From my recent post in the "Whats torque?" thread:
The rpm at which max torque is produced is the engine speed where each 4 stroke cycle of each cylinder produces it's maximum effort so if maximum torque is at 2000rpm then more power will be produced at higher revs, say 4000rpm, because there are more cycles to account for though the engine wont produce twice the power at 4000rpm relative to 2000rpm because each cycle is not as strong.
Torque will usually drop off as rpm rises above a certain point however as long as torque is not dropping off faster than the rise in rpm (if plotted as two curves) then power will continue to increase with revs. As soon as rpm reaches the point where the torque curves falls away more quickly that the rpm curve is rising then power will reduce despite revs continuing to rise.
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As a footnote, I use Haldon Hill just past Exeter to speed test my car( going UP the hill, obviously :P). It tops out at 80 in 5th gear (doing 3000RPM), but will only get to 75MPH in 4th (pushing a bit past 4000RPM). By my reckoning a diesel thus has more usable power at lower RPMs.
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It's torque that provides the action to accelerate a vehicle, and power is a resulting by-product.
With engines that don't have an ECU-engineered flat torque 'curve', there is a torque peak at certain RPM. If the engine is running faster than this RPM, then the factors which conspire to slow the engine down (road friction, air resistance, upward hills) try to force it to a region where it develops more torque, so it stabilises and carries on. If the engine is running slower than peak trorque RPM, then it will tend to peter-out and require a downshift.
A 4-valve/cylinder normally-aspirated petrol car engine without variable valve timing tends to produce maximum torque at around 4500 RPM, thus in order to get into the zone of really purposeful acceleration you have to use 3rd or even 2nd gear. Otherwise you put your foot down and then drum your fingers on the steering wheel while you 'wait' for the rev counter to finally climb into the zone.
In a turbo diesel, you are more likely to be cruising at, or above, the peak torque RPM to begin with, so you can just put your foot down.
All this is subject to gearing, of course, but with similarly-sized engines, a turbo diesel will nowadays produce about 167% of the peak torque of the petrol, but is unlikely to be geared even as much as 150% longer, so there is a net gain in the mid-range (turbo diesels are not so good at very low revs where they are off-boost, and drop-off sharply at RPM above their power peak).
It's horses for courses - during an overtake, you may have to shift up in a turbo diesel because you run out of revs, but in a 16V petrol you have to downshift first (and maybe shift up later anyway, if you'd had to drop to 2nd).
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Lots of interesting theory here .
BUT
If you are driving at say 30mph in a diesel in 3rd gear doing 2,000 rpm and acclerate hard
1. you will be close to maximum torque at 2,000rpm
2. Within the pwower band - say up to 4,500rpm in THIRD still, you will be doing 67.5mph.
So ignore most of this thread and use 3rd gear to accelerate from 30 to 60mph ...
All the rest is carp or petrol drivers' envy:-))))
madf
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"> It's torque that provides the action to accelerate a vehicle, and power is a resulting by-product.<"
Therefore the higher the torque output, the greater the accel, is that correct??
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I think the answer is yes it does = greater acceleration in terms of the rate of speed increase but not top speed (which is not what you are asking).
I would just appeal to diesel drivers though - recognising you feel you are endowed with torque and feel the need to overtake please try not to overtake petrol drivers and sit in front of them - your diesel exhaust smell is ghastly and clogs our pollen filters up.
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">I think the answer is yes it does = greater acceleration in terms of the rate of speed increase<"
So, Chedd's beloved TDCi Mundano with 260+ plus ftlbs will accelerate from 0-60 more quickly than - say - the 3.0 petrol V6 Mondeo with just over 200 ftlbs?
{he writes after furtively ploughing through Mondeo performance specs}.
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err -- I suppose it depends on gearing as well.
If I'm honest I am not to sure on this one so would bow to someone with greater knowledge.
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">I think the answer is yes it does = greater acceleration in terms of the rate of speed increase<"
perhaps see my post timed at 12:08 today.
So, Chedd's beloved TDCi Mundano with 260+ plus ftlbs will accelerate from 0-60 more quickly than - say - the 3.0 petrol V6 Mondeo with just over 200 ftlbs? {he writes after furtively ploughing through Mondeo performance specs}.
Missing the point there Mikcy, put your foot down at 1800 rpm in top in both and yes the diesel will accelerate quicker, probably up until around 90 mph when the V6 would be nearing its peak torque and horsepower would be building nicely. 0-60 is another matter.
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">Missing the point there Mikcy<"
Er.. no, absolutely on the point thanks Cheddybaby!
tunacat stated: "> It's torque that provides the action to accelerate a vehicle<"
If so, more torque = more accel, which is rubbish. More bhp = more accel, there may be more torque, there may be less torque. In many ways, torque is irrelevant, it's bhp at any given moment that counts.
