crankcase air leaks

Can anyone explain this to me
with a crankcase air leak engines will probably seize when highly reved.. is that true? usually a piston seize... is that true??... what is the seizure specifically caused by? the very lean a/f mixture? major heat produced by lean a/f mixture or no lubrication around the piston?

thanks.
started my bike up ... ran fine.. forgot to turn the fuel on.. no it didnt seize, but as the last of the fuel disappeared out the carb the engine rpm increased....
why?

I don't know the answer but its a good question!

Having seized a great number of pistons - and put holes in  piston crowns - I know seizures happen because of weak air-fuel mixture. With a 250 Special using a Gardner carb the engine ran perfectly at Brands on Wednesday with a Gardner Rep setting the carburation   yet three  days later at Snetterton it did as just stated above -- it seized at full bore on the (Old) Norwich straight and put a hole through the piston crown.
    Why should that be?
Differences of circuit,  ambient temperatures and height above sea-level are small -- opinion was that the (Old)  Norwich Straight was longer than any straight on the Brands Club-Practice circuit and time for  heat build up greater.  

Point is the engine goes at its best when running very close to its Lean A/F ratio limit... There are - now -  engine-fuel-injection  management systems using microprocesser control  which monitor the combustion chamber & exhaust parameters and richen the mixture when the temperatures go above set limits so the racer gets the best out of his motor with relaibility..  

But when you say "crankcase leaks" I visualise the crankshaft seals and if they leak the performance  suffers sufficient  -- I think -- that  seiure  would be doubtful -- yet still possible. If the leak is in the transfer-port areas  there certainjly would be a definite A/F weakening and siezure a reality....  

Higher compression ratios requre use of high octane fuels (which NEED richening compared with lower CR ratios) according to the increass of compression ratio ....

Hope that is of some use....

Cheers!

I can't say I've ever suffered a piston seizure caused by a crankcase air leak, but if that's what happens, my guess would be that weakening of the mixture is the cause.

I've alway considered that two stokes produce more power on a mixture which is slightly weaker than is good for them. In effect they are petrol cooled, though I'm sure there is a far more erudite explanation.

I qualified for the final of a 250 race at Mallory Park, but my 250 DMW's ignition failed as I crossed the line at the end of the heat. A friend offered me his 250 Greeves "Silverstone" for the final. The Greeves went very well - until it seized on the exit from Gerrard's on about the third lap. My buddy was meticulous in setting the carburation, but that was for the pace at which he rode the bike. Without wishing to seem immodest, I was riding quite a bit quicker than the owner, reaching maximum revs earlier, and holding them for longer. The settings which worked fine for him, were too lean to sustain the quicker pace.

Pity- we were well placed.

Leak that was described on here -- long ago -- was at the barrel to crankcase joint which had the piston seize ....

Loosely could be called a crankcase air leak... and makes my earlier notion of power-loss look a little silly but apparently it was the case....

Cheers!

By crank case leaks are you specifically thinking joint leaks such as case half`s, case/cylinder base and/or oil seal/shaft leaks?
These examples immediately raise the spectre of drawing air into the engine and adversely influencing the air/fuel mixture ratio, producing less than perfect stoichiometry.
The ideal fuel is octane (C8H18) as it burns perfectly with air in the balanced equation called…the stoichiometric equation.




From the above equations it is easy to see where the ideal air/fuel ratio is derived from. For safe power in a 2t engine we need to have the mixture a touch richer, this ensures that every molecule of fuel mates with every available oxygen molecule, around 14.7:1. Therefore, in mass terms, we need 14.7 kg of air to completely burn 1kg of petrol at “Lambda-1”, but for complete safety with a stressed, hot running, air-cooled Bantam engine, Lambda 8.5-9 is more optimal! In such engines internal evaporation of fuel in a richer mixture reduces thermal loading and helps to stave off rapid overheating and possible piston seizure, and with Bantams, every little helps!

