TORQUE

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Guy Croft
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TORQUE

Post by Guy Croft » Sun Nov 26, 2017 5:17 pm

TORQUE

Everyone talks about ‘more power’ these days; racing drivers always want more and TV motoring presenters love the big numbers. It must seem odd to begin a section about 'torque' by discussing 'power' but the two remain as misunderstood as ever. Must be an educational thing. But to understand and apply tuning principles and techniques effectively we have to understand torque production and what’s really going on inside the engine. But talking about power without understanding torque is very unfair to the engine and more often than not torque rarely gets a mention. Unless it’s to bury the engine’s reputation at a stroke with ‘it’s got no torque at all’ and, yes we’ve all driven one or two of them. The torque figures from many engines, especially turbo/supercharged (boosted) and very undersquare engines (small bore + very long stroke) say more about the car’s performance because than the power, because , by its very nature, torque is distributed across a wide section of the engine’s working range whereas a high power figure simply tells you it can do, say, 130mph when the tachometer reads 9500rpm in top gear. At this stage, let’s be quite clear, the purpose of our modifications to increase the torque output and the availability of it. Newton's 2nd law states: "the force needed to accelerate a body is proportional to the inertial mass of the body and the acceleration produced", ie: force = mass x acceleration (F = M x A). To raise the value of A we need more F, in other words more torque. Either that or shed some weight.
'Useful work’ as it's known in thermodynamics, is performed when a force moves its point of application through a certain distance. In the case of crankshaft motion, the distance moved by the applied force at point of application is the circumference through which the center of the crankpin/connecting rod big-end rotates ie: 2 x π x r where r is the crankpin radius of rotation, π is 3.142 and F is the net thrust that drives the piston down. I emphasise ‘net’ force at this early stage because there is friction everywhere opposing thrust, especially between the piston assembly and the cylinder and the inertia too, of the rod/piston. That which is left drives the piston. F is dependent on the relationship F = P x A where P is the cylinder pressure (which varies during engine operation according to throttle position and engine speed) and A the cross sectional area of the piston (bore area for simplicity as they're virtually the same). Thus the work done per full rotation is F x 2 x π x r. Now, since torque (T) is defined as ‘applied force applied x perpendicular distance from point of application’, much the same way as the lever arm of a torque wrench is used to tighten a bolt, the effect of the piston and connecting rod acting on the crank generates a torque on the crank and we can effectively substitute T for (F x r) giving work done = 2 x π x T, or 2π T in short, an equation which is of no particular use to us in itself, except as a means to an end.

