Vehicle & Technical > Series Land Rovers
3.3 Litre Perkins in a series 3
bezzabsa:
--- Quote from: "Miniman" ---So why would a 3.3L 60 BHP perkins do so then.
--- End quote ---
torque, pure and simple
Xtremeteam:
--- Quote from: "bezzabsa" ---
--- Quote from: "Miniman" ---So why would a 3.3L 60 BHP perkins do so then.
--- End quote ---
torque, pure and simple
--- End quote ---
yep
BHP sells cars,Torque wins race's,
Miniman:
But the perkins has less tourque than the TD.... or are we going to say because the tourque is lower down in the rev range....?
bezzabsa:
Right sorry about this, but this is a complete explanation. shamelessly 'borrowed'
Torque, Power, and Kinematics
The following discussion of basic physical principles involved in kinematics and dynamics of automobiles may bore some of you. It may also enlighten you. The discussion will be in no way satisfactory for those that study such physical phenomena on professional level or are true enthusiasts seeking advanced understanding. Rather it will be light and informative by just slightly touching the mathematics and physics involved. However, after reading the text and looking at the diagrams I hope that you will walk away with just a bit more understanding of the concept of power and torque.
In the consumer world of car salespeople, car magazines, internet groups dedicated to the wonder on four wheels, the words power and torque are heard a lot and used a lot. Ironically, very few people, according to my observation, know the precise definition and application of each terms as related to cars. What is better to have – more power or more torque? Are they related? Such questions puzzle many, but only few stray from pseudo-scientific explanations and myths. I will not try to answer these questions directly; rather I will give definitions and relations, letting you find the answers for yourself.
Let us begin:
In our universe we’re given three independent quantities: length, time, and mass. All other quantities are derived from the three main ones.
Let us label them as:
L – length
T – time
M – mass
Then speed, for example, is s = L/T. Force is f = (M *L) / (T * T). Note that these quantities are independent from our system of measurement. Speed can be measured in Miles per Hour or Parsecs per Millennium.
Let us now define what torque and power are as a function of the three fundamental quantities:
Power is the rate at which one does work or work per time.
Work is a force applied over a distance or force times distance.
Thus power is: p = (M * L * L) / (T * T * T).
Torque is force applied at a moment arm of a given length.
Thus torque is: t = (M * L * L) / (T * T ).
Comparing the above two formulas, one can clearly see that the only difference between the two is that power has one more time factor in the denominator. One is wise to notice this because if there is a quantity that has a formula 1 / T and that is physically relevant, we can multiply it by torque to get power.
Such quantity does exist and in our case, due to the geometry of force application, it is the rotational speed of the engine’s crankshaft or more simply, the engine speed.
Thus in our case:
Power = C *torque * engine speed.
What is that C doing there? That C is a factor that has to do with the measurement system that we’re using. If you happen to measure torque in ft-lb, engine speed in revolutions per minute (RPM), and power in SAE horsepower, C comes out to about 1/5250. If you happen to measure torque in meter-newtons, engine speed in radians per second, and power in Watts, C = 1.
Let us stick to our SAE motorheads measurement table. Then we have a formula relating torque and power:
Power = Torque * RPM / 5250
Great. Assuming it’s right, so what?
Well, consider we want to accelerate a car with mass m from 0 to speed s using an engine with constant power p. How much time will it take?
Well, the car will undergo an kinetic energy change of: k = (0.5 * m * s*s) – 0, or the difference between the kinetic energy of the car at rest and at speed s.
Now this kinetic energy change was brought about by work done by the engine. Since we know that power is work per time, we know that by dividing total work done by the applied power, produces the time needed to do the work and thus time to accelerate the car.
So what does this tell us? This tells us that it is the power that accelerates the car.
But what about torque?
Remember that:
Power = Torque * RPM / 5250.
Examine the formula carefully….
It says that power has two variable components – torque and engine speed. Naturally, the torque here is the "twisting force" developed at a particular engine speed. So there is more than just torque involved into accelerating the car. After all, you can apply torque all you want, but if the engine is not spinning, there is no work done, no power produced and you're not moving anywhere.
Another interesting feature is that at 5250 RPM the numeric magnitudes of torque and power figure will always match in our measurement system, irrespective of the number of cylinders, volume, valve train or any other such feature of the engine. Cool, huh? This is a very fundamental notion because it tells us that power ALWAYS grows faster with increase in RPMs than torque does. We’ll see how that is used in engine building later.
Now that we’re armed with this new understanding, let us examine a few idealized examples.
Consider an engine which develops exactly same torque T throughout its allowed RPM range. In this case torque is constant and:
Power = RPM * T/5250 which is a straight line with a slope T/5250 (see graph below).
How does this car behave? Smooth and predictable. The power increases linearly and the higher RPMs you attain, the more power you develop all the way up to the redline. This is the ideal curve for an engine – maximum torque everywhere in the range leaves only the RPM as the limiting factor. See graph below.
The next example is where torque is a linear function of the RPM.
Torque = a * RPM + b where a is the slope and b is the base torque developed when the engine is barely moving.
From our trusty power/torque formula we deduce that in this case:
Power = (a * RPM * RPM + b *RPM) / 5250.
Not hard to see that power is a quadratic function of the RPM. Notice that power rises much faster, as a square of RPM, than in the first example. Thus high power and acceleration can be attained, but you need to spin the engine faster. Notice that power is always rising faster than torque. See graph below.
So, what is important? Well, as you can see, at low RPM, if you want to develop lots of power, then you simply need lots of torque at those engine speeds. That is why you hear – high low-end torque being a good thing. But, in order to develop high low-end torque you either need a large engine or forced induction scheme. So the makers of small engines have focused their attention on the other side of the spectrum – high RPM side taking the advantage of the fundamental law that allows power to outgrow torque provided torque doesn't diminish too fast with rise in engine speed. There, it’s the high RPMs that make horsepower even if the torque being developed is only marginal.
What does that mean? If you’ve got an Integra and want to accelerate fast, you better keep you tachometer needle way higher than a 5.0L Mustang GT next to ya! :-)
So the next time somebody asks you what’s important and what to look for, you’ll know what to say.
Miniman:
So after reading that what it is , is because the perkins will only rev to 2600rpm that means the tourqu is stronger in a small amount where as the 2.5td say revs to 5000rpm the tourqu is applied over a longer length. It was all very confusing to me.......
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