Question about the trucks engine physics

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kriechbaum
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Re: Question about the trucks engine physics

#21 Post by kriechbaum » 23 Sep 2016 15:55

Max, nice ! Hope you can fix it.
Thanks.
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Max
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Re: Question about the trucks engine physics

#22 Post by Max » 11 Nov 2016 10:48

well, i'm back.

after some dig i found few things that could be related.
one of the most important issues i found is that we have engine brake curve relative to torque. this causes stronger engines to slowdown faster apparently. i suspect we have issue about data we used. i mean we have some get_torque() method which includes torque curve and engine braking curve computation, but elsewhere we have also engine flywheel inertia that affects it. interaction between these parts of the code is kind of hard to track (everything make just some coefs that fall into differential equation set black box and it "magically" returns coefs of balanced state for given frame).

so i have in mind little refactoring of the code. making it simpler and more respectful to data.
could you get me examples of braking torque of heavy duty diesel engines (eg the reliable picture of torque curve with that sloped line below zero torque)? such data are rather hard to find, i found just few examples so far: engine around 2000Nm has resistance torque near -300Nm on 2500rpm and it is linear (thus resistance torque = 0.12 * rpm). it defines natural slowdown of the engine without fuel, the engine braking. question is how this changes for bigger or smaller engines. it should be linearly dependent on engine displacement (volume), but i'm not 100% sure. i would like to make it dependent on torque but it won't definitely be linear. could you help us?
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natvander
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Re: Question about the trucks engine physics

#23 Post by natvander » 11 Nov 2016 12:05

Max I have a few here and I can hunt around for more. I PM'd you one for the MX engine (basic but it's a curve). I figured PMing them to you would leave this thread free of the clutter, but I can post the links here if you prefer. They'll be in my Dropbox folder so they won't be available forever.
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Max
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Re: Question about the trucks engine physics

#24 Post by Max » 11 Nov 2016 14:42

just little clarification: engine braking is not the same thing as engine brake.
engine brake is mechanism that is intended for braking fast (exhaust brake, jakobs brake, someone put here retarders as well). there is lot of info about them.
engine braking (drag, resistance) is passive friction and resistance which in inherent part of engine. working engine must overcome this force constantly. and that is what i found as incorrect in our code. for this term i have only little info about RL engines.
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Re: Question about the trucks engine physics

#25 Post by Smarty » 11 Nov 2016 18:02

I have some Cummins data sheets from as late as 1997 which list friction horsepower (ie mechanical losses) for 2-3 data points for engine displacements of 10, 11, 14 and 19L. Seeing as diesel engines have no throttling loss, mechanical loss accounts for the majority of engine braking torque. It's strongly affected by the size of the compressor and alternator/generator so a one-size-fits-all approach may be ill-advised. I'll try to organize and summarize this data over the weekend. I foresee spreadsheets and scatter plots. <3
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Re: Question about the trucks engine physics

#26 Post by Llewelyn » 15 Nov 2016 14:17

Y'know, that reaction speed doesn't sound so terrible for a heavy engine. One of the issues in many games is unrealistic treatment of inertia - especially older, larger truck engines would take a while to spool up to max speed and wind back down again, off load.

your video, Kreichbaum, does sound sluggish, but I think a lot of the in-game ones are too fast. Max appears to have found some inconsistencies though, so maybe we will see improvements. I assume you have coded the torque curve into the engine .sii file more or less correctly?

In the UK, the vehicle tests require the engine to be accelerated to max governed revs and then return to idle. With small diesels such as car and van engines, they're fairly fast responding but bigger truck engines are considerably slower, the rotating masses in the engine are much bigger, after all. Also, modern engines are made to have as low friction loss as possible, which probably contributes to a slower return to idle.

One of the issues I've always noted in truck games is that acceleration and braking are often exaggerated. Real trucks at max weight take a long time to get up to speed.

