Engine Braking and how it works...... ?

[Hawk]

Just thought this would be an interesting thing to know, because I'm pretty sure it is a misunderstood subject?

Also. Is engine braking in a 2 stroke engine generated the same as it is in a 4 stroke engine, and why is one engine braking(2 stroke) a lot weaker than the other engine braking(4 Stroke)?

So who knows how engine braking works in a motorcycle engine?

Hawk.

[Mac]

I'm not a mech engineer but I know that EB is generated by the compression phase in both 2 and 4 stroke engines.
On throttle the compression phase still exists but the expansion phase generates power that overcome this loss.
Sport motorcycles engines also have a huge compression ratio (over 12:1) similar to diesel car engines.

2 strokes have less EB because you have to compare similar power ---> smaller capacity values and these engines have no inertia coming from camshafts, valves etc.

[doubledragoncc]

Dang I wrote a good answer and hit the bloody back key so will do it short.

Basically it is said that a 2 stroke has little if any engine braking as they have practically no back pressure caused by closing valves as on a 4 stroke. As the piston of a 4 stroke travels upwards and the valves are closing or closed, it is having to fight against compression between the piston crown and the cylinder head ceiling. If you think of how it is when you push the plunger of a syringe downwards and put your finger over the hole, it gets harder to push the plunger until you remove your finger. The short period of time that valves are closed on a 4 stroke as the piston travels upward is enough to slow the engine down, thus causing engine braking. A 2 stroke does not have valves like a 4 stroke so the piston never has the physical fight as a 4 stroke piston does. It is very simple in the end. I should also emphasize that this only happens when the throttle is closed and therefore the ignition is now causing less sparking of the plugs, so less detonation and there is less fuel being fed to the cylinder.

I am a 4 stroke guy and it is why in GPB I stick to the big boys lol. I always need to brake sooooo much more on the 2 strokes and spend a lot of my time on my ass as you cant teach an old dog new tricks lol.

I made a butter fly valve back in 83 to allow me to add back pressure myself to save on brakes as I rode 1000 plus miles a day and it helped a lot. Later Yamaha designed the same kind of thing called YICS. I got the idea one night riding back to London from Wales as I passed a truck using its J-Brake to slow it on a steep hill.

I use EB a lot and it helps save on brake ware and heat fade.

I hope that clears it up a bit for you.

[Napalm Nick]

Inertia is a small part , mainly it is the compression and stroke length. When you shut the throttle the air intake is shut and a vacuum is pulled. 4 strokes of comparable power to a 2 stroke are usually about double the capacity also with higher compression cycles so when comparing say a 500 cc 4 stroke and a 250 cc 2 stroke (similar power) there is a lot more air moving in the 4 stroke (about double!) Twice as much vacuum being pulled in opposition to the rotation of the crank.

Inertia from amount of moving parts , 2 stroke crankbox size, valve open timings and other factors too I expect.

All I know is when I moved from a 2 stroke to my first big 4 stroke - I grabbed the same amount of brakes, and went down the gears about the same amount coming up to a roundabout and the bike stopped and I didn't !

Edit: cubic capacity police got me 

[HornetMaX]

It's not a trivial question.

I don't think inertia plays a significant role. Friction a bit, but it's marginal compared to the rest.

Essentially, engine brakes come from the fact that with a closed throttle the pistons are trying to suck air in from a smaller "hole" than when the throttle is closed.
That, summed with the fact that en combustion generates less energy, gives the overall brake.

Now, why 2s have less engine brake than 4s: mainly because usually we compare a 500cc 2s to a 1000cc 4s.
But even at same cc, I think 2s have less engine brake but the reasons are not so clear to me (exhaust resonance helping to pull gases from the cylinders, charge circulating beyween crankcase and cylinder, ...).

Maybe Allen can explain that

[Hey Nick, you have a typo: 250 vs 250]

[Napalm Nick]

Thanks yes not so much a typo as a wrong choice-o

Apart from that we seem to generally agree but its another of those dark science-arts.