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Old 2nd May 2006, 19:27   #16
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Originally Posted by skandyhere
acceleration is a result of certain torque being produced at certain rpm, and so depends on the torque and power curves at different engine speeds. it does not depend on torque alone. it depends on THE PRODUCT of torque and velocity - which is the same as bhp.
Acceleration depends purely on torque. Pick up any auto magazine which gives acceleration at speed at contant time intervals or time at constant speed intervals, plot them on graph and you will see the result.

Again this rule has been made by God.

RK

Last edited by jat : 2nd May 2006 at 19:35.
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Old 2nd June 2006, 14:56   #17
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Quote:
Originally Posted by jat
Acceleration depends purely on torque. Pick up any auto magazine which gives acceleration at speed at contant time intervals or time at constant speed intervals, plot them on graph and you will see the result.

Again this rule has been made by God.

RK

hey jat, this physics is simple. but is definitely given to misunderstanding.

maybe it's not possible to teach/learn much by typing. without wanting to imply that i know something and u don't, here's my good willed suggestion. catch hold of some guy who's cleared his JEE or a physics prof or somebody like that.

no offense, but the physics u'v presented is a little inaccurate.

... also do check that diesel cars have higher torque figures than petrol cars. and u do know that the petrols are quicker...

regards.
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Old 2nd June 2006, 20:56   #18
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Engine's power becomes dominant in high rev and high speed. In fact, what will be the car's max speed depends on engine's power.
The max speed of a car is dependant on many factors. Aerodynamics/Drag Coefficient, rolling resistance, engine capacity, gearing, power to weight ratio etc.

If power was everything, it wouldn't have been possible for a 175bhp bike to crack the 200mph barrier.

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I disagree

To compare two engines, we have to compare equivalent force available at the point of contact between the wheels and road. That means more power > more torque.
Actually he's right in a way. Petrol engines produce relatively less torque than diesel engines. What gives them the higher bhp fig, is the higher engine rpm at which the torque is produced.

Your torque curve decides, how much power the engine produces.

The Vtec produces 132nm (97.35 lbft) at 4700rpm. Now imagine a similarly sized F1 engine producing 132nm at 17000 rpm. How much power diff do you think there would be between the 2 engines at peak torque rpm??

Vtec - 97.35 * 4700 / 5252 = 87.11bhp@4700rpm
F1 Engine - 97.35 * 17000 / 5252 = 315.10bhp@17000rpm.

So you see, both engines produce the same amout of torque but, what finally matters is, where that torque is being produced.

PS : Peak bhp produced would obviously be more than what is calculated at peak torque rpm.

Shan2nu
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Old 2nd June 2006, 21:32   #19
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For example a let us assume one diesel engine is developing 80 ps at 4000 rpm and another petrol engine 100 ps at 6000 rpm. How do we compare? Diesel engine with 80 ps will have a torque equivalent of around 80/4 = 20 while petrol will have 100/6 = 16.7.
Haha, thats not torque dude. What you've calculated is an estimated average of "Ps" being produced at 1000rpm.

It's like this. 80 bucks for 4 Apples (20 bucks for 1 Apple). 100 bucks for 6 Oranges (16.7 bucks for 1 Orange). You're talking about the same thing from a different perspective. LOL

Shan2nu

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Old 2nd June 2006, 23:02   #20
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Default Interpreting the BHP and torque...

I am listing down the BHP and torque of 2 petrol and 2 diesel cars....

Baleno 91bhp@5500 130@3000
NHC 77bhp@5000 125@2700
Fiesta 68bhp@4000 160@2000
Viva CRDi 82bhp@4000 187@2500

Guys, how best to interpret the above data.
like which car is best for slow moving traffic, which for normal-pretty easy going traffic, which one for the highway cruising etc.....

i also see that the peak torque/power is at different rpm for differnt cars.....so whats best to have peak torque/power at lowest possible rpm?
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Old 2nd June 2006, 23:07   #21
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The following web site gives a simple explanation of torque and power:

http://auto.howstuffworks.com/question381.htm

Jat, power output does not just depend on the torque -- it also depends on the rpm at which that torque is produced -- see the equation in the website that relates power, torque and rpm. Intuitively, if the same torque is produced at a higher rpm, then that torque is produced "more number of times per minute", as explained in the website, so the power output is higher.

