Another Jab bites the dust.

[motzartmerv]

I realise its hard to judge from just one pic, but what are you saying could have been the cause?..Best guess??

 

 

[turboplanner]

Detonation caused either by an inlet manifold issue, or exhaust tuning issue.

 

 

[motzartmerv]

Ok thanx. I'll pass this info on.cheers

 

 

[Tomo]

Does anyone know if the through bolts that break are fairly consistent in location? Top, bottom, left, right etc... Almost makes me wonder if the thrust/force path of the piston isn't in line with the crank. So when it gets a detonation it puts a lot of side load on the barrel, cracking, then pulling the head off the bolt/nut.

 

 

[facthunter]

There are nearly always sideloads. The piston would only give equal loads on studs when the conrod is at TDC. The pushrods contribute too especially if one of the valve guides seizes up momentarily.. If the barrell crack is extensive the studs may remain intact and leave the flange attached to the case. . Barrel cracks may be due to fatigue or due to a single high load.. Nev.

 

 

[Tomo]

There are nearly always sideloads. The piston would only give equal loads on studs when the conrod is at TDC.

That's true... be interesting if they are similar through bolts breaking though wouldn't it.

 

 

[XAIRVTW]

Motz did this engine have the new 12 point nuts fitted? How many hours total time in service did the engine have?

 

Glad pilot & Pax were not hurt.

 

 

[facthunter]

It's quite likely Tomo. We should look for patterns of failures, and It might help to rectify the problem. Nev

 

 

[Hongie]

anyone know what the rod ratio is on this motor? Ie. if its a real short rod, this produces more sideloading on a piston. longer rods = equal less angle = less sideloading forces. Downside is a longer rod does put a bit more strain on the valve train.

 

just a thought

 

 

[facthunter]

They are short with a lot of angularity and consequently, sideload. This is done to keep the engine narrow, amongst other things. Nev

 

 

[turboplanner]

Many engines have high side loadings, but this just causes a bit more wear; think in terms of a force strong enough, that all of a sudden one day out of the blue it tries to wrench the engine apart - that's a force of tonnes!

 

 

[corvairkr]

Just thinking out loud guys I am a little confused here, it seems I have got my wires crossed or some previous posters have got there’s crossed in regards to detonation.

 

 

it has been explained in this thread as basically a huge force pushing down on the rising piston …I always believed this to be a result of pre-ignition and results in instant destruction ? pre-ignition is caused by a red hot carbon ember or other hot spot igniting the charge as the piston is rising.

 

 

And detonation as I understood it was the spontaneous detonation of the remaining unburnt fuel/air charge or “end gas “as it’s referred, under high engine load.

 

 

As I understand it depending on the size of the engine detonation in itself isn’t necessarily instantly destructive as it happens after ignition as the piston is on its way down and the “pinging” you hear is actual structure of the engine resonating and vibrating as a result of the sharp pressure increase of the end gas detonating , basically the larger the engine the more resistant it is to detonation, to a point.

 

 

That being said , in the relatively quiet confines of a car its easy to hear an engine detonating in an aircraft it’s a different story pretty much the only way to tell if you have a cylinder detonating or about to is close monitoring of cht and egt on each cylinder.

 

 

My question.............is this scenario plausible

 

 

Given that some Jabirus may have cooling issues (a factor for detonation) and Jabiru have acknowledged engines are detonating , say an aircraft has been flying lands refuels , changes students whatever the engine is sitting there heat soaking in the cowl, restarts either taxis to the end of the strip or run up bay all the while temps are getting a little warmer ….line up temps are high but in the green open throttle engine is now drawing hotter than normal air from the cowl increasing the fuel/air charge temperature (another factor for detonation )

 

 

Halfway down the takeoff roll a cylinder starts detonating the pilot is unaware, aircraft rotates and begins climb out , temp in cylinder is reaching critical stage the spark plugs heat range is beyond capacity and starts to glow red hot, pre-ignition occurs with the resulting catastrophic failure of the engine .

 

 

 

Like I said I am just thinking out loud I just find it strange a through bolt can fail so quickly without any indication .

 

 

Jason

 

 

 

 

 

 

 

 

[Hongie]

detonation/pinging is 2 (or possibly more) flame fronts meeting within the combustion chamber. one ignited by the plug, the other from hot carbon deposits, or compression in an excessivly lean mixture, to low an OCTANE rating, or other such as advanced timing

 

 

[facthunter]

Your summation is pretty right jason. I don't thik the "rattle" comes from engine components. More likely an explosion instead of a progressive flame front. If you compress/heat a gas which has fuel and oxidant mixed it can ignite throughout (all at once). it's ready to go anytime.

 

Direct injection engines cannot do this as the fuel is not there ( at combustible ratios) till near TDC. hence they are immune from detonation. Nev

 

 

[frank marriott]

How much more interesting and informative in this thread after the crap of page 1 to 4 lost it run. I know I promised not to enter into this area but how informative the thread has become after [obviously I was wrong] but informative questions and posts being made.

 

I'll stop again now and only read as I know I will get hate mail from this basic comment.

 

I can state that Winsor is completely wrong and go back and read my many posts - I am not supporting anything because I own one, I have made made wrong choises in my life before and admit to them.

 

At this stage I am yet to be convinced about the invincible other engine that doesent fail? I know Lycomings and Contentials [i have had them do so whilst I was flying them] do but maybe they are inferior to the great non mentionable.