">put your foot down at 1800 rpm in top in both and yes the diesel will accelerate quicker<"
So you keep reminded us, but repeat the same exercise at - say - 4000 rpm in 2nd and see what happens. When I'm old and grey, I'm sure I'll prefer a diesel as well.....
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And you drive everywhere at 4000RPM Micky?
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If so, more torque = more accel, which is rubbish. More bhp = more accel, there may be more torque, there may be less torque. In many ways, torque is irrelevant, it's bhp at any given moment that counts.
Mikcy, to say torque is irrelevant is simply wrong! Torque + revs = bhp so without torque you wont have any bhp.
A diesel produces loads of torque at low revs, they also produce a fair amount of bhp at low revs compared to a petrol engine however the torque drops off sharply over around 4000rpm so despite the revs rising if you hang it out to 5000rpm they are actually producing less bhp than at lower rpm. What allows a petrol engine to (generally) produce more bhp is higher revs and the ability to sustian it's torque curve longer so even if the torque is dropping off it is doing so at a lesser rate than the revs are rising so bhp continues to rise with the revs.
">put your foot down at 1800 rpm in top in both and yes the diesel will accelerate quicker<" So you keep reminded us, but repeat the same exercise at - say - 4000 rpm in 2nd and see what happens.
So there you go, that is the point, to overtake in a TD you dont need to change down and cos you are doing around 2000rpm you can at least double you speed, a petrol my need 4000 rpm so you cannot double your speed unless it has at least an 8000 rpm redline.
>>When I'm old and grey, I'm sure I'll prefer a dieselas well.....
>>
I might be slightly old (a relative term) and slightly grey though I dont prefer diesel, I simply recognise the strengths and weaknesses of both.
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">to say torque is irrelevant is simply wrong<"
My dearest Cheddski, that's not strictly true, I said that it was irrelevant in many ways. Tell me the max bhp and weight of a car and I can tell you something about the potential acceleration. Tell me the max torque and weight of a car and I can tell you nothing about the potential acceleration.
Bhp defines the energy available to do work (accelerate past the mimser), torque doesn't.
">So there you go, that is the point, to overtake in a TD you dont need to change down and cos you are doing around 2000rpm you can at least double you speed, a petrol my need 4000 rpm so you cannot double your speed unless it has at least an 8000 rpm redline.<"
Ah, but the OP asked the question: ">What do you suggest I do to get past as quickly as possible? Downshift, or just 'boot it'?<"
The technically correct answer is: keep rpm at max power and adjust gearing to suit. Foot down at 2000 rpm is not the fastest way to overtake in most TDs (assuming the gear ratios are chosen correctly and the gearchange is reasonably quick). The problem arises when there is a need to sustain power (mimser speeds up) and the bhp curve is falling, the resultant panicky gearchange with a marshmallow FWD box can leave the TDi driver in a dangerous position with reduced control. So the real world solution in many ways for the intrepid TDi driver is to floor it at 2k and overtake without a gearchange, but it's not the quickest overtake.
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That's a very good real-world answer Micky.
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So according to Micky if I'm doing 2000RPM (peak torque) in forth, its better to change down to third to increase the revs to say 2900RPM, then accelerate and change again at 4200 or so. What rot!
Come to Crewe Micky and I'll show you how to overtake in a chipped tdi, leave the pens, pencils, formulae and calculator at home, 5 and a half years experience of driving this car and seat of the pants tells me when I need to change gear or in this case not.
Where is this torque / bhp gauge everyone speaks of?
Do you own a modern turbo diesel Micky?
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It's simple:
acceleration = power/speed/weight
If speed and weight are constants, then accel increases as power increases, so max accel is available at max power.
Seat of pants = rate of change of accel (otherwise known as jerk)
>"Where is this torque / bhp gauge everyone speaks of?<"
A dyno, although a vague approximation can be made using an accelerometer/datalogger.
">Come to Crewe Micky and I'll show you how to overtake in a chipped tdi<"
It would take a lot more than the promise of a ride in a Tdi to get me to Crewe, although something with decent performance might persuade me ...... no, on second thoughts, I won't be travelling to Crewe (where is it anyway?)
">Do you own a modern turbo diesel Micky?<"
Yes.
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More formulae. Id suggest the "jerk" from 2900 - 4200RPM in third gear in my golf would be less than 2 seconds and youve added two half second gear changes.
Name and shame the diesel Micky
Crewes nice we have trains and............I'll get back to you;o)
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>>"acceleration = power/speed/weight"
I thought acceleration was Force divided by Mass...
And torque is a force (Newtons) applied at a distance (Metres).
Work is done when a force is applied *through* a distance (a revolution)
And Power is the time-rate of doing work (revolutions per minute).
So power is the chicken from torque's egg.
>>"If speed and weight are constants, then accel increases as power increases, so max accel is available at max power."
Max accel is indeed available at max power, i.e where the product of the torque and the revs is highest.
