Model Two-Stroke Radials
This is a topic that surfaces from time to time: is it possible to build a two-stroke radial configuration engine, and would it be simpler and/or more efficient than an equivalent four-stroke? Yet again, the answer--in no particular order--is yes, no and maybe. On this page, we'll examine what is required to make such an engine work and the options available to designers with their relative merits and disadvantages.
We'd best start the analysis by defining what we mean by the name "two-stroke radial". The "two-stroke" bit is easy: an engine based on the concept of having the four basic operations of inlet, compression, expansion, and exhaust all take place in 360 degrees of crankshaft rotation. The "radial" part is a bit more vague. The obvious definition would be an engine whose cylinders are disposed radially around the crankshaft, like spokes on a wheel. We'd better exclude the trivial case of a single cylinder engine, and twins, both opposed and 'V'--although both fit the simple minded definition. So for our purposes here, a "radial" will be an engine having three or more cylinders disposed radially around a central output shaft. That should do for now.
The people who pose the question typically have seen the remarkable power to weight ratio produced by modern, single cylinder two-stroke model engines. They reason that several of these connected together somehow would give n-times the power, and recognize intuitively that a 'star' configuration would produce the most compact package. I suspect the wow factor such an engine would elicit at the flying field enters into the equation as well (it would work for me ). The major problem to be overcome is induction and scavenging. The single cylinder two-stroke is so simple and compact because it is able to use the crankcase and primary compression as a pump to draw in the fuel/air mixture, and transfer it to the cylinder under pressure. This achieves the aim of replacing the last of the old, burnt charge with a fresh one in one revolution. Our radial two-stroke will have to accomplish this too.
The most common practice used by radial engines is to have a single, overhung crankshaft driving a Master Rod. To this are attached the requsite number of Slave rods (sometimes called Articulated rods). As the shaft rotates, each piston in turn is moved through its cycle. The net effect is that the crankcase volume remains essentially constant, and so is of no use to us as a pump. Hence we need another way of pressurizing the case, or of coupling the pistons. There are a number of ways of achieving this.
The Interconnected Singles
The simplest approach is to build our two-stroke radial out of totally separate, self-contained, single cylinder units. Their individual crankshafts are then coupled together in some way to a central shaft that drives the prop. Although not a production engine, the M&M Radial shown here indicates that the concept was being actively evaluated in the late 1930's.
The first commercial offering of this type that I've been able to identify was the Four Aces, a .60 cuin engine based on four Cox .15 cylinders--an ad for which is seen here. Like the M&M described by Bert, it is actually a pair of opposed twins. How the single carby fed the four crank cavities is not clear; reed valves probably. I also recall, but can't locate, a variation of this engine that fitted non-operational cylinders between the 'real' ones to present a more busy look [aside: if the name "Four Aces" signifies a 'winner', should the engine with the dummy cylinders have been called "Five Aces" for 'cheater'? ]. As noted in the ad, the designer took advantage of the gearing to reduce the prop speed and increase the available torque.
A far more serious implementation of this approach appeared in the 80's from the Japanese company G-Mark. Their five cylinder .30 cuin radial used gears and complex die castings. As seen in the photograph of a photograph here, each cylinder was fitted with an exhaust collector ring that vented forward into the gear housing, allowing the unburnt oil to provide lubrication. The MAN engine review  by Peter Chinn found the engine to be remarkably quiet, although an electric starter was required and it had a voracious appetite for glow-plugs; the latter hardly attributable to the configuration though.
Another variation on this theme came from SIC editor Bob Washburn before the inaguration of his magazine. Following an idea borrowed from the "Thompson Tarantula" that used geared Cox 049's, he designed and built a three cylinder radial based on Cox PeeWee .020 cylinders. This engine, called the "Triscamp", coupled the individual crankshafts using 1.33:1 gearing to reduce the prop RPM and increase the available torque. Like the G-Mark (from approximately the same time period), he used captured exhaust oil for lubrication, but fitted individual reed valves and TeeDee venturi assemblies for each 'engine'. RAW notes that he built eight of these and sold seven, qualifying this as a low-volume, "commercial" engine. Plans for it appeared in Home Shop Machinist magazine . Peter Chinn made a theoretical review of the engine and noted that (at that time) plans and instructions were being offered by Robert for $35 .
The G-Mark employed complex, precision die castings; RAW's engine used some rather precisely machined parts. But there have been one-off amateur-built engines that simply mounted up a number of off-the-shelf engines with a spur gear where the prop would usually go. All work. The down-side is that 'n' separate engines need 'n' crankshafts, and the moral equivalent of 'n' separate crankcases. These add weight and take up space. There are also losses associated with the gearing, and extra friction losses in the separate shafts.
Timing of such engines may be either simutaneous, or staggered. This decision is driven by the induction and transfer method chosen. The G-Mark uses a rear drum rotary valve to distribute fuel/air mix to each separate crankcase cavity in turn. Hence the firing order is 1-2-3-4-5 and the power impulses are distributed evenly. Another advantage of this scheme is that as the carburator is effectively only breathing for one cylinder at a time, it can have a relatively small choke area--as compared with that required for a simultaneous firing engine. An engine composed of completely separate units like our simple home-built could be arranged to fire in sequence, or simultaneously since each cylinder has its own induction system. The down-side to this is that we now have 'n' needle valves to adjust. But at least they can be individually adjusted. Obviously with a common mixture regulation point, each cylinder takes what it can get and gravity be damned!
