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Calculating engine braking


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#1 mariner

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Posted 08 June 2019 - 10:42

Engine braking can be pretty powerful, with a "Jake Brake " it can hold a 40 ton truck on quite steep grades. Hybrids ( F1 and road) have dynamic EBD  to blend the motor braking with foot brake pressure. 

 

But if you just have an old fashioned manual gearbox can you calculate engine braking with any accuracy?

 

Most of the variables are knowns - tyre rolling radius, final drive ratio, gear ratios , compression  ratio and engine size. 

 

So it should be easy but I suspect the problem is defining an effective CR which accounts for the cam overlap etc.

 

I suspect it might be better to just  use your mobile's built - in accelerometer  to measure de- acceleration directly - modern mobiles  have many neat uses , you can even assess ride frequency with one and a $10 app.

 

Either way I have noticed that you an use engine instead of hydraulic  braking most of  the time on the road if you want to.



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#2 Greg Locock

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Posted 08 June 2019 - 18:20

Jake brakes are different. With a normal engine I would say that if the throttle is closed you are mostly seeing FMEP (friction), there isn't much gas work going on after the first couple (?) of stroke to empty the intake manifold.



#3 PiperPa42

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Posted 08 June 2019 - 23:54

I use engine braking all the time, and it is part of the reason I really don't like using automatics. Even after driving around in a rental for a week, I still lift the accelerator and expect it to slow down. On the other hand I found the Ford Fusion Hybrid nice because the regen on deceleration felt like engine braking. 

I don't know anything about calulating the force though.



#4 Joe Bosworth

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Posted 09 June 2019 - 03:05

Marriner

 

You can make a very simple job of calculating engine braking HP.  Start with the principle that energy at point in time is equal to MV^2 and Hp is energy change per unit time.

 

Establish aero drag and rolling friction by finding a dead level road with no wind affects.  Time a roll down in neutral say from 80 mph to 50 mph and calculate the HP absorbed by drag and rolling.

 

Do the exact same test and from and to same speeds but this time in say 4th gear or there about.  Calculate the HP absorbed and compare to test 1.

 

The difference is friction and CR absorbed HP.

 

Reagrds



#5 Lee Nicolle

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Posted 16 June 2019 - 09:09

I use engine braking all the time, and it is part of the reason I really don't like using automatics. Even after driving around in a rental for a week, I still lift the accelerator and expect it to slow down. On the other hand I found the Ford Fusion Hybrid nice because the regen on deceleration felt like engine braking. 

I don't know anything about calulating the force though.

Errr, what is this I dont like autos because of the lack of engine braking? 

I use engine braking on autos all the time. Carrying heavy loads or towing them. Maybe you should try it!! Pull the gear lever back one or two slots!

I am sure regen does it too,, but a lot more complex and expensive to build and maintain



#6 Fat Boy

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Posted 17 June 2019 - 19:17

It's funny this comes up. I've been thinking about differentials a lot lately. We often run aggressive coast ramps in the diff to get braking stability. Because there isn't much torque required to spin the engine over you can't get all that much locking across the differential even when really trying to cinch things up.



#7 Greg Locock

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Posted 17 June 2019 - 23:04

Maybe you need a jake brake



#8 Nathan

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Posted 18 June 2019 - 00:10

I use engine braking all the time.

 

Ever personally replaced a clutch and brake pads before?



#9 Fat Boy

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Posted 18 June 2019 - 01:59

Maybe you need a jake brake

 

A little preload goes a long way, Mate. :cool: