Feasibility of small 4-cycles in practical usage?

The smallest commercially available 4-cycle for RC aircraft use was an OS FS-20, and in current production a Saito FA-30. They're OHV and use standard glow plugs, and they're approaching the smallest limit to combustion chamber size where you can fit practically large valves and a glow plug in there.

I'd like to fly 4-cycle on models much too small for even these engines, models that normally call for 2-cycles displacing less than one CC. General rule of thumb is a 4-cycle aircraft engine is about 50% larger in displacement to a comparably strong 2-cycle, so we're looking at something in the 1.3-1.5cc range.

Fuel wise, standard for engines that small is a rather hot blend. Base is methanol, castor oil mixed in for lubrication, nitromethane for extra kick and tunability. I go for a 35% nitro 20% castor blend on mine.

Do you guys think it's even possible to build such an engine and get practical amounts of power out of it? 40-60 watts or so would be enough. I'm thinking a flathead would be a good start vis-a-vis getting glow plugs into it, but that also introduces some inefficiencies(particularly at high RPM) that won't help much.

 

The whittle V8 is 10.6cc so devide that by 8 it has a single cylinder size of 1.325cc per cylinder. You could easily take those plans and develope a single cylinder version. Not sure what kind of power you can expect from it but it physically can be done. Maybe a Vtwin version or 2 cyl inline would do the trick.

 

It'd need to make enough power to swing a 6x4x2 or 6x5x2 prop at something like 8-10 thousand RPM to be useable where a Cox 049 2-cycle can be used(This would give a thrust/weight ratio of about 0.5:1, flyable but not exactly ripping holes in the skies). And it'd have to be feather light; spark ignition is just too heavy to fly a plane with an engine that small. typical flying wieght of these models is only something around 200-300 grams, and that weight budget has to include radio gear as well. Why using glow plugs is part of the plan.

 

Firs a 4 strokes has half the bangs per turns so off the bat has 1/2 the power of an equal displacement 2 strokes.
Second is not just a matter of scaling, as dimension go down accuracy has to go up.
A 1.5 CC square engine has a 12.4 mm bore
Is difficult enough to make a good valve seal at 8mm diameter, is a lot more difficult at 4mm.
You may use a smaller glow plug than a standard 1/4" they do exist.
You may gain some space in the head by making a conic combustion chamber/piston head but that nearly double the head complexity.
The power drained by the valve motion apparatus does not fall as fast as the displacement reduction.

If an industrial product like the OS FS-20 is the best a company with precision grinders can do, I believe assuming to be able to make an engine half the size in a home shop is over optimistic.

Anyway I wish you good luck, it will be a good project for the winter.

 

Hi Kevin,
I am considering the build of a 2,6 cc flathead fourstroke marine engine. I made a mockup model from metal and plastic to get an idea of the possibilities and impossibilities of making such an engine.
Now there are a few considerations regarding small flat head engines vs. overhead valve engines.
- With flat head engines, you have ample space for both valves and the glow plug. In a small OHV engine, the 1/4"-32 thread of the plughole will eat up a lot of space in the cylinder head leaving little room for valves.
-Specific power output of sidevalve engines is always lower than OHV engines. With a poor power to weight ratio,
you won't find any flathead aero engines throughout aviation history ( at least I could not find any).
Even the plane of the Wright brothers had an OHV engine.
-You might consider making a diesel engine, which does not need a glow plug. Of course there need to be a device to vary compression , a kind of plunger, but it can be much smaller than a glow plug

I would advice you to just go for it, after all, the proof of the pudding is still in eating it....

Greetings from Holland,
Jos

 

This is why I'm looking into a 1.3-1.5cc 4-cycle to replace a 0.8cc two cycle.

The piston, crank, conrod, not hard to do. Commercially available two cycles as small as just 0.1cc are on the market in both glow and compression ignition varieties. The hard part's induction and ignition; these small two cycles do not have a cylinder head at all. Instead, the glow plug is the cylinder head; even changed the nomenclature to reflect this. Glow Heads.

I want to use an OS F glow plug, and this is why I'm leaning towards a flathead rather than an OHV. A flathead makes it WAY easier to use a standard OS F plug. Could also be made to use Cox glow heads as well, though gas flow might be a bit funky here.

There's other factors involved. I'm fairly confident OS or Saito could make 1/2a sized 4-cycles a commercially viable product if they wanted to, but with demand for glow engines in general on such a steep decline that the smallest 4-cycle OS sells these days is an FS-56 it's no big surprise they're not trying. Saito's the only game in town anymore if you want a 4-cycle below 9CC or so that's brand new.

What's possible to make and what's profitable to make are entirely different things and the latter is what dictates what companies like OS and Saito are willing to offer.

Mmm, indeed!

Noice!

this is precisely why I'm leaning towards a flathead. If it's a flathead it becomes quite easy to shove an OS F glow plug in there and let it ride. Or use Cox glow heads. Or make it a model diesel; contrapiston becomes much easier to work with if the valves aren't also in the way.


I'm not opposed to making it a twin, either, if weight can be kept under control. The big challenges are power, weight, reliability.