 

Anyway if pages 5 6 and 7 are any indication then this thread will be worth reading again

 

I'll be there on Saturday Ross and we can sort this out in person.

 

FrankM

 

 

[M61A1]

Can't say that I entirely agree with that. While aviation engines are indeed designed lighter (because they simply have to be to satisfy the performance vs weight equation) and do have more demand on them than the average car engine, they also frequently use higher quality components, and components subject to much more rigorous quality control procedures than automotive engines. It is rare for aviation engines to fail when they are treated and maintained well and have proper components fitted, considering the demands put on them. Now cut to my useless EF Falcon of many years ago, which was always regularly serviced, driven responsibly, and experienced a litany of costly failures throughout its life, the final one being its death knell.

I should probably clarify what I mean- Generally it's not uncommon these days to see car engines go 300 000 k's without any work other than oil , filter & plugs (cam belts if fitted), which would roughly equate to 3000 hrs, how many piston aero engines see that without work.

 

 

[Gnarly Gnu]

if someone offered me a ride in a jab powered aircraft, i think i would give it a miss!

C'mon be brave Hongie! Just imagine you are one of the Wright bros stepping out into the great unknown....

 

 

[Guest pookemon]

How much more interesting and informative in this thread after the crap of page 1 to 4 lost it run. I know I promised not to enter into this area but how informative the thread has become after [obviously I was wrong] but informative questions and posts being made.I'll stop again now and only read as I know I will get hate mail from this basic comment.

 

I can state that Winsor is completely wrong and go back and read my many posts - I am not supporting anything because I own one, I have made made wrong choises in my life before and admit to them.

 

At this stage I am yet to be convinced about the invincible other engine that doesent fail? I know Lycomings and Contentials [i have had them do so whilst I was flying them] do but maybe they are inferior to the great non mentionable.

 

Anyway if pages 5 6 and 7 are any indication then this thread will be worth reading again

 

I'll be there on Saturday Ross and we can sort this out in person.

 

FrankM

*sigh* - A pity that people aren't entitled to their opinions unless and engine has tried to kill them...

 

 

[deadstick]

Maint background on this engine:last top end was done at tbo (2 before that) due nil compression. Engine performed well post overhaul and passed runin retorques and oil analysis. At 100 hrs weeping vertigrease like black sludge was found around the base of all barrels, jabiru were contacted and advice was to check torque thru bolts, nil movement evident. This was relayed to owner and jabiru, with jabiru stating the leaks are within tolerance but that if not happy the barrells could be lifted and new sealant applied with the owner electing not to do so as he had a fear of disturbing the thru bolts and believed that it would happen again. At each, service check torques were carried out with nil movement (tq wrenches set and calibrated) things to note: no indications of detonation on plugs, all leak downs good, t's and p's all within limits, and nice mixture indications on plugs. The thru bolt that failed at the base of the nut has clear fret lines.

 

My opinion only: the shims under the barrels idea introduces another weak spot, it has sealant on both sides (not good for rigid torques) and also offers a heat sink for differing thermal expansion properties. The engine was still running when it landed, and pulling the prop through you still get 4 cylinders albeit 1 with a leak sound and slightly reduced compression. Not sure I support the detonation theory as the cast pistons are poor quality and more of a weak link, I lean towards a resonance issue with these bolts. JMHO

 

 

[damkia]

I should probably clarify what I mean- Generally it's not uncommon these days to see car engines go 300 000 k's without any work ....

Not at 75-100% power continuously though....

 

As previously noted most automotive engines are lucky to see one to two short bursts of WOT in a day. More work = more wear.

 

 

[deadstick]

Direct drive aero engines are designed as low stressed, high torque, low rpm and generally speaking 75% equates to 2600 rpm. These engines are designed for the aero environment and would not perform well in a car.

 

 

[turboplanner]

OK then deadstick, how about Briggs and Stratton stationary engines on elevators where life cycle exceeds 1000 hours?

 

 

[turboplanner]

How much more interesting and informative in this thread after the crap of page 1 to 4 lost it run.At this stage I am yet to be convinced about the invincible other engine that doesent fail? I know Lycomings and Contentials [i have had them do so whilst I was flying them] do but maybe they are inferior to the great non mentionable.

 

I'll be there on Saturday Ross and we can sort this out in person.

 

FrankM

You could have fooled me Frank.

 

Taking shots at the Rotax engine is just trying to divert attention away from the discussion.

 

Your statement that at least two Lycomings and at least two Continentals failed while you were flying them sounds far fetched to me. Maybe you'd like to tell us a bit about that amazing sequence.

 

This is not a pi$$ing competition, there is a serious underlying risk which needs to be addressed.

 

 

[Tomo]

OK then deadstick, how about Briggs and Stratton stationary engines on elevators where life cycle exceeds 1000 hours?

Ever tried lifting a BnS Tubs!

 

Aero engines have the disadvantage of operating in drastically changing conditions, with minimal strength of components just to last till TBO! I rebuilt a 454 Mack engine the other day, each cylinder head weighed about 70kg!

 

 

[turboplanner]

If you argue the power to weight ratio Tomo, a designer would slim the weight down until fractures started to occur, then beef it up again until he got better than 97% life cycle.

 

I've gone away from the idea that the thru bolts are too light after reading about the number of cracked cylinders when I checked the statistics. To me that's saying more.