You made a number of statements in this thread equivalent to "And that rule is made by God (Nature)". This is a deep philosophical issue that cannot be discussed at length here, but one point of view is that NONE of these "truths" are made by God (who or what is God anyway); they are all man-made. In other words, what you call "Nature" may "exist" in some sense without human beings, but any *description* of Nature that we human beings can understand, such as, Newtonian mechanics (being discussed in this thread) is a human creation. Newtonian mechanics, or for that matter any theory that we humans create, only produce "truths" that are *postulated by humans* (as "Laws of Nature") and exist only in human minds. If human beings disappear, so will their theories, such as, Newtonian mechanics, and the truths they represent.

Note that even the equations of Newtonian mechanics are not ultimate truths anyway; they are only *approximations* that will fail in certain realms (quantum mechanics, relativity theory). So what is God-like about Newtonian mechanics? Newton is not God. Newton = man.
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Old 2nd June 2006, 23:29   #22
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Quote:
I am listing down the BHP and torque of 2 petrol and 2 diesel cars....

Baleno 91bhp@5500 130@3000
NHC 77bhp@5000 125@2700
Fiesta 68bhp@4000 160@2000
Viva CRDi 82bhp@4000 187@2500

Guys, how best to interpret the above data.
like which car is best for slow moving traffic, which for normal-pretty easy going traffic, which one for the highway cruising etc.....

i also see that the peak torque/power is at different rpm for differnt cars.....so whats best to have peak torque/power at lowest possible rpm?
It's really not that simple. Bhp and Torque figures don't always give you the complete picture.

Gearing plays a major role in deciding how the car accelerates from a given vehicle speed, how it behaves at low speeds or what speed is best suited for comfortable cruising.

Gearing can be altered for fuel economy, driveability or outright performance.

Shan2nu
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Old 3rd June 2006, 03:18   #23
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Quote:
Originally Posted by venkatrx
I am listing down the BHP and torque of 2 petrol and 2 diesel cars....

Baleno 91bhp@5500 130@3000
NHC 77bhp@5000 125@2700
Fiesta 68bhp@4000 160@2000
Viva CRDi 82bhp@4000 187@2500

Guys, how best to interpret the above data.
like which car is best for slow moving traffic, which for normal-pretty easy going traffic, which one for the highway cruising etc.....

i also see that the peak torque/power is at different rpm for differnt cars.....so whats best to have peak torque/power at lowest possible rpm?

Venkat, I was puzzled by exactly the same thing when I started seriously thinking about buying a car....how to interpret torque and power and make apple-to-apple comparison for different cars??? The best answer that I came up with was these curves, which I had referred in "true power" thread also:

http://www.team-bhp.com/forum/techni...ost127523.html

Torque and power numbers in car spec are nothing but two poits on the engine's torque curve. Former is of course the peak of it and the later indirectly gives another point which is necessarily at higher RPM and its magnitude is lesser (power = torque x rpm). But even after this you can compare only engines, not cars. Because there are many factors which actually decide driving force/acceleration on road at various road speeds. Major factors are 1. weight of the car 2. gearing 3. drag coefficient 4. road gradient.
Out of that, road gradient does not make sense when we are making relative comparison so ignore it. Similarly drag becomes significant only at high speeds, so leave that too. Effect of weight and gearing is always fixed, whatever may be the speed at which you are driving. These two are the factors that you must not ignore.

when you consider weight factor, all that you have to do is to divide two torque numbers by the curb weight + additional load to get torque/tonne for example. If you assume a healthy driver weighing 80kg as the payload and include weight specs, you get this-

(all numbers are N-m/tonne@RPM)
baleno-EIII: 122.5@3000 & 110.7@6000
NCH-ZX: 110.6@2700 & 95.3@5000
Fiesta-TDCi: 130.1@2000 & 97.2@4000
Viva-CRDi: 152.3@2000 & 115.8@4000

So far you have got two points in the first curve in my reference post. This just tells how good engines are for respective car's weights. Note that these numbers do not tell us anything about lowend torque (unless of course peak torque itself occurs at pretty low RPM)

Now, you have to map these to 2nd figure in the reference which will give you a real and exhaustive picture. This depends on gearing (not only gear ratios between input and output shafts of the gearbox, it also include other ratios along the whole drive chain). Lets say for the 1st gear, torque at wheels gets multiplied by factor n1. At the same time road speeds get divided by n1. similarly for other gears, such that n1>n2>n3>n4>n5. As you see, in the top gear force at wheels is quite less, but the curve gets stretched across a very wide speed range.

