[jat]

This topic has really become hot ;-)
I would like to clear the doubts as far as possible.

1- I cleared my JEE long time before. At that level then they don't teach us how the combustion is influenced by length, dia and ratio of length to dia of the holes of fuel injector's nozzles and how it effect the torque and power of engine, combustion efficiency, liner and piston wear etc.

2- Well I couldn't get the hold of physics prof but I was constantly involved with makers of engines (one of the jobs which I have done was to train people about power/performace calculations and analysis) including the guys who made the most powerful diesel engine in the world.

3- The maximum speed of car depends on "how the power from engine is trasferred to the wheels and the resistance of the car". Please check the graph presented. It describes the power at wheels in different gears. Transmission efficiency and ratios play very improtant roles. You may have max speed in 4th gear instead of 5th, and depends on where you on the resistance and power curve. And Honda guys are clever in doing so, that's why even when power torque data is similar with other cars, their cars perform better.

4- I have used torque equivalent and not torque for comparison. Just the get at the ratios of equivalent power output on some common base line and ratios have no units. If I had to used actual calculations, it would have run into number of pages and it would go above the head of most of the non tech guys. (I prefer SI/MKS system of units by the way and have mentioned it on another thread). Also, I have never mentioned that power output does not depend on rpm. Power is proportional to mean effeictive pressure x length of the stroke of engine x square of the bore of engine x rpm. Then how come, I can say the it is independent of rpm.

5- You can call nature or God or whatever you like, but laws of nature remains irrespective of whether human being is there or not. Say a building can be built only when human being is there, but anything dropped from the top floor will always fall the the ground even if human being is not there (nature's law). Similar torque power rules will remain same on another planet even if it not inhabited by humans.

6- And guys please see the graphs and then read through, I have clearly talked about the net effective torque and power at wheels through gears. Instantaneous acceleration depends on instantaneous torque but overall accelaration depends on mean effective torque. Had everythind depend on the instantaneous values then the alternating current passing through your domestic hotplate would not have consumed electricity and electricity bill would have been zero. But there for power calculation rms value is used, but when insulation strength is considered, peak values are considered. Similarly, if you see any engine torque-crank angle map, you will find it highly varying, but this values are used in desining crankshaft, camshafts, bearings etc.

Sorry for any miss-under-standing

[rks]

Originally Posted by jat

[...]
5- You can call nature or God or whatever you like, but laws of nature remains irrespective of whether human being is there or not. Say a building can be built only when human being is there, but anything dropped from the top floor will always fall the the ground even if human being is not there (nature's law). Similar torque power rules will remain same on another planet even if it not inhabited by humans.
[...]

For fear of inviting the mods' wrath, I will limit myself to just one brief response. You can have the last word if you like -- no further responses from me.

The viewpoint you have expressed above is the majority viewpoint; most of the top scientists today believe that there are truths independent of the human mind. But a significant minority of logiicians, philosophers, physicists, mathematicians, are dissenters who don't accept this.

"Anything dropped from the top floor will always fall to the ground even if human beings are not there" is NOT Nature's law. It is simply *your* ASSERTION, which you might have derived from, say, the postulates of Newtonian mechanics and Newtonian gravity. How do you know that these postulates of mechanics and gravitation will *always* hold, forever into the future? You have no PROOF, it is simply a set of unproven POSTULATES that you have here. So far we have found these postulates to be satisfactory, whenever we have made observations; but that doesn't prove that they will continue to be satisfactory forever into the future. So you cannot be sure that in a million years from now, anything dropped from the top floor of a builiding will fall to the ground. In fact if you think carefully, the notions of "up", "down", "top", "bottom", "left", "right", etc., that are tacitly or explicitly assumed to make sense in your assertion, may not really make sense without human beings (how do humans always know their left and right sides? Without humans what do left and right, or east and west, mean anyway?). In short, I claim that you need human "observers", in order for your assertion to make sense. Think about it, and take it or leave it.

In fact one of the most successful physics theories of modern times, quantum mechanics, does not really have an interpretation that makes sense without "observers" (although there are plenty of claims to the contrary). This is a topic of intense ongoing research and a lot of controversy. I claim that observers= humans, something that many physicsts (I am not one myself) don't accept.

Regards, rks

[venkatrx]

Originally Posted by santosh.s

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:

santosh, a great post indeed.

but to pick from the cars i had mentioned, how to go about it?

which is best for city traffic driving and why?

after 50,000 kms of driving, how would be the condition of a 'fiesta-petrol' car v/s 'fiesta-diesel' car? will the diesel car be too much screwed up?

[Shan2nu]

In fact the non-standard units used for torque in this context may be stated as "ps per thousand rpm".

"Ps" is a unit of power and not torque. 1Ps = 1.34Bhp. Thats why, mags either use Bhp or Ps to denote engine power. Torque is never reffered to, in Ps.

So, 20ps per 1000rpm is not torque. It's power.

20 * 1.34 * 5252 / 1000 = 140.75lbft or 190.83nm or 19.45kgm. (torque per....or rather at 1000rpm)

Shan2nu

[santosh.s]

Originally Posted by Shan2nu

So, 20ps per 1000rpm is not torque. It's power.

Shan2nu

shan2nu, I think you are getting it wrong. "ps per 1000rpm" is not power... it is indeed an unit of torque such that at 1000 rpm, its value becomes same as corresponding power value if expressed in PS. At 2000 rpm its value will become half of respective power number, again in PS, and so on. Isn't it?

[Shan2nu]

shan2nu, I think you are getting it wrong. "ps per 1000rpm" is not power... it is indeed an unit of torque such that at 1000 rpm, its value becomes same as corresponding power value if expressed in PS. At 2000 rpm its value will become half of respective power number, again in PS, and so on. Isn't it?

So, if "Ps" is the torque, what unit is used to denote the corresponding power value?

1ps = 75 kg meter per sec. Ps is the power and Kgm is the corresponding torque unit.

Shan2nu

[jat]

Interesting, you are rks and initials of my name are also rks ;-)

Originally Posted by rks

-- no further responses from me.

You should not do that. With discussions like this we definitely gain something. But you should not get upset.

Originally Posted by rks

"Anything dropped from the top floor will always fall to the ground even if human beings are not there" is NOT Nature's law. It is simply *your* ASSERTION, which you might have derived from, say, the postulates of Newtonian mechanics and Newtonian gravity. How do you know that these postulates of mechanics and gravitation will *always* hold, forever into the future? You have no PROOF, it is simply a set of unproven POSTULATES that you have here. So far we have found these postulates to be satisfactory, whenever we have made observations; but that doesn't prove that they will continue to be satisfactory forever into the future. So you cannot be sure that in a million years from now, anything dropped from the top floor of a builiding will fall to the ground.
Regards, rks

Your philosphy is quite right and interesting, but think, if anything dropped from height will not fall to the ground, then who needs cars. Everybody will be able to fly. And then the funda of torque, power, rpm won't be valid at all after a million year. And anyway, sun will die out eventually and so will the life on earth. Actually, our type of guys think of today only and not of future and that is why my discussion was limited to present day theories.

[jat]

Originally Posted by Shan2nu

So, if "Ps" is the torque, what unit is used to denote the corresponding power value?

1ps = 75 kg meter per sec. Ps is the power and Kgm is the corresponding torque unit.

Shan2nu

You are absolutely right.

[jat]

Let us do a comparison study of two motors (it can diesel, petrol, electric, pneumatic or any thing, both can be different type also), theoritically.
Both have absolutely flat power torque say from 10% rpm to 90% rpm or there normal operating range and there torque falls beyond this range. Now say motor "A" develops 20 kgm torque from 1000 rpm to 3000 rpm and motor "B" develops 10 kgm torque from 2000 rpm to 6000 rpm. Now both are connected to same load (say same type of truck) at 200 rpm to 600 rpm. Now which motor will pull faster? Neglect the transmission losses.

You will find that both are pulling the load with same acceleration and will maintain same speed within their operating range mentioned.

Now when the motors have different characteristics then to compare, you have bring them to some common base line. That is they are assumed to be connected back to back through gearing to get some common rpm and see which one is stronger. And there we use ratios of the torques. Which does not have any units.

[santosh.s]

Originally Posted by Shan2nu

So, if "Ps" is the torque, what unit is used to denote the corresponding power value?

1ps = 75 kg meter per sec. Ps is the power and Kgm is the corresponding torque unit.

Shan2nu

Interesting

I have clearly mentioned that power in my illustration was PS itself, not torque.
torque was "PS per 1000 RPM", let us call this unit of torque as "xyz" which is
1 xyz = 1 PS / (1000 rounds/minute)
= (75 kgf-m/s) / (1000/60s)
= (75*60/(2000*pi)) kgf-m

Note: I have used kgf instead of kg because "kg" in kgm is used as unit of force not mass, so its preferrable to use kgf which is the force equivalent to gravitational force of 1 kg mass = 9.8xxx N = 1kgf. Hope I am clear now.