Another alternate is to fit each "crankcase" with a reed valve. This too would permit latitude in the firing order, but supposing that the gears were arranged for 'big bang' operation, the inlet would have be able to supply enough for the total engine displacement at once.
So summing up, when compared with a four-stroke radial, we have extra friction losses in the separate crankshafts, losses in the inter-coupling system, extra weight to provide separate crankcases as pumps, and potential uneven fuel distribution problems. We've gained something that looks and sounds cool though.
If all pistons were timed to fire at the same time, our crankcase would become a pump again and we could dispense with the individual shafts and a lot of extraneous metal. One way this can be achieved is by using a multy-throw crankshaft and some staggering of the cylinder center lines. And yes, there has been a commercial two-stroke radial built like this! This engine--a three cylinder radial--came from the Japanese company Hiness, noted for their elegant twins and "in-line", or axial single. Click here for a full review of this engine.
Crankcase ChargedThe third approach is to use a conventional master/articulated rod arrangement driving a single throw overhung crank. We must now use some kind of blower to charge the crankcase with atomised fuel/air mix under pressure sufficient to achieve transfer and scavenging. The most obvious way of implementing this scheme is to use a take-off from the crankpin--similar to the drive for a Zimmerman valve--to spin some kind of blower that empties into to the crankcase. Placing the carby in the inlet to the blower provides the fuel and provides lubrication and a degree of cooling (compensating for the heat generated in compressing the air).
A design of this nature by Lou Ross appeared in Peter Chinn's regular column in RCM&E . As seen here, both three and five cylinder engines were planned to compliment the Ross "boxer" series. Sensibly, the design would share cylinders and pistons with the boxers. This gave the three cylinder engine a bore and stroke of .800" x .600" for a total displacement of 0.9048 cuin (14.83cc). Notice that the three rods all couple direct to the crankpin of the three cylinder engine. The article noted that the engine had run briefly with the four vane blower shown, but that a Roots type compressor was in development. As far as I'm aware, the engine was never developed into a commercial product.
At least one engine using this scheme did make it to market though. This was the Burger Radial from the late 1990's. The designer, Lee Burger, used seven K&B "Sportster" .28 cylinders, arranged to place the exhaust at the rear. The heart of the design lies in the blower--an aluminium disk slotted to carry four free floating steel vanes. This assembly spins in a cavity whose axis is offset from that of the disk drive shaft with the inlet situated at the point where the ecentricity is causing the volume between two vanes to increase, with the exit opposie where the air has been compressed. Centrifugal force (or centrepidal acceleration if you are a purist) throws the vanes out against the chamber walls so that air is drawn in, compressed and vented into the crankcase. The carby is upstream of the inlet, providing fuel draw and atomization (and lubricating the sliding blower vanes).
What is unusual about this design (which was subject of a patent application) is the low pressure in the crankcase, which is actually vented to the atmosphere between the lower two cylinders. According to Lee, the mixture is distributed to the cylinders by the design of his master rod assembly that provides a "sweeping" action. Don't see it myself, but you can't argue with success. The design appears to utilize fork type big-ends on the articulated rods. Strange as this all may seem, it works, and an unknown number of engines were sold at $1,195 each. It should be obvious that hand starting such an engine would not be practical as the blower and presumably, the master rod assembly need to be spinning at a fair clip in order to do their job.
Regretably the only information that I have on this engine came from a photocopy of a draft article destined for a magazine (RCM, I beleive), kindly supplied by Neville Palmer, New York (Neville was the US distributor for the original Model Engine World and a contributer to that magazine). The author of the article is not cited, so I'm unable to give appropriate credit.
Conclusion:Two-stroke "radial" engines are practical, but not so simple as they seem at first glance. We've examined three basic schemes for constructing such engines--there are probably more. All have their merits and their disadvantages.
For the tinkerer (no disrepect intended), simply gearing separate singles together is practical. This generally results in a rather bulky crankcase. A more 'realistic' appearance however requires considerable work. Some effort can be saved by the use of commercial cylinder units (like the Berger and Triscamp), but the remaining effort is still not inconsiderable. It should also be noted that the sound produced will not be the traditional "round engine" sound (produced by the odds and evens firing order of a four-stroke radial).
From the perspective of a model engineer, such an engine can be built to proove a point, or demonstrate an unusual concept. However the overall effort to build a four-stroke radial from plans, like the Hodgson, Edwards, or Morton M5 will not be a whole lot more, and the result probably more satisfying (note that you'll have to join the Yahoo group to view the Edwards plans).
|||Chinn, PGF: Engine Review: G-MARK .30, Model Airplane News, Volume 101, Number 3, September 1980, Air Age Inc, CT, p28.|
|||Washburn, RA: Letters, Model Engine Builder, Volume 1, Number 1, March 2005, Elmwood Publishing Inc, CA, p36.|
|||Chinn, PGF: Engine News, Aeromodeller, Volume 46, Number 551, December 1981, Model & Allied Publications Ltd, Herts, p621.|
|||Chinn, PGF: Peter Chinn's radio motor commentary, Radio Control Models and Electronics, Volume 12, Number 4, April 1971, Model & Allied Publications Ltd, Herts, p241.|