seem to be the more "polished".
Nope... 40mph. 50mph is only for
Right, So on a per mile basis, the work done is the force times the distance? So in theory the *consumption* is affected as the square of the velocity?
If you lump the two things together, you get that the frictional losses are constant..so there is a fixed amount of gallons per mile you need to burn just to keep moving irrsepective of speed, and the only way to reduce that is with skinnier tyres and smaller cars and engines, and pumping the tyres up harder..and a part which is related to the velocity squared, which starts at zero, and increase accordingly.
So given a *uniformly efficient* powertrain, in cruise the slower you go the less fuel you burn. Although the effects of aerodynamic drag are not that great up to 50mph or so.
The next point is where are engines most efficient?
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?
-- Nige Danton
Aerodynamic drag cubes with speed.
-- Nige Danton
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