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Oh and the biggest jerk in my car is the one behind the wheel... ;-)
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>>>>"If speed and weight are constants, then accel increases as power increases, so max accel is available at max power."
>>Max accel is indeed available at max power, i.e where the product of the torque and the revs is highest.
If you have a perfect CVT, then, for best acceleration, you set the engine at the speed for maximum power, and change the gear ratio accordingly as the car speeds up. (I have another spreadsheet which demonstrates this, but I don't have the time to dig it out and tart it up at the moment)
If, however, you have a manual gearbox, in any particular gear, you get maximum acceleration at the engine speed for maximum torque. However, in most cases, you do get better acceleration by changing down, because although you are further from the torque peak, you *more* than make up for it by having more torque multiplication by the gearbox.
For all this, and more, have a nosy at the spreasheet which Gordon has kindly linked to from the what is torque thread.
Number_Cruncher
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You're right of course NC -but in reality those CVTs must be some way from perfection: IIRC, all the figures I've seen for models with CVT options seem to have *poorer* acceleration than the manual box versions.
:-(
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CVT would'nt handle the torque, VWs 6 speed had reached its reliable limit when it was introduced for the PD TDis
No ones yet come up with the equasion for momentum lost during 2 extra gear changes or boost lost due to the wastegate opening.
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Yes, you're both right, there aren't any perfect CVTs. There aren't any perfect examples of anything, but you have to begin somewhere!
But, there's a point here, which is that most mathematical engineering work is aimed at producing a model - by analogy, you won't get a working merlin engine in an airfix model of a spitfire, you won't include every last physical effect in any model of a complex entity. In by far the majority of (sensible) cases, engineering models don't aim to replicate all aspects of the entity under assessment, only the salient aspects of it.
The only question worth asking of a model is; does it tell you anything that you didn't *know* beforehand, i.e., does it make any worthwhile predictions.
*know* - Sometimes, a quick model can confirm or deny a vague thought or inkling. I've lost count of the number of times a quick calc on half a page of A4, or a simple spreadsheet knocked up in an hour has completely changed my view and aproach to an engineering problem.
IMO a simple spreadsheet or computer model should enable the typical performance questions asked on this forum to be answered. If you need to include sub-models of turbos spooling up to speed after wastegate operation, then there is probably insufficient distinction between the two cars/engines/techniques, and no amount of modelling would give you a reliable answer.
If the decision is so finely balanced, possibly only the stopwatch would give an answer! But then without testing a number of the vehicles/engines/techniques, you only have one sample - not enough to form a general conclusion.
Number_Cruncher
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The cars on the drive, the keys in my pocket, why would I need a model?
Your a clever guy, and obviously a mathematics nut, if I had do a calculation on A4 for the programs I'd want to watch on TV I'd miss them all.
I enjoy your posts but have to read them 10 time before I can understand what you're saying, its not you, its me ;o)
What would you factor in for mathematical error plus or minus 10, 15, 20%?
And no ones mentioned gradient of the road yet. lol
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Thanks Dox -I've learnt a lot from your in-depth VW knowledge.
>>obviously a mathematics nut
I really struggled to reach an accpetable standard in maths because I came from a practical background, with Btec qualifications rather than A levels - a lot of my spare time at University was spent in remedial maths lessons.
>>The cars on the drive, the keys in my pocket, why would I need a model?
If you can use a model to see what will happen if you change a value, you can gain insight without spending much money, or cutting any metal. (say if you are considering an engine modification adding 10Bhp - what will this mean for your 0 to 60?)
>>but have to read them 10 time before I can understand what you're saying
Sorry about that. In another thread, I bemoaned the poor state of technical journalism, but as I'm not the world's best communicator, I know I would be rubbish at it!
>>What would you factor in for mathematical error plus or minus 10, 15, 20%?
The best way to do this is to carry out a sensitivity analysis. This means varying each input number by a given percentage, and checking which one makes the most difference. The values which make the most difference to the model's output are the ones which you might spend most time and money in defining with greater accuracy. Having said that, the adage Garbage in = Garbage out is very apt in this context.
Ah! Gradient is easy. The term m*g*sin(theta) is added to the expresion for drag.
Number_Cruncher
Number_Cruncher
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After rading your post I think I have fallen in love with my physics A-level teacher, figulatily but not literally. Only don't stop the day job and become a journalist, I can't quite understand the last equation.
Certainly for potential energy used to climb shoulden't it be M*G*Velocity*gradient
-Or is Velocity*gradient implicit in the sin(theta), I was never any good at trignometry.
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Torque means nothing without RPM
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Sorry mk, I missed your post.
I'm not dealing in anything as esoteric as potential!!
m* g * sin (theta) is just the component of the vehicles weight that lines up with the angle of the road, and as such is a force acting to slow the vehicle, so you can just add it to the other components of drag force, namely tyre rolling resistance and aerodynamic drag. There's no velocity term in there at all.
