Jabiru Gen 4

[anjum_jabiru]

Any reason why one of the cylinders would lose compression in  mid-flight?

[onetrack]

The last engine I had reconditioned (a few months ago - a forklift engine) the reconditioner wanted all the pistons to check for accuracy in dimensions, and to hone each cylinder specifically to each piston.

[RFguy]

Interesting history/info , Nev. 
I try to stay away from turning hot props. Bruce, how long has  the engine been shutdown for when you find it stiff to turn ?

One for the crankcase rebuild- place inspection ports so infra red cameras / sensors can be poked inside ....

 

[turboplanner]

39 minutes ago, anjum_jabiru said:

Any reason why one of the cylinders would lose compression in  mid-flight?

Several.

Engine running lean, combustion chamber (1000 - 1500 deg) temp rises and softens a valve stem. Valve tilts sightly and you've lost compression. It can then smack itself flat if the Combustion temperature is still high, tilt more and bash the seat on one side, snap the stem and cruise around bouncing on top of the piston, embedding itself in the top or taking out part of the top, or again with the combustion chamber too hot (usually running too lean) soften (burn) the exhaust valve eroding material off it until it leaks like a seive.  I think I've managed to achieve all of those things.

[RFguy]

overheated valves (as above) 

stuck ring.

hole in the crown. crack.

recession of bore sleeve into head follwed by cool down and shrink and loss of compression due to lack of head seal.

 

ha ha how many do ya want.

Edited April 15, 2021 by RFguy

[facthunter]

Usual  reason to lose compression would normally be a stuck valve, almost always the exhaust. OR the head broken right off. With the last you will most times get secondary damage to the head and piston crown.   Inlets are relatively trouble free. Cause,   grunge build up on the stem OR a part seizure. Crook oil (too much additive) or not enough running clearance respectively. IF you had stuck rings there would be a lot of blow by and also noises from the high pressure gas  blasting past the rings. (If this happens in a diesel you'd think a bearing was gone, it's so loud). Nev

[RFguy]

head come off ? !!!$#$%^#%^! 

[jetboy]

Regarding engine stiffness after running, which I still have a measureable amount of, for earlier engines this was caused by design of the cylinder base flange and is corrwected by AVDAL SR050 attached below.

 

My 2200 was built 2003, has the slotted piston crown and seems to stick the rings in the portion of their lands at midspan of the open slot areas below. 

 

A common cause of compression loss and valve damage can be head overheat leading to valveseats loosening and falling out then of course the valve gets jammed or hammered back

Synopsis of an Engine Failure.pdf 2200a burnt pistons.pdf AVDALSR050-1.pdf

[RFguy]

Oh this is gold . Nice post, Jetboy.

 

In "Synopsis"...... "See Figure 1. RPM history, from takeoff to landing. Max RPM 3330."

 

Oil is up to operating temp, CHTs have stabilized, and engine goes to maximum stress conditions - engine max RPM and WOT. 

 

"Burned pistons". well, nasty...

 

 

 

 

 

 

[turboplanner]

In addition to the varying temperature phases and their effects on pistons above, there are other factors controlling those temperatures.

The compression stroke is to compress the volume of charged gas trapped when the intake and exhaust valves are close. A byproduct of compression is heat, so the piston top gets a charge of heat.

 

Then you get the heat of combustion.

 

The expansion stroke reduces the pressure in the chamber, and a byproduct of pressure reduction is cooling (a refrigerator works by releasing gas through a valve over and over again).

 

So the piston top is rapidly cooled again.

 

This is overlaid by the efficiency of the intake system and the exhaust system.

 

Manufacturers have to build to a commercial price, so if you have the skill and patience it's not hard to double the power output of an engine.

 

Fitting an Intercooler cools the slug of air ready to go into the combustion chamber. This gives it a smaller volume, so on the intake stroke. More mixture enters the chamber, so the expansion is greater poducing more power.

 

Phil Irving was the early genius using better breathing to produce more power. If you visualise the breathing of a 4 stroke engine, a schematic might show a series of sausages or slugs of air/mixed air/exhaust flowing through the engine when it is running. These slugs reach sonic speeds, but when the valves close for the compression stroke the incoming charge slams into the closed valve and a sonic wave bounces back, taking the column of mixture/air with it. This can be back out into the plenum chamber. If we squeeze the plenum chamber in various places we can minimise the "waiting" volume of mixture and make it equally available to all cylinders. Or we can take the next step and fit separate intake pipes. These contain the slugs as they form. when the sonic wave bounces back it reaches a point where the intake cycle sucks the mixture back in also at sonic speed. The momentum creates a point in the tube where, if the carburettor is fitted at the turnaround length, when the intake needs a charge, it is ready to come down the pipe at sonic speed, and charges the cylinder with a lot more mixture, which when trapped, produces a lot more power. The exhaust is now the remaining blockage in the system, but the moving slugs of gas are also having a sonic effect at the exhaust where the sonic waves are going out, but then coming back in where they collide with the next outgoing wave. If the exhaust pipe is shortened to the point that the outgoing sonic wave exits the pipe instead of coming back in and slowing down the flow, then the exhaust gas can expand much faster, and as we know, expansion means cooling. 

