9914
The Manufacturability of the Rotapower® Engine
Paul S. Moller, Ph.D. Freedom Motors
Freedom Motors 1855 N 1st St., Suite B Dixon, CA 95620 www.freedom-motors.com
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9914
The Manufacturability of the Rotapower® Engine
Paul S. Moller, Ph.D. Freedom Motors
ABSTRACT introduced their rotary powered Evinrude RC-35-Q and Johnson Phantom snowmobiles. There are many elements of the charge cooled Wankel type rotary engine that make it inexpensive OMC also investigated liquid cooled housing marine to produce. OMC was able to show that they could models. OMC’s four rotor outboards raced six produce this type of engine at a cost competitive times in the summer and fall of 1973, winning every with their carbureted two-stroke engines. race in U class (unlimited). At the Galveston Speed Classic, they placed 1 st, 2nd and 3rd, lapping the THE PRODUCTION CHARGE COOLED WANKEL entire field three times (a fourth OMC boat rolled). WAS FIRST INTRODUCED AS A POTENTIALLY It was rumored that they once made a straightaway CLEAN, LOW COST, POWERFUL REPLACEMENT pass at 165 mph. FOR TW O-STROKES. THE CHARGE-COOLED ROTOR WANKEL TYPE In the late 60’s Outboard Marine Corporation ENGINE HAS A LOW PART COUNT (OMC) recognized the market value of an advanced, more powerful engine. This interest was When choosing an engine for a particular intensified by a growing concern that emission application or comparing the part count between issues would necessitate a clean burning, engines, the required power and torque environmentally friendly powerplant. OMC clearly characteristics of the engines must be considered. understood the obvious advantages of the rotary The displacement per piston and number of pistons engine over the reciprocating t wo-stroke engine. a particular engine has, determines this. These included low vibration, high power and very Horsepower can be gained by either increasing the compact design. Even more important, since the displacement size of the cylinders or by adding rotary operates on the four-stroke Otto cycle, it had cylinders. More cylinders provide better airflow to better fuel consumption and much lower emissions. each cylinder and allow the engine to run at a Four-stroke engines also have a broader torque higher RPM. However, more cylinders add parts curve and are less sensitive to exhaust and hence cost. As more cylinders are added the backpressure, which provides much better muffling. engine tends to have less vibration and better OMC’s knowledge of, and experience with, oil - torque characteristics. More cylinders may have a cooled rotor rotary engines, through an engine higher value in the eyes of the end user. development contract with Curtiss -W right, convinced them that this design approach would not Rotary engines, by design, breathe very well and be economically feasible (1). Instead, a charge- are not limited in RPM except for apex seal speed, cooled rotor version was chosen, not only for cost which should be below 50 ft/sec., typical of piston but because servicing a charge-cooled engine in ring wear speeds. Since the rotary engine power the field is much simpler. stroke has a duration of 270 degrees of crankshaft rotation and each rotor fires once per crankshaft The snowmobile was chosen as OMC’s first test revolution the rotary engine had the same peak -to- application because it was separate from their mean-torque ratio as a three-cylinder four-stroke marine market and presented a demanding piston engine. A two-stroke engine has a application (extended operation at maximum shortened power stroke so that as a practical power). OMC established the requirement that matter it also requires approximately three cylinders their rotary engine must equal or surpass any to match the torque characteristics of a single rotor competitive reciprocating two-stroke engine in engine. Therefore any part count should compare a terms of cost, performance, packaging, single-rotor rotory engine with a three cylinder, two manufacturability and effect on the environment, or four-stroke piston engine. When comparing the before it could be substituted for the two -stroke number of moving parts, the two-rotor rotary has engine in any application. In 1972, OMC three, the crankshaft and two rotors. This equals only 23% of the total moving parts of a six -cyl i n d e r two-stroke and only 2% of those for a six -cylinder The costliest components in the OMC production four-stroke piston engine. The rotary crankshaft is process were the rotor housing and the rotor. The a very simple component while the rotor and its seals for the engine were not expensive, according seals are no more complex to manufacture than a to OMC’s production manager (3). OMC confirmed piston and rings. that the crankshaft for the rotary was much cheaper to manufacture than the two-stroke crankshaft. The PART COUNT (Engines with similar torque characteristics) electrical and the fuel systems were also less R OTAPOW ER MAZDA WANKEL 2-STR OKE 4-STR OKE expensive. Assembly was comparable to the two - (Two charge (Two oil cooled PI STON PI STON cooled rotors) rot ors ) (Mercury V6) (Toy ota V6) stroke engine. OMC generally used standard Major 8 8 16 28 machine tools to produce the engine. Additional Part s production equipment included a plasma spray Mov ing 3 3 13 134 Part s capability to plasma coat the wear surface of the Tot al 163 242 225 312 rotor housing and an automated machine for cutting Part s side seal slots in the rotor.