The other issue, which is steadily improving in SCS's games, is the variation in performance between say a 300HP fleet truck and a 600HP+ top spec. In earlier games, lower spec trucks literally ground to a halt on any gradient, they had less pulling power than they should, while the top spec ones if anything were too impressive. More recently, in latest versions of ETS2 and ATS, it feels more realisitic.

Max, you're right, also, getting accurate torque curve data is not easy. I set out trying to do that, when the ability to vary the torque curve appeared in-game. Such ones as I found, I posted in engine data in the forum, for people to incorporate if they choose.
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Re: Question about the trucks engine physics

#27 Post by Frank » 15 Nov 2016 21:12

Hi,

I think I found some an example.

CAT C-16
15.8 litre
Capture.PNG
Capture.PNG (42.23 KiB) Viewed 843 times
Source: http://www.ciar.org/ttk/mbt/engine.cat.c16.pdf
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Smarty
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Re: Question about the trucks engine physics

#28 Post by Smarty » 15 Nov 2016 21:54

The curve you've highlighted looks to me like it's for an exhaust brake (e.g. a PacBrake), rather than just the passive braking/motored torque of the engine itself (which is what Max is referring to). It's good data though, and illustrates how much engine brake performance curves can vary, even for one model of engine depending on how it's equipped.

That site also has a sheet for the Caterpillar C15. The braking curve for the Brakesaver(1) on that is rather interesting, and contrasts a lot with other braking curves. It's yet another strong argument for data-driven engine brake torque curves, per engine. (I can dream, right?) ;)

(1) Brakesaver is a proprietary engine brake on some Caterpillar engines, which is basically a hydraulic retarder at the flywheel end of the engine. It fills with engine oil as retarding torque is demanded. It's kind of an interesting and unique system.
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kriechbaum
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Re: Question about the trucks engine physics

#29 Post by kriechbaum » 20 Nov 2016 08:39

Max, i am really happy that you searched a solution on that problem.

For engine braking i'd say we are on a truck simulator, and in my experience, a truck with 300 hps and an another one with 600 have the same engine braking level.


The engine braking level is most of the time more affected by the temperature of the oil and its viscosity. And it's almost impossible to notice it.
As Smarty said, yes different accessories on the engines can affect it, but in a standart truck, we know what those parts are, they are almost the same for one truck to an another one.
I think to keep the engine braking aroung the 2100nm of torque of the current version of the code would be nice and realistic, at least to my feeling of driver.

Hope this helps. Thanks guys again to help to solve that issue !

See you.
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Re: Question about the trucks engine physics

#30 Post by Cadde » 20 Nov 2016 15:30

300 hp vs 600 hp = same engine braking? Nope!

To generate more horsepower you need to do one or more of the following:
  • Increase bore. If you do this without also porting the head (making intake and exhaust ports larger) then inevitably you are going to have a reduced airflow into and out of the engine. This adds resistance. And there's now MORE contact surface between the piston and the cylinder. As well as a lot of other things.
  • Increase stroke length. This is same as increasing bore size. However, this also alters compression ratio. Higher compression, stronger engine brake.
  • Change cam profile. Longer intake and longer exhaust periods. Increases compression ratio or otherwise alters the flow of air/exhaust through the engine.
  • More air into engine (turbocharger etc) also increases compression ratio.
  • Change of air/fuel mixture in general. Again, changes characteristics of airflow through engine.
The difference between an active engine brake and a passive one is that the active one alters the exhaust cam profile or otherwise restricts air through the exhaust side causing a blockage which forces the engine to slow down.
The passive one is just regular resistance that is there even when you floor it. Given, if you don't introduce fuel/air into the engine then there's nothing to combust and the resistance can increase from the intake side of the engine.

For instance, driving an old car with the choke applied will rob it of a lot of power AND it will slow down quicker. It is also ofc harmful to the engine as it's running rich...
On extended hiatus.
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