You can understand from all these variations in pressure and temperature why EGTs don't give you much more than broad information after it's all happened.

If you transfer this to an aero engine, you can see we don't have enough space for precision intake, or multiple exhausts, but we still have a lot of options with mixture, plenum and partial exhaust work, the aim not being to produce more power, but to lower combustion temperature.

 

 

 

 

[skippydiesel]

Turbo me old  mate - Bit hard to follow -Lot of non engineering words to confuse matters - However I get your gist.

 

The "expansion" stroke usually referred to as the power/combustion stroke, does not reduce pressure in the chamber - if it did you wouldn't get much in the way of power being delivered. 

 

Intercoolers, sometimes called after coolers, pretty much only apply to exhaust turbo (possibly super) charged engines.

 

Forced air (turbo/super charged) engines do not "give a smaller volume" of intake air - they cool the air, allowing for a higher density/oxygen content. This  allows for more fuel to be burnt. Can also assist with scavenging and cleaner/less polluting combustion.

 

As for inlet/exhaust optimisation on aero engines - in general these "tuning" techniques work better on high rpm engines. eg Rotax 2 & 4 strokes (land based vehicles). Slow revving LyCons will get some benefit but it will be minimal.

 

The whole consent is known as Volumetric Efficiency - describing the ability of an engine to process the fuel air mixture.

 

 

[facthunter]

Gas in the ports doesn't reach sonic speeds, it's the pressure waves that do and that's what you can tune to with varying lengths.. Similar to waves on the surface of the ocean  where the wave speed has nothing much to do with the current  movement in the water underneath.. Nev

[turboplanner]

1 hour ago, skippydiesel said:

Turbo me old  mate - Bit hard to follow -Lot of non engineering words to confuse matters - However I get your gist.

The non-engineering words were to make it simple for more people to understand the principle. I wrote it on the run from memory because in about a decade of discussions on this subject these critical components of the engine cycle have been ignored, so we go round and round in cicles without a solution. By factoring them in, there can be an understanding that perhaps things like carburettor main jet size to suit what's going on might be very important. 

 

You can get a fully drafted and edited version in engineering terms by buying the Phil Irving book "Tuning for Speed", this morning selling for $2069.00 in hard copy.

 

Edited April 16, 2021 by turboplanner

[RFguy]

Turbs, WHen I was a lad (18) I experimented with asymmetrical constrictions (IE different resistance out than in)  in inlet ram tubes to change the behaviour of the inlet any in prescence of the reflection wave from the closing- closed inlet valve.

 

It ended up being VERY dependent on gas velocity , and had a very small sweet spot.

 

Going to much larger inlet open time  reduced the reflection pressure wave ALOT.

 

IE the inlet gas velocity was slowed down by the action of the slowing and reversed direction piston as it got through BDC in intake stroke, with the inlet open way into the compression phase from BDC to TDC.  that was no good for low RPM.....

 

golly the things you do when ya young and have plenty of time.

 

 

 

 

[turboplanner]

1 hour ago, skippydiesel said:

Turbo me old  mate - Bit hard to follow -Lot of non engineering words to confuse matters - However I get your gist.

 

The "expansion" stroke usually referred to as the power/combustion stroke, does not reduce pressure in the chamber - if it did you wouldn't get much in the way of power being delivered. 

 

Intercoolers, sometimes called after coolers, pretty much only apply to exhaust turbo (possibly super) charged engines.

 

Forced air (turbo/super charged) engines do not "give a smaller volume" of intake air - they cool the air, allowing for a higher density/oxygen content. This  allows for more fuel to be burnt. Can also assist with scavenging and cleaner/less polluting combustion.

 

As for inlet/exhaust optimisation on aero engines - in general these "tuning" techniques work better on high rpm engines. eg Rotax 2 & 4 strokes (land based vehicles). Slow revving LyCons will get some benefit but it will be minimal.

 

The whole consent is known as Volumetric Efficiency - describing the ability of an engine to process the fuel air mixture.

