Theory of Engine braking....

Does the compression cause the engine to brake? Or is it the internal
friction? If compression is the cause, what it the motor compressing if
the throttle bodies are closed? Does the weight of the crank or
flywheel have an effect upon engine braking? How do the R.P.M.s effect
engine braking? Why do two strokes have less engine braking? Did the
old Suzuki water Buffalo's have much engine braking? Would it be
impossible to lock the rear wheel if the plugs were removed and you
were coasting down hill at 30 mph with the motor off and you let the
clutch out in first gear?

Inquiring minds want to know...
cheers
ken

 

http://en.wikipedia.org/wiki/Compression_braking
http://www.everything2.com/index.pl?node_id=1181314
http://www.google.com/search?source...ls=DVXA,DVXA:2006-01,DVXA:en&q=engine+braking

 

Its commonly called compression braking deriving from "compression
release" one-way valves (commonly known as "jake brakes" on big trucks)
which vent the compression stroke resulting in the piston pulling
against a vacuum on what would have been the power stroke.

With throttle closed and engine being driven at higher RPM than it would
turn itself under those conditions, the intake stroke pulls harder than
it would at idle. This is what is commonly called compression braking in
"normal" 4-stroke engines. The braking effect is due to the attempt to
pump a vacuum. Generally the mixture is too lean to burn under this
condition resulting in missing, which sounds like popping. If enough
unburned fuel hits a hot spot in the exhaust pipe it ignites in the
exhaust pipe with a bang, often flames out the tail pipe.

2-stroke engines have little more than engine friction for "compression
braking" because the piston does not directly draw (suck) thru the
carburetor throttle slide. The carburetor feeds the crankcase thru reed
valves (or rotary valve). The backside of the piston serves as a pump to
draw mixture thru the carburetor into the crankcase then on the downward
stroke compresses that mixture (supercharger) to push it into the
combustion chamber. This layer of buffering minimizes "compression
braking."

In years past 2-strokes typically had two "spark plug holes" that one
could install a compression release for the purpose of "compression
braking." Keep in mind historically brakes haven't been as good as they
are today and as such the compression release was a safety feature,
which is one of the reasons they continue to this day on big trucks.

 

Thanks that was an excellent reply.
To clarify that for my pea brain, when I am experiencing engine braking
on my motorcycle, the majority of that force is caused by the pistons
drawing a vacuum against the throttle slides on the intake stroke.
Correct? On the compression stroke does that vacuum act to draw the
piston up? Or is that vacuum somehow eliminated before the
compression stroke begins?
cheers
ken

 

Your throttle slide isn't fully closed so something is drawn in for the
compression stroke. But there isn't much compression being done and
thats why its hard to ignite that mixture, because there isn't much of
it.

High performance MX 4-strokes are usually tuned to lessen the amount of
"compression braking" as most world class riders are coming off
2-strokes and haven't adjusted to compression braking. The effect can be
lessened with valve timing. Am not sure exactly how but am told a
CRF450R has less than a CRF450X.

 

Depends upon what gear the transmission was in. It was more of a
crankshaft inertia effect.

I had to watch the digital gear indicator to make sure I didn't
downshift my Water Buffalo into 1st gear, as the transmission had a
"granny" low gear that would let the
engine almost idle away from a stop. Getting into low gear accidently
would really slow the machine down suddenly.

I was riding with a friend who had a Honda CB-750A automatic. We rode
down a twisty canyon and that really upset him because the torque
converter didn't allow him any engine braking at all, the engine just
free wheeled when the throttle was rolled off.

But he was upset with me for just running away and leaving him behind.