Well I\'ve looked but as there are over 1500 posts referring to economy and no two posts that contain the words Torque, Revs and Economy in any meaningful sense, I give up. I\'m losing the will to live here!
Looking at this in simple terms, you could argue that driving at 1000rpm is more economical than 2,000rpm but it plainly isn\'t so.
In order to move your car torque has to be applied to the wheels. When cruising at motorway speeds you will be constantly making very small changes to the throttle position to maintain speed to accomodate changes in road conditions/engine load etc.
If your revs are well below the point of maximum torque for your car then any adjustment in load on the engine (a hill or a small increase in speed) will either require a downshift or for you to open the throttle by a significant amount. Either approach is going to burn more fuel. Next time you are out on a 60 limit road, assuming that you won\'t cause any hold ups, try putting your car in 5th and adjusting your speed to take the revs down to 2000rpm. Now without changing gear, try to marginally increase your speed, just as you would in the ebb and flow of motorway driving. In order to get any worthwhile change in engine speed you will find you are having to open the throttle by a large amount. Now try the same speed but in a gear that allows you to rev at around 3200-3500rpm and try the same exercise. The adjustment in throttle position is minute in comparison.
If your car is geared so the sweetspot in the rev range most closely matches the typical cruising speeds (50ish in 4th and 70-80 in 5th) you will be able to remain in gear and make those minor adjustments to your speed, drawing on maximum torque, with very little increase in throttle opening.
Labouring an engine at well below the maximum torque will require a significantly wider variation in throttle movement to accomplish a given change in speed/load conditions than maintaining revs closer to the sweetspot.
Although common sense suggests that low revs use less fuel, this is not the case. Excessive high revs will decimate your economy, but so will labouring the engine.
I am going to change the title of this thread to more accurately reflect the direction it will hopefully follow. As explained above, Dave, Mark and I are here to make sure that this site is not used in a manner that will breach the small print so I would ask that you bear this in mind when posting. We are unable to accept posts that \"name and shame\" unless there is clear and conclusive evidence already in the public domain. Unfortunately \"But I know it to be the case\" does not constitute proof and will not be accepted.
After all that, welcome aboard. We hope you enjoy the Backroom!
No Dosh
mailto:Alan_moderator@honestjohn.co.uk
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Seems odd that anyone could take exception to this topic - I've much more fatuous ones allowed.
Actually I've always wondered myself if the best speed to run an engine for economy is the maximum torque speed.
Any answer?
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Hi Alan,
The reason the original posts were locked is because some specific allegations were made against a manufacturer which are not supported by empirical evidence that is in the public domain. In other words, it was a case of naming and shaming.
As that breaches the Small Print of the Backroom the original post was removed. I will be moving parts of the original thread that were not in breach of the Small Print across to here a little later on.
No Dosh
mailto:Alan_moderator@honestjohn.co.uk
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Just a few current cars with mph/1000rpm:
Rover 75 2.5 manual 22.9mph/1000rpm (3050RPM @ 70mph)
Alfa 156 2.0 Manual 20.1mph/1000rpm (3500RPM @70mph)
Mazda6 22.5mph/1000rpm (3100RPM @ 70mph)
And an extract from a review of the torquey Alfa 156 1.9JTD
\"The significant benefits for the driver include a very \'relaxed\' and quiet engine (in 5th gear with the engine at 1000 rpm, speed is 44 km/h compared to just 34 km/h for an average 2.0 petrol engine).\"
This last example equates an average 2.0 petrol car to 21.1mph/1000rpm (3300RPM at 70mph) for the average 2.0 petrol engined car.
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I can assure you that my 1999 Mazda 626 2.0 did 85 mph in 5th gear at 3000 rpm, so there is a a car that performs as it should do. My big mistake was selling the Mazda.
The spec on my current car says the maximum torque is at 3750 rpm and maximum power at 5750 rpm
It also says the 0 - 62 mph time is 9.9 secs and the maximum speed is 127 mph.
In second gear the car is doing 9.5 mph at 1000 rpm. Therefore the maximum speed achievable in 2nd gear is 54.625 mph. To achieve 62 mph you have to change into 3rd gear. 62 mph in under 10 secs with two gear changes? I think not.
The maximum speed achievable? In fifth gear the maximum speed without taking into account any resistance is 123.6 mph not 127 but that doesn\'t take any account of air and tyre resistances. The drag coefficient is 0.33. Now I am not technical but perhaps you could suggest a realistic maximum speed taking into account the drag coefficient.
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(Tongue inserted into cheek)
realistic maximum speed 70mph, it\'s all you need to do
(duck below parapet)
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So using your approach the drag coefficient and the tyre resistance will reduce the car\'s ability to rev to the redline? It may offer more work for the engine but it doesn\'t stop the car doing its job.
If maximum torque is at 3750 then it is desirable that any cruising should be close to this engine speed for the maximum combination of economy and flexibility. It sounds very much to me that you car meets this criteria at 70-75mph in fifth.