THE OMC PRODUCTION EXPERIENCE WAS ONE The highest process cost associated with the rotor OF LOW COST DESPITE BEING PRODUCED IN housing was the plasma coated wear surface. The RELATIVELY SMALL QUANTITIES surface was first machined and then went through an elaborate preparatory process before being The Research Group at OMC built many different plasma-sprayed. The potential for damage during rotary engines including a 50cc single rotor, 825cc processing was significant. After plasma spraying, single rotor, 1300cc double rotor marine engine, the the housing sides were flat ground to remo ve over- 530cc snowmobile engine and a four rotor 2120cc - spray. Then it was finish-ground on a housing wear 360HP racing outboard. The only engine that was surface grinder (trochoid grinder) and, finally, it was placed into production was the air-cooled lapped to 4 to 5 micron finish. The plasma coating snowmobile engine. Forty thousand (40,000) of was difficult to apply evenly and therefore the these engines were built from 1972 through 1974. resulting coating was put on approximatel y twice as During the same period OMC built 1.5 million two- thick to ensure meeting the required thickness of stroke engines. the finish coating after grinding. This resulted in
extended grinding with a diamond wheel. The raw OMC undertook an extensive cost comparison materials for the plasma coating (tungsten-carbi d e ) between their rotary engine and a two-stroke were relatively costly and the over-spray resulted in engine of similar power. The two-stroke engine an additional 50% loss of the sprayed material. used in the comparison was produced at seven times the volume of their rotary engine. In a direct IMPROVEMENTS IN MANUFACTURING comparison, without accounting for these PROCESSES FURTHER REDUCE THE production differences, the cost of the rotary was ROTAPOWER PRODUCTION COST only 8 to 13% higher than their two-stroke. OMC concluded that if they were produced in similar Casting Technology volumes the production costs would be nearly In the years since OMC’s first production run of identical. (2) rotary engines, several manufacturing processes
have matured that will further reduce the production cost of the Rotapower engine. The most significant Two Rotor of these is the lost foam casting process. This Rotary Engine method of casting was just being developed as OMC was discontinuing their snowmobile engine production. However, OMC did introduce the lost foam process in their two-stroke engine production and continued to experiment with it as part of their continuing liquid cooled rotary engine development through June 1986(4).
Lost foam casting uses a foam pattern packed in sand. The addition of the molten metal burns away the pattern, leaving the “as cast” part. Because the Six Cylinder part is first replicated in foam, rather complex Piston Engine shapes can be cast. Cores, which represent internal features, are placed inside the foam. As a result, the core is fully supported during the pour CRANKSHAFT COMPARISON process, eliminating the typical problem of sifting cores. This process allows the parts to be cast very close By comparison, even a carbureted, charge -cooled to “as-designed” dimensions. The result is the Rotapower engine already meets the emission elimination or reduction of secondary machining requirements being mandated for both industrial operations and very low rejected parts. OMC used and recreational engines (6). Several opportunities sand casting for both their Wankel engine rotor exist to simplify the Rotapower engine further while housing and crankshaft, which is slow and improving its performance. For example, results inaccurate. They used shell molding for the rotor from a NASA-sponsored study, “Evaluation of and this is also replaceable by the faster and mo re Thermal Barrier and PS -200 Self-Lubricating accurate lost foam process. Coatings in a Wankel engine” (patented by Moller International) make a lubricationless engine Coating Technology possible(7). Recent developments have shown the Wear coating technology has significantly matured potential to eliminate the traditional ignition system since the OMC production and can provide very and replace it with an inexpensive proprietary non- economic alternatives for co-deposit plating in powered igniter that utilizes the unique geometry of which a composite coating is created by, for the Rotapower engine. example, plating nickel along with carbides. In the past this process was patent -controlled by Mahle in In an era where emission compliance seems to Germany. Variations of this technology are now translate to complexity and cost, the charge-co o l e d used extensively. Recently the automotive industry Rotapower engine offers four-stroke performance at has begun using this type of plating process to coat the cost of the s imple carbureted two-stroke engine. cylinder walls for low friction, high strength wear surfaces. REFERENCES
This technology can be directly applied to the 1. Ward, Harry and Griffith, Michael, “Outboard application of the wear coating to the Rotapower Marine Corp’s Production Wankel rotor housing. The advantages are several. The Combustion Snowmobile Engine,” SAE plating process is much faster and simpler than the 730119, January 1973. plasma spray process used by OMC. Control of the 2. Miller, George, Engine Program Manager thickness of the surface coat is greatly enhanced, and Chief Engineer, OMC (Retired). which substantially reduces both the amount of 3. Schroeder, Harry T., Production Manager, material used and the time associated with grinding OMC (Retired). the surface coat to the desired thickness. 4. Van Rens, Russ, Manager of Casting Operations, OMC. THE FUTURE OF PISTON ENGINES GROWS 5. CARB Staff Report, “Public Hearing to INCREASINGLY COMPLEX WHILE THE Consider Adoption of Emission Standards ROTAPOWER ENGINE CONTINUES TO SIMPLIFY for Marine Engines,” October 1998. 6. CARB El Monte Test Facility Data, 12/2/98, The original basis for OMC’s interest in the rotary Freedom Motors Emission Testing. engine was the expected stricter emissions 7. NASA Contract NAS3-26309, December standards. The standards anticipated by OMC are 1993. now a reality. Engines are coming under increasingly stricter emission limit mandates.
The technology growth path to comply with these mandates proposed by most piston engine manufacturers focuses on direct fuel injection. As a result of the harmful particulates created by direc t injection, some type of particulate trap will be required as well.
The cost of direct fuel injection significantly increases the cost of the engines. A catalytic converter is also expensive, if required, with its use of precious metals such as platinum . Weight and space are additional considerations in view of the importance of these in both recreational and auxiliary power unit (APU) applications.
FREEDOM MOTORS 1855 N 1st St., Suite B, Dixon, CA 95620 © 2018 All Rights Reserved www.freedom-motors.com