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Thursday, March 3, 2016

Broken Promises of the Wankel Engine

BROKEN PROMISES OF THE WANKEL ENGINE

Through the history of internal combustion engines, there has been plenty of evolution, but few revolutions. Talk of radically different designs always leads to a single name – Wankel. The Wankel rotary engine, most notably used in automobiles by Mazda, has been around since the late 1950’s. The Wankel rotary is an example of a design which makes sense on paper. However, practical problems cause it to underperform in the real world.

Original Article: http://hackaday.com/2016/03/03/broken-promises-of-the-wankel-engine

Invention and History

felixwankelFelix Wankel’s engine was conceived during a dream. In it, 17-year-old Felix was driving his car to a concert. When he arrived, he bragged to his friends that his car used a new type of engine – half turbine, half reciprocating. “It is my invention!” he told his friends. Upon waking up, Wankel became dedicated to building his engine. Though he never received a formal degree (or a driver’s license), Wankel was a gifted engineer.

Young Wankel’s checkered history includes membership in several anti-semitic groups in the 1920’s. He was also involved with the founding of the Nazi party. His conflicting views on the direction of the party lead to his arrest in 1933. Eventually released through action of Hitler himself, Wankel joined the SS in 1940. The end of the war saw Wankel spending several months in a French prison for his wartime involvement.

felixwankelWork on the engine resumed in 1951 with funding from NSU Motorenwerke AG. The first working prototype was produced in 1957. Dubbed the DKM 54, this engine had a rotor and housing which rotated on separate axes. The engine was capable of great rotational speeds, up to 17,000 RPM. Maintenance was a problem though. The entire engine had to be torn down just to replace the spark plugs.

Unknown to Wankel, Hanns Dieter Paschke was called in to build a simplified version. His prototype was called the KKM 57P. This much simpler design utilized a stationary housing. It pleased everyone except Wankel who remarked, “You’ve turned my race horse into a plow mare.” The KKM design was quickly adopted and licensed. This engine is the basis of the modern “Wankel” rotary engine.

Engine operation

Piston powered engines, chiefly the Otto and Diesel cycle, are current kings of the internal combustion mountain. Piston powered engines turn reciprocating energy (the up and down motion of the pistons) into rotational energy. Wankel flies in the face of all this. A simplified Wankel engine has only two moving parts: the rotor, and the eccentric shaft.

The rotor is a triangle shape, but the sides bow out. Many rotors are also use cupped faces to increase combustion chamber volume. The rotor rotates within a roughly oval epitrochoid-shaped housing. The rotor doesn’t just spin, it orbits on an eccentric shaft which is analogous to the crankshaft of a piston powered engine. A fixed gear mounted to the engine case meshes with a ring gear on the rotor. The gear ensures that the rotor rotates ⅓ turn for every 1 turn of the eccentric shaft.
rotary1

The points (or apexes) of the rotor create three chambers inside the housing. These chambers move with the rotation of the rotor. Fuel and air are pulled in through the intake port, compressed against the narrow side of the housing, ignited by the spark plugs. The expanding gasses push the rotor through the power stroke until the apex passes the exhaust port, which allows the spent gasses to escape.

The animation shows the process for one face. The genius of the Wankel engine is that the process is happening for all three faces in parallel. In effect, the engine has pipelined the combustion process. It would be fair to say that a single rotor Wankel engine is analogous to a three cylinder piston engine.

Commercial research and development

RotaryThere were numerous licensees for the Wankel engine. Just about every major manufacturer spent time researching the concept. GM created a two rotor prototype. Rolls Royce created a two stage model with low and high pressure rotors. A few companies put the Wankel into production. Curtis Wright built airplane engines, Sachs produced small air-cooled engines for everything from chain saws to snowmobiles. Norton created several motorcycles using the design. However, the only major manufacturer still working on Wankel engines for cars is Mazda. The RX series of sports cars has been synonymous with Wankel rotary engines for decades. The last model was the RX-8, discontinued in 2011. Mazda has not given up on the Wankel though, with concept cars such as the RX-Vision as proof of their continued research.

Reality sets in


So why aren’t we all driving Wankel-powered cars? The problem lies in the pitfalls of the design.

Fuel Economy: 

The Wankel’s combustion chamber is long, thin, and moves with the rotor. This causes a slow fuel burn. Engines try to combat this by using twin (leading and trailing) spark plugs. Even with the two plugs, combustion is often incomplete, leading to raw fuel being dumped out the exhaust port. The small 1.3 liter 232 horsepower two rotor engine in the 2011 Mazda RX-8 gets worse fuel economy (16 city / 23 highway) than the 6.2 liter 455 horsepower V8 engine used in the 2015 Corvette Stingray (17 city / 29 highway).

Emissions: 

The unburnt fuel, along with burned oil (described below) both result in terrible emissions from Wankel engines. The emissions problems are one of several reasons the RX-8 was pulled from production.
gmRotaryRotors use seals on the faces, seals around the central port, and most importantly apex seals. The apex seal rides the wall of the housing, sealing each of the three chambers formed by the rotor. The apex seals are under extreme thermal and pressure stresses as they travel around the engine housing. Failing apex seals are the primary cause of rotary engines going down for overhaul. YouTube is littered with videos showing the rotary overhaul process.



Much like piston rings, these seals have to be lubricated. However, due to the design of the rotary engine, there is no way to keep the oil lubricating the seals out of the combustion chamber. Mazda engines include an injector pump which pushes small amounts of oil right into the engine housing, as well as into the air intake. This oil is eventually burned, causing increased carbon and emissions over the life of the engines.
wankel-inside-kart-engine

Overhaul interval:


 Rotary engines in general don’t last as long as piston powered engines. As explained eloquently by Regular Car Reviews, the primary problem is with the seals. Browsing Mazda and rotary forums shows people rebuilding somewhere between 50,000 and 100,000 miles. However, this all must be taken with a grain of salt. The RX-7 and 8 are after all, sports cars. While some people treat them gingerly, many people drive these cars hard. Aftermarket performance parts like turbochargers will also negatively impact engine reliability.

787
The Wankel rotary story isn’t completely bleak. There are some advantages to rotary engines. As mentioned above, rotary engines create more power (albeit at lower torque) than equivalent piston powered engines. They also are more reliable in the short term. With fewer moving parts, there is simply less to break. Rotary engines also tend to fail gracefully. With failing apex seals, rotary engines lose power, but will still get you home. Piston powered engines tend to fail catastrophically, blowing holes into engine blocks, spraying oil and parts all over the place. Rotary engines do well on the racetrack, that is, when they are allowed. Many racing classes (notably F1) have banned rotary engines. Of those allowed, the most notable is the Mazda 787B, which won the 1991 24 hours of Le Mons race.



What does the future hold for the Wankel rotary engine? Most likely more of the same. Mazda will continue to support the engine, and it will continue to be used in some niche fields. However, it would take a major advancement in materials and design to correct all the issues that have thus far relegated the Wankel engine to a footnote in the history of internal combustion.



Broken Promises of the Wankel Engine