Calculating Torque/hp etc

[Carsnz123]

hi there

I'm computer programing as a project for school and I thought it might be fun to create an engine power calculator to go along side the 100's of others on the internet. my question is can you calculate torque with some formula involving the bore, stroke and compression pressure of an engine rather than using the " HP=2pi(T)(RPM)/33000 " formula? That involves knowing the HP or the torque to calculate the other, what if you don't know either and need the torque to sub in to the formula?

I know that:

torque = force*distance

and

pressure = force/area

so working out the torque of a 230cui inline 6 dodge (3.25 bore, 4.625 stroke, 150psi compression) should go (I'm going to work in metric but I'll convert to imperial at the end)

force=pressure * area

f = 1034000pa * 0.005352sqm(area of 1 piston)

f =5534 Newtons

T = f*d

T = 5534*0.0587m(half stroke)

T= 325 Nm

T= 239 ftlb

My result is a lot higher than the 177 ftlb that the workshop manual is saying so what an I doing wrong?

Cheers

Carsnz123

[Spinneyhill]

Where is the shop manual quoting the torque? I think it is usually at the fly wheel but may also be at the rear wheels and there are losses in the system. For example, friction (oil is not a perfect lubricant, and it degrades with use). You are also turning the crankshaft against seals at each end, where there is friction. There are losses due to the engine having to pull ("suck") in the fuel mixture with a carb or the air with injected fuel (that is where blowing the mixture, or at least the air, into the engine helps). Some power is lost in running the engine, e.g. in turning the cam shaft - you have to compress the valve springs. The fuel efficiency is variable: in warm temps the air is less dense so there is less oxygen in the combustion chamber and thus less power produced. Might the compression pressure vary with air temp? Thus the quoted numbers have an implicit set of conditions and methods (i.e. a standard, such as DIN or SAE or ISO) at which they are calculated and measured.

You will also be aware that people interested in performance (e.g. of a dragster) play with them on a dynamometer to optimize the output. Engine efficiency and thus power output changes with many variables, including ignition and valve timing, plug gap, points gap and age (if used), heat of spark, position of spark in the chamber, combustion chamber shape, the oil used, the timing advance curve, the calorific value of the fuel, the air temperature, even the humidity (phreatic combustion helps). Electronic ignition is better than magneto which is better than coil, not least because it is easy to change the timing curve on a good CDI system.

Here is the challenge for you, to work out which of the loss variables have the most effect and to try to allow for them. In some branches of engineering and mathematics (e.g. fluid mechanics) the variables are examined to determine their relative size and effect, and the minor ones are often ignored so a reasonable solution (within a few percent) can be obtained relatively simply.

Excellent project, by the way! Keep us abreast of your thinking.

[Rusty_OToole]

There are many formulae for calculating horsepower and torque. None completely accurate. The best of the lot, and quite accurate in most cases, was a formula invented by Frederick Lanchester in 1906. An engineer commented that if you adjust slightly for improved metals and technology, it was still accurate. This was in the fifties.

[Spinneyhill]

Can you give us a reference to read about it Rusty?

I found this:

http://grandprixengines.com/Note_35.pdf

I am not sure if this is the "rating rule" you mean.

I think the compression pressure used by CarsNZ123 is not really relevant to engine output. It doesn't produce power - burning the fuel does. So you need to look for other formulae.

Edited May 8, 2014 by Spinneyhill (see edit history)

[Mark Shaw]

It seems that one could use the actual engine rotating assembly weight, torque at the crank (with a torque wrench to establish mechanical drag) in combination with air/fuel energy rating, displacement, and RPM to determine engine output HP. The fuel burn efficiency would also effect the formula, but based on emissions data, one could also calculate that variable.

[Carsnz123]

it seems that by my research and your help the main factors i need to research is the force the fuel being used burns with (pressure in the combustion chamber), the efficiency of combustion and the drag of the engine mechanicals. ignition type,timing and spark plugs probably will be included in the efficiency factor. Spinney that grandprix engine website has a lot of very interesting history and data and is very helpful also I got the torque for the straight six from the workshop manual for one of the passenger cars so i suppose it might be torque at the rear wheels.

[Rusty_OToole]

Can you give us a reference to read about it Rusty?

I found this:

http://grandprixengines.com/Note_35.pdf

I am not sure if this is the "rating rule" you mean.

I think the compression pressure used by CarsNZ123 is not really relevant to engine output. It doesn't produce power - burning the fuel does. So you need to look for other formulae.

That must be it. I read about it in an English motoring magazine dated to the mid fifties. Have not seen it in years and doubt I still have it. The author stated that the Lanchester formula was the most accurate devised for estimating horsepower, and was still valid 50 years later with a slight upward adjustment to a constant, to reflect improved fuel, metallurgy etc. I suppose he must have been referring to the number 369.

I don't suppose any formula would hold up, for a "freak" engine with a stroke 2 1/2 times the bore, hopped up to the point where it would not run more than an hour. The same would go today for a racing engine vs production engine, or a supercharged or turbocharged engine.

