November 1999 R/C Modeler
Vol. 36 - No. 11
Site Sponsor: R/C Modeler
Type: Single cylinder, 4-stroke cycle, Rotary Cylinder Valve, ringed piston, glow ignition
Bore: 31.8mm (1.252")
Stroke: 25.2mm (.992")
Displacement: 20.02cc (1.22 cu. in.)
Compression Ratio: 10.5:1
Operating Range: 1,200-5,000 rpm
Weight: (including muffler) 34.75 oz.
Fuel Consumption: .75 oz. per minute
Manufacturer: RCV Engines, Ltd., 6 Haviland Road, Ferndown Industrial Estate, Wimborne, Dorset, England BH21 7RF. Phone: +44 (0) 1202 877044, fax: +44 (0) 1202 861210
Price: $549.00 plus shipping
In the 31 plus years that we have been writing the Engine Clinic column and doing the engine reviews, we have taken a look at a lot of unusual engines. One of the very first of these was the Aero 35 marketed in 1963 that utilized a horizontal piston on the same plane as the crankshaft. The fore and aft motion of the piston actuated the crankshaft by means of an "L" shaped connecting rod with a ball socket at the elbow that rode on the backplate. Another ball socket, fit into the crankshaft, provided a revolving, rocker arm type motion. We took a look at this engine and its prototype companion the Savage 60 in an old-time engine review back in the March 1976 issue. Another non-conventional engine would be the O.S. Wankel that we reviewed in the February '71 issue. More recent unusual engines would include the BOMO B20 twin (May '87 issue) that utilized the camshaft to drive the prop at a 2:1 reduction ratio and the Erickson FE-120 (November '98 issue) that utilized a migrating combustion chamber rather than a conventional piston and sleeve. These are only a few of the more unusual engines we have taken a look at over the years. To the list, we can now add our review engine for this month - the RCV (Rotary Cylinder Valve) 120 whose smaller brother, the RCV 60, we brought to your attention in the December '97 issue. Although the RCV 120 does use a conventional piston, wrist pin, rod, and crankshaft assembly, the sleeve or cylinder, as RCV refers to this part, is free to rotate within the finned aluminum, outer housing, in turn, driven at a 2:1 reduction ratio by means of spiral cut bevel gears - the drive gear on the crankshaft and the driven gear as part of the cylinder. The prop shaft is an extended part of the cylinder and, by turning at half crankshaft speed, allows the engine to develop massive torque and big prop-turning ability, resulting, in turn, in increased prop efficiency.
The RCV engines are the brain child of Englishman and long-time modeler, Keith Laws, whose background includes involvement in the full-size internal combustion field, particularly engine management systems. During the development stage, four prototypes were built - the first being a small .10 cubic inch displacement size engine to prove the theory of concept. This engine, however, had the propeller bolted to the crankshaft rather than the rotating cylinder. As the engine ran successfully, a second larger prototype displacing .29 cubic inches was built which was the first prototype to feature the propeller bolted to the cylinder. Following the success of this engine, two more prototypes were built, the third displacing .50 cu. in., and the fourth .60 cu. in., with design changes and modifications incorporated during this development period. Keith Laws built the first three prototypes himself after having purchased a small lathe and teaching himself how to operate it. However, for the fourth prototype, the machine work was handed over to his friend, John Oliver, manufacturer of the "Oliver Tiger" engines whom many of our readers may be familiar with. John Oliver also added his engine expertise as an engine manufacturer in finalizing the design for production. Now, with a successfully working and proven prototype design, Keith Laws found a British Engineering company interested in producing his engine for whom John Oliver, again, built several additional prototype engines. With the design perfected, the first .60 displacement size production engines were marketed in England in early 1998. Following the RCV 60's good acceptance by the R/C fliers, RCV decided that a larger version would be in order.
With the background and some basics covered, let's take a look at the engine's design and construction features.
