How is variable engine braking implemented in a vehicle ECU?

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  • Thread starter cosmik debris
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  • #1
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Hi All, The ECU on my motor bike allows you to select different amounts of engine braking in it's various riding modes. I imagine that the ECU just opens up the throttle bodies a little so that at closed throttle the engine still delivers a bit of torque. Doing this will increase the idle speed as a side effect, so how is the idle speed controlled, in this case lowered? My best guess is that ignition timing is used to keep the idle speed down. Any ideas?

Cheers
 

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  • #2
berkeman
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Hi All, The ECU on my motor bike allows you to select different amounts of engine braking in it's various riding modes. I imagine that the ECU just opens up the throttle bodies a little so that at closed throttle the engine still delivers a bit of torque. Doing this will increase the idle speed as a side effect, so how is the idle speed controlled, in this case lowered? My best guess is that ignition timing is used to keep the idle speed down. Any ideas?

Cheers
Sounds like it's just a valve timing adjustment during deceleration. Why would a temporary valve timing adjustment during deceleration affect idle at all?

Are the valve actuators electric solenoids? Or is the only adjustment via overall cam timing adjustment somehow?
 
  • #3
jack action
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I imagine that the ECU just opens up the throttle bodies a little so that at closed throttle the engine still delivers a bit of torque. Doing this will increase the idle speed as a side effect,
Not if you set a (very?) lean air-fuel mixture. Then opening the throttle reduces the intake manifold vacuum, thus mimicking a diesel engine.
 
  • #4
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Sounds like it's just a valve timing adjustment during deceleration. Why would a temporary valve timing adjustment during deceleration affect idle at all?

Are the valve actuators electric solenoids? Or is the only adjustment via overall cam timing adjustment somehow?
No variable valve timing on my bike.

Cheers
 
  • #6
I'm very interested in this question, and would like to know more about the implementation on your bike. How do you select from the ECU options provided? Is there a switch?

When I owned a 2008 BMW 328i, with a stick/clutch, I noticed that the engine braking was very much reduced from my experience with earlier cars, and pretty much negligible and not worth the bother. To be honest, I had not owned a stick car for some time at that point, but downshifts for deceleration were once my standard practice and there was just no point with that car. The engine spun up very freely.

Now I own a newish Miata with a stick and it has the expected amount of downshift engine braking, and much more so than the BMW. This has made me curious about what produces engine braking and how it could be so different between different vehicles.

My working hypothesis is that the BMW's variable valving (both timing and lift) are responsible for the difference. The BMW uses valve lift as a throttle and has no throttle body butterfly valve like carbureted engines do. So far, so good...

You say that there is no variable valving in your bike, but how can you be sure? That might be worth exploring.

I'm thinking that strongest engine braking (apart from the bearing friction, oil pumping, battery charging or AC compressor loads) results from the pumping of air (without much fuel) through the engine and out the exhaust, which would be strongly affected by valving.

Any thoughts on this would be very welcome.
 
  • #8
Wow! Thanks much; that’s pretty eye-opening.

Has there ever been a two wheel drive motorcycle? It seems that all wheel drive would eliminate that downside of engine braking and give control back to the driver.

This reminds me that mid-rear engine configuration is thought to have a significant advantage in minimizing weight transfer to the front wheels for braking.

It seems to me there is quite a bit of variation in opinions about how engine braking actually works, by default, not counting ‘interventions” of various types. I’m pretty confident that air pumping is a very significant (and varied) component of it. There is even a commercial product for enhancing engine braking by opening the exhaust valve at the top of the compression stroke so its energy can escape into the exhaust instead of being returned as forward thrust on the downstroke. I will confess I’m still studying about Diesel engines and “jack braking” and its prohibition for noise making.

Thanks again!
 
  • #9
OCR
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  • #10
OCR
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