So, taking extremes;
theta = 0, level road, there is no drag force resulting from the gradient.
theta = 90 degrees, the road is vertical, the drag force resulting from the gradient is equal to the weight of the car.
Number_Cruncher
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alternatively you could just look for a gap and welly it.
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If, however, you have a manual gearbox, in any particular gear, you get maximum acceleration at the engine speed for maximum torque. However, in most cases, you do get better acceleration by changing down, because although you are further from the torque peak, you *more* than make up for it by having more torque multiplication by the gearbox.
NC, it is not strictly torque multiplcation by the gearbox, the effect of the gear ratio change is independant of the torque and power peak and simply effects the speed of rotation of the road wheels relative to the crankshaft. Rather the lower gear ratio allows higher rpm for the given road speed hence although the engine is further from it's torque peak it's power is increased because (if plotted as two curves) the torque curve drops at a lesser rate than the rpm curve rises so more power is produced at higher revs (up to a point of course).
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>>NC, it is not strictly torque multiplcation by the gearbox
Cheddar, it is. Really.
In terms of power, a gearbox does nothing. Ignoring small losses, power out = power in.
In terms of torque, again, ignoring small losses, torque out = torque in * gear ratio. -> a pure multiplication of torque.
In terms of shaft speed, speed out = speed in / gear ratio.
At heart, we are using different words to describe the same phenomenon.
There are two methods which you can use to estimate the performance of a vehicle - one is based on torque and the difference between tractive effort and drag force(my preferred approach), and the other is based on the difference between engine power and the power required to overcome drag at various speeds. Despite having different numbers internal to the calculation, they both predict the same response - as one would expect!
Number_Cruncher
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>>NC, it is not strictly torque multiplcation by the gearbox Cheddar, it is. Really.
At heart, we are using different words to describe the same phenomenon.
You are right in you last statement I think. My point going back to your assertion................
>>However, in most cases, you do get better acceleration by changing down, because although you are further from the torque peak, you *more* than make up for it by having more torque multiplication by the gearbox. >>
............. was that the key benefit of changing down is the increase in power derived from the increase in revs, power being a factor of torque and revs, in a close ratio gearbox the torque multipliaction between gear ratios may be quite small however a small increase in revs can mean that the engine is producing much more torque, much more power or both, torque multplication by gear ratios is linear, torque output at the crank is far from linear as illlustrated by the massive step between 1500 and 1800 rpm on most TD's.
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Yes, that's right - it depends on the balance between how "peaky" the torque curve is, and the step between ratios.
In many cases with a standard gearbox, and a broad, flat(ish) torque curve, you get the benefit of changing down because the step in ratios gives you more torque multiplication than the torque reduction you suffer by being off peak, so the nett effect is more tractive effort, hence better acceleration.
Number_Cruncher
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">If you have a perfect CVT, then, for best acceleration, you set the engine at the speed for maximum power, and change the gear ratio accordingly as the car speeds up.<"
Hurrah!
">If, however, you have a manual gearbox, in any particular gear, you get maximum acceleration at the engine speed for maximum torque.<"
No, no and no! Consider a manual gearbox with 100 ratios linked to an infernal compression engine with a 2000rpm powerband, 20 rpm per ratio means that each ratio can be specified to hold the engine at max power, it doesn't matter where the peak torque value of the engine lies, because we can hold the engine at max power and we have your CVT (essentially).
">However, in most cases, you do get better acceleration by changing down, because although you are further from the torque peak, you *more* than make up for it by having more torque multiplication by the gearbox.<"
And therein lies the root of torque confusion, 200bhp is 200bhp no matter what one does with the gearbox, but torque is a meaningless quantity without further information.
"My car has more torque than yours"
"Where? At the crankshaft? At the gearbox output shaft? At the rear axle?"
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>>No, no and no!
Yes, yes, and yes!
Micky, having made my point about the acceleration of a perfect CVT, I was then talking about what happens if you stay in any particular gear of a standard manual gearbox. If you stay in, say, third gear, you do get maximum acceleration in that gear at or very close to the torque peak of the engine - it is where you get the maximum tractive effort.
Please have a close look at this logic - it is right!
If you choose to change down a gear, you move away from the torque peak, but you more than make up for it by the torque multiplication offered by the lower gear. This is why, when you have a choice over which gear to be in, it is better to choose a low gear, and be near the power peak.
Number_Cruncher
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You can't have it both ways, max accel cannot occur at max bhp and max engine torque (which might not be the same as max drive shaft torque). And don't forget, acceleration = power/speed/weight
">but you more than make up for it by the torque multiplication offered by the lower gear.<"
Which perhaps indicates the transient nature of torque and why torque should be ignored in any meaningful critique of an engine's performance.
More bhp will always out accel less bhp if weight is constant, torque is irrelevant. Torque doesn't even have it's own units, it has to borrow weight and distance.