One can answer many questions by looking at this figure-

1. pick-up: higher the wheel force available at a given speed, higher is the pick up. different curves also show how much drive force is available for each gear at any given speed.

2. drivability: if the curve raises and drops quickly, it spans for shorter range on speed axis. It indicates that if you want to lower or increase your speed, the engine has to be revved and slowed down too much and that too rapidly resulting in jerky drive. (Like the 1st gear) As you up-shift if the next curve drops too much then you will feel under-powered after shifting, which is typical of underpowered, high FE vehicles.

3. gear shifts: If there is lot of overlap between adjecent curves with comparable magnitudes, then you have more flexibility of choosing lower or higher gear across a wide range of speeds. If its the other way, then you will need frequent shifting. Also intersection points are nothing but shift points for max. performance.

4. highway cruising: curves corresponding to 4th/5th gears, which are usually overdrives answer that. Here you need to consider drag also.

Rule of thumb is higher and flatter is better. Since curves for individual gears are directly derived from engine curve, the rule obviously applies to it. "flat" part is more evident in lower gears, as you have to usually cover more RPM range in these gears. In slow moving traffic the "low end" part of "flat" becomes important. Therefore such cars are better for cities.

Actually, I don't think it is practical to get these detailed curves for each car. What we can get is just two points for each gear. In fact it was tough for me even to find out gearing information itself for various cars! I finally gave up on that. However, I had made an excel sheet which calculates torque/tone points for a lot of different cars, which I compared graphically.
I assumed that when I am comaring petrol to petrol or diesel to diesel, and weights and speed targets are also roughly same, then gearing should also be comparable. So direct comparison of engine curves itself should be good enough. The conclusion was that in 1.6L and lower petrol category, baleno and ikon(1.6) are the best performers... real josh machines Accent petrol did look better on paper, but my impression is that these are over-quoted numbers... just unbelievable (based on many people giving such feedback), so I simply excluded this particulat car

Now let me address diesel versus petrol comparison- As the numbers mentioned earlier make it clear, diesel engines have higher torques than petrol counterparts, but the RPM range across which they can maintain it is significantly lesser than petrols. But since cars have similar speed targets, overall gearing for diesels must be taller to extend their road speed to match petrol ones. Due to this, drive force at wheels gets significantly reduced. If top seed for petrol and diesel cars is same, but diesel has lower power, then it essentially mean that 4th/5th gear torque at wheel for diesel has to be reduced below corresponding torques for petrol. So diesel is sluggish compared to petrol. However, tall gearing make diesel car more drivable in low gears (due to lesser RPM range and high torque)

Just by looking at torque numbers of diesels, one tends to think that it has got very impressive pick-up. That would have been true if the gearing was same as the petrol counterparts. Then diesel would be really fast as compared to petrol but it would struggle to reach 100kmph (assuming lower power rating), whereas petrol would easily pass 150. This hypothetical diesel car could have also taken much more load, may be tow another car quite easily which petrol car can't do, in spite of higer power. The moral of the story is that we can not directly compare petrol versus diesel torque/power numbers, unless you know gearing.

.... I am tired now, this is probably my biggest post to date...without any cut-pastes!

Last edited by santosh.s : 3rd June 2006 at 03:21.
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Old 3rd June 2006, 08:20   #24
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Quote:
Originally Posted by Shan2nu
Haha, thats not torque dude. What you've calculated is an estimated average of "Ps" being produced at 1000rpm.

It's like this. 80 bucks for 4 Apples (20 bucks for 1 Apple). 100 bucks for 6 Oranges (16.7 bucks for 1 Orange). You're talking about the same thing from a different perspective. LOL

Shan2nu
not quite, shan2nu.

well known law...force x velocity = power. when it comes to rotary power, it's torque x rpm = power.

what he's done is simply calculated the torque figure at the rpm where max power is produced. and max power and max torque are never produced at the same rpm. which is why the arithmetic exercise carried out there is not relevant. the max torque would always be higher than the torque produced at the rpm he's used in computing it.
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Old 3rd June 2006, 10:53   #25
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not quite, shan2nu.

well known law...force x velocity = power. when it comes to rotary power, it's torque x rpm = power.

what he's done is simply calculated the torque figure at the rpm where max power is produced. and max power and max torque are never produced at the same rpm. which is why the arithmetic exercise carried out there is not relevant. the max torque would always be higher than the torque produced at the rpm he's used in computing it.
Ok, if your theory is right, then what are 20 and 16.7? Torque i guess.......but in what form? nm, kgm or lbft?

