-
Post count: 2134
The characteristics of torque and power are often enthusiastically discussed and I am frequently amazed by the lack of understanding of these basic parameters.
Just like intelligent people get caught out with the "get rich quick" schemes because they prefer to ignore the common sense, risk - reward relationship, people often believe wild claims on horsepower or torque improvements, fuel consumption reduction etc. just because it all sounds so wonderful.
I am not a mechanical engineer but much to the surprise of my teachers, I did pass science in standard 7 and with this benefit and background I will try and explain the relationship between torque and power in simple terms.
Most people are probably familiar with the concept of force. In our SI system, this is measured in Newton, so named after the famous scientist Isaac Newton who made valuable contributions to our understanding of such matters. Newton explained that in nature a simple equation governs the behaviour of objects subjected to force: Force [N] = Mass [kg] x Acceleration [m/s2]. Today, one hardly needs to be a scientist of Newton's calibre to realise that a motorcycle of given mass will accelerate faster when more force is applied through the accelerator.
In rotational systems the concept of "torque" is used to describe rotational force. Torque is simply the force multiplied by the moment arm or in equation form: Torque [Nm] = Force [N] x Moment arm length [m]. Everybody has seen this simple equation in action when changing a wheel on a car. By applying a force at the end of a wheel spanner the wheel nut is loosened (or fastened) with a torque equal to the force applied to the spanner multiplied by the length of the arm of the wheel spanner. In case of a sticky wheel nut, requiring more torque to loosen, one can either increase the force, or find a longer wheel spanner.
Incidentally, the wheel nuts on modern cars need to be fastened to a torque of 100 to 110 Nm. If your tyre fitment centre do not use calibrated torque wrenches to fasten wheel nuts I suggest you find a professional setup that does. I have learnt the hard way that a stripped or stuck wheel nut can make a deep dent in your wallet, never mind the unthinkable risk when your wife gets stuck in the middle of nowhere unable to change a flat tyre.
Whilst not the time to describe how an engine works, combustion of an air - fuel mixture creates pressure above the piston, which exerts a linear force through a connecting rod, producing torque in the crankshaft, not unlike our example of wheel spanner and muscle power above.
Fundamentally then, all that even the fanciest engine on the planet can produce is rotational force or torque. The picture illustrates how linear force is transferred by the piston through the connecting rod to create rotational force (torque) in the crankshaft.
Every engine is unique in the sense that it produces a certain spread of torque (or rotational force if you prefer) across a usable rotation speed / rpm (revs per minute) range. The maximum torque output is obviously a key parameter but more about the engine's characteristics can be learnt by examining a graph of torque produced versus rpm.
By examining such graph of the mighty BMW K1200S, it is clear that at 2 000 rpm about 75 Nm of torque can be produced under full throttle. At 3 000 rpm, almost 100 Nm is developed and this builds up to a peak of 130 Nm at 8 250 rpm and drops again to 100 Nm at 11 000 rpm. This is an impressive torque distribution, starting with a moderate number just above idle, quickly building up to a high percentage of maximum torque, building up to an impressive maximum number and then falling off slowly near the maximum usable engine rpm. More than 100 Nm torque is developed all the way from 3000 rpm to 11000 rpm - brilliant!
It is important to realise that the torque versus rpm graph depicts the maximum torque available under full throttle at every point in the revolution range. Using the example above, if one cruise along in top gear at 3 000 rpm at say quarter throttle, the engine will only develop approximately 25 Nm (or 25% of the 100 Nm potential maximum at 3 000 rpm). If the 25 Nm is enough to overcome friction losses, rolling and wind resistance, a steady speed will be maintained. If one then opens the throttle to its maximum position, 100 Nm of torque will be produced of which 75 Nm will be available to accelerate the motorcycle. If one change down gears to a ratio 2.75 times lower than top gear (assuming such gear is available) the engine revolutions will increase to 8 250 rpm. A 30% increase in total engine torque (to 130 Nm) will be achieved but with the downward gear change the engine torque will be multiplied with a factor 2.75 through the gearbox to 357 Nm at the back wheel. More than 330 Nm will then be available to produce the eyeball flattening acceleration so typical of powerful motorcycles.