[santosh.s]

Correction....
"= (75*60/(2000*pi)) kgf-m" in my previous post was a mistake while cut-pasting from scratch pad, you should read it as:

= (75*60/1000) kgf-m
= (75*60*9.8xxx/1000) N-m

[santosh.s]

Originally Posted by venkatrx

santosh, a great post indeed.

but to pick from the cars i had mentioned, how to go about it?

which is best for city traffic driving and why?

OK, let us take example of baleno and NCH-ZX. Before going ahead let me correct a typing mistake in baleno-EIII numebers- 2nd RPM (peak power) is at 5500 not 6000 as I had mentioned. So we have

(all numbers are N-m/tonne@RPM)
baleno-EIII: 122.5@3000 & 110.7@5500
NCH-ZX: 110.6@2700 & 95.3@5000

Note that we are comaring just engines and assuming gearings are fairly close to each other. Also we don't have any accurate torque curves either. So we are extracting as much from these numbers as we can....

It is obvious that peak torque puts an absolute maximum limit to the torque curve. So if we don't have any other data, then all that we can say is that cure has to be below a horizontal line which is equal to peak torque. But we have another data which is peak power. This puts another limitation on the curve, i.e. at any point the power can not exceed this number. Therefore it is implied that power corresponding to peak torque has to be less than peak power or at the most equal to it. It further implies that below peak torque RPM, max. torque limit prevails, as the max. power limit is ruled out. Now imagine we plot the max limit on torque curve - from zero RPM through peak torque RPM it is just a straight line = peak torque. As you continue, we reach an another RPM between peak torque and peak power where max. power limit take over, which can be calculated mathematiacally. For baleno-EIII, it is 4970.2 RPM and for NCH-ZX it is 4308.3 RPM. After that, the max. limit on the curve is governed by max. power limit, which takes form of inverse proportion to RPM, i.e. K/RPM where K is a constant = 608850 for baleno-EIII and 476500 for NCH-ZX. You can say that at 5500 where baleno claims to produce 110.7 N-m/Tonne, NCH-ZX can theoretically produce not more than 86.64 N-m/Tonne. As you move ahead, baleno is redlined at 6500, don't know about NCH-ZX.

Regarding lower RPM's, anything below idling RPM doesn't have any practical significance because at that speed engine will start knocking or will stall. Generally the torque of any engine is very less at idle, after which it raises to peak torque in a particular way depending on engine characteristics. As a matter of fact, baleno reaches a very good level of roughly 105 N-m/Tonne right at 1000 RPM which just little less than NCH-ZX's max. limit across a large range from 0 to 4308.3 RPM, beyond which its even lesser! This is based on baleno's torque curve published by MUL for EII model, for EIII it is slightly reduced as you move from mid to high RPMs. At 2700 RPM where NCH-ZX is "guaranteed" to produce 110.6 N-m/Tonne, baleno is somewhere close to its peak itself, may be around 115-120.

The verdict is clear, baleno is head and shoulder above NCH-ZX in terms of being flat and high which also includes low end, so be it city or high way baleno is better. As I mentioned earlier, we are not considering gearing. But we know that-

-operational RPM range of two cars is quite similar
-operational road speed targets are also similar
-weight factor is already accounted for, since we have always used torqes as "per tonne"

therefore, gearbox designers for baleno must have had more flexibility as compared to NCH. At high speeds we should consider drag also. But again both cars are known to be aerodynamically good, don't really know which one is better. Though, front projected area is also similar (I guess baleno may be lesser, which is beneficial in reducing drag)

Now, lets take Viva-CRDi instead of NCH. Knowing gearing would be better here. Just for theoretical analysis, lets assume we are using baleno's drive chain but Viva's engine. In order to get similar road speeds for various gears, we would need to connect a speed multiplier from engine to the gearbox, which is essentially torque buster in other words. Let say we use such a ratio which brings it's max power point to that of baleno. Then we can modify Viva's spec as follows, which then can be fairly compared with baleno. So original specs were-

(all numbers are N-m/tonne@RPM)
Viva-CRDi: 152.3@2000 & 115.8@4000

and transformed specs would be
110.76@2750 & 84.22@5500

this is very close to NCH-ZX, and it is impressive for a diesel (of course, it is due to CRDI technology). But remember one thing, though we have virtually stretched Viva's RPM range, actual engine would operate within its original limits. That being considerably lesser, Viva is going to be more drivable than NCH. Now, actual gearing used in Viva may be different. They could have improved any of the parameters like low/mid/high speed performance, max speed, drivability etc. at the cost of some others.

