All the power is in the head. Part 1 - Power & Torque

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Guy Croft
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All the power is in the head. Part 1 - Power & Torque

Post by Guy Croft » June 24th, 2006, 2:04 pm

All the power is in the head.

By Guy Croft BSc

Part 1: Power and Torque and what they do

Everyone talks about ‹Å“needing more power¢ž¢ these days. Racing drivers always need more. Most people talk about it quite blithely without having the faintest idea what it really is. Put simply, power is the rate at which a machine does what's called in thermodynamics 'useful work'. Torque is rarely mentioned except to disparage an engine by saying ‹Å“she¢ž¢s got no torque at all..!¢ž¢

Talking about Power without understanding Torque is very unfair to the engine. Power depends on torque, not the other way round!

Power is a time-dependent thing. More work in the same time or the same work in less time means the machine is producing more power. In bygone days power was literally measured in 'horsepower', and machines were matched against the 'power' of our 4 legged friends. Much effort was expended (quite pointlessly) in trying to correlate the 'power' of a horse - based on how plowing he could do in a day -to the power of an engine. Marvellous calculations were developed to demonstrate how engine cubic capacity and bore/stroke etc would give the true determination of the power of an engine. All nonsense we can now see.
But this was long before the 'brake' dynamometer was invented and anyone had thought of a concept called ‹Å“torque¢ž¢.

Doing 'useful work' as far as an engine is concerned means producing 'power'. Engine power is typically stated in brake horsepower (BHP) or Pferdestarke (PS). As far as a car is concerned it is a measure of whether the engine can perform 'useful work' at a rate sufficiently fast to hold a certain vehicle speed against opposition from aerodynamic drag and friction - bad things that are trying to bring the vehicle to a halt.

100 mph in a vehicle weighing 1 ton might need 120bhp, to do 120mph you might an engine with 140 bhp. So - what is acceleration? In other words how do we get from one speed to another? Here we have to turn to Newton's 2nd law: the force needed to accelerate a body is propertional to the inertial mass of the body and the acceleration produced, or more simply force = mass x acceleration. To put it another way, to raise the value of A we need more force to propel the vehicle - or else shed some weight.

To understand what this force is and where it comes from, we need to examine the ‹Å“power flow¢ž¢ in the drivetrain from the tyres acting on the road backwards to the power plant
- The interaction between the tyre and the road gives the tractive force to move the vehicle relative to the road. (How this works is in itself quite interesting and there is quite good explanation at : ... lling.html )
- The devices acting to turn the wheels are the axle - or driveshafts, the drive coming from the gearbox and main prop shaft to the differential.
- The drive into the gearbox comes from the flywheel and clutch assembly
- The flywheel is driven by the crankshaft
- The crank is driven by the connecting rods, which in turn are forced by the motion of the pistons.

The net force that drives the piston down is the combustion gas pressure in the cylinder acting on the piston crown ¢‚¬Å“ this force F being dependent on the relationship F = P x A where P is the gas pressure and A is the cross sectional area of the piston. I say ‹Å“net¢ž¢ because there is piston and ring friction at the the cylinder walls and the inertia of the piston and upper rod mass trying to resist this motion. What¢ž¢s left after losses drives the piston.

In an engine that is running well, the pressure on the piston rises but rapidly - in controlled fashion - after ignition, as the mixture burns, and drops as it pushes the piston down the bore, until the exhaust valve opens at which point there is no useful pressure left to do any work on the piston. The effect of the piston and connecting rod acting on the crank generates a torque on the crank. Torque is the leverage needed to turn a shaft against an applied load, which, in the case of a car is a question of overcoming the inertial mass of the vehicle, those frictions imposed by the drivetrain, and the vehicle¢ž¢s aerodynamic drag ¢‚¬Å“ which, over 80mph is usually by far the biggest component.

Torque (T) in a reciprocating engine is the force F acting along the rod axis multiplied by the perpendicular distance from the rod to the centre of the crank. Thus, in terms of mechanical advantage T is totally dependent on the engine stroke and rod length. Torque is typically measured in Nm (Newton metres) or lbf ft (pounds-force feet), usually called 'foot pounds' for brevity.

