Future small aircraft engines.Wich way?

[Niels Abildgaard]

Lohring

First picture is exhaust side.Very much car technology.Two slide bearings and divided conrod foot.Thrust bearing standard ball bearing.Lots of oil everywhere and some is cooling underside of piston.
Next picture is inlet side where crankcase pumping is part of scavenging system.Standard thrust ball bearing and a big roller bearing on crank disc.
Diameter 200 mm more or less and 2400 rpm gives a dn number of 480000.
Accepted value for steel rollers 600000 and 2000000 for ceramic rollers.

 

[lohring]

Very nice. An overhung crankshaft is a lot more tolerant of manufacturing imperfections and the all rolling element bearings don't need much oil. Two of the overlooked advantages of crankcase scavenged engines are connecting rod big end and piston cooling. High output engines need oil cooling of both without the fuel and oil flowing through the crankcase. That often burns more oil than needed for lubrication.

Even the small 26 cc engines I deal with are getting to the limits of their big end bearings. Life is limited at over 20,000 rpm as I found out. The blue overheated rod end is the first sign of a problem. The cage then fails allowing the needles to skew breaking the rod end. One solution is a high strength cage with two needles in each slot to carry the high loads.

Lohring Miller

 

[makoman1860]

Very nice. An overhung crankshaft is a lot more tolerant of manufacturing imperfections and the all rolling element bearings don't need much oil. Two of the overlooked advantages of crankcase scavenged engines are connecting rod big end and piston cooling. High output engines need oil cooling of both without the fuel and oil flowing through the crankcase. That often burns more oil than needed for lubrication.

Even the small 26 cc engines I deal with are getting to the limits of their big end bearings. Life is limited at over 20,000 rpm as I found out. The blue overheated rod end is the first sign of a problem. The cage then fails allowing the needles to skew breaking the rod end. One solution is a high strength cage with two needles in each slot to carry the high loads.

Lohring Miller

Lohring,
We spent a lot of time on cage and roller design. There are so many tiny details of the design and manufacturing of that type of system if you want it to live. I personally would never use a cantilever type crankshaft, too flexible.

 

[Niels Abildgaard]

A more conventional method of two stroke crank construction.

http://global.yamaha-motor.com/news/1999/0323/images/map1e.gif

It is same method as the two cylinder two stroke Trabant made in millions in DDR.

For my 105 160 mm version cold (inlet) side

 

[makoman1860]

That would seem much more reasonable of a design. If it were me I would eliminate the through hole in the crankpin if you plan on heat treating the crankshaft, unless you plan to hone/polish the hole. They tend to be sources of cracks. I may be a little confused by your drawing, how close is the main bearing to the crank cheek? It looks to be quite far away.

 

[lohring]

Lohring,
We spent a lot of time on cage and roller design. There are so many tiny details of the design and manufacturing of that type of system if you want it to live. I personally would never use a cantilever type crankshaft, too flexible.

I tend to agree especially as engines get larger. However, bearings on both sides don't necessarily make the crankshaft stiff enough. The crankshaft our race engines use comes from a weedeater engine that started out at around 2 hp 20 years ago. Its design (and I believe materials) are unchanged even though it's now used in engines that develop over 8 hp at a lot higher rpm. This crankshaft and a similar design used on a 35 cc engine we worked on flexes enough to cause flywheel strikes even with .015" (.4 mm) clearance to the magneto coil pickup. The solution has been two bearings on at least the output side. The crankshaft bearings need a lot of clearance to prevent seizing from heat and heavy press fits. In effect we are treating each half of the crankshaft as an overhung crankshaft.

A long time ago we souped up Homelite overhung crankshaft engines. Both the rods and crankshafts of these engines are too light duty to stand much more than their design power and rpm. However, they cost around 1/3 as much as the Zenoah engines we now use.

Lohring Miller

 

[Till]

Niels, that crankshaft design will wobble unter load conditions because of the bearing layout. There will be lots of undesireble sideeffects, including periodic noise (and wear) of the bevel gears.

And I think that the amount of power supplied to each prop is not equivalent (at least with asymetric timing). You will need some kind of damper on the geared shaft to reduce torsional vibration.

 

[makoman1860]

I tend to agree especially as engines get larger. However, bearings on both sides don't necessarily make the crankshaft stiff enough. The crankshaft our race engines use comes from a weedeater engine that started out at around 2 hp 20 years ago. Its design (and I believe materials) are unchanged even though it's now used in engines that develop over 8 hp at a lot higher rpm. This crankshaft and a similar design used on a 35 cc engine we worked on flexes enough to cause flywheel strikes even with .015" (.4 mm) clearance to the magneto coil pickup. The solution has been two bearings on at least the output side. The crankshaft bearings need a lot of clearance to prevent seizing from heat and heavy press fits. In effect we are treating each half of the crankshaft as an overhung crankshaft.