The root cause of the confusion regarding torque is the difference between torque at the crankshaft and torque at the final drive shaft, or even the tyre casings on the driven wheels. Ignore torque (wherever it is measured) and look at bhp. I might even post an equation ...... at some stage.
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Remember I am talking about what happens in any particular gear.
Taking the (incorrect) equation which you have posted a few times
acceleration = power/speed/weight
First, I'll correct the error, and clarify the terms a bit
vehicle acceleration = engine power/ vehicle speed/ vehicle mass
re-arranging slightly
vehicle acceleration = ( engine power / vehicle speed ) * ( 1 / vehicle mass )
Now, if you stay in one particular gear as per my post, there is only a constant (derived from gear ratios, and the rolling radius of the tyres) between engine speed and vehicle speed - lets call the constant C
vehicle acceleration = ( engine power / (C* engine speed) ) * ( 1 / vehicle mass )
Re-arranging again,
vehicle acceleration = ( engine power / engine speed ) * ( 1 / (C* vehicle mass) )
So, everything in the second bracket is just a constant
and the first bracket is just engine torque!!!
vehicle acceleration = engine torque * ( 1 / (C* vehicle mass) )
vehicle acceleration = engine torque * A constant which depends upon what gear you are in.
So, in any gear, using your formula, as the basis for the calc, max acceleration occurs at the engine speed for max torque.
Number_Cruncher
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">First, I'll correct the error, and clarify the terms a bit<"
Where's the error?
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>>Where's the error?
It is that mass and weight aren't the same thing.
Mass is typically measured in kilograms, and is purely a measure of the inertia of a body, while weight is a force, and is measured in Newtons.
If you know the mass of a car is, say, 1000kg, then its weight is m * g, 1000 * 9.81 = 9810 N
In you equation, you have power/speed - which depending upon the application is either some force or torque. According to Newton's Second Law, acceleration = force / mass, rather than force / weight as in the original equation.
Looking at the numbers in a model can be helpful - I usually use MATLAB, but there is an Excel sheet which GordonM kindly posted on my behalf which might help clarify the situation - one of the graphs in there shows how tractive effort varies with both engine speed and gear ratio.
Number_Cruncher
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">It is that mass and weight aren't the same thing<"
Yes, surprisingly as it may seem, I am aware of that, but in this context the use of the term "mass" confuses the issue when we are talking about cars where the term "kerb weight" is used in preference to "mass". Horses for courses.
I always enjoy out-pedanting a pedant, so:
">If you know the mass of a car is, say, 1000kg, then its weight is m * g, 1000 * 9.81 = 9810 N<"
is not strictly correct, because (pedantically) g is not always a constant.
The use of the terms power and speed is crucial because these are terms that are closely related to cars. Force isn't.
If the objective is to overtake as quickly then why would we select a gear (or gears) that prevented the engine from operating at max power?
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In correcting Micky's equation, I missed one subtelty, although the point and validity of the post remains essentially unchanged.
I wrote;
>>First, I'll correct the error, and clarify the terms a bit
>>vehicle acceleration = engine power/ vehicle speed/ vehicle mass
I should have written;
vehicle acceleration = available power/ vehicle speed/ vehicle mass
Where available power = engine power - road load power
Where road load power = drag froce * vehicle speed.
The point of this is that you can re-plot the engines power curve in any gear as a function of road speed rather than of engine speed. So you can build up a graph of engine power in all of the available gears. If you then add onto this graph the road load power, the maximum speed that the vehicle is capable of is where the engine power line crosses the road load power line.
This is exactly analogous to the graph in the spreadsheet, where maximum speed is given by where the tractive effort curve crosses the vehicle drag curve. The two methods give exactly the same performance prediction, they just use different "language".
Number_Cruncher
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So let me get this right, you've changed "my" equation:
acceleration = power/speed/weight
to:
vehicle acceleration = engine power/ vehicle speed/ vehicle mass
and then to:
vehicle acceleration = available power/ vehicle speed/ vehicle mass
Well, I think we all know that acceleration in this instance means "vehicle acceleration". But if you really want to be pedantic then perhaps we should introduce vectors in case the vehicle has fallen off a cliff. Or even captured by one of those twin rotor helicopters with an enormous electromagnet swinging beneath ;-)
I used the term "power", you changed it to "vehicle power" and then to "available power". In the context of this thread, that is pedantism in the extreme. And as one pedant to another, well done ;-)
As far as I am aware, most vehicle data sheets refer to weight not mass because we live on planet Earth and weight units are close enough.
The end result is: at any moment, if weight (mass) and speed (or should that be velocity?) are constant then the vehicle with more (engine) power will out accelerate the vehicle with less power, irrespective of (engine) torque.
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Available power really isn't pedantry, because it introduces the concept that the car can't just keep accelerating ad infinitum, there is a maximum speed where the available power is zero - just as analogously, at the same speed, the drag force is equal to the tractive effort.
