Saturday, February 07, 2009

Primer: The Wankel or Rotary Engine, with Emphasis on Mazda's Contribution

No one asked for this post, but I wanted to assemble a short introduction to the rotary or Wankel engine. It will be useful to me for a later post.


Rotary engines (REs) are internal-combustion engines that don't have reciprocating pistons; instead, they have "fat triangles," or rotors, which spin inside peanut-shaped epitrochoidal housings. Numerous fascinating animations of the rotary combustion process are available online. Here's one, for example; here's another.  To understand the process, don't watch the rotor; watch one of the chambers---called working chambers---created by the gap between the rotor and the housing.  The rotor is mounted on an eccentric shaft, and it both revolves around its center and orbits the shaft's center.  As the rotor spins, the working chamber expands and contracts, mimiking the traditional strokes of a 2-stroke piston engine:  intake, compressesion, combustion, and exhaust.

A rotary has several advantages over reciprocating engines:
  • Most importantly, well-designed REs put out more torque and much more power than similarly size 4-stroke piston engines with similar induction.* For example, the above-mentioned RX-8's powerplant, which is naturally aspirated, produces 159 lb-ft of torque and 232 BHP of power, all from 1.3 liters of displacement. That's 178 BHP/l. By comparison, 100 BHP/l is considered very high specific power output for naturally aspirated reciprocating engines. The reason for this superior output becomes obvious when you count the number of combustion cycles per crank-shaft (or eccentric shaft) revolution per cylinder (or rotor). In a 4-stroke piston engine, for each piston, there is a single combustion cycle for every 2 revolutions of the crank shaft. In a Wankel, each of the 3 faces of the rotor see 1 combustion for each revolution of the rotor. However, the eccentric shaft rotates at 1/3 the rate of the rotor, so there is 1 combustion for each rotation of the eccentric shaft, for each rotor. Thus, an N-rotor RE fires twice as often as an N-piston reciprocating engine, for a given engine speed.
  • Rotaries have very few moving parts, making them small, light, and durable. There are no valves or connecting rods; each engine is comprised of just an eccentric shaft---analogous to the crankshaft in a piston engine---some number of rotors---usually 2, but sometimes 3 or 4---and 3 apex seals per rotor.
  • Rotary engines run extremely smoothly. The main cause of this smoothness is the rotary, rather than reciprocating movemen of the engine. Additionally, the overlap of the combustion cycles on adjacent faces of the rotor smooths power delivery, when comparing an N-rotor Wankel to an N-piston reciprocating engine. Lastly, the twice-as-high rate of combustion cycles, as mentioned above, smooths power delivery, comparing and N-rotor to an N-piston. These last two advantages are mitigated when comparing the common 2-rotor Wankels to multi-piston reciprocating engines.
  • Also, since the rotary motion doesn't stress the parts the way reciprocation does, rotaries can reach very high engine speeds. For example, the Renesis engine in the RX-8 redlines at 9000 RPM.

Like everything else in engineering, the rotary engine also has disadvantages. I'm sure you guessed that based on the fact that the vast majority of vehicles on the road are piston-powered.
  • Rotaries tend consume fuel at a rate disproportionate to the amount of power they put out. This problem has several causes. First, the long, thin working chamber---analogous to the combustion chamber of a piston engine---has a hig ratio of surface area to volumn, yielding poor thermal efficiency; energy released during combution escapes the working chamber as heat, rather than being used to push the rotor through its orbiting and revolvling path. Secondly, REs designed to date have low compression ratios, and some hydrocarbons escape the combustion cycle unburned. Lastly, sealing the working chamber at its sides and its apex is more difficult than sealing a reciprocating engine with piston rings, so some fuel escapes unburned
  • Rotaries have tended historically to emit pollutants at a rate disproportionate to their power output. This problem stems from the the low compressin ratios and poor sealing, both of which result in the release of unburned hydrocarbons. Modern rotary REs, by which I mean the Renesis, engine have managed to meet contemporary emissions standards, however; THe RX-8 is classed as a Low-Emissions Vehicle (LEV) by California.
  • Although rotaries have high specific torque outputs, the peak torque is typically fairly small compared to the peak power. This phenomenon is related to the fact that power is given by torque times engine speed (RPM) ---with an appropriate multiplier to give you the units you want. Most drivers "buy power but drive torque": they compare cars based on the peek power (a scalar) but experience a vehicle's performance based mostly on the torque curve (a vector), especially at low engine speeds. As a result, a rotary with a "reasonable" peak-power number will feel "weak" to many drivers. This particular shortcoming can be partially overcome by keeping the engine at higher revs, in the "meaty" part of the power band.
  • Since rotaries burn some oil; indeed, a small amount of oil is injected into the working chamber. Thus, the owner must check and fill the oil regularly. The RE in the RX-8 reportedly burns much less than a quart in the 3000 miles between oil changes, so the refilling burden isn't onerous. However, this oil burning also means that REs must be lubricated with conventional oil; synthetic oils have flash points that are too high; they do not burn and instead leave behind residue. Thus, rotaries cannot take advantage of the beneficial properties of synthetic oil. I'm not sure if it's possible, but I'd like to see a design that uses synthetic oil to lubricate the engine but also has a small reservoir of conventional oil that feeds the oil-metering pump. Perhaps some lubricating oil reaches the working chamber by a route other than the OMP, making my solution impossible to implement.


