Engine braking calculation

[GregLocock]

Has anybody got a handle on the typical torque you'd get from engine braking? In particular I am very interested in the effect of throttle opening, ignition off.



Cheers

Greg Locock


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[ivymike]

To be clear, you're not interested in the compression braking offered by diesels with variable valve timing, nor in that available w/external exhaust restrictors?

 

[kirrer]

With only low-throttle, no-ignition, the only net torque comes mainly from friction and pumping - you can estimate this as appx. 1 bar for a 4-stroke NA engine at no-throttle. Then, the torque can be calculated from T=MEP*V_displacement/(2*PI*N_c), where N_c is the number of strokes per power cycle (2 for 4-stroke, 1 for 2-stroke). So, on a 5.0 liter NA gasoline engine, you would expect about (100,000 Pa)*(0.005 m^3) / (2*3.1415*2) = 39.8 N-m (29.4 lb-ft) of torque, probably lower since 1 bar FMEP/PMEP combined figure is likely high for those operating conditions. You can modify the numbers using estimates from your application, but you won't get very far 'braking' by this method --> a majority of the work which could help you (compressing intake gas) is lost during the expansion stroke, since the cylinder acts as an air-spring.

What ivymike alluded to with the diesels & the exhaust/compression braking effect - ie, open the exhaust valve at TDC after the compression stroke - would add significantly more torque, since all the compression work is lost. But, this requires an external mechanism (ala Jake Brake or similar).

 

[GregLocock]

Conventional SI racing engine, say 500 hp 5 litre SI NA EFI, no variable nuffin. In particular I am interested in the effect of throttle opening at a given rpm, ignition off. My thoughts are that at zero throttle there is no pumping work past the butterfly, little past the intake valve, and some in both directions across the exhaust valve. At WOT there is zero across the butterfly, and then some past each valve in the usual direction, and quite a lot of work done on quite a lot of air in the cylinder. At 50% throttle then there is some work done at the butterfly, and then a reduced amount of everything else further on.

This applies near red line. As the air velocity drops at lower rpms I imagine the differences converge quite rapidly.

Cheers

Greg Locock


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[140Airpower]

Greg, I think throttle open (WOT), no ignition is the "motoring" condition and is usually measured and given as a friction torque and HP figures, or it is the main friction component to which you would add fan and possibly water pump contributions. It should be the difference between brake torque and indicated torque, BMEP and IMEP.
BTW, in my experience it is less than with a closed throttle, but I never tried part throttle conditions which could be greater.

 

[patprimmer]

I can only add some real world seat of the pants comments which you probably already know.

If you lose ignition at speed, it will coast a lot further in gear with the throttle open compared to the throttle closed or engine braking in a lot more on a closed throttle compared to open throttle with no ignition.

Regards
Pat
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[140Airpower]

I think some value of part throttle should give the greatest braking due to the pumping work. The maximum work is, no surprise, the amount of airflow times the pressure drop. A peak should occur in between WOT and closed throttle. But again, I've never seen this. Like Pat, my experience is that the WOT condition has less engine braking than the closed throttle condition.

 

[Lou Scannon]

Depending on how accurately you need to know, the engine braking torque can be inferred from measurements conducted along the lines of SAE
J1263 Road Load Measurement and Dynamometer Simulation using Coastdown Techniques.
You would need the neutral coastdown data to get the road load of the vehicle; then, instead of the full throttle acceleration, you could do your closed threttle deceleration measurements. QED.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

This will also include driveline friction, but it may be appropriate to include that. Any ideas how to separate the two?

 

[BigClive]

GL - I am not quite sure what you mean by "zero throttle". This means to me that the throttle plate is completely closed. Certainly in this state there are no (or very little) pumping losses. But conventional engines don't normally have a throttle plate that closes completely - it is always cracked open a little for the idle setting. This give a typical 20 (or so) mm of Hg manifold pressure at idle. But suddenly returning the throttle plate to the idle setting when the engine is at high RPM causes the manifold pressure to drop to 25-27 (or thereabouts) of mm Hg. So engine braking would be at a maximum when the accelerator is allowed to close up to idle the opening at high RPM.

It is very noticeable with a bench-test engine that the engine's starter motor has to work hard to turn the engine over with the throttle at idle. Adjust the throttle plate so that it is completely closed and the starter motor can turn the engine over much more easily.

 

[BigClive]

Oh Dear - I meant "inches" not "mm". It is a pity that this forum doesn't have an "editing" function for people like me with finger trouble.

 

[Lou Scannon]

140Air, the driveline friction is captured in the coastdown measurements (transmission in neutral. Since it is also present in the closed-throttle in-gear decel, the difference between the two is essentially the engine braking torque. Obviously the accessory loads are included in the braking torque measured this way, but I can think of several ways to deal with this if necessary.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, I think driveline friction depends mostly on engine power (drive or drag) so does not fully show up in neutral. However, this may be a moot point if you are interested in the total effect of engine braking and are not trying to isolate engine friction.

 

[ivymike]

I'd estimate 12kW pumping loss with the throttle closed, and 1.3kW with the throttle wide open. That's essentially using the MEP method above, and doesn't count any effect of valve timing. That doesn't count FMEP from the mechanical side.

I know that small engines' starters seem to labor more with the throttle open, but I took that "laboring" to be the sound of increasing and decreasing speed due to compression/expansion of the charge, and not necessarily indicative of average power consumption.

 

[Lou Scannon]

140Air, that is a valid point, you are quite right.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[BigClive]

Ivymike - pumping losses at idle are something I take quite an interest in - especially trying to minimise the losses. Is your estimate of 12kW at idle RPM? What capacity engine? (Now that I think about it - seeing that this thread is about engine braking, the 12kW is probably at high RPM)?
On a different matter - I assume that you are aware that "Ivymike" was the codename for the first hydrogen bomb test? Or just a coincidence?

 

[ivymike]

I didnt save my calcs, but I think I used 3000rpm. (engine braking, but not at an extreme)

I just thought this handle was "the bomb."