>>As far as I am aware, most vehicle data sheets refer to weight not mass...
For public consumption, this is probably true - that doesn't make it correct though. To the layman, the two concepts of mass and weight are interchangeable. To an engineer, there is almost an order of magnitude difference between the two different concepts - so the units definitely aren't close enough!! I think it's fair to say that we are discussing a technical point, rather than the level of trim fitted to the GXL model; so obeying good practice with respect to units and physical concepts is required.
A good example of units that are close enough for many practical purposes are imperial tons and metric tonnes.
The end result really is that all things being equal, mass, speed, etc, then an engine with more power must also have more torque.
I know that you are mad keen on converting everyone to a power-centric view of the motoring, but both measures have their uses, and it's no accident that both are routinely quoted.
Number_Cruncher
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">To the layman, the two concepts of mass and weight are interchangeable.<"
Yes, because most laymen (and laywomen!) live on earth ;-)
">For public consumption, this is probably true - that doesn't make it correct though. <"
You're absolutely right, but we must try and live in the real world with the uneducated rabble ;-)
">I think it's fair to say that we are discussing a technical point,<"
Then we must consider velocity not speed. And losses, what about the losses?
">A good example of units that are close enough for many practical purposes are imperial tons and metric tonnes.<"
Ah yes, net ton, long ton and metric ton. Or was that tonne? Or US ton?
">The end result really is that all things being equal, mass, speed, etc, then an engine with more power must also have more torque.<"
No, no and no ;-) If we are referring to the engine, then why bring speed and mass into it? Why not just consider the engine.
">I know that you are mad keen on converting everyone to a power-centric view of the motoring, but both measures have their uses, and it's no accident that both are routinely quoted.<"
And that's where the problem lies, bhp can be routinely quoted and routinely understood (probably), torque is routinely quoted and routinely misunderstood.
This thread is probably reaching a conclusion (phew), Patrick Glenn's Cartest programme is worth dabbling with, I don't know if it's accurate in any way, I can't confirm that it won't empty your bank account and run off with your life-partner. But I used the full version it to develop a virtual car fitted with an engine that developed huge torque, it kept stalling. Odd.
www.cartestsoftware.com/cartest2000/index.html
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Micky, I have a magazine that tested a chipped Golf TDi like mine, was 115BHP now 145BHP. They compared it against a standard Golf V6 4motion 204BHP. The TDi beat the V6 on 3 or 4 of the acceleration tests (out of 5).
Using your logic how can this be?
The magazine was VW Audi Car or VW Driver 2001 the car was an ABT tuned Golf.
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Micky, I have a magazine that tested a chipped Golf TDi like mine, was 115BHP now 145BHP. They compared it against a standard Golf V6 4motion 204BHP. The TDi beat the V6 on 3 or 4 of the acceleration tests (out of 5). Using your logic how can this be? The magazine was VW Audi Car or VW Driver 2001 the car was an ABT tuned Golf.
What acceleration tests? A horse will out accel your Golf on five tests out of five:
0 -1 mph, 0 - 2mph, 0 -3 mph etc.
But that's because the gear ratios on your Golf aren't designed for peak performance in the 0 - 5mph range.
Assuming weight is the same, accel from - say - 30mph with 145bhp will be a slower than accel from 30mph with 204 bhp. Physics.
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">Max accel is indeed available at max power<" Hurrah!
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">Id suggest the "jerk" from 2900 - 4200RPM in third gear in my golf would be less than 2 seconds<"
Wrong units, try again ;-)
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I'll now make it my mission to clear my exhaust of soot with a pointless excursion to the rev limit everytime I overtake a civic just incase it should be you type's'
Everyone should have a mission in life, don't you agree type's'? ;o)
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So that's why the BBC weather forecasts include a pollen level warning...:-)
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What\'s for you won\'t pass you by
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Agreed - I'll keep an eye out for you passing me.
Just some friendly banter by the way.
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I enjoy our little spats type's', I look forward to reading your posts ;o)
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I don't - you have this wonderful ability to end your threads with a closing comment that stops me responding to the subject matter after you have posted. 'it's brilliant.
It leaves me sat at the keyboard floundering for words.
I aim to get even through practice though.
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mk124
I don't know if you missed my question / request for info which I posted above, but just in case you did, I was interested in more info on the Doblo.
Thanks
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Coming back to the original question....
If you are in one of the early Mondeo diesels - (the 1.8) take great care, Third is geared too low (you hit the rev limiter) and when you change up to fourth the engine is turning too slowly so it will not accelerate at all - caught me out coming back from Edinburgh one day years ago - had to abort the overtake. Are there any of those left?
In the newer engines (VAG/Honda/Ford) no problems!
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The early 1.8's fitted to Mondeos and Escorts are best avoided. Horrid noisy things.
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I don't care what technique you use to overtake me in a diesel, provided that you don't subject me to all that diesel exhaust soot in the process!