 

 

The Power stroke starts with the gas mixture confined to a tiny space above the piston. As the piston travels down, the gas expands and the byproduct of expansion is cooling.                                                                                                Intercoolers and aftercoolers are two different products. The Intercooler cools the air before it reaches the turbo pump section and the Aftercooler cools the air after it comes out of the turbo blower. I was just explaining the principle that by cooling the air you can stuff more in during the compression stroke.                                                      The principles I discussed work for all ICE engines at all rpm. You can tell an engine builder knows what he is doing by listening to the engine ticking over at 500 rpm.                                                                                                              Although I did mention power increase due to better breathing, the aspect of what I was discussing here relates to combustion process and it's various temperatures, but high and low which overlay the other piston temperatures discussed in earlier post; it's a bigger moving target and you can change the piston temperatures by allowing and engine to breath in better and breath out better (non-technical terms I know), and it makes a lot of difference if through using a plenum intake chamber, one cylinder breathes better than another, and the same with a common exhaust manifold.                                                           

[turboplanner]

1 hour ago, facthunter said:

Gas in the ports doesn't reach sonic speeds, it's the pressure waves that do and that's what you can tune to with varying lengths.. Similar to waves on the surface of the ocean  where the wave speed has nothing much to do with the current  movement in the water underneath.. Nev

Yes, I just rushed the story, tryingf to use sausages to visualise that the flow through an engine was like a series of sausages starting/stopping/starting etc.

[turboplanner]

19 minutes ago, RFguy said:

Turbs, WHen I was a lad (18) I experimented with asymmetrical constrictions (IE different resistance out than in)  in inlet ram tubes to change the behaviour of the inlet any in prescence of the reflection wave from the closing- closed inlet valve.

 

It ended up being VERY dependent on gas velocity , and had a very small sweet spot.

 

Going to much larger inlet open time  reduced the reflection pressure wave ALOT.

 

IE the inlet gas velocity was slowed down by the action of the slowing and reversed direction piston as it got through BDC in intake stroke, with the inlet open way into the compression phase from BDC to TDC.  that was no good for low RPM.....

 

golly the things you do when ya young and have plenty of time.

 

 

 

 

I was just focusing on how the movement and expansion/compression of the gas during the cycle changes to end temperatures dramatically and is an intergral part of finding how to optimise aircraft engine cooling. 

 

Valve timing and inlet exhaust diameters with carburetted, naturally-aspirated engines is a fascinating experience in itself. On one engine I was using over 180 degrees overlap. It didn't work too well below about 4000 rpm and at times would backfire and spit carbies off, but the power band increase at 8,500 was explosive - a very tiny sweet spot. If you got ahead you could usually place in the top three, but if you got into traffic on a corner and forgot that sweet spot, the car would do a 180 instantly.

 

There are optimums for intake pipe diameter too. People buy the biggest throat carbies they can and are worse off than they were with throats half the size.

[facthunter]

Whenever gas expands, it will lose heat. P1V1/T1= K There's still a lot of pressure when the exhaust valve opens especially at high power settings and your EGT's will be around 1100 C  Nev

Edited April 16, 2021 by facthunter

[skippydiesel]

1 hour ago, turboplanner said:

 

 

 

1 hour ago, turboplanner said:

 

1 hour ago, turboplanner said:

The Power stroke starts with the gas mixture confined to a tiny space above the piston.

 

The fuel air mixture is certainly "confined" however this is a result of being inside a rigid cylinder with one end (piston) reducing the volume (pressurising the air fuel mixture. The ignition of the air fuel mixture initiates a burn (expanding gas) which drives down the piston delivering the power stroke.

 

As the piston travels down, the gas expands and the byproduct of expansion is cooling.   

 

Noooo ! - the gas expands because its burning (hot hot) any cooling, is completely overcome by the fuel/air mixture burning at high speed 

 

 Intercoolers and aftercoolers are two different products. The Intercooler cools the air before it reaches the turbo pump section and the Aftercooler cools the air after it comes out of the turbo blower.

 

The air coolers in a boosted engine, will always operate after the  air "pump"  (usually an exhaust powered turbine but could also be a belt /gear/chain powered screw pump). The combination of pressurising the air and in a turbo charger the heat from the exhaust side of the system, raises the air temperature & reduces density . Lower density = less O2/M3. To recover some of the lost efficiency, an air cooler (usually air to air but can be an air to liquid) i s used between the turbo and the inlet of the engine.

 

I was just explaining the principle that by cooling the air you can stuff more in during the compression stroke.

 

In a 4 stroke the air enters the combustion chamber during the inlet stroke not during the compression stroke. In a 2 stroke the air enters during the later stages of the power/erly stages of the compression stroke.