PIC 5 - Although being a 4-stroke engine, a minimum of parts are utilized. Note split (2 piece) shim used to provide proper mesh of reduction gears. Rear bearing not shown (not removed from finned housing). Front bearing shown to right of prop drive washer.
Crankcase: The two-piece crankcase and cylinder housing have been machined from billet aluminum, utilizing CNC machining equipment, and joined together with four socket head cap screws with an "O" ring providing a leak-free seal. Four mounting tabs have been machined as an integral part on the rear of the crankcase for radially mounting the engine. We found all machine work to be top quality with considerable attention paid to deburring, smoothing sharp edges, etc., something we always like to see in an engine and something that is often overlooked in many production engines.
PIC 6 - One piece reduction gear and cylinder unit. Hole in reduced diameter section aligns with intake/exhaust ports in the outer housing timing ring and also allows access to the glow plug for ignition.
PIC 7 - The back view of the cylinder showing combustion chamber at end of cylinder and intake/exhaust/glow plug port.
Crankshaft/Bearing Assembly: The crankshaft is of conventional design having been nicely machined from mild bar stock steel and utilizes a constant thickness counterbalance milled away on either side of the crank pin for counterbalance action. A hardened steel crank pin has then been pressed in and welded in place. We are guessing by the Plasma method where little heat is transferred to the crank pin and surrounding metal, although there was some discoloration in this area. The counterbalance would balance out the full weight of the connecting rod, wrist pin, and about 2 grams of the piston's 12 gram weight. This resulted in one of the smoothest running 4-stroke engines we have tested to date. How much of a part the fore and aft rather than up and down piston movement contributed here is difficult to say. Support is provided by a 1.125" o.d. x .500" i.d. (non-metric) steel caged bearing at the rear, and a 1.141" (29mm) o.d. x .390" (10mm) steel caged, single-sealed bearing at the front. The nose of the vertically mounted crankshaft extends through the top of the housing and provides a means of starting the engine from behind the propeller - one of the main features of the engine besides its streamlined and compact shape. A socket head cap screw holds the crankshaft in position and also serves as the electric starter drive, i.e., an adapter is connected to the starter that, in turn, engages the hex indent in the cap screw. Although being able to start the engine from behind the propeller eliminates any chances of the aircraft jumping forward if you forgot to retard the throttle, you still have to be very careful when applying the starter, as your knuckles are still in close proximity with the backside of the propeller.
PIC 8 - Back view of cylinder outer housing, huge rear bearing, and timing ring. Note "O" ring for sealing.
PIC 9 - Piston, wrist pin, and rod assembly. Note short skirted piston used to hold down overall length of engine. A single, floating, plastic end pad retains solid wrist pin in piston. Rod bronze bushed at both crank pin and wrist pin ends.
A 1.250" dia. spiral cut bevel gear is positioned between the two bearings and this whole assembly mounts in a removable carrier housing that is a snug slip fit into the crankcase, with an "O" ring used for sealing (see photograph). A split shim, i.e., composed of two halves, provides proper meshing of the gears. By using a split shim, the shim can be installed or removed without completely removing the carrier unit from the crankcase while determining the proper gear clearance. The shim used in this case was only .001" thick.
PIC 10 - Irvine Jet-Stream carburetor and 90¡ manifold. Purple anodized aluminum body and brass needle valve and end cap make for a very attractive appearing carburetor.
PIC 11 - Crankcase and removable crankshaft/ drive gear carrier housing. A split shim (see photo of all parts) adjusts gear mesh. Note "O" ring for sealing.
Piston, Wrist Pin, And Rod: These parts are of conventional design with the piston having been machined from bar stock aluminum. The piston utilizes a considerably shorter than normal skirt in conjunction with a shorter than normal rod, evidently to hold down the engine's overall length. A single 2mm (.078") wide, unpinned, low tension Dykes ring provides the compression seal. For those not familiar with a Dykes ring - this is a ring that has an "L" shape cross section. The foot of the "L" fits into a groove machined in the piston and the top edge of the ring is flush with the top edge of the piston. The Dykes ring relies on combustion pressure to expand the ring for sealing, rather than spring pressure, and does not require any, or just low, wall tension which, in turn, eliminates ring drag following the combustion stroke.