Flatheads can make stupid amounts of power if they're poked with a stick. Guys are getting 30+ horsepower out of 5HP flathead briggs engines in go-karts and Flathead Ford V8s will spit out well north of 400hp with the right go-fast bits in 'em. A lot of the bonuses found in these engines is found in increasing CR; that's not necessarily important running on glow fuel. Something around 6:1 is all you need for adequate performance on glow fuel and high-nitro blends love lower CRs anyway. 6-7:1 is pretty much ideal for a 35% nitro blend like I use in my 1/2a aircraft.

As long as it can breathe reasonably well at 8-10 thousand RPM it would probably do ok on 35% 1/2a fuel.

That requires a pretty high CR, though. David McIntyre has run commercial 4-cycles on model diesel fuel; Saitos are pretty close(Close enough that they'll start with an electric finger but no glow driver), OS needed the glow driver to kick and sounded like a bag of hammers falling down a metal staircase.





It might simplify ignition a bit, especially if a good CR can be reached for it to run as a 'fixie'(Some 2-cycles back in the day were sold this way too), but I'm not sure the added weight from requiring much more substantial parts to survive the knocking would be worth. Might be a good experiment tho.

One of these days I want to take a Detroit 1-71 and scale it down to something in the 30-40CC range, then shove that into a 1/5 buggy. Kinda unrelated to this(No doubt it'd run fine if it was machined with acceptable tolerances) but still be a fun project. And a true diesel, too, heh.

 

Whittle is known for the V8 but also had the Robin which was a single of the same cylinder displacement as well as a boxer 4. Plans can be found in SIC.

Geroge Luhr built much smaller engines.

FS-20 was not the best OS could do, it was what they could sell. How many people would buy a 1.5cc four stroke? Not enough, as there was a prototype by RJL which was never produced. http://www.mecoa.com/museum/never.htm

Just start building. It will run. making power though means high RPM. You may end up making your own glow plugs or modifying them like the Whittle designs.

Diesel works but adjustable compression is very useful if you want to fly. I've converted many OS to diesel and flown my FS-48S conversion quite a bit.

 

I am intrigued by the persistent use of the word "cycle" instead of "stroke". A two-cycle engine would only run for two revolutions if it was a two-stroke or four revolutions if it was a four-stroke. A cycle is the complete "suck-squeeze-bang-blow". A 2-stroke engine does a complete cycle in just two strokes of the piston or one revolution. A 4-stroke engine takes four strokes of the piston to do one complete cycle, or two whole revolutions. There have been many miniature 4-stroke engines designed and built. One reason for using a 4-stroke engine over a 2-stroke for model aircraft use is noise.

 

A 4-stroke engine has a lot more than half the power of a similar sized 2-stroke engine. The cylinder on a 2-stroke is never filled with a full charge. A lot of the fresh charge also goes straight out of the exhaust. In a 4-stroke, the valve timing more or less inhibits this, so the cylinder has a much better charge to burn.
A 4-stroke also has more torque than a similar sized 2-stroke.
Jack

 

And every single person using the term understands, it's just how things are commonly spoken.

 

Are you trying to be pedantic for a reason?

Lessee, why one might use a 4-cycle in an RC aircraft over a 2-cycle.

* Better fuel economy. Glow fuel costs $30 a gallon and planes tend to fly roughly twice as long on a given fuel load if a comparable 4-cycle is used in place of the 2-cycle. My NexSTAR is a fairly extreme example; on an OS 46AX two cycle(The recommended engine by Hobbico and what was included in the RTF versions even though mine was an ARF), the 270cc fuel tank it carries gave roundabouts 10-12 minutes of flying with ample reserve for landing under power. Swapping that engine out for a Magnum 52RFS 4-cycle increased my typical flight time from 10 minutes to nearly 30. I timed it; 33 minutes and change to run the tank till it quits.

* Larger props are more efficient. Swapping the 46AX out for a 52RFS meant I went from a 10-5 3-blade to a 11-7 3-blade. Total thrust stays roughly the same but the larger prop turning lower RPM helps with efficiency. I fly at around 35% throttle on the Magnum 52; the 46AX wanted about 1/2 throttle.

* Quieter. Kinda related to your point, though you didn't specify what aspect of the noise you were referring to. My Magnum 52 is worlds quieter than the 46AX ever was.

* More realistic sound. Real aircraft generally aren't powered by 2-cycle engines and 4-cycle engines tend to be a reasonably close facsimile. Especially with Cessna-like aircraft, which most trainers are.

* More aerodynamic. 4-cycles don't require massive aluminum tumors hanging off the side, which means they are much less of a drag generator than a comparable 2-cycle would be. More applicable in a cowled up aircraft(Like the Phoenix Typhoon I'm building over winter!) where the drag of the jug itself isn't all that much of a consideration, but the exhaust system poking out into the airstream is.

* Easier starting. 9 times out of 10 I have to fight my 2-cycles to get them to run in the correct direction. My 4-cycles...physically cannot run backwards. I can just bump them against compression in reverse with the glow driver attached and they kick right to life; my 2-cycles bounce a few times and it's potluck which direction they actually run in when they properly start.