 

The principles I discussed work for all ICE engines at all rpm. You can tell an engine builder knows what he is doing by listening to the engine ticking over at 500 rpm.    Although I did mention power increase due to better breathing, the aspect of what I was discussing here relates to combustion process and it's various temperatures, but high and low which overlay the other piston temperatures discussed in earlier post; it's a bigger moving target and you can change the piston temperatures by allowing and engine to breath in better and breath out better (non-technical terms I know), and it makes a lot of difference if through using a plenum intake chamber, one cylinder breathes better than another, and the same with a common exhaust manifold.

 

You have lost me here - Yes engines will be able to burn more fuel, delivering more power, for a given capacity, if they can "breath" better. Your plenum chamber idea ?? - In my limited experience the plenum chamber is a "box/large duct" that contains filtered air (colud be cooled &  pressurised), that then "feeds" into the carburetors/inlet manifold (injected fuel system). The plenum does not in itself do anything much.

 

1 hour ago, turboplanner said:

 

                                                 

 

1 hour ago, turboplanner said:

 

                                                           

 

[RFguy]

All that aside

BRUCE

I think jts good you extended the air flow to the base of the cylinders, give them every bit of chance to get heat out. 

My guess is the top and sides of the bores are doing all the cooling work they have some airflow, with the bottom side staying hot and not doing much cooling work, and perhaps lending itself to a hot side of the piston due to a lack of cooling

. Let's see. even side, skirt thrust is on.. the top . Odd side, skirt thrust is on the bottom. I think that's right . 

 

As is said, different shapes of pistons lend themselves to different cooling performance ...Does anyone know if the Gen4 solid skirt pistons are round at cold  ?

 

Short of keeping the bores cool and hopefully having good piston-bore cooling contact area (shape , clearance) , a jet of oil from inside is the only good option I can see to get piston temps down (apart from suitable mixture richness and possibly max power limits imposed). If clearances are excessive, piston temp may be excessive due to lack of contact area with the bore case. By the time the piston gets big enough that it is having good contact area, it may be bl00dy hot...  (IE it may reach soem equilibrium- IE expand until the heat can get away) . they are my guesses.

 

Given the Buick V6 holden engine is cast iron block (water jacket), the piston expansion problem can't be too great in the steel cylinder jab engines, as long as the pistons are not allowed to get too hot. As the Aluminium expands at almost twice the rate.

 

In a modern vehicle engine, , like the Buick V6, do the undersides of the pistons get a good splash, or jetted ?

 

 

Edited April 16, 2021 by RFguy

[facthunter]

No they aren't jetted . Oil will cool but you need a lot of it and in excess it will cause loss through the breather and be a bit of a brake on the engine. Slotted pistons depend on a temperature difference between crown and skirt.The cylinder getting to the temps they do creates a difficulty there. Those pistons are compromised in an aircooled motor, particularly when the whole thing is quite hot and alloy is expanding much more than steel and it's shut down. nev

[RFguy]

OK on that.

 

In terms of reducing piston temps, for this engine, any other suggestions ?

 

Worthwhile  for cruise/takeoff  to use some EGR and or varying timing ?

 

I know that advancing timing reduces EGTs but increases cylinder pressure and temps.

retarding timing increases EGT, and decreases the other two, very roughly....

 

EGR  does need to be servoed pretty well, looking at all the charts I read, but I cant help thinking that  you might as well just put a throttle early stop (not quite wide open) or limit RPM and acheive similar.

-glen

 

 

[facthunter]

Piston temps will be helped if you don't use slotted pistons and reduce the ovality to about .003" Your skirt clearance will have to be determined and will be a lot more that with the slotted piston, when cold. IF you get it right it will be very little when hot but must always be SOME or she will lock up. It will also need some taper. .Nev

[RFguy]

It seems that, getting clearances right  and the oval, slotted  piston just the right fit in the bore at some workload/temperature is rather tricky indeed. Not to be guessed.

 

If the cold oval pistons are round at just a small range of operating temperatures, where the expansion has provided the correct fit in the bore for cooling, and not too tight during conditions that deviate slightly from the optimal, this is very tricky indeed (from my POV).

Too much clearance , poor cooling. IE small contact area, small heat transfer area ...

That is : IE then piston expands to fit to where the bore removes heat. But too much clearance where might mean the piston is red hot by the time it gets there.  Then another problem, if the piston temp is far over the designed temp where the ovality is very low, (let's call that round), then the expansion will keep on going, with the minor axis of the oval (ellipse ) now larger than the cold major axis. uh oh.

 

I am amazed the concept works  and can only summize that the oil film of varying thickness  takes up the slack, as do the rings.

 

 

 

 

 

[facthunter]

What I covered above  and heat a piston to about 220 on the crown  top with a strong gas flame gives an indication of likely expansion..  From all the data I have had  and Pistons I've made. (No failures) you'll need about .010" at the bottom of the skirt. (the usual place to measure it). This is with a solid piston.. Expect the motor to be noisy cold. Nev