Wanted to mention one more imprtant point that even torque curves of engines are nothing but max. limit that it can produce at corresponding RPMs. But it is more of a static data. It takes some time for engine to transition from one RPM to another. So when you are changing speeds quickly, you won't get all the torque shown on the curve! How well or sluggishly it responds determines so called "rev happiness" of the engine.

after 50,000 kms of driving, how would be the condition of a 'fiesta-petrol' car v/s 'fiesta-diesel' car? will the diesel car be too much screwed up?

Answer to this question will depend on many other factors other than just torque and power numbers, like entire design and materials used to construct these engines...I guess its too complicated, so better I don't make any comment since I don't have enough expertice

[rks]

Originally Posted by Shan2nu

"Ps" is a unit of power and not torque. 1Ps = 1.34Bhp. Thats why, mags either use Bhp or Ps to denote engine power. Torque is never reffered to, in Ps.

So, 20ps per 1000rpm is not torque. It's power.

20 * 1.34 * 5252 / 1000 = 140.75lbft or 190.83nm or 19.45kgm. (torque per....or rather at 1000rpm)

Shan2nu

I agree that "ps per thousand rpm" is not the standard units in which torque is expressed. But if you see the equation
Power=torque*angular velocity,
the units of torque should be (power/angular velocity). Now the angular velocity can be expressed in radians per sec, which is the standard; but it can also be expressed in terms of rpm or "1000 rpm" -- the units of the angular velocity should always be the reciprocal of time, and you can see that this criterion is satisfied by the non-standard "1000 rpm" as well.
So "ps per 1000 rpm" has the units of torque. But all said and done, it is always better to use standard units as you point out, in order to avoid confusion.
Regards, rks

[rks]

Originally Posted by jat

You should not do that. With discussions like this we definitely gain something. But you should not get upset.

I am not upset at all. The only reason I said "no further replies" is that this is way off-topic for this forum and such discussions will always be endless, with neither party conceding ground.

Your philosphy is quite right and interesting, but think, if anything dropped from height will not fall to the ground, then who needs cars. Everybody will be able to fly. And then the funda of torque, power, rpm won't be valid at all after a million year. And anyway, sun will die out eventually and so will the life on earth. Actually, our type of guys think of today only and not of future and that is why my discussion was limited to present day theories.

The point of this philosophy is that "theories" are all we have. These theories contain "axioms" or "postulates", and what are called "rules of inference", using which we deduce theorems from the axioms. Ultimately, all the axioms and theorems are nothing but temporary declarations made in the human mind-- there is no other absolute truth. This philosophy also leads to a new logic that flatly contradicts classical logic (which is based on the kind of assertions you make -- mainly, that there are truths independent of the human mind). The subject is deep and interesting, but very frustrating as well -- if you go against the majority viewpoint, you will quickly become a pariah and get completely ignored by the mainstream scientists.
Regards, rks

[jat]

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?

Lot depend on your driving style. If you are the person who loves to cruise in low rpm, then the cars with max torque in lower rpm will be good; but if you are the guy who loves to rev their car engine, then cars having max torque in higher rpm would be better. Thus in city traffic where it is stop and go, cars with max torque occuring in lower rpm range would be better.
How to interpret the data?

91 bhp @ 5500 rpm = 116 Nm
Percent of max torque = 89%
Percent rpm at which it occurs = 54%
Percent rpm over it is available = 45%

77 bhp @ 5000 rpm = 108 Nm
Percent rpm at which it occurs = 54%
Percent of max torque = 86%
Percent rpm over which it is available = 46%

68 bhp @ 4000 rpm = 119 Nm
Percent rpm at which it occurs = 50%
Percent of max torque = 74%
Percent rpm over which it is available = 50%

82 bhp @ 4000 rpm = 144 Nm
Percent rpm at which it occurs = 62%
Percent of max torque = 77%
Percent rpm over which it is available = 37%

Lot depend on the gearing also, but from above you can see that Baleno has got flatter torque characteristics when compared to NHC and over smoother accelaration and drivability. As the max torque occurs at almost same point for both cars, the initial pick up feeling would be almost same. But depending upon the weight etc, the actual acceration would be different. We assume the gearing pattern is same, that is the ratios between 1st, 2nd, 3rd etc is same. If accelaration through all the gears is same for both cars, then top speed for Baleno is going to be higher, and if top speed of both car is going to be same, the NHC pick up is going to be very sluggish.
Similarly Fiesta is going to offer better drivability due to its broader or flatter torque characteristics, but its top speed will suffer as the torque at max power is quite less compared to Viva (if both car has same external constraints).

Thumb rule, flatter torque, less gear change required.
Lower the rpm for max torque, easier initial pick up, that is if you are guy who changes gear early, then good for you.