The units of torque as far as we're concerned are Nm (Newton metres) or lbf ft (pounds-force feet), the latter more simply referred to as 'foot pounds' for brevity but so often wrongly referred to in print as ft lbs (or worse). In real terms torque is the product of force F acting in the direction of the rod axis (whatever its orientation) during the firing stroke between top and bottom centre, and the perpendicular distance from the crankpin (center of the big end) to the centre of the crank. The latter varies as the rod moves down the bore on the firing stroke. From this simple analysis it's obvious that the ‘instantaneous torque’ ie; the turning effect at any moment in time varies considerably throughout the firing cycle and we’re not interested in that in itself - all that matters is the net output at the flywheel and the cyclical nature of torque means we can only measure it on a dynamometer - though it's worth remarking that advanced simulations can predict it off-engine with a high degree of accuracy. The 'net' torque output is essentially the ‘sum’ produced by the firing cycle of all cylinders minus the mechanical losses. When we load up an engine on test to see what it’s capable of we are measuring an average (or ‘mean’) value of T at the flywheel by applying a brake to the output shaft from the flywheel and seeing essentially what load is needed to try and near-stall the engine, hence the name ‘brake dynamometer'. This electric or hydraulic braking mechanism applies load to the crankshaft (without actually stopping the engine - though if the applied load is excessive this will happen) and the lever arm on the body of the dyno swings against a potentiometer or spring balance, the resultant deflection of the lever arm giving us our torque measurement.
For brevity gasoline is usually referred to as the 'fuel' although strictly speaking the fuel is the air/gasoline mixture, referred to in this book as 'charge'. Burning it in a controlled manner generates the power stroke and the more air, thus the more gasoline in the right proportion we can add and the more pressure is developed during the power stroke. Getting the gasoline in effectively is definitely the easy bit as it will be carried in the airstream. The greater the working range over which we can get the engine can do this, so the more 'powerful' it will be. Air is governed by the rules (laws?) of aerodynamics and whilst its low viscosity means it can be made to go where we want, its low inertia causes it to misbehave readily in aerodynamically 'bad' regions. Less air, less torque. The crankcase assembly may well be the thing that translates cylinder pressure into work but all the power is in the head: cams, valves, inlet and exhaust systems. All, we hope, working synchronously. They are the hardware of volumetric efficiency (Vr), the ratio of actual mass of air ingested in any one cycle compared with the maximum that would occupy swept volume (Vs) if that were fully filled at atmospheric pressure. Thus it's a measure of the filling event and the higher the range of good Vr, the better. Measuring it requires careful instrumentation of a running engine and few undertake it because frankly, if torque is low, one can conclude with virtual certainty that Vr responsible.
On any engine type, maximum Vr coincides with peak torque and drops over that speed as the cylinder filling degrades. The higher the speed the more engine cycles per second and the higher the demand for air, thus the highest intake air velocity and massflow is at peak power, not peak torque. And as the speed climbs the output at the flywheel is increasingly hindered by mechanical (mainly frictional) losses that grow as the square of crankshaft speed. On an n/a (normally or naturally aspirated) 1600cc unit if you get 125lbf ft max torque (and you can ratio that up by cubic capacity to assess other engines), you've got 100% Vr: the engine is taking in a complete 'swept volume's worth' of air on every cycle at that speed. Conversely, if torque is low you have to re-evaluate the head and intake components and find out why. Though the position of peak torque can be set and varied by tuning methods, maximum torque on all engines is produced at full throttle when the restriction to incoming air is a minimum. Boosted engines develop more torque and a broader range of it than equivalent n/a ones because the air is pumped in under pressure. That can give a peak ratio significantly over 100% (or orders higher) and whilst it's true to say that the better the head setup the more powerful they will be, it's the capability of the device creating the pressure (turbo or supercharger) that really controls the output. They are not ‘pressure-limited’: their output is restricted only by the thermal and mechanical load the engine can withstand.
Consider 2 variants of normally-aspirated setups of the same engine with the same gearing tested in the same car: at constant throttle setup ‘A’ and ‘B’ are able to achieve and maintain a steady 100mph. But whereas A takes 10 seconds to accelerate from rest to 60mph ‘B’ can do it in half that time. Under acceleration, as the driver changes up through the gears from rest, if he maintains full throttle the cylinders will ingest more fuel and both engine speeds will rise. Where they differ is highly tuned B’s ability to build up speed quicker in every gear and get the car to 60mph faster. B’s cylinders consistently fill with more fuel in the time available, than those of A, giving higher cylinder pressure in every induction cycle to force the crankshaft rotation. If you want to win, think ‘torque’ and the distribution of it. Torque is an important analytical tool used to describe the ability of an engine to do its job against a load. It doesn’t matter what the load is, generator, propeller, hill, 0-60mph target and whether the vehicle is 35ton lorry or a racing car.
Brake mean effective pressure (BMEP) is a useful commodity. It is an indicator of the engine's ability to induct and burn charge and on boosted engines attainable BMEP is a measure of how well the engine is coping with the extreme temperatures and stresses.

BMEP is calculated from:
BMEP (Pa) = 4 π T
Vs x 100000
Where Vs is total engine swept volume (ie: cubic capacity) in cubic meters, T is torque (Nm). To use BMEP as a yardstick for your motor you have to know what various engines can produce. An F1 engine might develop 15.5 bar (approx 225lbf /sq in), an extremely well tuned 16v 2 liter race or tarmac rally unit 14.3-14.4 bar, a National level competition 8v engine 13 + bar and a fast road unit, say 10-11 bar.

Example
A 1640cc 'all steel' 8v full-spec fuel injected big valve tarmac rally n/a SOHC unit (see P5) develops 180.7Nm torque
4 π T = 2270.7
Swept volume is 1640cc (divide by 106 to convert) = 0.00164 cu m
BMEP (Pa) = 2270.7/ 0.00164
= 1384573.2 Pa
= 13.85 bar (200.9 lbf/sq in)

This is a high result, indicating Vr over 100% at the peak torque of 5160 rpm and clearly high port velocities. But in truth, any such unit giving over 100bhp per liter is going to be a good one. Reverse-engineering the formula by inputting the highest BMEP you are likely to get can often separate fact from fantasy where torque figures are concerned. 16v engines produce higher BMEP than 8V ones because they 'breathe' and burn way better - 2 small valves flow better than one big one and permit a more compact combustion chamber.
Guy Croft, owner

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