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L\'escargot.
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I presume L'escargot you drive an LPG converted Prius, complete with coachwork conversion to keep your bicycles in the back and only use it for essential journeys? No? thought not.
At least the petrol heads no longer insist diesel drivers can not overtake their sewing machine powered motors.
Keep it up soot boys, we're getting through sssssssslllllllllooooooowwwwwwwllllllllllyyyyyyy ;-)
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Micky said: "more torque = more accel, which is rubbish."
No, I don't believe so, Micky. BHP is just a figure, a measurement of the power being produced at those particular revs. Torque is the actual motive force, and what is needed to *increase* the car's speed from its current value.
The torque at the tyre's contact patch on the road depends on the effect of the overall gearing from crank to tyre tread applied to the torque the engine is producing at those revs. When this is all calculated-out, if the torque at the tyre tread is higher, the car will put-on additional speed at a greater rate.
The maximum BHP will dictate the car's top speed, and has some bearing on 0-60 test times where the car is set-off with the engine at peak power and attempted to be held there, but in normal 'rolling' acceleration, it's torque that's doing it.
If the engine happens to be 'riding atop' (going just a bit bit faster than) the crest of the torque curve when this accelerative effort is called upon, rather than having to 'scrabble up to' the crest, so much the better. This is why a turbo diesel will accelerate faster than a normally-aspirated non-variably-valve-timed 4-valve petrol engine in typical mid-range-revs driving.
In typical mid-range-revs driving, *is* the lower gearing of the petrol-engined model sufficient to give it equal or more torque at the tyre tread than its diesel-engined variant? *That* is the question we need answering.
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">Torque is the actual motive force, <"
No, no and no ;-) Torque is not a force
The equation for acceleration is: a = power/speed/weight. Take two cars, same weight, same tyres, same everything except one has 400bhp available at 30mph, the other has 200 bhp available at 30 mph, which can accelerate faster?
">In typical mid-range-revs driving, *is* the lower gearing of the petrol-engined model sufficient to give it equal or more torque at the tyre tread than its diesel-engined variant? *That* is the question we need answering.<"
The real question is: which one accelerates faster from a given speed at a given rpm, the answer is: the car with more power at that rpm.
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Yes, and the reason it has more power at that RPM is because the engine is exerting more *torque* at that RPM.
:-)
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The early 1.8's fitted to Mondeos and Escorts are best avoided. Horrid noisy things.
All pre-CR/PD four pot diesels are nothing more than tolerable to drive in my experience. Most are horrid. SWMBO's old 1998 XUD (the supposed "king" of the pre-CR diesel) engined 306 D-Turbo was beautifully smooth, but if you put your foot down with less than 2,000 RPM on the dial it simply refused to accelerate at all. It would then kick you in the backside at 2,100 and die at 4,000. Horrible! The Ford unit has a much wider spread of power, but sounds and feels like an out of balance twin tub washing machine. At least the stereo drowns out the noise. The old VAG 90PS TDI goes well, but is hardly any more refined than the Ford unit. Swings and roundabouts.
At the end of the day, you buy an old-school diesel as a tool. All of them, whether Ford, PSA, VAG or any other give you 45 mpg+ economy, easy DIY maintenance, no possibility of four figure fuel injection repair bills, 200,000 miles+ of reliable A-B pottering, and a low purchase price. If you want an enjoyable drive, you buy a petrol version, or stump up for a newer generation diesel.
All IMHO, of course.
Cheers
DP
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I cannot agree with DP's comments on pre CR 4 pot diesels being ALL horrible.
The 1.4 diesel as in early 106s - NOT the 1.5 - is smooth , quiet and refined - when warm. When cold it's clattery. But 50bhp means the skin will be safely adhering to the rice pudding as it is woefully slow (but ok in towns as a runaround)...
And some Mercedes diesels were ok...
madf
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I had the pleasure of driving a Golf TDI hire car up the motorway today and my question is this :-
How do you manage to stand the noise and vibration these engines generate over long distances.
I think I have vibration white arm and my ears are still ringing from the noise. (Not half as much as they will be when the diesel drivers read this).
It was terrible - and I have heard people say that modern diesels are more refined !!!!!!!
Once my body has stopped shaking I think I will nip out in the refined smooth turbine like accord just to re-acquaint myself with what a car should feel like.
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How do you manage to stand the noise and vibration these engines generate over long distances.
Never had a problem with this
I think I have vibration white arm and my ears are still ringing from the noise.
Again, not an issue.
It was terrible - and I have heard people say that modern diesels are more refined !!!!!!!
Maybe...I like mine as you can tell there's something under the bonnet. Same reason I will never own a 4 cylinder motorbike. Yeuch.
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I do have to ask what exactly your post has to do with overtaking in a diesel car though.
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Maybe...I like mine as you can tell there's something under the bonnet. Same reason I will never own a 4 cylinder motorbike. Yeuch.