PIC 12 - The assembled crankshaft carrier unit and crankcase.
A rather small 5mm (.197") hardened steel, solid wrist pin is a tight slip fit in the piston. One side of the piston has been step-bored to prevent the wrist pin from passing through, and a plastic scuff pad on the insertion side prevents cylinder wall scoring.
The connecting rod, like the piston, has been beautifully machined from bar stock aluminum, completely deburred, and bronze-bushed at both the crank pin and wrist pin ends. Knocking off the sharp edges on the connecting rod is something you do not often see in model engines and, although a cosmetic feature, it does show that considerable attention to detail has been made.
Rotary Cylinder Valve: Now we get to the heart of the engine and most complex part - the rotary cylinder valve. The hardened steel-driven gear, cylinder, and prop shaft have been machined in one piece. A reduced diameter section at the end of the cylinder contains a single port similar to that used with a front induction 2-stroke engine; however, in this case, the port leads to the combustion chamber. This single port serves three purposes - intake, exhaust, and access to the glow plug for ignition. As the cylinder rotates, the cylinder port aligns with three ports in what is called a "timing ring." The three ports in the timing ring lead to the carburetor, exhaust outlet, and glow plug which, in turn, determines the intake, exhaust, and glow plug timing. Although the overall description of the engine may sound a little complicated, the engine is actually very basic and results in a 4-stroke engine that does not require a camshaft, valves, rocker arms, pushrods, or other valve train-related parts and, with the exception of the gears and two additional bearings, does not contain any more parts than a conventional 2-stroke engine.
PIC 14 - Small but effective muffler can be positioned to either side or rear of engine.
PIC 15 - Drive unit that attaches to electric starter for starting engine.
Being naturally curious of the engine's timing, we attached our degree wheel to the end of the crankshaft (not the propeller shaft) and, after determining top dead center (TDC), took the following readings. Intake port opens 20¡ before TDC and closes 60¡ after BDC - the exhaust port opens 60¡ before BDC and closes 20¡ after TDC, and the glow plug port opens 120¡ before TDC and closes 80¡ after TDC. The intake and exhaust timing figures are pretty much what we would expect to find in a 4-stroke engine utilizing poppet valves. The 60¡ opening of the exhaust port is actually a little later than some of the more recent conventional 4-strokes we have checked with, some opening as early as 90¡ before BDC. However, by delaying the opening of the exhaust valve, the combustion pressure is retained for a longer duration, resulting in increased low end torque which is what the RCV 120 is all about.
A 12mm (.472") o.d., by .500" deep, rounded bottom combustion chamber has been machined into the top of the cylinder bore intersecting with the cylinder port. We measured the combustion chamber volume at 2.1cc which computes to a full-stroke compression ratio of 10.5:1. This compression ratio seemed to be ideally suited to the engine, as even when loaded down with the larger prop sizes with the mixture leaned considerably past peak rpm, there was no indication of detonation "rattle," as is common with conventional poppet valve 4-strokes.
The rotating cylinder is supported by a humongous 5mm (2.165") o.d. x 35mm (1.378") i.d. steel-caged rear bearing and a considerably smaller 21mm (.827") o.d. x 12mm (.472") i.d. double shielded front bearing.
Carburetor: A standard 61 size Irvine "Jet-Stream" rotating barrel, 2-needle carburetor with an 8.5mm (.335") dia. intake is used. The carburetor, in turn, connects to the engine with a 90¡ machined aluminum elbow. The aluminum body has been given a purple anodize finish which, along with the brass end cap and needle valve parts, makes for a very attractive carburetor. Of course, I have a thing about purple anodize, having used this same color on the heads of my Lee 45s back in the late 50s and early 60s. The carburetor, in conjunction with the engine's overall design, performed flawlessly. Although we tried adjusting the idle mixture slightly richer and leaner in order to obtain the lowest idle speed, the setting, as received, proved to be perfect - more than likely due to the fact that the engine had seen running time prior to our receiving it.
