Jabiru Gen 4

[facthunter]

"Slipper"type pistonns and "W" and "T" slot types alter that temp pattern. Detonation also can result in the centre of the piston getting so hot it sags or gets a hole in it, because it melts. That fills the lower part of the engine with aluminium spray. Particularly a problem with 2 strokes requiring a complete strip of all antifriction bearings. Ball and roller.  Remember the piston top  is nearly 1/2 of the combustion chamber surface area and gets much hotter in close proximity to the  exhaust valve. Also most supercharged aero engines run extra oil jets to the piston undersides, as well as extra strengthening in other places. Nev

Edited April 12, 2021 by facthunter

[Old Koreelah]

5 hours ago, RFguy said:

Which means before goign crazy on cooling the bores all the way round, one has to be confident the piston temperature is down also, otherwise cooling the bores may have unwanted repercussions...

Now he tells me!
On the basis of this and other Jab engine discussions I recently enlarged my ram air ducts to cool the entire length of the cylinders- not just the top third.

I also fitted snug ducts all round the barrels.

At the same time, I re-directed oil cooler air out a separate exit under the cowl, expecting this to greatly increase “suction” of cooling air thru the top half of the engine.

Results have been disappointing: CHTs have barely changed, presumably because that air now has to spread out and cool more hot metal.

 
With no way to monitor temperatures of the steel barrels, I have no real way of knowing if I’m making progress. 

[RFguy]

Hi OK.

yeah . I still beleive cool the barrels. But it is tricky, relying on the steel to expand also and high piston temps to avoid the piston being oversize.

If you dont cool them at all, piston temp will go way up until they get too big for the bores. (really bad) 

if you super cool them, the piston temperature will be way down. lots of clearance.  (not great)

-lots of clearance though likely means larger oil path or air path and thus less hear transfer.. and hotter pistons...

Equilibrium will take care of that. IE the pistons may heat up to fill the bore and minimise the oil path ..until they get enough heat out.  But i in that case the pistons may not heat up in a nice even way. but out of my experience depth there with varying piston temps with varying shape and loads.

 

The steel bores cannot store as much energy, so they may cool down faster, that and they are exposed to the outside world.

That may go some way to explaining tight pistons on shutdown. maybe.

specific heat 4032 ally at 25C is 870J/kgK. specific heat of 4130 at 25C is 477. -hmm not as much a difference as I thought.  2x

The steel is a poor conductor compared to the Ally about 3x, so the heat may be more likely retained in the bores by the nice steel insulator jacket. educated guessing though ...

The oil film presence would I am sure affect such simple musings.

 

*needs that oil spray jet in the crank case to provide a 2nd means of piston cooling...*

 

How's your oil temperature now ? 

 

 

Edited April 12, 2021 by RFguy

[Old Koreelah]

2 hours ago, RFguy said:

...needs that oil spray jet in the crank case to provide a 2nd means of piston cooling...

RF that would be nice to have, but sounds like a big modification..

2 hours ago, RFguy said:

How's your oil temperature now ? 

Oil temps stable at 85C, with the outlet flap half closed.

 

Just got home from a camping trip off grid, so catching up on correspondence. Hope to fit the latest modification of my RH ram air duct on Wednesday. No.1 head is too close to the prop and since widening the ducts, I have to get more air in and rapidly slow it down before it changes direction down thru the fins. Will report on progress, but I hope it’s cooling well enough for my trip to the Coonabarabran spectacular this Saturday.

[onetrack]

Is there any reason you can't fit piston cooling jets to the Jab engine? Get-M Garage make piston cooling jet installation kits for the LS, BB and SB Chevy's, BB Mopar engines, and some Ford engines.

Watch the installation video on Youtoob and you'll see there's nothing really complex about their design and installation.

The only question might be, if there's enough "meat" in the bearing support region of the Jab block, to accommodate the squirters.

 

https://www.youtube.com/watch?v=9Cljwe1dkdc

 

 

[Bruce Tuncks]

A very interesting discussion rf and facthunter. Thanks.

Here's my guesses... My old 700 hour engine runs fine but the stiffening up is so marked when you hand-turn it hot that I think this cannot be happening when running or the engine would seize. Could it be that some internal heat-soaking is taking place after the engine stops? When taxying to the hangar after a flight, the cylinder head temps are all about 130C. I understand that an aluminium piston in a steel cylinder would tighten up on heating, actually your calculations rf show this to be less than I thought. But that heating pic of the piston shows a big amount of post-shutdown heat  movement from the piston top will happen.

[RFguy]

Hi Bruce

well my calcs did day there was a big bit of unknown variable which was how fast will the bores cool down...

 

If the pistons are 97.5mm starting (the assumption for this post)  and you are running 0.2 total clearance (book)  at 20C, the pistons when they get to  20 +102deg  = 122 degC they will be interference fit in the bores !

Of course the bores expand, but at a ratio of 65% for the same temperature, but of course they will be cooler (air flow)  and not expand as much.  

