Technical Q. What forces affect engine braking?

[TLRMan]

A jake brake on a sport bike... would probably make it sound like a HD!

Hmmmm.....

For those wondering..Diesels don't have throttle plates, so when you lift off the pedal, all it does is decrease fuel being injected into the cylinders....Diesels naturally have higher compression engines, so what happens is the piston compresses the air, and acts like an air spring, so the vehicle doesn't like to slow as quickly as a throttled engine...Enter the Jake brake....
Most are just an extra valve that's opened by an extra cam lobe actuated by a solenoid, thus reducing the compression of the engine.....

[13]

isn't a jake brake just the opposite, a valve that closes/restricts the exaust

I'm pretty sure a jake brake opens an exhaust valve at the right time so the compression in the cylinder won't force the piston back down with the throttle closed. It just lets it out the exhaust.

[TLRMan]

I'm pretty sure a jake brake opens an exhaust valve at the right time so the compression in the cylinder won't force the piston back down with the throttle closed. It just lets it out the exhaust.

I do have my invisible button on....

[akira700]

Paul: "Why does a twin engine brake so much more than a four?"

I think its more than just closed throttle plates.
Upping the idle also increases the number of engine revs...
What would happen if you opened the plates yet didnt increase
the revs? Would this help? Is this what the new 600RR engine does?

I think it also has to do with piston surface area. ??
Twins have a larger bore for given displacement.
So the rear tire with clutch engaged is pushing on a bigger area
during compression stroke.

Or maybe less frictional areas due to there being two less
cylinders? or both?

About the two stroke... ??
Maybe they have less engine braking effect
because of the exhaust ports are in use every cycle
instead of every other ...

I'm just trying to make an educated guess here.
We need to recruit some higher powers into this thread.

[TLRMan]

Paul: "Why does a twin engine brake so much more than a four?"

I think its more than just closed throttle plates.
Upping the idle also increases the number of engine revs...
What would happen if you opened the plates yet didnt increase
the revs? Would this help? Is this what the new 600RR engine does?

I think it also has to do with piston surface area. ??
Twins have a larger bore for given displacement.
So the rear tire with clutch engaged is pushing on a bigger area
during compression stroke.

Or maybe less frictional areas due to there being two less
cylinders? or both?

About the two stroke... ??
Maybe they have less engine braking effect
because of the exhaust ports are in use every cycle
instead of every other ...

I'm just trying to make an educated guess here.
We need to recruit some higher powers into this thread.

I'm going home now.....
Red-6-out...

[akira700]

Sorry TLRman
Dont get a insivibility complex cause of me !
I'm just an idiot who thinks out loud.

Anyway about 6 posts happened in the time it took me to hit submit !
I'm understanding it more but not completely.
thanks

[TLRMan]

Sorry TLRman
Dont get a insivibility complex cause of me !
I'm just an idiot who thinks out loud.

Anyway about 6 posts happened in the time it took me to hit submit !
I'm understanding it more but not completely.
thanks

All in fun brother....

[rebelpacket]

I smell another crazy debate thread coming on.

Engine dynamics always seem to be in constant sort of argument. Its always perplexed me, because with a library of mechanics and engineers on the internet

Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression braking is a common legal term for the same mechanism. Large trucks use a device called an engine brake to increase the effectiveness of engine braking.

Engine braking - Wikipedia, the free encyclopedia

Pretty much what TLRman already said...

Makes sense to me. Changing the ECU should have no effect on a 2001 ZX6R. This kid's feeling about engine braking is either an adjusted idle due to the ECU change, or completely placebo about a new fancy part on his bike.

[Paul_E_D]

Well, what confused me was that TLRman so confidently said it was not compression doing the braking. This definiton seems to contradict that...

[TLRMan]

Well, what confused me was that TLRman so confidently said it was not compression doing the braking. This definiton seems to contradict that...

In a diesel the compression defeats the braking because diesels DON"T have throttle plates...read...airspring....
What kind of engine do you wish to discuss..?

