Another Motorsport question I can't answer for my lad!!!

[silverback mike]

Ph maestro's!! A question I quote below!!!

Can you ask on PH please how engine manufacturers determine engine torque, power and specific fuel consumption of an engine in test conditions using an engine dyno (not a rolling road)

I hang my head in shame and can't answer it to the level he's into, help gratefully received!!

Mike

[MDMA .]

http://www.explainthatstuff.com/how-dynamometers-w...

[silverback mike]

Much obliged mdma!!!

[MDMA .]

You could possibly read for weeks about it and still not get your head around it. Easy if you understand maths and physics, bit harder if you don't.

[silverback mike]

Further enquiry from brains......

I need to find out how engine developers (not car developers) engine developers. How they go from taking an engine on a dyno to max revs to getting a figure of torque. What calculations do the computers do to get that figure. Engine dynos are attached directly to the crankshaft. So the calculation will include RPM somewhere along the line.


Erm.........

[MDMA .]

As per James Watt, estimation

You're best reading up from the beginning. The HP figure came about from the old steam engine days based on what a horse could do in a days work. Again, estimation ( sic - over ) as to sell bigger and better steam engines with higher HP outputs. Cleaver marketing from day one really.

Edited by MDMA . on Thursday 17th November 21:24

[bitwrx]

The things they measure on the dyno are speed and torque. They multiply them together to get HP.

Someone will be along in a bit to fill in the details, but that will probably do for a layman's explanation. Certainly it's all I feel I need to remember from uni, but then I'm not a very good engineer...

[poppopbangbang]

silverback mike said:

Further enquiry from brains......

I need to find out how engine developers (not car developers) engine developers. How they go from taking an engine on a dyno to max revs to getting a figure of torque. What calculations do the computers do to get that figure. Engine dynos are attached directly to the crankshaft. So the calculation will include RPM somewhere along the line.


Erm.........

No calculations as such as torque is a measurement not a calculation. Although you may do some math to go from the load cell output voltage to torque in an electronically controlled system.

If you have a 1ft bar on a pivot and you put 100pounds of weight on one end then you have 100 ftlbs of torque. All dynos work on the same principle with a way of presenting a load via some sort of brake and a load cell at a known distance from the centre of the braking load. In this way you have a direct measure of torque and can convert this to any unit you require. Generally you will measure RPM via either the speed at which the input shaft to the brake is rotating or via the flywheel/crank pulley. In this way you can quote a torque at an RPM


Not my pic but one pinched from google images that illustrates it well!

Once you have torque and RPM the formula Bhp=Torque(ftlbs)*RPM/5252 will give you brake horse power.

Plotting a curve is just the result of many measurements at specific RPMS to generate a point cloud on an X/Y graph and interpolating between them. The more points and less interpolation the more accurate the measurement.

Generally to ensure results are relatively comparible the raw measurements will be corrected in some way, usually to something like SAEJ1995 - http://standards.sae.org/j1995_199506/ in this way the result is corrected for ambient temperature, humidity, altitude etc. which means measurements taken on different days and in different conditions should be comparible with previous measurements.

In answer to your how do we go from max revs engine to a measure of torque - you would also specify a throttle position. Assuming a WOT measurement then the engine speed will be increased to the RPM we wish to meaure at, load will be placed on the engine as the throttle is increased to maintain the engine speed in steady state. The load cell value will then be recorded to generate a measure of torque at the required RPM. The formula above gives you BHP and this would then be corrected for atmospherics to present the final value.

[poppopbangbang]

With regards specific fuel consumption you would generally use brake specific fuel consumption (BSFC). BSFC = Grams per second of fuel consumed / power produced in Watts. This will give you grams per joule which can be converted to grams/KWh for more usual comparisons.

BSFC is useful as it provides a comparsion of efficeny but also as it is time based (grams per second) it provides a fixed period over which the calculation is based. As BHP is a measure of work done this is essential for a compariable measurement.

[silverback mike]

Thank you very much chaps

[Otispunkmeyer]

silverback mike said:

Further enquiry from brains......

I need to find out how engine developers (not car developers) engine developers. How they go from taking an engine on a dyno to max revs to getting a figure of torque. What calculations do the computers do to get that figure. Engine dynos are attached directly to the crankshaft. So the calculation will include RPM somewhere along the line.


Erm.........

Dynos have a load cell. On an eddy current dyno it is essentially a big electro magnet. They pump in some volts to resist the engine. Control loop is used to close the loop on engine speed to keep a target speed by altering the strength of the magnet (which is just one type of dyno controller mode, there are others).

The resistance to the engine in turn has a reaction force on the outer casing which would want to turn if not bolted down. It's stopped from turning by being held down by a small arm with a load cell attached. You then just calibrate this with some known torques (they usually have a load arm and some cal weights for this) and away you go.

Torque and RPM are directly measured. Power is calculated along with many other parameters. Fuel will be provided by a fuel conditioner and a measurement system often comprising gravimetric and coriolis meters for accurate direct measurement. Specific fuel consumption is then trivial to work out.

[TheExcession]

Great to see you're still around Mike

I love threads on topics like this - a proper school day.

Otispunkmeyer said:

coriolis meters

I'd never heard of it (which surprised me) and had to Google it.

Here's a nice little vid on the coriolis effect.

Really interesting how phase shift measures the mass flow and frequency measures the density.

(OT - my back ground is in electromagnetic waves - I attended a lecture a few years back given by a mathematician investigating waves at the 'sea side'. 99% of the mathematics went over my head but in chatting to him afterwards he summed it up by saying "In the lab with a flat bottomed tank we can describe and predict everything we observe with a huge level of certainty - once the floor is sloped or bumpy it all falls apart..." )

Great thread - thanks everyone.

[silverback mike]

Still about chum hope all is well.

Thank you very much for all the input folks! The ph power is still strong after all these years!!!