Torque is a 'cyclical' commodity because:
1. We depend on the gas pressure in the cylinder to drive the piston down, but useful work is only performed during the early part of the firing cycle.
2. The length of the lever arm varies as the crank rotates
3. The frictional and inertial loss in the cylinder varies.

We can only measure an average (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 ‹Å“brake¢ž¢ the engine, hence the name ' brake dynamometer'. In practice a dyno will an use eddy current or hydraulic rotor to apply load without actually stopping the engine. The amount of 'leverage' developed when the load is applied causes the dyno to swing and it is measured via a lever arm on the dyno acting on a load measuring cell, this simple system quite literally gives a torque measurement in ‹Å“foot pounds¢ž¢. The measured torque at the flywheel is essentially the sum of the torque produced by the firing cycle of all cylinders minus the losses (frictional and inertial) during each cycle, and maximum torque is always produced at full throttle - when the fuelling and ignition for a particular engine is fully optimised.

The ‹Å“net torque¢ž¢ of course that we see at the flywheel, is only a fraction of what we might hope fr due to frictional losses in the rotating parts. Inside an engine the other friction losses come from oil drag in the bearings, intermittent and microscopic metal to metal bearing contact, crank and cam seals, windage (air resistance in the sump), oil drag on the crank and rods, valve guide and seal friction, belt or chain drive and pulley bearing friction, cams and auxiliary shaft friction in their housings.
(And to make matters worse never forget that even the best gasoline reciprocating engines waste 70% or more of the energy of the fuel by heat rejection to the cooling system and down the exhaust pipe.)

More cylinders give more torque. Flywheels traditionally were always used to store energy ie: 'store power between firing strokes' and thus give the crank assembly the momentum to drive the piston past bottom centre back up to top centre, but these days on relatively short stroke lightweight petrol (and diesel engines) they are getting lighter and lighter even on production cars and competition engines always sacrifice any 'smooth running' benefit in favour of the reduction in power loss that a light flywheel can give. Formula One engines have so many cylinders (V8, V10, V12) that they don¢ž¢t need a flywheel at all, only a simple titanium disc to hold the clutch.

Flywheel horsepower depends on torque and is calculated, not measured from the torque measured from a dynamometer according to:

Brake horspower (bhp) = 2 x Pi x n x T

where Pi = 3.142, n is engine speed (rev/min) and T is torque.

Going back to our earlier question.
To accelerate the vehicle we have to speed up the acceleration of the piston on the power stroke. This can only come from higher cylinder pressure.
In other words, however you look at it - we need more torque.
03_Piston reciprocating motion.GIF
Sketch shows the lever arm relative to the crank centre and how it varies with crank angle 'Theta'.
03_Piston reciprocating motion.GIF (175.53 KiB) Viewed 22657 times
05_cyclical torque .GIF
cyclical torque...what the dyno sees. It cannot be measured at any point, only calculated from knowledge of cylinder pressure, a thing made possible only recently by the development of high speed transducers.
05_cyclical torque .GIF (9.18 KiB) Viewed 22634 times
09_Torque-power curves.GIF
actual power/torque plots from Tom's 2 liter Fiat shown above. Note the flat upper power curve, this setup determined by very accurate software development, resulting in an engine far faster than many others with more power.
09_Torque-power curves.GIF (63.36 KiB) Viewed 22633 times
08_Lancia Montecarlo on rolling road dyno.GIF
Geoff Ward's stunning (and rebuilt) Lancia Montecarlo on test at Northampton Motorsport's rolling road. 170 bhp engine is GC StII 2 liter 8v.
08_Lancia Montecarlo on rolling road dyno.GIF (123.98 KiB) Viewed 22617 times
Tom Mcdermott's 2 liter big valve tarmac rally engine on test at Cork CIT, build by Mike Fitzgerald to Gc spec, 4-2-1 exhaust and big-wing wet sump for Mk 3 Ford Escort
tmcd2.JPG (163.79 KiB) Viewed 22639 times
Nova dyno _01R.jpg
Darrren McCarthy's fuel injected tarmac rally 1600 Vauxhall sohc engine on test at Cork CIT dyno. Full spec GC head. The overhead cam/rocker actuated valve train needs huge turning torque and saps a lot of power. Power is confidential - but quite good.
Nova dyno _01R.jpg (44.72 KiB) Viewed 22571 times


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