# Physics of a diesel engine revving up and slowing down

1. Jun 6, 2013

### gtzpower

Hi everyone. So I am trying to write a very simple engine simulator for a game, and I have a question about how diesel engines accelerate and decelerate when a machine is in neutral. I have found some videos of a machine at full throttle and then the throttle released, and the engines acceleration pretty much matches the deceleration.

I have been using known torque values for the engine, and simply adjust inertia of the engine until the acceleration matches what I have seen in the videos. Then when the throttle is let up, I am using engine brake torques to slow the engine down. Unfortunately, something is wrong with this as I require very high brake torques to decelerate all of that inertia at the same rate it is accelerated. Furthermore, Wikipedia and many other resources indicate that diesel engines have practically no engine braking (unless a compression brake or an exhaust brake is in use) because the compressed air acts like a spring.

Does anyone have any experience with large diesel engines in heavy equipment/large trucks? Maybe these engines don't typically output full torque when revving in neutral? Maybe a compression brake is enabled in neutral? Maybe my physics calculations are missing a lot of variables? :) Any help is appreciated!

Thanks!

Last edited: Jun 6, 2013
2. Jun 6, 2013

### Staff: Mentor

An idling engine (whether diesel or spark-ignited) isn't producing anything near the rated torque value. The torque-at-rpm values that you see are maximum torque assumes that the cylinders are being filled with a full charge of fuel (diesel) or fuel-air mixture (throttled, spark-ignited) so generating maximum cylinder pressures - it's the maximum load the motor can handle at that speed - think "pedal to the metal". If you were to operate an idling, unloaded motor that way, the torque would continue accelerating the crankshaft until the motor overrevved and destroyed itself.

What you're seeing is an engine developing only enough torque to overcome internal friction at idle, because it's throttled such that cylinder pressures are no higher than needed to generate that minimal torque. And then it's not surprising that internal friction slows the engine down about as much as small increases in torque speed it up.

3. Jun 6, 2013

### Spinnor

Not quite symmetric for this diesel,

Last edited by a moderator: Sep 25, 2014
4. Jun 7, 2013

### gtzpower

Hi Nugatory, thanks for the reply.

So are you referring to the torque produced while an engine is idling (because I am referring to full throttle revving), or are you saying that even full throttle revving produces less torque when the transmission is in neutral? If the latter is the case, do you know how the torque would be controlled? Is it based on load, and the computer starts injecting more diesel when it detects load?

Hoping you can help clear this up. Thanks!

5. Jun 7, 2013

6. Jun 7, 2013

### 462chevelle

and torque would be based off load because without load there would be no need for turbo boost.

7. Jun 10, 2013

### gtzpower

Thanks 462chevelle and Nugatory! You gave me some stuff to search for, and it sounds like I just need to factor in load when generating torque from my engine. Thank you so much for helping me understand that

8. Jun 10, 2013

### 462chevelle

if you wanted some real numbers I could probably get you some screen shots test driving a 6.0, 6.4, or 6.7 ford diesel of the load, nm torque, rpms, and boost off of ids if you like. may not be for a couple days but I could probably get something for you to understand.

9. Jun 10, 2013

### gtzpower

Thanks a lot, if its not too much trouble that would be nice to look at. If its a pain, don't worry about it. I have actually implemented load based torque rise in my game already, and it seemed to solve the issues i was having :) But if its really easy to share your imagery, then It's always helpful to have more data.

Thanks again!

10. Jun 10, 2013

### 462chevelle

alright. can I post pictures here like that does anyone know or should I just screenshot them and email them to you?

11. Jun 10, 2013

### gtzpower

if you cannot post them, my email is my username here @gmail.com. Thanks!

12. Jun 12, 2013

### 462chevelle

I got the screenshots this morning. its on an 09 f350 4 door longbed 6.4 auto. ill try to post them later.

13. Jun 12, 2013

### 462chevelle

hope these work. if they do ill add some more. I don't know how theyre going to fit on here. and I just realized that the ones I took, I didn't wait long enough for the mgp to pick up on the computer. some of these pics, are right when the turbo starting to spool up good

in this second pic here. the turbo boost on the guage and the seat dyno was around 15-20. the pid lags about 3 seconds on mgp, frp, map, etc. type pids

14. Jun 12, 2013

### 462chevelle

here are some more.

15. Jun 17, 2013

### gtzpower

Thanks a lot 462chevelle! You have been most helpful! I can clearly see how the torque output changes with load here, confirming what you have been saying all along :).

Thanks for taking the time to post these :)

16. Jun 17, 2013

### 462chevelle

no problem, if you want some with mgp higher I can get some. I have a couple 6.4s to drive tomorrow morning