Posts Tagged ‘torque’

This is a nice project to try out.  It is quite simple, especially for those of you using an OBD 2 compliant petrol car.

If you have a car computer, and want to make a power meter here is how to do it:

Step 1 Figure out how much fuel you are burning. (MAF reporting gasoline vehicles)

  • Using an elm tool or whatever you have to read OBD data, request MAF – PID 0x10 (this is mass airflow) [g/s]
  • Now divide this MAF value by 14.7, this is the magic stoichiometric ratio that fuel needs to burn in air.  For the sake of an engine power calculation we don’t need to worry if the engine is running rich, it is only able to physically burn at this ratio, so any additional fuel is not turned into useful work at the crank.
  • The result of this equation gives fuel mass [g/s]

Step 1 Figure out how much fuel you are burning. (Non-MAF reporting gasoline vehicles)

  • If you don’t have a MAF sensor reported on your OBD you will have to do a speed density calculation, this is less error prone if your car is NA (non turbo).
  • Note engine size in [cc]
  • Gas constanst R = 0.082057 [L atm mol^-1 K^-1]
  • Molar mass of air = 28.75 [g/mol]
  • Estimate Volumetric Efficiency  (this is a definate source of error, browse the web for expected VE figures) for now assume  a vtec engine should be getting around VE = 0.9 at 6500 rpm and at idle assume VE = 0.4, do interpolation to find VE for all other engine speeds, or make a lookup table.
  • Prepare to request the following PIDs:
  • Engine speed (PID 0x0C) [RPM] (raw/4)
  • MAP – Manifold air pressure (PID 0x0B) [kpa] (raw = value)
  • MAT – Manifold air temp (PID 0x0F) [degC] (raw + 40)
  • Work out airflow with the following formula (adding 273 to MAT converts to degrees Kelvin)
  • Airflow [g/s] = (MAP/100) * (Eng_speed/120) * (EngSize/1000) * (1/(MAT+273)) * (1/R) * 28.75 * VE
  • Fuel flow [g/s]  = Airflow / 14.7

Step 2 Estimate engine power

The net calorific value of petrol in the UK is about ~ 43 MJ/kg depending on what fuel station you go to!  The rather poor efficiency of our beloved internal engine is ICE_eff ~ 0.33 (33% is thermal loss to the cooling system and a further 33% goes to bearing and pumping losses).

Power [kW] = 43 * fuel flow [g/s] * ICE_eff

Power [bhp] = power [kw] * 1.34 (look at my previous post to calculate torque)

There we have it.  Have a fiddle with the engine efficiency value, it should not really exceed 0.36, let me know how you get on.

I’m talking about that age old argument between diesel fans (who like to talk about torque) and petrol heads who claim power is most important.  My vote is power all the way, and here is why…

Torque:

Say we need to tighten a nut up.  Doing this without a spanner will not get you very far, why? because you need lots of torque.  Torque includes two things-  force AND distance.  For example, slamming a door by pushing on it next to the hinge will not make it slam, that is because the force may be large but the distance between point of rotation and acting force is small, so the torque is low.  Infact even an incredibly strong man with no tool will not be able to tighten a nut as much as a child with a spanner.

So our strong man ‘Arnie’ tightens the nut using only his very strong fingers.  Now a child comes along to undo the nut, without a spanner he’s helpless.  With a spanner, the child now applies a comparitively small force to the spanner but this force acts a long way away from the centre of the nut the longer the spanner is, the easier (or less forece) required to undo the nut.  Try this with a door.  See how hard it is to close the door if you push near the hinges, compared to if you push near the handle.

Taking this example and applying it to a car, the engine has now become our strong man/weak child and  the car’s gearbox is considered to be the length of the spanner.  The nut we were tightening is of course the road applying an opposing force on the car’s wheels.

Power:

Say we need to tighten up 100 nuts, engineers refer to this job as ‘work done’ (which is equivalent to ‘energy’).  Doing this without a spanner would probably be a lot quicker, why, because you don’t have to tighten the nut up by travelling through a large arch/distance.  Speed is the relationship of distance : time.  If we have less distance to cover and we turn nuts at the same rotational speed it will always use less time.

If you don’t believe me, drive 10 miles at 10 miles an hour and then do only 1 mile at 10 miles an hour- the latter surely won’t use up as much time!

So if Arnie and the child both complete the same amount of work in the same amount of time, they are said to have the same power.

Transition to the car example.  Two cars of equal weight have to tow a caravan up a hill.  One car has a lovely (sarcasm) torquey diesel engine, the other has a racy motorbike engine.  The work done will be the same, (same hill, equal weights).  So which one will get there first?  Well it’s all about power.  Power is of course, work done per unit time.  So if you get your work done quicker you finish first.

Interestingly, these vehicles make power in very different ways.  Rember Arnie and the child, well the diesel engine is our strong man- he can afford to spin things slowly because he has lots of torque (strength).  The motorbike engine and child have relatively little strength but a gearbox/spanner provide a torque/strength multiplication.  Like all things in engineering, if you fix one issue, another one is introduced.  By using the gearbox as a torque multiplier you must operate more quickly to keep your work output high.  This is why motorbike engines have to rev so much.

For those interested, a new school SI unit engineer like myself uses the following formula to work out power in kW:

Power [kW] = Torque [Nm]  * EngSpeed [RPM] * (2*3.142/60000)

In the brackets above we are converting engine speed into radians per second and watts into kilo-watts.

To convert kW into brake horsepower, BHP = kW * 1.34

So, What’s Best?

If you want to win a drag race, you need lots of power.  Simple, that is the answer to the question.  If you want to tow your caravan to the top of the hill before all the other caravans you still need to have the most power!  The subtly is the gearbox that denotes the relationship between road speed and engine speed.  Towing caravans up hills requires lots of torque at the wheels even to get rolling.  This can be done by a weak or low torque engine geared to do many more revolutions for each turn of the cars wheels.  Or perhaps a high torque engine, geared to do similar revolutions as the wheels it is driving.  In a towing race, the higher powered engine will always win!  And it doesn’t matter which engine produces more torque (provided they both have a suitable gear box) .

Most diesel fans will claim that this means lots of revving of the less torquey engine to keep up.  Yes this is true, but so what- thats why you have a gearbox and a red line almost double that of the average diesel!  Also, in favour of the smaller less torquey petrol engine is the weight advantage.  By minimising weight you reduce the amount of work needed to complete a race/sprint.

The only real advantage of high torque is longevity.  By creating  power at low engine speeds, service life is dramatically enhanced.  It is also possible to observe fuel savings by keeping engine speed low, as frictional losses are less significant at low speeds, and are zero when the engine is stopped!

In Summary

Cars, trucks boats have all been designed to speed things up.  Whether it’s getting to work quicker in a car compared to walking.  Or perhaps transporting building materials from A to B.  These jobs took a lot longer before the benefit of powerful engines.  Low power means less work done for a given period.