[John Welden]

If the parts could realistically be made, how small of a V8 could theoretically run? I'm talking real cam, rods, pistons, crank, valves.

Would like to hear about the problems with going small as you see them.

Thanks,
John

[stevehuckss396]

George Luhrs built a single cylinder that has a bore of 1/8 and a stroke of 5/32. It is a 4 stroke with spark ignition.

Convert that to a V8 it would be .0153 CID.

[One of Many]

If the parts could realistically be made, how small of a V8 could theoretically run? I'm talking real cam, rods, pistons, crank, valves.

Would like to hear about the problems with going small as you see them.

Thanks,
John

Starting with the smallest part or feature you, your talents and machines are capable of reproducing, then go from there? The other thing to consider is attributes that are not truly scalable. These can be environmental aspects of ambient air temps, fluid flows and thermal issues.

Much of this in the scale model engine realm seems to be just getting it to run for short periods is good enough. Besides the artistic/craftsmanship joy of building something this complex. Making it useful is secondary, beyond an impressive show of skill, eye and ear candy.


The smallest runner I know of is Jim Moyer's 1/6th scale V8. There are tons of pictures to get an idea how small things get that can be produced consistently and still survive in the running assembly.

Moyermade Site

Keep a bib handy. The pictures can make you drool!

DC

[imhuge999]

The biggest issue for me has always been the ignition. You can't scale voltage.

I favor CI engines but it doesn't matter much considering I have never tried to make a very small engine.

[John Welden]

The biggest issue for me has always been the ignition. You can't scale voltage.

I favor CI engines but it doesn't matter much considering I have never tried to make a very small engine.

Are you saying a tiny spark plug wouldn't work?

[One of Many]

Are you saying a tiny spark plug wouldn't work?

The spark plug will work, but the distributor normal function cannot always be scaled due to spark gap. The spark voltage can be reduced to minimise spark jump, but could foul the plug more often. Some of that tradeoff is a function of compression ratio.

Next might be external oiling system. Besides lubrication, the oil also carries away heat and flushes wearing abrasives. A scaled sump, filter and pump might not serve either fuction well.

DC

[dynosor]

If the parts could realistically be made, how small of a V8 could theoretically run? I'm talking real cam, rods, pistons, crank, valves.

As an engine gets smaller, many of the tolerances required to run it are reduced proportionally. Your ability to make parts to a tolerance depends on your machine tools and skill, not on the requirements you are trying to meet.

When you shrink and engine too much, you simply cannot meet the tolerances that would yield an engine producing enough torque to overcome the collective friction between all bearing surfaces; let alone produce useful power. A large question is how to achieve good cylinder sealing and low ring friction?

Although roundness and surface finish can be tricky at a small scale, the least of these problems is maintaining diametral clearances in the journal bearings, between pistons, rings and the cylinder bore: Producing a crankshaft and camshaft that are straight to 0.001" becomes more difficult as these parts get skinny because they tend to spring while you are cutting them. You can reduce this problem by making fixtures to restrain shafts and skinny parts from springing during manufacture. Unless the rotating shafts are straight within bearing clearance, they will tend to bind and stall the engine.

Journal bearing clearance is not fully scaled down because oil film thickness does not vary much - in that sense you have proportionally more tolerance to play with. However, achieving 0.001" straightness across a 4" length shaft can still be tricky.

My personally assumed limiting factor is making small bolts and nuts for connecting rod big end bearings - nothing smaller than M3 for me, thank you.

[John Welden]

Great info, thanks guys.

I'm not too serious about this project, I just like to think about it once in a while. At work I've got the machines to go ultra small and I'd like to think I have the skills needed. Maybe someday I'll get around to trying.

Thanks,
John

[Mago]

The replies so far have highlighted the obvious problems in scaling down a full size engine althouh most can be avoided if you are prepared to accept a compromise ie bolt and screw sizes and ignition and spark components.
If a very small engine will do, try the Whittle V8 serialised in Model engineer a few years ago. Drawings and construction notes are still available.
It is air cooled, Glow plug ignition, wet sump and the smallest screw is 12BA.
A cnc mill is helpful for all those small parts.

Something a bit bigger is Ron Colonna's Offenhauser. An in line four with radiator cooling with water pump, twin oil pumps and electronic ignition.
A completed model you would be proud of.

I have 70% completed the whittle and about 10% the Offenhauser.

Making the V8 becomes very boring when you have to spend ages making 16 tiny rockers or 16 little valves etc, etc, hence the Offenhauser, variety.

Good luck with your quest and modelling

Mago