Most Bantam engines will be running a little rich on mainjet so a small amount of air being sucked in will do little harm. But when running right on the combined survivable limits of comp-ratio, ignition timing,  mixture and so on, then it doesn`t need much of a push to go over the edge! Common sense would suggest that you don`t put the engine in that situation in the first place? It is also worth remembering that fluctuating case pressures both suck and blow, so only a limited period of crank rotation creates the conditions to be able draw air any way. I well remember when competing at Snetterton decades ago, the annular spring on the timing side oil seal failed and chewed up the seal lip. There was no difference in engine performance during that race that I noticed and only came to light when failing to re-start the engine for the next race. The bearing cage was spinning rapidly enough to form an effective seal during running, but with zero case pressure at the push re-start, it was a no-go!
Whilst there might be air being drawn into the case from a leak, there is no reduction in the oil deliverd via the carburettor so seizures from lack of oil are unlikely. Providing of course that you have the correct oil ratio in the first place and, a top quality race oil best suited to the Bantam engine, it won`t sieze. Extra heat input to the piston crown from a more efficient, complete combustion with a more oxygenated process might promote piston expansion and a seizure. But, those oxygen molecules have to have more fuel to react with and in most cases it is a rich mixture that leaves spare fuel available. Excessively weak mixtures, around 18:1,  have so little fuel to combust that power drops dramatically, and so heat production is also down! Compromise for reliability and longevity!
It is crank flywheel energy that keeps the engine turning over at tick-over level of running, that stituation requires only a tiny amount of fuel to maintain in overcoming static compression and friction. Carburettors need to have a reservoir of fuel to enable the engine to respond rapidly as the slide and needle are raised. This requires sufficient fuel to be available in instant response as engine revs rise, which in-turn means a larger fuel flow than tick-over requires. So with the fuel tap closed the fuel level in the float bowl drops in relation to the primary jetting, reducing excess fuel from entering the engine. So the slow running is nicely cleaned up in relation to the demands being made so it is this response that can be felt. Inclined carbs make this situation worse with the fuel level effectively rising in respect of the primary outlet, and in the worst cases fuel can, unaided, trickle into the engine. Lectron type carbs don`t suffer the inclination problems as there is no primary circuit in their design.
Decades back when we ran Amal TT carbs with a remote,in my case, SU float chamber it was standard practice to switch off the fuel supply and run the fuel level down untill the sweet running spot was achieved. Failure to do this could lead to flooding before the engine subsequently fired up. Mind you, the poor ignition spark-energy from the primative battery/coil/ contact breaker system didn`t help us one bit either.
Not sure if you questions have been satisfactorily answered Nigel, been given a go anyway, but someone might benefit as a consequence?

Trevor

Interesting that -- Trevor!

So there is a dynamic compression within the crankcase even though the cranklshaft sealing was kaputt! sort of suggests that at say 9,000 to 12,000 rpm the 150 to 200 crankcase-compressions per SECOND are too quick for the leak to happen which I can believe ...
... which would mean - if both sides' seals faile -- there´d be less friction drag from the seals ....

Someone must have measured the friction drag of C/S oil seals and I wonder how much that comes to in relation to piston and ring drag and engine friction amount overall....??

Hmmm -- I retire ....

John,
Some time back I did a lot of searching on-line for information regarding friction of garter seals and potential torque loss. Seal manufactures provide a lot of very general info but generally it promotes only their specific products. Different materials gave small differences in losses as did physical dimensions, running temperatures, lubrication, shaft surface finish and rigidity of the installation.
Labyrinth seals are the best from a reduced friction application but they also require some engineering to install and perhaps more lubrication than the average Bantam installation can provide. I did find some research stuff, SAE, laboratories and such like but none of the tests were conducted at the high, 10,000 or more rpm level of a racing two-stroke engine with the complications that comes in their wake. So on the basis that every one is in the same boat, I left well alone, and concentrated on making more power than trying not to lose what I had.

