Need some hints and tips on how to build engine

[Adaware]

I'm in the process of designing a big multi sylinder model engine but first i will make a four sylinder engine to test my design and seek improvement.

The sylinder bore will be 4cm and so will the stroke, a square sylinder as you might call it. I will be an DOHC engine with four valve technology because i like that idea and it doesn't seem any harder than a two valve layout. I have made some drawing and need some opinions regarding design and so on, also i'm not sure on what clearances i must have between the parts like the crank and bearing, sylinder and piston etc

I will use:
Stainless steel valves
Aluminum pistons, rods, head, block and many small parts
Pressed steel liners
Bronze valve guides
Cast iron piston rings?? need help here...
Bronze bearings?? need help here..

I will use a Okuma 3-axis CNC mill with a additional A-axis, Okuma CNC lathe. We also have a Okuma Multus 5-axis but that's kind of defect for a couple of months....

A picture of my combustion chamber as it is designed today with some squish area, the sparkplug will be a standard CM-6 because it's cost effective. The valvestem is 4mm, the diameter of the valve is 14mm and the ports are 12mm.


And i know, my english isn't so good but i think you understand

[TOTALLYRC]

I'm in the process of designing a big multi sylinder model engine but first i will make a four sylinder engine to test my design and seek improvement.

The sylinder bore will be 4cm and so will the stroke, a square sylinder as you might call it. I will be an DOHC engine with four valve technology because i like that idea and it doesn't seem any harder than a two valve layout. I have made some drawing and need some opinions regarding design and so on, also i'm not sure on what clearances i must have between the parts like the crank and bearing, sylinder and piston etc

I will use:
Stainless steel valves
Aluminum pistons, rods, head, block and many small parts
Pressed steel liners
Bronze valve guides
Cast iron piston rings?? need help here...
Bronze bearings?? need help here..

I will use a Okuma 3-axis CNC mill with a additional A-axis, Okuma CNC lathe. We also have a Okuma Multus 5-axis but that's kind of defect for a couple of months....

A picture of my combustion chamber as it is designed today with some squish area, the sparkplug will be a standard CM-6 because it's cost effective. The valvestem is 4mm, the diameter of the valve is 14mm and the ports are 12mm.


And i know, my english isn't so good but i think you understand

Your english is fine.
I like the drawing you posted. Normally the intake valves are noticeably larger than the exhaust. Cast iron rings are used in full size engines but I don't know what is used in small scale engines. I have seen some small engines use the bare bore of the aluminum rod on the crank, but I am unsure of the main bearings. If you search the web you will find out a lot of what you are looking for. One place I think is strictly IC but I don't have the web address.

[Adaware]

The problem with the valves is that i can't make them bigger because they won't fit in the head. Only way i can have bigger intake valves than exhaust is to make the exhaust valves smaller. I don't think this will be a problem anyway, this is not a race engine and it is easyer to make identical valves then to different types. It's just a engine that will function and run smooth, atleast that was the plan
Bare aluminum on steel don't sound so good i think but who knows, maybe it's sufficient for a model engine

I Googled after the stictly IC forum but no positive hit yet.... help?

[tstom]

If the engine will be just for display operation that is to say not do any actual work the aluminum on steel will be fine Most American small engines (lawn mower engines etc)are made this way I would recommend .0005 inch clearence per 1 inch of crank journal diameter
The other option would be to size the rods and crank so you could use some commercially available bearing

[dynosor]

As you know, there are many alloys of aluminum with different properties:

You could probably get a way with 6061 for all the aluminum parts, or only use this alloy for general structural parts and ones that are subject to corrosion (water cooling).

2024 is stronger and harder than 6061 and is good for connecting rods and the crank case, but not for any part in contact with water, such as the cylinder head and cylinder block of a water cooled engine. 2024 contains copper, and while not as hard and slippery as bronze, it should make a decent bearing if large enough. Just make sure the steel journal running in it is highly finished and well lubricated.