A single or twin cylinder bike would certainly harden you to the more minor vibes of a diesel ;-)
Cheers
DP
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I am pleased to see my question in the torque thread has generated some debate ;-)
I have since done some anecdotal testing and found that in my car (Ibiza PD130) the best acceleration seems to come from around 2400rpm - about 500rpm above max torque. This means you can almost double your speed before changing down, although staying above 4000rpm for any length of time is a bit pointless when there's another wave of torque waiting to be experienced.
type 's' I am interested and surprised to read about your experience with the golf - can I ask how highly it was revving on your jouney? The ibiza sits at about 2000rpm at 80 and creates considerably less noise / vibration / harshness than similarly sized petrol cars, although obviously it's never going to be the last word in refinement!
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Never really paid too much attention of revs - it was an auto dsg jobby so I just left the car to it.
Whilst it has not much to do with overtaking - the only point I was making (badly) was that when flooring it to overtake I did actually find it very noisy and rather unpleasant.
I was being a little facetious in my post but the point is still true - I would struggle to go any disctance in one day after day - especially after driving the accord petrol.
Having said that a colleagues 320d is very refined.
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Was it a hire car? I loathe autoboxes as they sound ridiculous (BUUUUUUUUUUUUUUUUU-uuurrrrrrrrrrrrrrrrrr - double yeuch.) so I'll give you that point. ;o)
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It's true what the say though about hire cars being the fastest on the road - regardless of engine and box.
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First - this has been a most interesting thread, loads of technical information and good debate.............but, isn't it all a bit too academic?? Don't most of us choose the appropriate gear for overtaking by "feel"? Does anyone look at the rev counter, assess the torque and power available in the other available gears before overtaking? For a start, every overtaking manoeuvre is different, one never knows the response of the "overtakee", so how can advice be given on particular methods? I haven't a clue at what revs maximum torque/power is available on my car, sometimes I cruise past without dropping a gear, sometimes I drop a gear and sprint past - but I can't recall looking at the rev counter, I just "feel" whether it is necessary. That's the case in my present car (diesel!!) with a rev counter, it certainly was the case in previous cars (petrol and diesel) without rev counters.
I can also say that in hire cars/vans/ minibuses I have never felt the need for advice on overtaking techniques and it usually just requires some careful driving until one gets the "feel" of what is available from the engine in terms of speed, power acceleration etc (and often it depends on the load, number of passengers, whether atrailer/caravan is on the back etc). Do I need to know the bhp/torque at different revs of the Sprinter I am hiring next week to help move house for my daughter? Not so sure I need to know whether it's even petrol or diesel --- except for when I fill up!
Anyway, please continue the debate - it's interesting!!
--
Phil
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">Don't most of us choose the appropriate gear for overtaking by "feel"?<"
But that's why most people cannot overtake quickly and effectively. And that annoys me, I have to forgo overtaking opportunities because the semi-mimser in front can't be bothered to change down to pass the tractor before the skip lorry heaves into view.
">Does anyone look at the rev counter, assess the torque and power available in the other available gears before overtaking<"
Forget torque. All my cars have white tape on the rev counter marking peak power.
Actually, that last bit was lie, but my BEC has shift lights which tell me:
1) Change up now, peak bhp
2) You really must change up now, peak bhp was a while ago
3) Please, please change up now, I might explode.
">For a start, every overtaking manoeuvre is different, one never knows the response of the "overtakee", so how can advice be given on particular methods?<"
Overtake as quickly as possible, with as much power as possible ... and then give the overtakee a "thank-you wave" in the style of Mr Moss.
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">the only point I was making (badly) was that when flooring it to overtake I did actually find it very noisy and rather unpleasant.<"
Perhaps the engine was running at the rpm that generates max power?
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The car must have had the seat massage option ;o)
Mine only feels rough at idle, maybe its the DSG box transmitting vibration or maybe because its had some abuse.
Noticed you said it was fast though :O)
Thought you had a Civic? I used to like the Accord, not keen on the new chisle shaped one though
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No - got the accord dox - chiselly shape one.
It's not a bad car though (touch wood).
You are right about the Golf being fast - I was impressed with it's turn of speed - the only thing letting down in all honesty was the response time from the auto box - it was a bit scarey when pulling out of a junction onto a busy road and it not responding quickly - once it was off though it was like a scalded cat. My mate says his 320d auto is the same when pulling off quickly.
I used to have Golfs and it was nice being back inside one - I still like em.
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I don't like autos, I like to be in control.
Used to share a lift with a guy with a BMW 330, nice but juicy. He was amazed how I could accellerate past lorries and tractors in my humble Golf diesel.
You've said nice things about the Golf now give us the punchline ;o)
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No punch line I'm afraid dox - I think I mentioned before Golf GTI & Type S were shortlisted - went for Honda because of previous poor dealer service from the VW boys.
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