[TLRMan]

Ok,
in a 4 cycle gasoline engine, with the throttle open, inducts air, compresses it, and makes power...close the throttle plates, no air in the cylinder, so how can you compress air if it can't get in? If it did, then the engine would work as an airspring, and there wouldn't be as much engine braking that you see now.

2 cycle engines don't have engine braking because of compression, >>> open exhaust port....<<<<

Diesels have no throttle plate, so they always will induct air into the cylinders...so compression defeats engine breaking again...Using a Jake brake releases the compression so you can HAVE engine braking.

So what do we have here? Engine compression DOES NOT WHAT SO EVER HAVE ANYTHING TO DO WITH ENGINE BRAKING, IT DEFEATS IT.

Now you are gonna tell me that the piston doesn't stop when arriving at top dead center, and bottom dead center too?

You must be specific on your engines....

[rebelpacket]

Ok,
in a 4 cycle gasoline engine, with the throttle open, inducts air, compresses it, and makes power...close the throttle plates, no air in the cylinder, so how can you compress air if it can't get in?

Ok... so the throttle plate and no air is getting in..... How does the bike idle with the throttle at rest and the plates closed?

You are correct. There is a great deal of vacuum created during engine braking. But it is paling in comparison with the forces required for compression. Both forces act to slow the rotational forces of the motor. C With the motor at an open throttle, air and fuel will still be pulled in by vacuum forces, but the combustion is exponentially more powerful with each stroke of the motor, forcing the next piston up with more energy. With the plates closed, at idle that combustion force is reduced, and something has to continue compressing that air.

Edit, Mark I mean no slight at wealth of knowledge at all. This is just engine dynamics as I've come to understand it.

[TLRMan]

Ok... so the throttle plate and no air is getting in..... How does the bike idle with the throttle at rest and the plates closed?

You are correct. There is a great deal of vacuum created during engine braking. But it is paling in comparison with the forces required for compression. Both forces act to slow the rotational forces of the motor. C With the motor at an open throttle, air and fuel will still be pulled in by vacuum forces, but the combustion is exponentially more powerful with each stroke of the motor, forcing the next piston up with more energy. With the plates closed, at idle that combustion force is reduced, and something has to continue compressing that air.

Edit, Mark I mean no slight at your knowledge at all. This is just engine dynamics as I've come to understand it.

OK do some math.....how many CFM of air does you favorite bike flow at full throttle?
when you close the throttle, do you think that same amount of air is inducted in, or do you think, that depending on cam timing, and rpms, the velocity doesn't even come close to that. I'll give you cam overlap, and reversion, but never will it intake the amount of air into the engine with the throttle closed...and you cannot ever slow and engine down with compression....if you get compression, fuel and spark, what happens?

at idle, either throttle plates are close, and there is an idle by-pass, or your throttle plates are cracked ever so lightly...

Hey I'm having fun!

[rebelpacket]

Ah-ha! I see what you are saying.... Air and fuel can only flow so quickly through a small hole for your idle circuit. Since there isn't as much air to compress, most of the drag comes from the piston trying to PULL air through the intake. Lightbulb!

Hey, I learned something today!

Simple analogy: Take an empty soda bottle, and make a small pinhole in the cap. Try to suck the air out of the soda bottle as fast as you can. Now take the cap off and try. Which one's quicker? Now relate that to a closed throttle plate, and an idle circuit pulling air and metal parts flying around...

[TLRMan]

Ah-ha! I see what you are saying.... Air and fuel can only flow so quickly through a small hole for your idle circuit. Since there isn't as much air to compress, most of the drag comes from the piston trying to PULL air through the intake. Lightbulb!

Hey, I learned something today!

Well at least someone got it....

Cheers Alex!

[tls25rs]

OK I have absolutely no leg to stand on with anything I am about to say from an engineering stand point.