Acccording to me, if a car produces 80bhp@4000 rpm, it should have a torque figure of 105.04lbft@4000rpm and if an engine produces 100bhp@6000rpm, it's torque figure should be 87.53lbft@6000rpm.

So, where do 20 and 16.7 fit in?

Shan2nu
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Old 3rd June 2006, 11:22   #26
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Quote:
Originally Posted by Shan2nu
Ok, if your theory is right, then what are 20 and 16.7? Torque i guess.......but in what form? nm, kgm or lbft?

Acccording to me, if a car produces 80bhp@4000 rpm, it should have a torque figure of 105.04lbft@4000rpm and if an engine produces 100bhp@6000rpm, it's torque figure should be 87.53lbft@6000rpm.

So, where do 20 and 16.7 fit in?

Shan2nu
(105.04/87.53) = (20/16.7)

Units don't really matter if all you are interested in is a comparison of the respective torques produced. You could say that he has got this ratio as 20/16.7. Or you could say that his torques are 20 and 16.7 in some non-standard units that nobody uses.
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Old 3rd June 2006, 11:37   #27
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Or you could say that his torques are 20 and 16.7 in some non-standard units that nobody uses.
U can't just use your own units man. I can't say my car produces "20" at 4000 rpm and "16.7" at 6000rpm. It doesn't make sense. Tomorrow, someone will come and say "My car produces 2@4000rpm so, it's as torquey as your car. Afterall (2/1.67) = (105.04/87.53)

It would make more sense, if you used the formula 80 / 4 * 5.252 = 105.04lbft. The constant is there for a reason.

Your resulting figure has to comply with world standards.

Shan2nu

Last edited by Shan2nu : 3rd June 2006 at 11:42.
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Old 3rd June 2006, 12:06   #28
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I second rks, for comparison all you need to do is use same units on both sides, it doesn't matter what unit you choose. it can be standard on otherwise (invented )

I have tried to explain interpretation of diesel and petrol numbers in last 2 para of my previous post... in case you have patience to go through!
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Old 3rd June 2006, 12:09   #29
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Quote:
Originally Posted by Shan2nu
U can't just use your own units man. I can't say my car produces "20" at 4000 rpm and "16.7" at 6000rpm. It doesn't make sense. Tomorrow, someone will come and say "My car produces 2@4000rpm so, it's as torquey as your car. Afterall (2/1.67) = (105.04/87.53)

It would make more sense, if you used the formula 80 / 4 * 5.252 = 105.04lbft. The constant is there for a reason.

Your resulting figure has to comply with world standards.

Shan2nu
Well, of course you can' just say that your torques are 20@4000 rpm and 16.7@6000 rpm. If you did that, I would ask "in what units?", and then you would explain that what you did was to take the power in ps units and multiplied it by 1000 and divided the result by the rpm. From which I can calculate the torque in standard units that I am familiar with.

Another example that occurs frequently in physics is concerning the velocity of light c=3*(10**8) m/sec. Often you will see an analysis in physics (say, relativity theory) in which c=1 has been assumed; no units are specified for c, and c simply disappears from the various formulae that appear in the analysis. If one keeps in mind that c has been set equal to one, then all velocities that appear in that analysis have to be multiplied by 3*(10**8) to get the units in m/sec, and similarly, various derived quantities, such as, power, torque, etc. have to be suitably modified. This is just a book-keeping exercise. The danger is in the fact that since c simply disappears from the various formulae, one may lose track of the actual units of the derived quantities. So it may be better to keep the velocity of light as the symbol "c" in the analysis.
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Old 3rd June 2006, 12:31   #30
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Quote:
Originally Posted by rks
(105.04/87.53) = (20/16.7)

Units don't really matter if all you are interested in is a comparison of the respective torques produced. You could say that he has got this ratio as 20/16.7. Or you could say that his torques are 20 and 16.7 in some non-standard units that nobody uses.
In fact the non-standard units used for torque in this context may be stated as "ps per thousand rpm".
Regards, rks
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