A long time ago we souped up Homelite overhung crankshaft engines. Both the rods and crankshafts of these engines are too light duty to stand much more than their design power and rpm. However, they cost around 1/3 as much as the Zenoah engines we now use.

Lohring Miller

Hi Lohring,
I get what you are saying. It seems that the root cause of the issues you have is the crankshaft design, not the general layout of a "full" crank. I am curious if your need for excessive clearance comes from the same issue. In effect you are bracing the crankshaft with the main bearing design as kind of a Band-Aid it seems. Are the rollers on the crankpin straight, or barrel? Is there enough room ( in the case and pocketbook) for a new crankshaft design with a larger crankpin?

 

[lohring]

Motorcycle racers have the issue mostly under control with the exception of the rod bearings. In both cases the secret is in the details, not the basic idea. I'm sure another crankshaft design would be better, but how can you beat $55 for a crankshaft and connecting rod assembly?

On a previous topic, I found the drawings of the OPOC (previous incarnation of Eco Motors) small engine with piston scavenging pumps. It was a very clean design, but they never got the reed valves working well, even though they were a standard air compressor design.

Lohring Miller

View attachment Reed_Valve_New_Concept 3-9-06.pdf

 

[lohring]

Crossed chain I feel is a bad idea, even a direct coupled system with gears will be difficult with the torsional harmonics. ---

If you take power off both crankshafts, the connecting gears or chain should carry very little load since it's just being used to time the two cylinders. This should be possible by staggering the propellers if their radius is less than the crankshaft center to center distance.

Lohring Miller

 

[Niels Abildgaard]

And I think that the amount of power supplied to each prop is not equivalent (at least with asymetric timing). You will need some kind of damper on the geared shaft to reduce torsional vibration.

Torsional vibrations come in a very narrow rpm band .
If it cannot be engineered outside 1500 2400 rpm,I will counter it with the generator/starter fixed on one of the cranks.Modern electronics can easily do that.Asymmetric timing serves no purpose.The 800 horsepower snowmobile engine shown has symmetric timing.

 

[makoman1860]

There is always going to be asymmetric loading on an OP 2 stroke. The blowdown event is above one piston only which tends to set up a gas dynamic in the cylinder. OP engines are anything but simple, if they were, everyone would use them (if the application deemed it worthwhile), its not like its a new idea. Most engine companies have experimented with them and found that for most situations, there is not enough there worth developing. The gen 1 ecomotor suffered a lot of design flaws that made it impractical. Reeds are very very complex things to get to work well, especially stainless ones for long life. Most companies throw in the towel and go with a composite reed that has a short life but at least wont destroy the engine. I wish the fellows at EM well on their adventures, however I sadly see that venture slowly disappearing as they find nothing to gain in the design for light motive power.

 

[Niels Abildgaard]

Many outboard engines use reed valves at 5000 to 6000 rpm and the proposed Junkers at 2400.
Are outboard reed valves a problem ?
In that case I will opt for a rotating disc valve.

 

[lohring]

Attached is a paper on some of the work Eco Motors founder did for his truck APU engine. It's a preliminary copy of an SAE paper. I have a more detailed paper on their truck engine, but it's too big for this forum. Like any real engine development, there's lots involved. They had to address a lot of the issues mentioned in this thread.

Car companies may change body styles every few years, but engine designs live on for decades due to the development needed even for a long proven basic design. Large and medium size aircraft piston engines haven't been developed at all for at least 50 years because there's no market. Two strokes really only dominate the smallest sizes and large marine two strokes due to cost and low weight for the constant high load power (really, even at 100,000 hp). The big BRP outboards are an example of what might be done in moderate sizes, but their development bankrupted OMC.

Lohring Miller

View attachment 2005 SAE epc Paper 05P-669 Truck APU.pdf

 

[Till]

Attached is a paper on some of the work Eco Motors founder did for his truck APU engine.

Well there are already small power plants established to supply trucks, especially for the refrigeration units (at least in Europe they are). So they clearly aimed at the military market. Military? Funding comes from the taxpayer → if there’s a problem, continue to throw money at it until it goes away or the project becomes obsolete… Just take a look at the engine: Lots of new, unproven and expensive design = very little chance for commercial success.

Large and medium size aircraft piston engines haven't been developed at all for at least 50 years because there's no market.

There is a market but american jurisdiction makes it a highly dangerous minefield for potential investors.

 

[makoman1860]

Well there are already small power plants established to supply trucks, especially for the refrigeration units (at least in Europe they are). So they clearly aimed at the military market. Military? Funding comes from the taxpayer → if there’s a problem, continue to throw money at it until it goes away or the project becomes obsolete… Just take a look at the engine: Lots of new, unproven and expensive design = very little chance for commercial success.