Performance: Although, as just mentioned previously, the engine had seen running time, we did not know how much, so it was given an additional 30 minutes of break-in time. RCV specifies that the fuel contain 10% nitromethane and a minimum oil content of 15%, with no more than 6% of this being castor. More castor than this can result in rough running. Morgan's 10% Omega met these requirements and was used for the testing. The engine came equipped with an O.S. "F" glow plug which still looked like new at completion of the testing, even though we had intentionally over-leaned the engine on several occasions when checking for detonation, etc. The majority of our testing was performed with composite APC propellers. For comparison purposes, we also checked the engine's performance with three Zinger wood propellers - 10" pitch being the highest pitch we had in these sizes. The recommended prop sizes are either an 18 x 12 or 20 x10. We used a variety of prop sizes to determine the engine's torque characteristics. The air Temperature was a cool 60¡F, the Relative Humidity 48%, and the Barometric Pressure a shade low at 29.82 inches of mercury.
18 x 10 APC - 5,200
18 x 12 APC - 4,950
18 x 14 APC - 4,700
18 x 16 APC - 4,650
20 x 10 APC - 4,800
20 x 12 APC - 4,500
20 x 14 APC - 4,300
18 x 10 Zinger - 4,950
20 x 10 Zinger - 4,550
22 x 10 Zinger - 4,150
As can be seen, the engine is a real torque monster with a torque curve similar to that of an electric motor, i.e., flat. As an example, there is only a 500 rpm spread in rpm between the 18 x 10 and 18 x 16, and only 400 rpm between the 20 x 10 and 20 x 14. In fact, due to the RCV's high torque characteristics which is a twisting force, RCV recommends that aircraft with wings attached with rubber bands not be used. Only those aircraft with the wings bolted on are satisfactory due to the high torsional or twisting force applied to the fuselage.
Our engine had a terrific compression seal and, as such, our trusty Sullivan 12-volt starter couldn't turn it over. Fortunately, we also have a 24-volt Sullivan "Super Hi-Torque" starter which spun the engine easily. Starting the engine from the rear is definitely a different experience, but one you can quickly adapt to. The engine always fired up instantly and was a real pleasure to run with no particular problems encountered during the testing. We were particularly impressed with the very broad high speed mixture adjustment range of several turns between a rich setting and maximum rpm, especially after having gotten used to some of the more conventional 4-strokes that have a mixture adjustment range of two or three clicks between overly rich and too lean.
Out of the box, the engine would hold a 1,350 rpm idle with the 18 x 12 and tick over at 1,250 rpm with the 20 x 10 with almost instant acceleration. Experimenting with a slightly richer and leaner idle mixtures did not result in a lowering of the idle speed. Deceleration to idle was a matter of only two or three seconds.
The engine comes with a small muffler that can be positioned to the rear or either side. The muffler is basically a perforated tube inside an outer housing. The muffler, in conjunction with the engine's low rpm operating range, resulted in a low 89 dB reading with both the 18 x 12 and 20 x 10 APC propellers measured over asphalt 3 meters (9.87 feet) from the side of the engine with the exhaust pointing straight back.
We also checked the thrust being developed with the 18 x 12 and 20 x 10 APC props with the American Hobby Products "Thrust Finder" we reviewed in the October issue and recorded 8.6 and 8.8 lbs., respectively.
The RCV 120 is an exceptionally well-built engine with considerable attention paid to minor details. Obviously, a lot of blood, sweat, and tears went into its development. If you have a model in mind that requires a large prop turning at lower rpm, and, in particular, a model requiring a cowled installation without a cylinder or cylinders hanging out, the RCV 120 is definitely worthy of your consideration.
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