Back-Torque

lordpantsington

From Bristow's post about the engine.ini:

RPMTorque=( 0, -32.3, -32.3)
RPMTorque=( 500, -32.1, -20.0)
RPMTorque=( 1000, -33.4, 3.0)
RPMTorque=( 1500, -35.2, 29.8)
<snip>

The table above defines the torque characteristics of the engine. On each line there is
- The rpm at which these values apply
- The maximum Back-torque, or drag, in Newton-metres from the engine at closed throttle
- The maximum Torque in Nm from the engine at full throttle
These torque values will be less (closer to zero) at part-throttle openings.
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With all the mods around you would think this info is readily available.
What causes it?
Where are people finding Back-torque? It certainly isn't on a dyno sheet.
Can it be calculated/synthesized?
How were the values decided upon in the original rFactor shipped cars?

I was messing with trying to build up an equation but then my brain hurt. I roped a few others into my conquest, but then they hurt too.
TIA
pants
 
My friend Ylid from SimRacingPL some time ago made a formulae treating engine (when working with throttle off) as an air compressor. The formulae takes into account engine capacity, its compression and rpms. You can draw a nice engine brake curve in Excel with it. Because it only calculates compression from pressure and not mechanical friction (which is present - you won't rotate engine with just 2 fingers ;-) ), I suggest to add additional torque. Something like 10-20Nm should be fine albeit I never had an opportunity to measure that. That torque also should keep the car steady when you stop it, put into 1st and release any brakes. Without that, the car will start to move (as there is 0Nm calculated brake torque at 0rpm).

EDIT: Check the attachment.

Also, you probably have to make a correction in the file, because I use localised polish Excel and because of that, use different names for some formulaes.
In the compression formulae, there's something like this: POTĘGA(value;value) - that's polish name for Excel POWER formulae. If what I provided, won't work then you should change that name to what will work in your Excel (POWER for english version).
 
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Oh my goodness, Thank you so very much. That spreadsheet is fantastic. Give Ylid a huge thank you when you talk with him next.
 
No problem but remember, I don't know to what degree that approach can be close to what you have in real world. I hope and think it should be close or at least plausible, but I can't give you guarantee for that.
For me, that approach works well in my mods so I'm happy with that. I don't have real data anyways, so better having that than nothing (or some values taken on rough guesstimates!) ;-)
 
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File works fine in open office. The numbers that it gives are almost linear. Because of this I doubt the accuracy of the formula. I am definitely not seeing where the constants 4.344x10^-6 and 0.2857 are coming from.
 
This has been a two year nemesis, Still learning.

I read here http://www.advrider.com/forums/showthread.php?t=578018 that
Engine 'braking' in a gasoline engine is due to the energy required to pull the pistons down through the intake stroke against a closed throttle. Exactly why diesel engines require additional equipment to generate engine braking as they have no "butterfly" valve in the intake manifold.
Click to expand...

POR!
 
LesiU, do you know the pumping process this formula is based on? I also doubt that an engine running off-throttle works like a piston-rod air compressor because the air into the cylinders is not released during compression stroke but it's compressed and then expanded again to its original volume instead. Somehow it works like a spring. It doesn't involves any energy loss regardless of what displacement or compression ratio are. I'm not saying the formula is incorrect, don't know where it comes from, it may be based on another sort of compressor or I might be wrong. Thanks for sharing btw.
 
Pistons are working like when you are on throttle so they are releasing compressed air. The "only" difference is, that in off-throttle air intake is closed. Braking force comes from compression stroke, over and over again.
That what you said, comparing to a spring, is for Diesel engines, FWIK. They have to have additional device that open exhaust valves to eliminate that spring effect and provide braking forces.

I will provide a description, where that two coefficents came from (I have to translate that from polish to english ;) ).
 
Thanks for your effort!

About engines, sorry but I disagree; they do work like when on-throttle so they only release air in exhaust stroke, not during compression stroke like air compressors and some Diesels do.

Air compressed doesn't means much in conventional engines since the energy you have to spend for compressing the air is the same that you will get from it pushing back the cylinder throughout the "power" (although not powered) stroke. That's the spring effect I was referring to. Moreover, in most multi-piston engines there is always one piston that is "pushed" down while another is compressing air, so forces are canceled out.

As you said, in petrol engines the inlet closure is what makes the big difference, the main contribution on overall back-torque comes from air-flow obstruction at throttle plate(s). That's what makes me feel that back-torque curve should look like a quadratic relation with engine speed with it's maximum at 0 rpm, as it's mainly caused by an aerodynamic drag, but this is just a guess.

Sorry about my English, hope you can understand me.
 
Been doing a few searches on this here and there, and as much as hate to admit it this is the conclusion I've come to: You can't synthesize this with an equation/spreadsheet. What happens is fundamental to each individual car, ie. must be measured, much like the torque produced @100% throttle.

Even though the scope of the paper is TC, if you look up SAE paper 2002-01-3359, page 11 you'll find the only Torque vs RPM vs Throttle % graph I've ever seen. Sometimes you get lucky looking for things.

Basically what is happening is that the throttle controls the Volume of air entering the engine. When shut, less air, so whatever compensates with less fuel added. Exactly how much less? GOOD QUESTION! I think only an engineer of a car can answer. Apparently the shutting of the throttle also creates a slight vacuum in the intake manifold thus creating drag on the crank as the piston moves from TDC to BDC. Air also has inertia and is compressible so flow from the closed manifold into the cylinder also isn't straight forward. Flow is based on shape of intake, valve shape, timing, yadda yadda yadda basically this turns into a giant complex thing. The only way I think it can be estimated is with CFD.

If one had access to a dyno and a specific car/engine, I would think you could similar results by restricting the intake opening.
 
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