I agree with the poster that suggested 1/2 thousands of an inch (12.5 microns) radial bearing clearance per inch diameter. If the engine has multiple bearings in line (more than 2 or 3), the alignment and straightness achievable may favor doubling that clearance to reduce friction in the newly built engine.

Ideally, the piston should be made from a high silicon alloy because these have the lowest thermal expansion. For models, even 6061 will do as long as you allow sufficient clearance for expansion. From memory, 6061 has half the thermal expansion of cast iron and the difference in expansion for a 1" piston diameter is 1/1000" per 100 degrees C temperature rise. I would suggest 1/1000” radial clearance per inch diameter on most of the piston length, but twice that at the top land (above the ring groove) because this area is the hottest.

If the piston was made from high silicon alloy (12+%) you can use half of these clearances, but machine-ability becomes difficult.

While 7075 aluminum alloy is very strong, this alloy loses strength at high temperature, so do not use it for the piston or connecting rod.

Your cylinder head model looks good and using the same valve size for inlet and exhaust allows “mass production” of the same part. You need to include the volume of the combustion chamber in your compression ratio calculation, along with any clearance cuts in the piston for the open valves. CR of 8 or 10:1 seems a reasonable place to start depending on the fuel used...


Piston ring making:

http://www.modelenginenews.org/cirrus/pistons.html#3

http://www.modelenginenews.org/feeney/pg8.html

http://www.modelenginenews.org/techniques/piston_rings.html

http://www.5bears.com/pistons.htm


Alloy ifo:

http://kb-silvolite.com/article.php?action=read&A_id=1

http://www.amarfounders.com/aluminiumalloycasting.html

http://books.google.com/books?id=Lskj5k3PSIcC&pg=PA627&lpg=PA627&dq=piston+alloy+aluminum+silicon&source=web&ots=Fsy0eg5yeY&sig=5QyJfCxP_Dx6-OEug1OAqN7HpO4&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPA623,M1

[Adaware]

You guys are amazing

I could only see compression rings in the articles, how about oil control rings?

I have som blocks of aluminium but the quality is belived to be in the 2000 series, i shall investigate further to find it out. The blocks originaly comes from casting equipment for big aluminium bolts so i don't think the quality is so bad.

I will try to have as much water cooling as i can make, i need to find a way to have water channels in the head and i think i have an idea there but it's difficult when you are making the head from a solid block. One way i can go past this is to make induvidual sylinder heads, four heads on a four sylinder engine but that's alot of work and i'm not shure if i can make all the bearings to align. What do you guys think?

[neilw20]

If you are going to run the engine, stainless steel valves might be a problem.
Stainless steel is a very bad conductor of heat.

[dynosor]

You are quite correct that model engines typically don't have oil control rings. One reason is simplicity and another is the large scale use of 2-stroke fuel even with 4-stroke engines (oil is in the fuel).

Some sump lubricated model engines running regular petrol do have oil rings: http://members.optusnet.com.au/barberdeb/Amsbury%20-%20V8%20120cc%20no%20castings/ See PDF #3.

In PDF #4 the clever Australian shows how to fabricate a cylinder head out of sheet or plate material. PDF #1 shows the "block" being fabricated in a similar way - I think the main bearings were line bored after fabrication.

There is no reason why your engine cannot be made with a single piece crankcase containing all the main bearings (accurate to each other) and separate cylinders housed in one or more "blocks" attached to the crankcase. There should be no problem with separate cylinder heads - cooling ducts can be linked by means of hoses.

I don't think stainless steel valves are a problem for a model engine - very common. The Amsbury V8 uses mild steel valves and pulls a model train. The question is how hard will it work and how long does it need to last?

[stevehuckss396]

I Googled after the stictly IC forum but no positive hit yet.... help?

Strictly I.C. was a magazine that ran for some 14 years. It was devoted to the model engine hobby. All the back issues may still be available. I bought the entire run about a year ago.

http://www.strictlyic.com/


The magazine that replaced it is called Model engine builder. Also very good.

http://www.modelenginebuilder.com/

[Adaware]

What about valve seals, is it necessary?
What can i make this of, rubber o-ring of some sort?

