quick engine braking question

[yorkie_chris]

Originally Posted by flymo

So likely that there is some braking effort provided by both the cylinders and the engine case venting restriction, but I would guess that the compression stroke of the cylinders will provide the bulk of the braking.

Remember you still have the power stroke going on there, so the compressed gas returns much of the work done on it in compression.

[flymo]

Originally Posted by yorkie_chris

Remember you still have the power stroke going on there, so the compressed gas returns much of the work done on it in compression.

Thats true, I would expect on a closed or reduced throttle the fuel would be cut or at least reduced dramatically so reducing the power produced. There's probably more power generated by the compressed air after tdc as you mentioned, just a question of how much that cancels out the work done in compression.

I wonder then if the friction of the moving parts of the engine is generating a significant portion of the engine braking.

It does seem that larger pistons in something like a twin generates more engine braking than an equivalent sized 4 cylinder engine though which could still be attributed to friction and at least some compression effort and crankcase air displacement. Probably a bit of all these adds up to the total.

Interesting.

[yorkie_chris]

As a trivial aside, a carbed bike dumps a lot of fuel in on a closed throttle. There is the TES valve on the side of the carb. (transient enrichment system)

This will increase engine braking, not power, as very rich mix won't burn (what you never learnt this playing with combustion cannons as a kid? ), also, fuel will evaporate under heat of compression. I seriously doubt it would get to the point of condensation on the expansion stroke, so will be a route out for heat.

Also, larger pistons, or imbalance of crankcase volume? A four can simply swap the gas volume between pairs of cylinders, a twin has a greater volume change.

I detected no chane in engine braking between being restricted and unrestricted