Trevor

Piston and ring(s) are major friction source of torque loss and I was in awe when I read of Maurice Quincey changing the ring (60thou thick) on the Walsh Bantam between practice and race and between races .... a lá Bill Lomas´s trip Down Under.
That Bantam had the "Norms" upside down: using alcohol fuel and high compression ratio it would still rev hish --on and on... Bill said in his article ....


I found it annoying to have to change the rear sprocket at Brands between practice and race and when running both Bantam and the 250 ABS ...

Interruption -- must close... Tcshuss!

The meter-maid dropped in to read the heating meter and in practicing her English kept calling me "Darling" so I´ve forgotten what it was I was on about....


PS -- I added this several hours later by editting  -- as a Last Word...

I have known piston seizures -- even with 4 stroke engines -- to occur with rapid shut off of throttle from a long period of  full bore -- several Star riders discovered that too late!

With a 2-stroke engine it could be sudden deprivation of oil combined with "Heat Sink"  which happens with 4-stroke motors -- Trevor might have the last word to add  -- after all!!

John,John, Trevor, thanks for the replies and the information.

I´ll repeat the bit I added as an edit up above...


In the old days (uhh!) when a race bike -- particularly a two-stroke -- which had been pushed hard was rapidly shut off it was very likely to seize ....

I can think of two well-known international/natioanl racers who died as a result of this....

I see nowadays some riders shutting off with such confidenece that they stand up on the footrests....


Is it the same reason -- why the reliabilty, regarding piston seizures is almost non-existent.

I achieved a good level of reliability by Running-In pistons, with Icarus-!, on Brands practice days. It was a matter of riffle-filing down the high-spots time and time again -- an agonising, painstaking pass-time...

Yes John, and sadly, too many good people and their machines have suffered grievously from the consequences of part-throttle engine seizures!
Full throttle, flat out scenarios usually are ok, but at part throttle case pressure is a lot lower but cylinder pressure is still high with exhaust flow still functioning. Hot cylinder gas reverse flows to the case and a mili-second later is drawn into the cylinder, and over heats the piston with obvious consequences. It only needs a few cycles of the engine to enable this exponential chain of events to take place.
KTM, when running their 125 bikes at GP level with Harald Bartol in charge, introduced a small fuel pump that fed mixture to the cylinder wall bellow the exhaust port to try to avoid this from happening. By combining the throttle position with revs and gear positions, sensors, linked via the engine management computer, messaged the pump and would be automatically activated. The down side was that winding the gas back on left the engine response a bit `wooly`.
It always seems counter-intuitive, but imagining that a case leak will suck air and go lean is not in most cases what really happens. Leaks compromise pumping, and the bigger the leak the less fuel/air mixture is pumped, thus less power is produced and temperatures drop accordingly! Take the leak far enough and the engine slows, then stops?
The picture speaks for its self, but interestingly also shows the two stage exhaust power valve in position in this one the last of the works barrels.

Trevor

This is the image that was meant to be posted but windows 10 seems to have a mind of it`s own at times, not convinced it was a good move to install, any one else had niggles with it?

Trevor

Thanks Trevor!

Still on Woíndows XP -- and suffering....

Trevor, i had windows 10 on a free download.. luckily i was able to remove it as i found the security aspect not to my liking. i think there is a 1month time scale within which you can  revert back  to windows 7..

John, i downloaded ubuntu 14.04 free operating system on my old windows xp laptop. i kept the xp to run along side it. i found that trying to set up the printer on ubuntu beyond my capability and used xp to do any printing/scanning i needed.

We've been using Windows 10 for almost three months. My Missus initiated the change from Windows 7, and chose the options, back in November. Our son, who works in IT, checked it out when he came home during the Xmas hols, tinkered with it a bit, and said it was all good.

No hassle, no problems.

Thanks Nigel but ubuntu sounds like something growing wild in Africa and I am faring not too badly at the moment ....

My computer expert reckoned my volume of Bits (or Bytes) is inadequate to fend off the high powered cyber advertisers who can hook onto such frail systems as mine and cause a stall by the security system trying to oppose entry ...

Something like that!!

Let´s get back to, "What is a crankcase leak?"

Cheers!

JayBee....