OK, I think that it is time to switch it up just a little bit here on Autospies. As we all claim to be car buffs and aficionados, I wanted to inform and shed some light on the misconceptions that plagues one of the most unique, reliable and underrated engines in the industry. It is a technology that has proven itself time and time again but is never recognized or given accolades. It also will fill in a niche that I feel we are missing on this site.
Ladies and Gentlemen I present to you the Rotary Engine.
A couple of months back there was an article on an RX-8 recall and it created debates as to how unreliable and inferior the rotary was to the piston. So I am here to give a real perspective on an ignorant stereotypical view.
This is the first of a few installments that I will try to use to insight our on-line public as to the marvels of the Rotary. So, to start us on this journey a little history is in order.
Editorial is a composite of my knowledge in addition to other informational articles.
History of Rotary Machines
The rotary combustion engine (RCE) must not be confused with "rotary" aircraft engines which are piston cylinders arranged in a circle. The whole engine rotates. These came into vogue in World War I.
There are three main types of true rotary engines:
1. Wankel types based on eccentric rotors,
2. Scissor action types using vanes or pistons, and
3. Revolving block types ('cat and mouse' type).
Designs for rotary engines were proposed as early as 1588 by Ramelli, though it took the development of the Otto cycle engine in 1876 and the advent of the automobile in 1896 to set the stage for a proper rotary combustion engine. Furthermore, it took Felix Wankel to catalogue and organize 862 configuration pairs, of which 278 are impractical. Wankel investigated 149. Prior to 1910, more than 2000 patents for rotary pistons were filed.
Wankel-Type Engines
The Wankel RC Engine is an Otto Cycle Engine (Four "Stroke").
Wankel engines use cycloidal gearing, an old and unusual form of gearing used in watches, Roots blowers, screw compressors, and pumps.
A Rotary Combustion Engine (RCE) as invented by Felix Wankel and developed by Walter Froede differs from a Piston Engine in four primary ways:
• The RCE Rotor supersedes the piston engine's piston.
• The RCE Eccentric Shaft supercedes the piston engine's crankshaft and connecting rods.
• The RCE Peripheral Housing supersedes the piston engine's cylinder.
• Intake and exhaust ports in the housings eliminate valves, camshafts, and timing belts.
The Housing shape is an epitrochoid curve (more properly, peritrochoid). It is generated by rolling a circle around another circle. The rolling circle is half the size of the generating circle. Drexel provides an extended discussion of the mathematics of the epitrochoid curve and the rotor shape. In other words. Its only moving parts are a three-sided rotor turning on a straight spindle; there is neither crankshaft nor camshaft. The rotor is not fixed to the spindle, but turns it by means of an internal gear on the inside of the rotor engaging a smaller conventional gear on the spindle. The rotor is positively located by the spindle and the geometry of the rotor and engine chamber. There is still some vibration, as while the centre of gravity of the spindle does not move, that of the rotor does. The angular momentum and kinetic energy of motion of the rotor also both vary, producing more vibration, see engine balance. A Wankel engine fires three times for every revolution of the rotor, so a single rotor is in some ways equivalent to a six-cylinder reciprocating engine.
There are two main types of RCE
• KKM Kreiskolbenmotor or planetary rotation motor (PLM), distinguished by one stationary peripheral housing. The rotor moves in an orbit and propels an eccentric shaft. This is easier to manufacture, cool, and maintain than DKM. Intake and exhaust passages are better. Better cooling is achievable. It is more compact. Modern rotaries are KKM types.
• DKM Drehkolben Machine or single-rotation engine (SIM), the first RCE, has the distinctive feature of an inner rotating housing and rotor moving in circular motion around a fixed central shaft. This requires disassembling the motor to change spark plugs, perhaps the main reason it was discarded. However, DKM is the smoother of the two, and high rates beyond 25,000 rpm are possible. Bearing loads are lighter.
The Wankel rotary was immediately recognized by the international engine community as an outstandingly simple approach to achieving all the functional characteristics of the efficient but complicated four-stroke piston engine. This recognition led to a world-wide frenzy to develop reliable and efficient models of this engine initially by Curtiss-Wright, Mercedes Benz, NSU, Fichtel-Sachs and later by Outboard Marine Corporation (OMC) and Mazda. Curtiss-Wright and then John Deere, who purchased the Curtiss-Wright technology, spent over $1.5 billion on Wankel-type rotary engines, mostly in anticipation of replacing turbine engines in military applications. By 1980, more than 1,500 U.S. patents were issued on Wankel-type engines. These have since expired and can be used by anyone. The total estimated expenditures on Wankel-type rotary engines exceed $3 billion dollars. Mazda alone spent well over one billion dollars on Wankel rotary engine development and production facilities.
In general, the attributes of the Wankel rotary were outstanding, but in the automotive industry it was competing against the four-stroke piston engine with a 100 year history and hundreds of billions of dollars behind it. In addition, tests suggested that the Wankel rotary had poorer fuel consumption at lower power settings (at that time sealing was not as good as the piston engine at low speed).