This is all rather futile with the computer programs available today, for calculating or estimating horsepower.

[Rusty_OToole]

Published horsepower and torque figures are measured at the flywheel not the rear wheels. Here we go again, but even horsepower ratings measured with an engine running on a test stand, are not always the same.

In the US, the Society of Automotive Engineers set the standard. It was supposed to reflect actual horsepower measured on a brake, corrected to standard temperature and atmospheric pressure.

Car makers were free to run their engines sans accessories like mufflers, air filters, generators and fans. They could also advance the timing for max power even though no engine would run for long with the timing advanced so far.

They also were not under oath when it came to what they published. In general, Chrysler was pretty accurate but GM and Ford would publish whatever they felt would sell cars. Exaggerated HP ratings were common.

Except in the sixties when the most powerful engines were often under rated to give them a better class break in NHRA drag racing. The Dodge racing hemi 426 cu in V8 had a published HP of 425 but actually produced 476 on dyno tests.

This changed in the early seventies. They changed the ratings from gross to net HP, meaning as installed in the car with all accessories and factory recommended settings. This cut HP by 25%, more on some models.

Meanwhile the English, Germans and Japanese had their own systems. There may have been others as well. This does not even count the many formulas invented to estimate HP, just the systems of actually measuring it.

In case I have not sown enough confusion, the definition of HP is not even accurate. It was invented in the late 18th century to give steam engine manufacturers a means of comparing their engines to some well known standard when selling their wares. They chose the horse as something everyone was familiar with.

They tested some horses at lifting weights by a system of ropes and pulleys. Then increased the load by a theoretical 1/3 when they did their calculations, to be sure their engines would always be able to outdo the horse and their customers not be disappointed.

But this begs a lot of questions regarding how big a horse, how old, in what kind of health and condition. It also ignores the fact that a horse working at maximum power must stop to rest periodically while an engine does not. So, any horsepower calculation is necessarily inaccurate.

[Spinneyhill]

This is all rather futile with the computer programs available today, for calculating or estimating horsepower.

Yes, maybe. But the young lad is trying to make his own, starting from the position of knowing not much. I applaud and encourage him. Curiosity is what leads to discoveries. Along the way he is likely to discover for himself the importance of the variables in working up an engine for his 1925 Chrysler speedster. And learn a fair bit about cutting computer code.

A couple of other factors you might need to consider are the swept volume and the rotating mass. Remember that the power stroke is most efficient when the piston is half way down (crankshaft throw horizontal) so there must be something in there to account for changing efficiency during the power stroke. And on top of that, the fuel doesn't all burn when the power stroke starts: a flame front advances around the combustion chamber. This is why chamber shape and spark plug location have an influence. In general, high octane fuels burn slower than low octane fuels. That will affect the combustion pressure available - you could probably draw a graph of pressure vs. piston position. Hopefully you can come up with one or two variables that account for these things in a general, near enough, way. If you can get within 10% you will be doing very well.

If you don't use horsepower, you avoid the ambiguities mentioned in Rusty's post about it. Stick to the metric system - it is based on good physics.

[Carsnz123]

thanks for the advice and info guys. tonight's focus has been on combustion chamber pressure and burning of fuel.

i found out that the flame front speed of petrol in an engine is roughly 16.5 m/s and the optimum air/fuel ratio for petrol is about 15:1. When petrol burns it releases 5370.27 KJ/mol. I'm not entirely sure what is in petrol http://www.atsdr.cdc.gov/toxprofiles/tp72-c3.pdf this has alot but the combustion of octane goes 2C8H18 + 25O2 --> 16CO2 + 18H2O.

so we have a product ratio of 8CO2:9H2O.

H2O takes roughly 2 times as much energy to raise it one kelvin as CO2.

according to some rules of expansion i cant quite remember all gasses take up an extra 1/273.15 space when they gain an extra 1 kelvin are at the same pressure.

so after some long calculations i couldn't be bothered typing out i calculated that the products of combustion expand 135% from the excess heat of the reaction as long as all the energy goes to the gasses and we all know it doesn't. Just clarifying an engine works on gas expansion doesn't it?

now going back to flame front speed and using my dodge engine as an example, the flame has to travel 120mm to get from the spark plug to the furthest away part of the combustion chamber. with the engine at 1200 rpm ( max torque speed ) it takes the flame 0.00727 seconds to travel that distance and in that time the the engine has rotated 52.33 degrees and the piston has traveled 34.16mm down the cylinder. now i have to work out the relervence of that in relation to pressure in the cylinder.

still got lots of work to do and any help is welcomed and greatly received.

Carsnz123

Edited May 10, 2014 by Carsnz123 (see edit history)

[Rusty_OToole]

135% expansion sounds low.

[Carsnz123]

I thought that as well but the math works. I'll have to check with my chemistry teacher on Monday