* Better idle/low speed/trasition. Two cycles like to 'load up' and need to be cleared out when they're left idling, or at low power. Four cycles generally don't have this problem and, as long as the mix is within 90% of correct, they'll be much more reliable at low speed. And often they'll idle down lower besides.

* Less prone to failure due to a lean run. In my experience, two cycles will quite happily continue to run well after they've been leaned excessively, especially on the top end, which will burn them up right fast in a hurry. My 4-cycles quit on me within seconds if I twist the HSN too far while on the ground. It's nigh impossible for me to run one lean enough to burn it up.

* Lighter. In some cases that huge aluminum tumor hanging off the side of a 2-cycle makes it heavier than the 4-cycle replacing it. 'Twas the case with my old Super Tiger GS45ABC. It weighed 100 grams more than a Magnum 52RFS in ready-to-fly configuration, even though it weighed 150 grams less without a muffler. But you'd never fly it that way; be way too loud, inefficient, prone to ill temperment.


Generally speaking, I won't fly a 2-cycle without a good reason for it. And usually that reason is 'I can't get a 4-cycle small enough for this model'. Which ties into why I'm even asking about the feasibility of making one this small that's capable of flying something in the first place. Smallest one I can get is 3CC or so, smallest brand new 4-cycle I can get is closer to 4cc, yet I have planes calling for just 1cc engines, planes which would never balance out with one of those large 4-cycles.

Rule of thumb is about 30% more displacement to equate the two, though in some airplanes it gets closer. I only went up by 12% in displacement when I swapped my NexSTAR to 4-cycle, but being a trainer with a flat bottom high-lift wing, it didn't much care. If anything it flies better than it did on the 46AX; I cruise at 35% power whereas the OS had to run at half throttle or so. That might be even better if I could run a 12" prop...the 4-cycle I got LOVES a 12x6...but ground clearance issues forced me to use a 3-blade 11" prop.

 

Model aircraft rarely run high performance engines. In that case all the above is true, but these days arguments for electric power make it increasingly rare to run any internal combustion engines except on very large models. Even jets run electric ducted fans in modest sized planes. Electrics even dominate in model boat racing where maximum power does matter. Electrics are superior in operating cost, ease of use, and controlability.

I have spent a long time building model boat racing engines. Despite several design exercises with four strokes, we always decided that a tuned pipe two stroke had a lot more power in the same displacement. A four stroke would need to be seriously supercharged to get close to the same power. In classes where that was allowed there are now small turbochargers that would be suitable for a 60 cc two cylinder two stroke. That combination would again generate more power than a supercharged two stroke, especially in model sizes. Mechanical issues with valves also limit four strike rpm. We decided small four strokes can't compare to a tuned pipe two stroke for power. YS comes as close as anyone with their supercharged four strokes. They are great for pattern flying and are as simple as you can get in a supercharged four stroke. They are still more complicated and expensive than a similar displacement or weight two stroke.

Lohring Miller

 

A four stroke also has a higher compression ratio therefore better thermal efficiency.
But size for size they won’t make “more” torque than a two stroke.
They will generally run over a wider rpm range with a more linear output.
A two stroke tends to have a narrow power band.

 

Electrics are not superior to engines when half the point of flying the model in the first place is having an engine on it. And that's the case for me. I'm well aware that I could have unlimited vertical at half throttle in every plane I own if I went brushless, yet I go the other way. I put engines on electric models. The engines are, themselves, half the reason I even fly.

Electric also massively struggles to beat glow and gas on flight time. The energy density of lipos is so much lower than of methanol/nitromethane(itself less energy dense than gasoline!) that I can fly my NexSTAR for 3x as long as the next guy who's got his converted to brushless power, just on weight reasons alone. He can't shove enough lipos into the thing to fly it for half an hour; if he somehow manages to get it balanced I can just put a larger fuel tank on board and now I've got one hour or more aloft. Only time electrics can approach that kind of duration is if they're in an airplane optimized specifically for that task, whereas wet fuel planes regardless of specification generally have no trouble carrying a half-hour fuel load 2 or 4 cycle.

 

Out of curiosity, what did you decide to build?

 

Reference an earlier comment, the first cross channel flight (Bleriot) was made with a side-valve or 'flathead' engine. The Anzani 'fan' engine had a cam operated exhaust valve and an atmospheric inlet valve. Such an arrangement might be practical for a very small engine, as the valves are arranged so as to minimise the dead volume between them, allowing for a decent compression ratio. The inlet valve could possibly be a steel shim reed type, which tend to work better on smaller scales.

 

I must agree with DickG. I live in Australia and engines here are always referred to as either 2 stroke or 4 stroke.

 

Regarding flying flatheads: the original Continental Four, as prototyped in the Hendee Mfg. toolroom (Indian Motocycle maker) in , early 30's was a 40hp flattie! Lifetime member NAAFH, Flatheds Foourever!

 

There is also a modern flathead aero engine, the D-motor.
In light aviation, the flathead had the advantage of being able to fly home on 3 cylinders if it drops a valve.
Ohv engines tend to destroy pistons, rods and heads when a valve fails.