There is a market but american jurisdiction makes it a highly dangerous minefield for potential investors.

Niels,
If long service life is what you are after, rotary might be a good path. Metal reeds are not an issue when designed and manufactured well. The scope of doing so is out of the realm of all but a couple companies in the world.

There are new aviation engines every year, certified and experimental. The reason that you see little change in design is that if you had a clean sheet to start with, and all the functional requirements of the current engines, a new design would look pretty much like what we have now. Its not that there is a fear of change, there just isn't much to gain. Many people falsely assume that an automotive type engine is somehow better than an aviation engine. The reality is that application dictates design. Auto engines make poor aviation engine, and vise-versa. Honestly the fuel burn on an O-320 Lycoming at 150hp is amazing , and you will never find an automotive engine that weighs the same, with the same BSFC, and lifespan. Two different sets of needs, and requirements. Trying to "set the airplane engine world on fire" is nothing new. People have been trying for at least 70 years. A look back through aviation tech magazines from the 1930's on shows the same ideas we have now, experimenting with new engines. All have failed, i.e. you cant fix what is not broken.

 

[Niels Abildgaard]

Hello Markoman.
I guess You are not investing in the future Junkers?
The O 320 makes awfull noise by turning a suitable prop at 45 rps.
If You reduce rps ,power goes down,and if You reduce prop diameter ,thrust efficiency goes down.
It yaws and rolls aeroplane,especially during start and aborted landing.
It uses/needs a criminal fuel and it rattles all the time.
Good engine?
You must be joking.
On the other hand I will be very interested in real measurements of fuel consumption for present day engines.

 

[makoman1860]

Hello Markoman.
I guess You are not investing in the future Junkers?
The O 320 makes awfull noise by turning a suitable prop at 45 rps.
If You reduce rps ,power goes down,and if You reduce prop diameter ,thrust efficiency goes down.
It yaws and rolls aeroplane,especially during start and aborted landing.
It uses/needs a criminal fuel and it rattles all the time.
Good engine?
You must be joking.
On the other hand I will be very interested in real measurements of fuel consumption for present day engines.

Hi Niels,
Haha no I'm not . Honestly the O-320 powered planes I am used to have little prop noise, but much of that is in the blade design. Yaws and rolls....well I guess to me that's just part of flying that I never gave a second thought to. Since when is premium auto fuel criminal? Rattles? Sure its not the smoothest being a big 4 cylinder, but I have felt worse. I am not saying that the O-320 is the best 150 horse out there by any means. The 6 cylinder Franklin and Continental are much smoother. I will give you this though, when you are flying over the mountains, there is no better feeling then knowing the engine will keep running, and that comes from a trusted design. I've worked in the engine design field for some 15 years now, and quite honestly the more I learn, the more I cringe at many of these "experimental" engines. The level of detail, development, and testing to get an engine into production is staggering. The work involved in understanding the control, tracking and processing of the components is mind boggling. This is why few have managed to survive. Jabiru is doing OK....but have had issues since they don't have the capability to do in house development, and I really wonder how long they will be around. They have this trouble building a pretty much "standard" engine. I really am not trying to be a wet-blanket here. If you make it work, all hats are off to you.

On the fuel consumption side, what engines are you interested in? Marine engines come the closest to the duty cycle of aviation.

 

[Niels Abildgaard]

Hello Markoman.
It is a shame to let a mental game (Design a better mousetrap , aircraft engine etc ) degenerate into the usual (Old american iron is best).
Next step down is that someone tells me that Harley Davidson makes motorbikes.
What would interest me was if You had a fuel consumption map for a o 320?

 

[makoman1860]

Hello Markoman.
It is a shame to let a mental game (Design a better mousetrap , aircraft engine etc ) degenerate into the usual (Old american iron is best).
Next step down is that someone tells me that Harley Davidson makes motorbikes.
What would interest me was if You had a fuel consumption map for a o 320?

Hi Niels,
Ha no I do not consider a Harley to be the best at anything (maybe marketing). Honestly I still believe a air assist DI "V" type 2-stroke would be a better road to travel down. If torque reaction is a big issue, then a concentric counter-rotating prop setup could be used. A traditional ( non OP type ) combustion chamber would allow you to run stratified up to almost 75% power. This is something the OP type design wont allow you to, its been tried and you just cant get the injector where you need it. Keep the power density down to about 60 hp/litre and it can be made to have a 3000hr TBO at 75% duty cycle without having to invent much. A V4 will be much easier to keep a gearbox on than a V2. An I3 would be good up to 100 horse and is excellent for pulse tuning ( as is a V6 as 2 I3's ). A good look at outboard engines will give some ideas. Both Mercury and BRP are the experts in this field (big 2 stroke, long life, high duty cycle, light).