Although maximum power comes at 5750, the engine doesn\'t stop producing power at that point. In second gear your car will reach 62mph at 6500 revs which corresponds with the redline on many cars of its type.
In a similar vein, a top speed of 127 based on your stated gearing of 21.5mph/1000rpm comes in at 5900rpm.
All the current competitors to this car that I was able to find data on in a 10 minute search offer near identical gearing.
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My current Volvo V70 140 BHP does 90 MPH in 5th gear at 3000 RPM, but it is an Auto (does this make a difference?).
However this is all academic because the thing is so underpowered it is preferable to do 70 for a peaceful journey!
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We need to consider the specific fuel consumption of the engine here - eg kg/hr fuel burnt / kW power produced. This has a minimum at a particular revs, and usually (for reasons I have yet to fathom) occurs at the max torque peak. This is good on diesels as this is usually at around 2000 rpm, which makes for nice relaxed cruising and economical driving around town. In petrols, the torque peak is usually nearer 4000 rpm, which (if the max torque/min specific speed rules still apply) makes for less economical cruising and driving around town. This effect is marked enough to be visible on the consumption computer in my mate's A4 TDi - at around 40 mph it is more economical to drive in 4th at nearly 2000 rpm, than in 5th at only 1400 or so RPM (all other things being equal of course!). I did try before to find some info about engine theory, but didn't really succeed - maybe I'll try again some time.
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RichardW
Is it illogical? It must be Citroen....
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Richard - interesting to hear what sounds like a scientific description. A few weeks ago I asked the diesel question you answer in your last-but-one sentence, about my 306 HDi which has no consumption computer. I can cruise on a level 30mph road in 4th at an indicated 35 and 1500rpm, but the engine is just starting to tell me it would prefer to rev a bit higher (~2000 in 3rd).
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Richard - So by that reckoning, if my 1.6 Petrol Escort has peak torque at around 4000rpm, shouldn't that technically mean that i should get excellent consumption at 95mph in 5th?
Or are these calculations (which i am not trying to dispute) not taking into account the drag resistance at these speeds?
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For best economy you need to be at the lowest revs which will allow the smallest throttle opening for the given conditions.
Imagine driving downhill, throttle virtually closed. The engine is only taking enough to keep it turning at that speed so you want to be doing the lowest revs possible because there's no load.
Imagine driving up a steep hill. You can probably do 40 mph in 5th gear but you'll have to have the throttle open quite a long way to make sure you don't lose speed. 4th gear may mean more revs but the throttle is not open as far.
As I said, you want the lowest revs which will allow the smallest throttle opening, that's the balance you've got all the time whether going uphill or on the flat.
There's no need to be at peak-torque revs unless the hill is really steep! The revs are higher so the engine requires more fuel.
Gareth
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Not exactly, Hawesy. It's saying that if you drive @ 95 you will get the best consumption in 5th gear. Likewise any other speed in other gears @ 4000 rpm. [that may mean 2nd gear for 30mph - and I'm not sure I believe that].
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please, please, no more - this one is SO boring. It should be in
Technical, surely...
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Good point. I will move this to technical later this evening.
No Dosh
mailto:Alan_moderator@honestjohn.co.uk
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Before you move the thread, how about asking for the reintroduction of vacuum meters as an economy aid. I had these on all of my cars until the last. They measure the manifold depression. The best economy is at the highest vacuum. It will surprise you to see that the gears mean something. The last car in which I had one fitted was a Volvo 440, and it was called an economy meter. Try going up a hill without changing down and you will see the economy plummet.
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Honda fitted this to the 1.5 civic for a while. They called it the VTEC-E.
All it did was cause traffic jams on hills as across the country, thousands of octagenarian civic drivers slowed to a halt when they saw the light go from green, to amber, to red. Not one of them bothered to change down as that would mean (shock horror) revving the engine.
HJ picked up on this in the Car By Car breakdown: \"1.5 VTEC-E sometimes seen at head of long traffic queue as elderly driver tries to keep in economy range.\"
I had one of these for a few days when my company car was in for repairs a few years back and I recall that the green light typically stayed on at around 2200 - 3500 rpm, ideally on a light or trailing throttle. Being a VTEC the torque band was that bit wider than most current multivalve cars, having a bit more low-down grunt.
I got bored of that after 24 hours and spent the remainder of my time with the car trying to remain firmly in the red whilst squeezing every last ounce out of the sewing machine of an engine. Handed it back wheezing like a goodun and took my proper VTi VTec back with a grin.
Thanks for reminding me of that, Hillman.
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"...measuring depression" - the ideal note on which to end this thread. Even Mats was fun!
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In the days of the Mobil Economy Run the serious competitors used to get into top gear as quickly as possible and labour everywhere at as low a revs as possible - including accelerating from transmission snatch speeds.
