Engine Braking

[docc]

I've been reading Code's book, Twist of the Wrist . It was assigned to me by fast -guy Goran and I'm trying to be a good student.

 

Code declares that engine ( or 'compression') braking is a mistake. He cites several reasons, one of which is the increased wear on pistons and the crank.

 

I admit, I love to row up and down the box and hear the song. Does it really eat up the motor? Doesit eat up the clutch?

[Baldini]

Docc,

 

There's a saying brakes are cheaper than gearboxes -

But they ain't so much fun unless your racing.

Brakes are for overtaking.

I use engine braking all the time always have done & I ain't gonna change now - so you maybe pay for it over a lot of miles - if you use it it's bound to wear out. Keith Code's book is good but don't take it too seriously - mostly he's talking about racing situations/laptimes & that's just different to the road. A lot of stuff he says is applicable but some of it not.

 

Stick to the way you like to ride but let anything the book says maybe inform you.

 

KB,Cymru

[Steve G.]

So engine braking is not supposed to be used in race conditions? Well, I'm not a racer, so can't confirm, but there has to be a reason why ALL makers of repliracers these days have special limited backslip clutches to allow more controlled engine braking.

On the street, engine braking is an integral part of the whole package, and I think you will be a faster rider as you keep the engine up on proper song.

Ciao, Steve G.

[pete roper]

I've never heard such utter piffle!

 

Loads on any of the engine componentry that is affected is much less on the over-run than it is under load. Some pistons are designed with a side that is more cabable of taking thrust and this is ALWAYS the side that will be taking the power loadings, not the compression loadings. Guzzi pistons are not usually, to the best of my knowledge, thrust face critical.

 

As for other parts? Oil pressure is constant dependent on revs. It doesn't matter if the motor is under load or not. Gearboxes on the other hand have to cope with power loadings and compression loadings and I think that it will be obvious to everyone who has looked at a helically cut Guzzi five speeder that the shaft that floats in these boxes only has one thrust bearingand that is for taking end thrust under load. at the other end it has a weedy little single row ballrace to take the thrust on over-run.

 

With a racing two-stroke there is bugger-all engine braking anyway but it is a bad idea to spend too much time with the throttle closed fro the simple reason that when the throtle is closed the crank bearings and piston will be getting insufficient lubrication. With any pressure fed four stoke it simply isn't an issue!

 

Pete

[Steve G.]

I forgot to mention my thoughts on the engine wear thing. I don't agree with the theory that engine braking will wear out the engine sooner. The only thing that will wear out quicker maybe, is the clutch, and very possibly, the crank thrust washers, but this will take years upon years.

Ciao, Steve G.

[emry]

but there has to be a reason why ALL makers of repliracers these days have special limited backslip clutches to allow more controlled engine braking.

 

You nailed it. The clutches allow for more controlled engine braking by allowing the clutch to slip under decelleration so that the rear wheel does not begin to slip and hop. Remeber when most racers close the throttle they are near red line. Engine braking can easiily over power rear wheel traction when all of the weight is shifted forward. The fancier clutches, even have different springs and ramps that can be changed to vary the amount of load before the clutch begins to sliip. Some racer like Tom Kipp were known to like no engine braking!!! He would downshift from 5 to 2 and then just let go of the clutch going into a turn, letting the slipper take car of the rest.

[Lex]

You nailed it. The clutches allow for more controlled engine braking by allowing the clutch to slip under deceleration so that the rear wheel does not begin to slip and hop.

 

Ah, guys, I think you have that backwards. The slipper clutch pretty much eliminates engine braking (not for reasons' wear, I have to agree with Pete and Steve, that is "piffle") but for a couple of reasons. They need to keep some link between the rear wheel and the engine (so you can down shift again) but I understand it is absolutely the minimum they can arrange.

 

The first reason, and I think most important, is that under heavy braking most racing motorcycles are carrying about 99.9% of their weight on the front wheel. Any resistance to rolling would lock the rear wheel. This takes away one of the two "gyroscopes" that make motorcycles work. Also, when the rider starts to ease off the front brake and lean the bike into a corner (generally still braking harder that we do on the street at any time) it would not be good to have the real wheel stopped.

 

The second is that the rider is VERY heavily task loaded in the braking zone. By not having to think about engine braking he can downshift early without either locking the rear wheel or sending the revs out of sight. That leaves him free to think about traction, brake markers, other riders, corner lines, etc.

 

Based on some things I pulled out of one of Kevin Cameron's articles I think the MotoGP bike are so sophisticated they have gone past just slipper clutches. Valentino Rossi said something like "we would need a different setting (amount of slip, I assume) in each gear, you can't do that with just a slipper clutch". The MotoGP bike are bringing the FI into the picture, adding just enough fuel and air to counter the friction in the engine (i.e. engine braking) at a given RPM. Cameron calls it a "throttle kicker".