If torque is the rotational equivalent of force, more is certainly better, so how do we get an engine to produce more torque? The Americans say that there is no substitute for cubic inches, and this is still the painful truth for normally aspirated engines.
Many factors influence the torque characteristics of an engine but the two most important are engine capacity and compression ratio. Compression ratio is the ratio of the volume of air in the combustion chamber with the piston right at the "top" end and the total volume of the chamber with the piston at the "bottom" end. With the available pump fuels, compression ratios in petrol engines can only be increased to a practical maximum of about 13:1 before detonation or knocking becomes a problem. Over the years compression ratios crept up with the use of electronic knock control and other clever arrangements but we do not often see ratios of more than the number quoted above. In fact, 11:1 or 12:1 are already on the high side even for performance engines. That really brings us back to the good old Yankee saying that if you want more torque, you need more cubic inches - a "bigger" engine. Super- or turbo charging and special fuels can certainly also do the trick but for this discussion we will stick with pump fuel and normally aspirated four stroke petrol engines commonly used in motorcycles.
To further examine the torque characteristics of various engines I compiled a table of a dozen or so well known motorcycles from the humble BMW 650 GS to the mighty K1200S and Suzuki GSXR K5.
Click to enlarge
Just to add diversity, the list includes engines with 1, 2, 3 and 4 cylinders and some motorcycles that date back to the sixties and seventies. The torque produced range from about 60 Nm (BMW 650 GS) to 130 Nm (BMW K1200S). To compare apples with apples, I divided the torque developed by the cubic capacity of the relevant engine, showing the results in the second last column. This yields a remarkably close spread with an average of just under 100 Nm produced per 1000 cc engine capacity. The ones that differ dramatically from this average either run very high compression ratios (K1200S with 112 Nm/1000cc @ 13:1 and GSXR with 118 Nm/1000cc @ 12,5:1) or very low compression ratios (BMW R90S with 84 Nm/1000cc @ 9.5:1 and Honda CB 750 with 83 Nm/1000 cc @ 9:1).
To normalise matters further I then divided torque per 1000 cc by the compression ratio to arrive at a number that I call the specific torque output, as shown in the last column. This produced an even more remarkable number with an average of just less than 9 Nm/(1000cc x Compression Ratio). The significance of this number can be seen by examining the almost identical numbers of the BMW R90S and K1200S despite 3 decades of development. As can be seen, the awesome BMW M5 and the Spartan City Golf also yields almost identical specific torque numbers despite the factor 10 price difference and vastly different roles and target markets. Even from the land of the rising sun the latest Suzuki GSXR develops almost exactly the same specific toque as the 40 year old Honda CB 750.
The brutal reality is that despite decades of development, modern electronics and clever inlet, exhaust and valve timing systems, or utilising any number of cylinders, engineers can only achieve roughly 10 Nm/1000cc x CR. So never let any salesman or proud new owner ever again tell you that this or that pathetic low power engine is developed for high torque. Crap!!! An engine's torque is proportional to its displacement and compression ratio and not even the F1 guys with almost limitless budgets, no pollution or noise can produce much more than about 10 Nm/1000cc x CR. Full stop!!!. In fact, with the information at the beginning of this paragraph your 9 year old son can calculate the maximum torque of any normally aspirated four stroke petrol engine to a very high degree of accuracy. No magic in there.
A group exploiting the general public's poor understanding of the basic principles are the aftermarket manufacturers of air filters, exhaust systems, engine management chips and other contraptions designed to liberate cash from your wallet. While nothing sounds sweeter than some of the aftermarket exhausts (which for me is reason enough to part with hard-earned cash), the wild claims of huge beneficial effect on torque are all blatant lies. In fact the opposite is often true but this is another discussion.
