Material choice for combustion engines and the limitations of homemade engines

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David Nolan

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So, I thought I was pretty experienced when it comes to machining, I naively thought that one day I could even build an engine that I could use to power my ultralight aircraft. I've learned a lot since then and am now significantly less arrogant but also significantly less confident that my ambition is realistic even if I am willing to work on it as a long-term goal.


Originally I thought the issue would be machining the accurate components which is no menial task in and of itself, it is one however that I felt with time and dedication I could overcome and have for the most part.

What I neglected in my foresight is the composition and pedigree of the engine material I would require. So my question, what is the correct grade of alluminium for this 'lofty' ambition, can it be sourced locally to avoid months of waiting and expensive shipping rates and will it hold its own for its application.


Regularly I use 6086 T6 alluminium for regular stuff cause it's easy to get where I live in Ireland. I'm not optimistic about this at all, My concerns are that I will not be able to source the material, or afford the right spec of alluminium to build an ultralight engine. Which as you can imagine needs to be somewhat reliable when you are 'x' amount of feet in the air.

Piston engines aren't great as it is for an aircraft but one made of unreliable or heavy material is just not a good idea. I feel I should be more concerned with the actual flight itself but I don't think I'll get to that point so if anyone can offer any advice that would be great. I know generally this forum is for the 'model' engine but I'm sure all of you have at least considered putting your engines into practical applications or have done so.
 

Asm109

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The proper material choice comes from designing and engineering your parts. When you know the stress levels in your parts, you can then choose a material that can resist those stresses. If the parts are too heavy, you pick a stronger material and design the part to have acceptable stress levels.
 

mayhugh1

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"Regularly I use 6086 T6 alluminium for regular stuff cause it's easy to get where I live in Ireland." ...

Just curious, what is 6086 aluminum and why is it so easy to get in Ireland?
 

lohring

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I would need a lot more information on design and production methods. A New Zealander, Neil Hintz, builds a two cylinder autogyro engine along with many other experimental engines. He posts on the bucket foundry section of kiwibiker.com as well as other places. He posts under the name Flettner

Lohring Miller
 

Mechanicboy

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Take material from car parts:
Driveshaft: cylinder, crankshaft, camshaft.
Piston: cast a piston of old piston.
Brake disc or camshaft of cast iron (except cam who are impossible to machine): piston ring or piston if lapped piston/cylinder without piston ring.
Stainless steel rod: poppet valve.

Except material from car...
Scrap aluminium: cast a engine block/cylinder head or machine a whole engine block/cylinder head.
Bronze: valve seat, bearing.

This is whole what I had build own model engine. ;)
 

David Nolan

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"Regularly I use 6086 T6 alluminium for regular stuff cause it's easy to get where I live in Ireland." ...

Just curious, what is 6086 aluminum and why is it so easy to get in Ireland?
Apologies I meant 6082, I will change that
 

Tim Wescott

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Each bit is going to have it's own requirements. I'd expect that if you were going to machine the whole thing from scratch that you'd be using a lot of 2024 and 6082 (or 6061) aluminum, along with various alloys of steel.

Personally, for an engine that I was going to actually use I'd plan on converting a fairly new motorcycle engine or similar powerplant. At least I'd want to use off-the-shelf parts for wear items (i.e., pistons, rods, cylinders, etc.) Building a set of pistons once for a unique motor sounds fun and interesting -- building it all over again at every overhaul, instead of popping down to the local motorcycle dealer sounds a lot more tedious.
 

The_reach

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For consideration, there are numerous light aircraft engines out there that are based on the venerable VW flat 4 engine, not the most advanced machine by a long way. On ebay there's plans for making a flat twin from VW parts for a light aircraft, definitely worth a look then see about improving from there?

 

Mechanicboy

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Jens, using scrap sources may be fine for making models, but the OP wants to build an engine that he would be trusting with his life.
Charles,..
I have not had any problems with my engines even though they are over 20 years old and work just as well as when they were new. It is not the reuse of car parts, it is to obtain materials from car parts that contain alloys that are necessary for the engine to last a long life where one does not have access to the nearest material dealer.

Mine modellmotor.jpg
 

Peter Twissell

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Hello David,

I am another microlight pilot and engine builder.
First I'd like to point out to all that the engine of a microlight aircraft is technically considered as an accessory. Unpowered landings and engine failure are part of the pilot's training and are regularly practised.
So to engine materials:
There is no single correct answer to your question. For example, in my radial engine (not for flying) I have used cheap 5000 series alloy for the crankcases, 2014 for the connecting rods and 2618 for pistons.
The crankshaft and cam ring are EN24t steel.
Each material is selected to meet the requirements of the part. I did some fairly simple stress calculations with large safety factors and adjusted the part designs accordingly for my engine, but that results in a heavy engine (about 120kg for 100hp engine!).
I assume you're looking for about 50hp and under 50kg?
I would suggest a two cylinder engine, based on an existing design.
Please keep us up to date with your ideas and progress.
There are plenty of knowledgeable and experienced builders here and you will not be short of help and advice.
 

Peter Twissell

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On the subject of limitations of home made engines:
Designing and making your own parts allows you the freedom to select materials and design elements best suited to your application. Commercially produced car and bike engines are built to a budget and are not designed to operate at 80% of full power for long periods.
As another example, I am also involved with building drag racing bike engines. These are based on 1960s Triumph twins, but as running progresses, boost is added and fuel nitromethane content is increased, parts fail and are replaced with home made upgrades. This is a very different application to your microlight, as we only need to run for a few seconds at full power, but we are making 240hp from engines originally rated at 34hp.
The point is that there is a lot of room for improvement.
 

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