I remember an article, in one of the hundreds of motorcycle magazines that I have read in my years (actually fairly recently, within the last two years), probably written by Kevin Cameron explaining engine braking and how it truly works in relation to modern four stroke engines. In this article the author explained that though it may feel as though a twin has more engine braking it is in fact a four cylinder of similar displacement that has the greater engine breaking. If anyone could locate that article I am sure alot of questions would be answered.

Personally I would think a lot of it has to do with frictional losses internal to the engine. Typically a four cylinder engine is spinning at a significantly higher RPM then a twin when the rider rolls off the throttle for an engine braking scenario. As the frictional losses are increased by a ridiculous factor as the speed increases I would think that the higher the RPM the more engine braking produced. Take for example your typical R6/GSXR-600/ZX-6 etc. revving at 14000+ RPM and then look at your typical SV650 revving at roughly 8000 RPM and look at the differences when rolling out of the throttle for a turn. More than likely the higher revving of the R6 etc. will put more of a braking force when the fuel is taken away then the lower revving of the SV in the same situation.

This is also part of the explanation of a point someone brought up earlier. The manufacturers are opening up the crankcase between the cylinders to reduce pumping losses when moving both air and oil back and forth between cyclinder bosses in the crankcase under deceleration. These frictional losses if reduced will allow an engine to rev more freely both up and down the rev range reducing both horsepower lost up top and engine braking on the back side.

Mind you this is purely speculation on my part I am neither a mechanical engineer nor did I stay at a Holiday in Express last night so take anything I have said with a grain of salt

[TLRMan]

nope...input is good....Yes because of the cross holes in the crankcases parasitic drag AKA pumping losses is reduced...Christ, I was doing that back in the 70's. Nothing new in racing..... I was also running ATF in the gearbox of my TR3 Yamaha too, not the sludge everyone else was running....As for frictional loses, you get the prize, although I used to make my own pistons with the wrist pins located higher, so I could use a longer rod, and that reduced some drag also...It afforded me a little more RPM, because it slowed the piston velocities down as well as allowed the piston to reside around TDC just a little while longer. When I say a little while, we mean milli seconds... Oversquare engines live a shorter time at the top, so head and piston design is key to good combustion. The piston in my TLR misses the cylinder head by just under .035 of an inch.

So, I'm just gonna sit back and see what comes of this thread now....Just make sure you say what type of engine you are talking about...2 cycles don't have valve trains they have to spin around, most run on magnetos (racing), and gear boxes are smaller, so if you really want to get into the engine braking thing, we can write a book about it.
I guess we could dig up some drive line dynamics data and discuss what affects does a reciprocating engine has on it's crankshaft given at samples taken along a 360 degree sweep. Now we will really know whats going on huh?.....and Yes Degsy, the piston does stop at the top and bottom....

[a13x]

My 1000rr feels to engine brake just as much as my old superhawk, at least I think...

You my friend, are crazy.

[wylee]

I'm getting blind, I only saw the part about a valve that introduces air


isn't a jake brake just the opposite, a valve that closes/restricts the exaust

I thought a jake brake opened a valve to put the power stroke out the exhaust instead of pushing the piston down.

[wylee]

You my friend, are crazy.

The engine braking is one of the things I dislike about the TL.

[13]

You my friend, are crazy.

I know it sounds crazy. But near redline, the 1KRR has a ton of engine braking.



That avatar is quite freaky.

[Paul_E_D]

Well at least someone got it....

Cheers Alex!

Thanks for the discussion! I'm a decent wrench, but I don't really grasp the dynamics at work...

[beet]

Come on thats the best part of TLing!!!!!!

The engine braking is one of the things I dislike about the TL.

No wonder you can't catch the bird.

[beet]

I think the twins, cause of the has less gyro then a four makes it have more engine braking! A four seems to keep spinning, also why a four gets more HP
just like running a heavy fly wheel @ the drag strip.

[wylee]

No wonder you can't catch the bird.

You bastage! That was below the belt!