By 1974, when the Wankel rotary was entering limited production, the automotive industry, which really drives the four-stroke piston engine industry, was facing two concurrent hurdles:
• The energy crisis of 1974, which emphasized the need for immediate fuel economy improvements.
• Pollution reduction mandates were initiated and qualifying a new engine along with the existing piston line was more than the automotive industry felt capable of financially undertaking.
As a result, other than limited production projects by Suzuki, Yanmar Diesel, NSU, OMC, and Fichtel-Sachs, only Mazda created a volume production facility and continues to produce engines. However, Mazda confined their Wankel engine primarily to a sports car (RX-7) and the low engine production numbers never allowed the engine to be produced economically within their high-volume production facility. Hence the RX-7 became rather high priced and limited in its sales.
A two-rotor design has been adopted by Mazda to further reduce vibration, and three- and four-rotor designs have been used in racing, notably the 4-rotor Mazda 26B engine that powered the winning car in the 1991 24 Hours of Le Mans race.
Wankel-based engines have a remarkable record of reliability. Ingersol-Rand achieved over 40,000 hours without an overhaul. Rotary Power International (RPI) rotary engines (took over from John Deere) are guaranteed for a minimum of 10,000 hours, and most of the OMC engines produced in the mid-1970's are still running today without an overhaul. Mazda racing engines operate an entire racing season without an overhaul while piston competitors are overhauled after each race.
Advantages
• Light weight and compact.
• Smooth: no reciprocating motion.
• Extended power "stroke" rotation of the output shaft: 270 degrees vs. the 180 degrees of a piston.
• Fewer moving parts: no valves, connecting rods, cams, timing chains. Intake and exhaust timing are accomplished directly by the motion of the rotor.
• Flat torque curve because no valves are used.
• Cooler combustion means fewer oxides of nitrogen. Catalytic converters lessen this advantage.
• Separation of combustion region from intake region is good for hydrogen fuel (technology in testing now).
The Rotary is a true phenomenon in the making, but it is not without its setbacks.
Disadvantages
• High surface to volume ratio in combustion chamber is less thermodynamically efficient. The Wankel's long and narrow chamber makes for long flame travel, but this is countered by the Mazda's two spark plugs (three on some racing engines).
• Higher fuel consumption. Thus Mazda has been successful with the RX-7 sports car, a market where fuel consumption is less of an issue. Only 16 years after the first engine ran the 1973 oil crisis devastated the RCE before it had sufficiently developed to become more economical.
The Wankel-type engine is less fuel efficient at very low power settings, which is where automobiles in the developed countries operate much of the time; i.e., 10% power at 60 MPH, 2% power at 20 MPH. In developing countries where engines used for transportation operate at the higher end of their power capability and where many two-stroke engines are used, the Freedom engine is far superior in fuel efficiency.
There is one last issue that has hindered the success of the Rotary, which is due in major part to many owner lack of fundamental knowledge of how to take care of the engine which leads to excessive abuse.
In practice, the main problem of the Wankel engine has proven to be the seals, and all proposed designs have some of the same potential weaknesses.
Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the reverse is the case. As well as the rotor apex seals, the rotor must also seal against the chamber ends. Worse still, these two sets of seals must somehow join at sharp corners at the ends of the apex seals.
An additional problem is that the seals at the Wankel rotor apexes meet the chamber walls at an angle that varies plus and minus ~26 deg; during the cycle, while a piston ring meets the cylinder walls at a constant angle. As well as making the seal design itself more difficult, this means that while multiple rings are easily fitted to a piston, a corresponding approach is impossible with the Wankel apex seals. Piston rings are not perfect seals. Each has a gap in fact to allow for expansion. Moreover the sealing at the Wankel apexes is less critical, as leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the crankcase. However, the less effective sealing of the Wankel is one factor reducing its efficiency, and confining its success mainly to applications such as racing engines and sports vehicles.
A further problem caused by the shape of the seals in Mazda engines is that carbon particles can become trapped between the seal and the casing, completely jamming the engine and requiring a partial strip down to rectify.
Let it be known that the issue of the seals can easily be overcome by taking simple precautions.
1. Wankel engines should never be started and run unless the engine will reach operating temperature; most such instances of jammed engines occur when a car is started and moved a few yards, e.g. from a garage to a driveway. In these situations it is better to push the car and not start the engine or just let it run for 5 mins before turning it off.
2. Also like any other engine when brand new, it can not be subject to extreme abuse, most owners will freely rev the engine high and race the car right of the dealership lot (it needs time to settle in like any other car).
3. Lastly, DO NOT use SYNTHEIC OIL. The Wankel design is based on burning regular oil as well as using the oil to lubricate the seals. Synthetic oils will kill engine life within no time.
In this new age, technological advances are growing at a fast pace everyday. Once the Rotary has been able to overcome the hindrance of fuel efficiency and carbon buildup (solved by the introduction of Hydrogen), I can assure you that you will be hearing more about this engine in years to come.