 

For pistons at say 200C, a likely 0 to 50% power piston temperature, the pistons will be 97.85mm .

this means the bores would need to be at least 97.85mm. If the bores started at 97.7, to be 97.85mm

 

The pistons could easily be 200C at shutdown time., which means the bores would need to be hot. 

How hot ?  They need to be 20 + 118 deg C  = 138 deg C to fit the pistons.

 

but this is rather more complex because of the thermal coupling between the piston , the oil film and the bores and the air !

The pistons when the cleanrance is low will , if the oil is doing the job, have better heat transfer to the bores, and cool faster.

 

So we have a issue of :

1) allowing the bores to get  hot to be big enough to accomodate hot pistons

2) but wanting to cool the bore casings to provide higher difference temps and hence better heat removal from the pistons

 

The top  of the piston, oil rings up,  from the modelling pics is alot hotter than the bottom of the skirts,

I'd need to see a thermographic picture to see what the heat is doing with the bores- ie hot end, cool end, etc

 

 

 

 

 

 

 

 

 

 

[facthunter]

None if this effect will occur with solid pistons. Make sure you fully comprehend how the "W" slot works (or doesn't).  When the whole piston gets hot is when the difficulty arises . Having the bore too cool won't happen. You will never get it as cool as liquid cooled one is and IF you did all would be well regarding the way  the piston works. Except they are weak.  Nev

[RFguy]

Nev, when shutdown, oil not splashing around, no more heat in the chamber, that is an opportunity for heat soak, which would produce a more homogonous temperature in the WHOLE piston.   Is that the where difficulty arises you refer to ?

[facthunter]

NO You are getting it wrong.  Look I'm likely to be flat out for most of the day I'll get back when I can. Read again what the process is. When the crown is hotter than the rest  the piston becomes less oval . so the diameter at maximum stays well below what it would be otherwise. The heat can't travel where the slots are as there's a gap.  so the crown is much hotter than the skirt. THAT is used to distort it. When the temp is equalised the distortion doesn't happen  to work against the expansion (of the skirt) that goes with being hot. Nev

Edited April 12, 2021 by facthunter

[RFguy]

got it

[RFguy]

Ovality : As I understand it, the ovality  on the thrust axis assists the piston not going too large and maintaining roundess at operating temp,  by the hot areas having the greatest room for expansion (perpendicular to the thrust axis where the slots in the crown to skirt are).

 

In the case of the shutdown engine , because the heat flow from the CC is not there, we have the opportunity for the piston temperature to become homogonous, and reach an equalibrium, because the high difference  temperatures caused by combustion heat are not present.  And that would make it more likely for the whole piston to expand - IE the hot goes into the cooler (major axis on the ovality) in order to acheive equilibrium.  Simultaneously, you have the bores cooling.

 

 

 

Edited April 13, 2021 by RFguy

[Mike Gearon]

This is something I had no idea about. Bloody interesting.

 

piston Design

Pistons are designed with features which perform specific functions during engine operation. The piston head or crown receives the majority of the initial pressure and force caused by the combustion process. The piston pin area is exposed to a significant amount of force due to rapid directional changes. It is also subjected to thermal expansion caused by the transfer of heat from the head to the body of the piston. The piston pin area is subject to more thermal expansion than other areas of the piston. This occurs from the thermal expansion properties of cast aluminum alloy and the mass in the piston pin area.

Some pistons are cast and machined at the factory into a cam ground (elliptical shape). An elliptical shape is an oval shape in which one-half is a mirror image of the other half. These piston shapes provide an advantage in conforming to the ever-changing dimensions of the cylinder bore. The piston is designed to be an elliptical shape when cold. As the engine reaches operating temperature, the piston pin bore area expands more than other thinner areas of the piston. At operating temperature, the piston shape becomes a circular shape, which matches the cylinder bore for improved sealing and combustion efficiency. 

Some pistons are designed with a taper, with the smallest diameter of the taper at the piston head. The taper shape compensates for thermal expansion and thermal growth. Thermal growth is the increase in size of a material when heated, with little or no change back to original dimensions. The taper design allows the piston to move freely in the cylinder bore regardless of the heat applied to the piston head.

Some Briggs & Stratton engines use a barrel-shaped piston skirt. The barrel shape provides a smoother transition during directional changes of the piston. The piston rolls into the cylinder wall when changing direction at the end of a stroke. This reduces noise, spreads the force of the directional change across a greater surface, and reduces side loading on the piston skirt.

Some piston designs have the piston pin offset from centre in the piston. The proper orientation of the piston pin offset is marked by a notch or an arrow on the piston head. The mark on all Briggs & Stratton pistons should be facing or closest to the flywheel on all one- and two-cylinder engines. The offset piston pin design offers a quieter running engine by reducing piston wobble and related noise. This results in truer linear movement of the piston in the cylinder bore.

http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Graphics/Piston%20Assembly.jpg

Figure 6- Piston Assembly

Each piston design must have a provision for returning oil to the oil reservoir and the crankcase. During operation, a significant amount of oil is accumulated in the piston oil ring groove. This oil is returned to the reservoir through piston windows or through a machined channel near the piston pin.