Up to now we only focused on one aspect of engine performance i.e. torque or rotational force. The next parameter, namely power, is just as simple to understand but somehow many people completely miss the boat. Power is the rate at which torque is doing work. Technically, Power = Torque x Rotation rate, but usually some constant is included to correct for a convenient choice of units. In our SI system we use the following formula:
Power [kW] = Torque[Nm] x Rotation rate [rpm]
9550
Using the formula above and the torque curve of the K1200S, the power at different rotation rates can easily be calculated. At 2000 rpm, 15 kW is developed, at 3000 rpm, 30 kW is developed, at 5000 rpm - 38 kW, at 11000 rpm - 123 kW, at 10250 rpm - 115 kW and so on. Nice to see that our calculations correspond to the published power graph!
Just as in the case of torque, the power output of an engine is dependant on two main factors, namely torque and rotation rate. From the equation above, to develop more power you either need more torque or more rotation rate (rpm) or both. For a given engine producing a certain amount of torque, the only way to produce more power is to rev it higher. Once again F1 engines provide a nice example. Torque is effectively restricted by the capacity limit (under the rules) and the compression ratios (practical considerations) so the only way to higher power levels is to rev these engines to stratospheric levels. This year with the 3000 cc V10 engines we consistently saw more than 18 000 rpm! Torque and power levels are closely guarded secrets in F1 but these engines are rumoured to produce 675kW despite developing not much more than 350 Nm of torque. The secret of course lies in the stratospheric revs. Next year, capacity of F1 engines will be restricted to 2400 cc which will instantly cut torque by 20% but the smaller and lighter pistons will certainly make 20 000 rpm possible. So my prediction is that F1 power levels will only drop by about 10% despite the 20% reduction in capacity and torque.
So there you have it. In order to have an engine with high torque output, you need decent capacity and high compression ratios or preferably both. In order for an engine to produce high power you need loads of torque produced over as wide a rev range as possible. An engine that develops torque only in a narrow band in the lower end of the rev range is almost useless to man or beast. Despite the praises sung by diesel prophets and others of the merits of "low down torque", such characteristics will soon promote you to the slow lane and snails pace on a steep hill. Power does not exist without torque and rpm and there is no such thing as an engine that magically develops reasonable torque but then fails to deliver at least moderate power levels as well…… except for Harley Davidson of course! Modern techniques have however (slightly) influenced the time lag to move from one torque level to the next by improving flow and through the combustion chamber/s and optimising friction and inertia. The word "responsive" is often used to describe this phenomenon.
I can think of no better X-mas gift for the power hungry than a Suzuki GSXR. Despite using only 999 cc the Japanese engineers manage to squeeze impossible amounts of torque out of this engine using a high compression ratio. The 118 Nm combined with the low mass of the bike will certainly rip arms out of sockets under acceleration. Furthermore, the Japanese engineers managed to arrange things such that the engine happily spins to 12000 rpm while still producing stump pulling torque, resulting in an impressive 133 kW of power (remember our equation of Power = Torque x Rotation rate?). No wonder a top speed well in excess of 747 take off speed is achieved.
Like in the case of the K1200S, the combination of brilliant torque produced over a wide rpm range results in blistering power and these machines will flatten the steepest mountain pass with contempt. At the same time it will blow any theories on the benefits of low down torque right out of the window. I have never ridden one of these Japanese wonders but just looking at the numbers I am sure that like a K1200S it responds to throttle input in a most satisfying manner.
Next time you walk past a parked motorcycle (or car for that matter) you can make an informed guess about power, torque, acceleration and top speed by just looking at the capacity of the engine and the position of the red line on the tachometer. For modern engines it is fair to assume reasonable compression ratios, so the larger the engine capacity, the higher the torque, (and thus acceleration with a given vehicle mass) and the higher the rpm red line the higher the power (and therefore top speed). With good power and torque, steep uphills should not be to taxing on the patience. Simple.
While my standard 7 level mechanical engineering is surely only scratching the surface, these simple principles provide one with a basic understanding of the constraints and you need not swallow all marketing hype or braai-side stories as gospel.
- You must be logged in to reply to this topic.