Nearing completion og drawings but a few details is left unknown, i want to have a dry sump system but have no idea how such pump looks like inside. I will google around a little bit

[dynosor]

You may find o-ring material that can stand 250 degrees C, and might work for the inlet valve, but the exhaust gas is way hotter than this - my avatar shows the glowing exhaust manifold of an engine run hard on a dynamometer. Exhaust gas temperature can exceed 1000 degrees C.

Commercial model engine valve seats are often bronze and many home-made engines have been built with steel, or no insert material in an aluminum cylinder head. In general you want to use a material that will not melt, burn or be pounded out of shape by the valve.

It is good practice to use dissimilar metals for valve and seat to avoid them "cold welding" to each other - when this happens on a small scale the valve and seat are rapidly eroded and the smooth and true contour needed for a perfect seal are lost. Because the valve and seat run hot this propensity to weld is even greater, but you can use the same grade of steel for valve and seat provided one is heat treated to be harder than the other and that difference is not lost at operating temperature.

Stainless steel tends to gal, seize and weld to itself more than carbon steel. If you use 300 series stainless valve material, don’t use exactly the same material for the seat. If you used 400 series stainless, you could heat treat the seats to about 10 Rockwell C harder than the valves prior to final grinding.

Again, it is a question of how hard the engine will work and for how long. Commercial engines use more durable materials:


From: http://www.aa1car.com/library/ar993.htm

"CHOOSING THE "RIGHT" REPLACEMENT VALVE SEAT

The original equipment manufacturers use a variety of seat materials, including cast iron, iron alloys, nickel alloys, cobalt alloys (stellite) and powdered metal (which generally contain no chrome or nickel, only vanadium and iron). Most OE seats in passenger car aluminum heads are a high grade of cast iron or powder metal. The better (more expensive) materials are usually found in high output and turbocharged engines, with hard seats and stellite being used mostly in diesels and industrial engines.

When replacing a seat, you should use one that is at least as good as the original if not better. Hard seats are a must for high temperature, high load and dry fuel (propane or natural gas applications). In fact, most seat suppliers have special alloys specifically designed for dry fuel applications. But hard seats are not required for light duty passenger car applications. Even so, many aftermarket seats are made of premium grade alloys or heat treated iron to provide improved longevity and performance."


See also:

http://www.tpub.com/content/construc...s/14264_92.htm

http://en.wikipedia.org/wiki/Stellite

[NC Cams]

Regarding bare aluminum on steel - Forged and cast aluminum uncoated pistons have been run on cast iron bores for years. Iron sleeves would be better thatn steel from an ease of build standpoint. If you did a harden and draw to the point that the steel liner was Rc 32 or higher, it might work OK.

If this is a dry sump or glow plug fueled engine, or even a dry sump spark plug fired beaset, YOu can probably get away with a single compression ring. PROVIDING you run a snug bore fit of the piston and use the bottom of the piston skirt as your "oil control" ring. THis is done by leaving the bottom of the skirt fairily sharp to act as a "scraper" to skive off the excess oil.

Good piston alloys would be 2018 for good hot strength under high load or 4043 which is a silicon alloy that can be fit much tighter than 2018. Check out the SAE handbook for basic info on how to design pistons with regard to material, heat treat, clearance, groove shape and specs.

Valves are a former specialty of mine. WOrked for a major Engine valve maker.

For light duty use, you could use 1050 or 4140 for intakes. YOu could use 4140 for exhausts providing you did some hardening to the part - Probably in the region of 40-42 Rc.

Austenitic stainless steel of the proper type is an EXCELLENT exhaust valve material. Why? Higher hot strength than martensitic materials at exhaust valve temps.

Stainless exhausts materials such as Silchrome 1 and Silchrome XB are not readily found at your local metals store but are used none the less by the OEM.s Totally overkill for your model. IF I recall, Briggs Stratton used 1050 intake and 3140 or 4140 exhaust valves for their lawn mower engines. I doubt you'll get anywhere near the thermal load of a Briggs hence their valve materials should be fine.