 

Groeten,

 

Lex

[Guest Fonzarelli]

There is actually some truth to engine braking being hard on the engine.

 

It is actually better for an engine (connecting rods and pins) to be under a compressive load than a tensile load. This would mean under throttle. The connecting rods are much stronger in a compressed state versus a tensile state.

 

A tensile state is what is seen when an engine is wound up an then allowed to decelerate under a closed throttle. During this closed throttle, there is very little (if any) compression stroke or power stroke and certainly much less exhaust to expell so there is no opposing force on the other end of the piston keeping the tensile loads in check.

 

Under very high RPM's, this is a very high stress on the engines reciprocating parts.

[pete roper]

Which would mean that rods would break on the over-run rather than under load I think not! Generally rods will let go at high RPM and under power. It's the accelerative and decelerative forces that will do the damage. (If the rod is inadequately designed or has some sort of stress raiser in it.

 

Rods usually separate just bellow the little end eye simply because in most cases this is the weakest part of the rod. Once the operator is off the throttle the motor will start decellerating immediately and stresses will be reduced so unless you are very unlucky and the rod had already started to separate it is highly UNLIKELY that it will actually fail on the over-run. It's important to remember that things are happening VERY fast inside your engine, at 6,000RPM the piston is being asked to stop dead and accelerate in the opposite direction 200 times a second, the rod on't just snap in 1/200ths of a second, it will happen over a number of cycles but since load is heavier when the piston is being forced down by 100 atmospheres of pressure on the power stroke than when it is simply fighting a low gas pressure in the cylinder on the over-run and the only slightly over 1 atmosphere in the crankcase if it's going to snap it'll snap under power!

 

If you want evidence of how ineffectual engine braking is try rolling off the throttle on a steepish hill. You'll find that with the throttle closed the braking will be there, but not particularly strong. Now try hitting the kill switch and opening the throttle to WFO. The braking will increase markedly because now the piston is having to compress a lot more gas than it does if the throttle is closed. Note, if you try this DON"T then whip the kill switch back on with the throttle still fully open unless you want to risk serious damage or fall off the back!

 

Pete

[Guest Fonzarelli]

Which would mean that rods would break on the over-run rather than under load   I think not!

Yes, thats exactly what I mean. The G load on the connecting rod is far greater on the direction change with NO pressure in the cylinder. The connecting rod is at it's weakest under TENSILE load, when the engine is under a high RPM NO throttle condition.

 

When the piston is trying to change directions at TDC (or BDC for that matter) the combustion event, compression event or even the exhaust event helps to lower the G load seen by the connecting rod.

[Guest Fonzarelli]

It's important to remember that things are happening VERY fast inside your engine, at 6,000RPM the piston is being asked to stop dead and accelerate in the opposite direction 200 times a second, the rod on't just snap in 1/200ths of a second, it will happen over a number of cycles but since load is heavier when the piston is being forced down by 100 atmospheres of pressure on the power stroke than when it is simply fighting a low gas pressure in the cylinder on the over-run and the only slightly over 1 atmosphere in the crankcase if it's going to snap it'll snap under power!

 

Pete

But, the extreme pressure created by combustion is channeled through the rod and into the crank where it is used to drive the motorcycle. The energy seen under a high RPM no throttle decelereation has to be absorbed by the rod, piston pin, and crank in order to change the direction of the piston.

 

It is this extensive, tensile loading that breaks rod bolts and rods.

[jrt]

Note, if you try this DON"T then whip the kill switch back on with the throttle still fully open unless you want to risk serious damage or fall off the back!

 

Pete

Well, I guess I know what I'm doing this weekend!

[Lex]

But, the extreme pressure created by combustion is channeled through the rod and into the crank where it is used to drive the motorcycle. The energy seen under a high RPM no throttle decelereation has to be absorbed by the rod, piston pin, and crank in order to change the direction of the piston.

 

Fonzi,

 

As far a I know you have all your physics correct but I think what you are forgetting is that the piston takes the incredible stress of being yanked from TDC every other cycle regardless of the position of the throttle. These are four strokes, the piston comes to TDC against very little resistance on the breathing cycle. Closing the throttle may increase the number of times the piston comes to TDC with little/ no load but it doesn't increase the peak stress.

 

Cheers,

 

Lex

[Guest Fonzarelli]

These are four strokes, the piston comes to TDC against very little resistance on the breathing cycle. Closing the throttle may increase the number of times the piston comes to TDC with little/ no load but it doesn't increase the peak stress.

 

Cheers,

 

Lex

Please also keep in mind that during the initiation of the intake stroke, the piston is pushing exhaust from the cylinder, so it does see some loading.

[Motomonster]

I used to do a lot of engine braking. Then I took the advanced safety course and they talked me out of it. Now I brake to decelerate or pull in the clutch and slow naturally. THe only time I use engine braking now is when I'm going downhill and keeping my rpm's low to avoid speeding up due to gravity.