Piston windows are a series of small holes machined into the oil ring groove surface of the piston. The oil ring collects excess oil from the cylinder bore. Piston windows allow oil in the oil ring groove to drain into the oil reservoir. 

Another common method used to return oil to the oil reservoir is through a machined channel near the piston pin. Oil collects in the rear of the oil ring groove and is routed back to the oil reservoir through the channel ending at the piston pin. This provides a path for oil to return to the oil reservoir along the outside surface of the piston when the machined channel is exposed to the oil reservoir at BDC.

[RFguy]

the pictures of the Gen4 piston I have seen do not have the thermal slots under the crown (which promotes circular growth at operating temperature  from ovality at room temperature) But the ally bores expand much more than the steel ones for a particular temperature. (but they also have much bigger cooling fins and higher heat conductivity) 

 

 

 

 

[facthunter]

One reason to attend to the steel cylinder temps is Lubrication.  IF the wall goes BLUE it's not likely to have a satisfactory film of oil either and what about distortion from uneven heat.?  Blow by is a feature of these engines. 

 The Gen4  has attempted to address many of these issues .

 Sort conrods produce more side thrust also on the skirt of the piston  More reason for a solid design. Nev

[pmccarthy]

This has been very interesting to me both for aviation and motorcycle engines. Thanks everyone.

[Thruster88]

All the major manufacturers of air cooled steel barrel or liner engines such as Continental, Honda bike and industrial, VW and lycoming all use solid skirt pistons. Why?

[RFguy]

Is that a steel liner in aluminium heatsink air cooled jacket ?

[Thruster88]

10 minutes ago, RFguy said:

Is that a steel liner in aluminium heatsink air cooled jacket ?

Yes for the Honda 

[RFguy]

of course , the lack of a thermal slot could mean they just  have more webbing to the pin   .  (to control heat flow) 

its all relative. 

IE if the thermal resistance is 10 across the thermal slot, and the thermal resistance to the pin  via webbing is 2, the heat will go to the lower number,--- the webbing and pin-

 

if they take the slot away, so the thermal resistance to that path thrust side is lower and now  "5" , and they beef up the webbing so the thermal resistance into the webbing that attaches the pin (pin axis) is  now "1" , then the ratio of heat transfer will be the same as if the slot existed, approximately.  

 

 

 

 

 

 

 

Edited April 13, 2021 by RFguy

[facthunter]

Some pistons have webs to support the top and take load to the gudgeon pin boss.. Most are for strength and rigidity Short conrods are for reducing engines size. This gives more side thrust as a result requiring stronger design and also causes more mechanical noise . Steel sleeve (rare) has more lubrication problems than cast iron . Cast iron is like a fruitcake and holds oil in it's pores. Steel depends on the hone pattern being retained . Works better on a nitrided surface. Cast iron is more likely to keep it's dimensions than steel but  it's rare in aero engines being weak in tension  and consequently adds weight  Volkswagen is Cast Iron..Nev

Edited April 13, 2021 by facthunter

[RFguy]

anyway Bruce

my take on the phenonomen of stiff to turn over after shutdown is :

- cylinder bores contract

- pistons heat soaks , leading to more homogonous piston temperature,  expanding on axis more than it would usually in service. 

 

piston temp is somewhat proportional to total fuel-air charge per second (RPM x throttle ).

The numbers I find for piston temps for wet vehicle pistons are usually at 6000 rpm ish.

 

this is an interesting snippet : (wet vehicle bores)

 

Edited April 13, 2021 by RFguy

[facthunter]

Load and RPM (Frequency and intensity). Much the same as spark plug temps. react . The location of the exhaust valve will move the max temp location unless it's centrally located . (never happens in practice). Area of piston skirt in good contact with the cylinder wall will affect heat loss  rate and therefor temp attained. Slipper pistons or large ovality will reduce heat transfer. an oval piston only has line contact A slotted piston will transfer less heat from the crown than a solid one., because you have cut off the path over much of its skirt.Nev

[RFguy]

The other thing Bruce, as not all Jabs do it :

1) maybe it is an indication of an unhappy piston ? 

2) unusually  tight cold clearances. (particular combination of pistons/bores) 

 

at full soaked cold is it easy to turn?

 

[facthunter]

It's my understanding the bores were made for the maximum oversize of those pistons (plus.040"?) you couldn't rebore the cylinders and fit bigger ones. Early ones were made in class sizes of small increments and selectively assembled in the car engines various bores. In the later models of that engine this practice ceased and all pistons are "whatever they are" and so are the bores but based on nominal sizes.. The running clearance was made larger so that no engine could be too tight whatever was assembled in any combination.  This of course results in engines with generally looser running fits which are more mechanically noisy as they were found to be in practice.

  If you were assembling any motor with new pistons you would hone each cylinder to have the minimum fit specified (or any fit you might choose" and keep them paired once that is done.

  I can't imagine any time you would have a physically tight piston fit cold. Nev