I am not convinced that economy is determined by how much the throttle is opened.
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What's your point, Hacko? Is it just that you find the idea of driving economically boring?
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not at all, Andrew-T - in fact, I practise it myself as much as poss. It was just that the original grudge against a certain make of car. ND. became a pain. The change of emphasis to economical driving made it much more interesting. For what it\'s worth, I don\'t think you need a gauge to tell you when is the optimum time to use the gears for best consumption.
Hey, a thought: if you remove mats, the weight reduction will give you more to the gallon, surely....
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Sadly Kevin hasn't come back to play yet. Shame really, as this thread may help him in the long run.
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shame on me - sorry DD, forgot not to name!
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shame on me - sorry DD, forgot not to name!
As its got "ND" next to the amended name, I don't think somehow it was me who did the editing!!
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...well within reason.
It is hard to get your head round this these days with five speed gearboxes and high torque diesels adding to the confusion.
Years ago it was easily possible to check this out with many luxury cars as they had a standard geabox with an additional overdrive on top, sometimes third too.
In this era many mid range luxury cars had a standard top gear figure of around 20mph/1000rpm. The overdrive figure was often 25-27mph/1000rpm.
So it was easy to get steady speed figures both for top and with the o/d switched in.
As an example the Reliant Scimitar GTE had an overdrive giving about 26mph/1000rpm against its top gear of 21mph/100rpm. This overdrive gave the GTE a very relaxed touring ability and excellent fuel consumption for its engine size.
Interestingly the speeds at which the overdrive decreased the fuel consumption the most were between 55mph and 70mph....i.e. between 2100rpm and 2700rpm. Well below the maximum torque figure which was reached at 3000rpm.
Also steady speed figures were recorded down to 40mph in top and o/d top, even at this lowest speed (equating to just 1500rpm in o/d top) the higher o/d gear still *improved* the consumption figure by 3mpg.
Funnily enough we owned a car that was about as far at the other end of the scale as you can get...a Fiat 850 Sport Coupe from the late 1960s. This was an amazing tiny rear engine coupe with "Alfa-like" styling and the most incredible tuned 903cc engine. It would rev freely to 7000rpm with a bike-like wail.
Its gearing in top was just 13.8mph/1000rpm meaning that its design top speed was at 6600rpm, and on a gentle downslope it would creep over 7000rpm to almost 100mph.
With no engine in the front a friend once held it up by the wing while I changed a puncture!
With that rear engine and semi-swinging rear arms I remember it was very prone to spinning, if you gained motoring experience on one of these you'd never complain about the "teeny" chance these days of a modern FWD car oversteering because of fitting "new tyres to the front".
M.M
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So does this bottom out to cruise at constant speed in as high a gear as possible but don't let the engine labour and change down to accelerate or go up hills.
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Hacko, you might be right about the mats. But they could also come in useful in the snow ...
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we're on a slippery slope again, AT
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"With that rear engine and semi-swinging rear arms I remember it was very prone to spinning, if you gained motoring experience on one of these you'd never complain about the "teeny" chance these days of a modern FWD car oversteering because of fitting "new tyres to the front"".
MM,
Brings back memories of a Renault Dauphine Gordini that a mate used to borrow from a woman "friend" he was teaching to drive.(Don't ask!) It would spin on dry roads - and was "quite exiting" on snowy forestry roads in N Yorks. Oh, the deserted roads of the '60s - in those days you drove at night with your lights on full beam and very occasionally dipped them. How often do you get to use full beam these days? Can't recall ever thinking about driving economically then either!
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Phil,
I think you meant "quite exciting", not" exiting". On the other hand, since you were on snowy forestry roads, perhaps "exiting" *is* the appropriate word!
Coming back to the base question ... On a test bed, the most economical constant engine speed would normally be the speed at which maximum torque occurs. However this would not necessarily apply 'on the road' where wind resistance and other factors come into play. A car with max torque at 4000 revs and gearing of 30 mph/1000 revs in top gear would almost certainly not be at its most economical at 120 mph!
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The words "maximum torque" don't mean a thing unless you have the necessary maximum throttle opening to produce that maximum torque. I think you will find that best economy is achieved at minimum throttle opening in the highest gear possible for the slope of the road you are travelling on. (Not counting coasting in neutral, of course!)
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L'escargot by name, but not by nature.
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At max torque you get max specific economy - so it is the most efficient speed in relation to fuel consumed per kW produced, but you need lots of kW to drive at 120mph, so time or distance based economy looks bad.
I had a thought about this the other day - people often complain that they can get no where near the manufacturers consumption figures (esp in petrols) and I began to wonder if the tests were done by keeping the engine on the max torque peak (and hence min specific consumption)? Whilst this would make for acceptable driving on a diesel where it occurs at about 2000 rpm, no one is going to drive around town in 1st at 4000 rpm in a petrol!
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RichardW
Is it illogical? It must be Citroen....
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