V12 Model Aircraft Engine

[dynosor]

I was planning to have a mature design, proven by means of a running engine before posting anything about my V12... Anyway, the pictures are screen captures from my evolving SolidWorks model.

The engine is just over 11 inches long and is intended for a scale model airplane. Bore and stroke are 1 & 3/4 inch respectively to produce a displacement just shy of 120 CC.

The idea is to produce usable power at 6000 RPM, rather than the much higher speeds typical of single cylinder models with this bore and stroke. This reduces stresses and should produce a sound with the second harmonic of a Rolls Royce Merlin V12 at 3000 RPM – better than the whine of a typical model engine.

The two combustion chamber designs shown result in a compression ratio of around 10:1 . The intent is to use glowplug model fuel with spark ignition - glowplug model engines typically have compression ratios in the range of 7:1 to 9:1 .

Many of the design elements were deliberately simplified to facilitate milling on a benchtop size manual machine. A rotary 4rth axis will be used to produce the crankshaft and crankcase regardless of manual or CNC control. Some parts, such as the connecting rods and cylinder heads exist in variants optimized for manual or CNC machining.

The design is far from finished. The valve-train exists only in my head. I have not yet decided if the overhead cams will use bucket tappets or rockers to take the “wiping load” exerted by the cam lobes. The crankcase will also be lightened to achieve a more rounded external appearance.

I have a number of different connecting rod designs. COSMOSXpress stress analysis shows that the conrod's piston pin bearing is strong enough, but it is the weak point in the design. The factor of safety can be brought up by making the conrod wider at the small end bearing - a width that would match the bearing area of the rod and piston where they contact the piston pin. This would require a variant of the piston design.

The stem of the conrod is stronger than it needs to be and it can be hollowed out and lightened as shown in the picture without reducing the factor of safety below 5.

[stevehuckss396]

Nice job on the drawings. I was wondering if you planned on cutting your valve seat right into the aluminum. Most model builders drill/ream a hole and then press in a one piece seat/guide of bronze or steel. If the engine is to be flown, might be worth the extra effort.

Keep plugging at it and post pictures after you start making parts!!

[dynosor]

The real thing:

http://www.youtube.com/watch?v=4qHov03402w" target="_blank">http://www.youtube.com/watch?v=4qHov03402w

http://www.youtube.com/watch?v=4qHov03402w" target="_blank">http://www.youtube.com/watch?v=4qHov03402w

A good facsimile:

YouTube - V12 Modellmotor

YouTube - V12 Modellmotor

Attached Zip Files:

1. Test animation file - yes, I know it would be better with all pistons and rods going, but it was just to test how to capture an animation file...
2. Mustang fly-by and start-up sound files

[dynosor]

Thanks Steve

The plan is to run the valves directly on the aluminum cylinder head - if that doesn't hold up, the cylinder heads will be reworked or re-made with appropriate valve seat/guide inserts.

The crank's main bearings will also be run directly on the crankcase aluminum. Ditto for the connecting rods bearings - no bronze bushes.

[BillTodd]

What a wonderful sound

[dynosor]

Haven't quite figured out where I want the engine mounts to be - this is one possibility. Looks better than the chunky, heavy rectangular rendering, I think?

[Adaware]

Nice work there

The cool thing with programs like SW is if you get your design right on a V2 engine model you can duplicate it to a 16 sylinder faster then you can say "ilovemodelengines".....

Care to share some specs with us?
Bore, stroke etc?

I presume you're goin' to make exhaust and inlet ports with ball mill?
What kind of cooling?
Lubrication?

[Kipper]

P51....Beautiful! Engine looks nice too...Potential for some lost wax into plaster castings?

What overall length is the engine?

[stevehuckss396]

Originally Posted by Adaware Nice work there The cool thing with programs like SW is if you get your design right on a V2 engine model you can duplicate it to a 16 sylinder faster then you can say "ilovemodelengines".....

I remember getting bored last year and stretched a V8 into a V100. Strange looking block for sure. Took about 4 minutes.

[dynosor]

Thanks Adaware and Kipper

Bore: 1"
Stroke: .75"
Displacement: 116 CC
Compression ratio: 10:1 or less
Water cooled - all passages not yet shown
Lubrication will be by means of 2-stroke model fuel and oil ways to the crankshaft - "belt and braces" approach

Adaware; you are right about using a ball mill for the valve ports.

Crankcase is 9.6 inches long - certainly room for lost wax casting to reduce machining - nothing that says it can't be scaled up.

The engine size was determined around the M3 screw :
I don't like to cut smaller threads than M3, and the big-end bearing caps are held on with tight fitting M3 screws. The conrods are .25" thick, with a square cross-section around the M3 screws.

With a crankpin diameter of .5" and two .25" bigend cross-sections, the cylinder bore diameter has to be at least one inch to assemble the engine - the conrods are profiled and relieved slightly at the big end to allow them to pass through a 1" bore – see screen capture below.

The stroke is .75" because I have 1.5" diameter 41L40 steel on hand and chunks of 6061 aluminum for the crankcase that will allow a minimum wall thickness of .25” – again to accommodate M3 screws. The main bearings in the crankcase are .25" wide - also the accomodate M3 screws holding the crankcase together. That, and I want to use an endmill no smaller than .25" diameter to rough out the main bearing journals on the steel crankshaft - the finished width of the crank's main bearings is .31".

The crank webs are .25" wide because that fits in with the overall layout of the engine - cylinder bore spacing is 1.4"
The minimum side clearance between rotating and static parts is .025"

The main bearings are .656” diameter because this is the nearest standard reamer size to match the .5” diameter crankpins – I scaled the crank and main bearings based on the 6 cylinder inline engine in my car, taking into account the ratio of cylinder bore diameter to main and crankpin bearing diameters.

My model design is “safer” than my car engine because the forces on the conrods are reduced compared to the torsional and bending stregth and stiffness of the crank journals and webs – conrod force is proportional to the square of the piston diameter, while torsional and bending strength are proportional to the 4rth and 3rd power of the main & crankpin diameters and the crank web depth. The crank is much stronger in shear than it needs to be.

Of course, the V12 crankpins are longer than on an inline 6, and that eats into my safety margin a bit, but I am willing to risk it.

My design assumes a peak cylinder pressure of 500 psi. Got that value from this interesting study on conrod design optimization:

http://www.steel.org/AM/Template.cfm...entDisplay.cfm

I didn't want to make the engine much larger than this because I am using a benchtop mill. Also, unless you use castings, machining time, material cost and waste become huge.


.

[Kipper]

I've just taken on a job casting suspension and wheels for a 1:6 scale Stuart tank (Honey) .....I bet a V12 would sound nice in that!

[dynosor]

Originally Posted by Kipper I've just taken on a job casting suspension and wheels for a 1:6 scale Stuart tank (Honey) .....I bet a V12 would sound nice in that!

This V12 is close to 1:6 scale Merlin, minus the complexity of the supercharger and the gear reduction to the propshaft...