The rotary engine was invented by Felix Wankel, a German,** in 1957, and it is often called the Wankel engine. I prefer to call this engine design a rotary, not out of disrespect for its inventor, but because I feel that "rotary" is more descriptive. Numerous manufacturers, including Mercedes-Benz and General Motors, experimented with rotaries in the 1950s and 1960s, but few rotary-engined automobiles reached the market until Mazda embraced the rotary.

Mazda launched the Cosmo in 1967 and has been the leading proponent of the rotary ever since. When the oil crises hit in the 1970s, they had nearly eliminated piston-powered vehicles from their model line-up. They even offered a rotary-powered pickup. Because of the poor fuel consumption of the rotaries, Mazda suffered rather badly during the that time. But, thanks in part to investment by Ford, Mazda survived.***

Mazda remained committed to developing the rotary, but they recognized that the engine's particular advantages and disadvantages were best suited to use in a lightweight sports car, a small niche in the automotive industry. So, for about 30 years, at least in the US, there has only been 1 internal-combustion-powered automobile that you could buy that didn't have pistons. For many years, that car was the RX-7; 3 generations of the '7 were produced. Currently, the lone rotary-powered car available is the RX-8.

The rotary engine has also made significant contributions in motorsports, as you might suspect given the high specific power produced by this engine type. In competition, rotary-engined cars are typically classed based on an effective displacement given by the true displacement multiplied by a factor between 1.5 and 2. Perhaps the most famous rotary-engined race car was the Mazda 787B, which, propelled by a naturally aspirated 4-rotor Wankel, scored the overall win at the 24 Hours of Le Mans. The following year, the governing body outlawed Wankel engines. To this day, this victory represents the only overall win at Le Mans by a Japanese manufacturer, or indeed any manufacturer outside of Western Europe or the United States.

The Present

The current single production rotary-powered vehicle Mazda's RX-8 With the '8, Mazda attempted to broaden the appeal of the car by adding 2 rear seats and unusual suicide doors by which to access them. The '8 is powered by the 13B-MSP Renesis engine. The Renesis---a portmanteau of "RE" and "genesis" displaces 1.3 l, hence the "13" in the name. The Renesis differs from the 13B that powered the RX-7 for many years partly by its slightly tweaked geometry but more significantly by the port location. The exhaust ports are no longer located peripherally; instead they are positioned on the sides of the rotor housings. ("MSP" stands for "Multi Side Port.") These changes yield improved emissions and fuel economy over the 13B. Unfortunately, I don't see how you can make a 3- or 4-rotor RE with side ports unless you leave space between the rotors.

The Future

The next production rotary engine from Mazda, the 16X, is currently being developed, and it powers Mazda's exotic Taiki concept car. The 16X offers several improvements designed to addressed the usual shortcomings of REs: low torque, poor fuel economy, and potentially poor emissions.
  • The displacement of the engine is 1.6, as the name indicates. The rotors have increased diameter with decreased width. The increased stroke analogous to increasing the stroke of a reciprocating engine---Mazda calls the 16X a long-stroke RE---and it increases the torque at all engine speeds. The new aspect ratio improves the thermal efficiency of the engine, keeping the energy produced by combustion inside the working chamber and not letting it leak out as heating of the rotor housing. This change should improve the fuel efficiency of the engine.
  • The 16X has direct injection. DI has been common in Diesel engines for year, and it is now becoming popular in petrol engines. In DI, the fuel in injected directly into the cylinder or working chamber rather than upstream, in the intake path. This injection method, when used with appropriately sophisticated injectors and engine-management systems yields very pieces control of the fuel delivery. The result is improved power and torque, efficiency, and emissions. The downside to DI is extra cost and complexity.
  • The 16X also uses aluminum side housings to reduce weight. That change should slightly improve the performance, efficiency, and even emissions any vehicle carrying this engine.

Conveniently, despite the increased displacement, the 16X is the same size, externally, as the Renesis. This, it can be dropped into the current RX-8 with few changes. Many "rotar heads" expected or hoped that the 16X would have reached dealers in the engine bay of the 2009 RX-8, but they were disappointed. Perhaps it will appear in 2010, or perhaps it won't show its face until the next RX car is released, whenever that turns out to be.

What about further in future? Mazda's rotaries have shown the ability to run on several different fuels, and Mazda is using this ability to experiment with cleaner, greener rotary engines:
  • A DI version of the Renesis called the Hydrogen-RE can run on hydrogen or normal gasoline with the flick of a switch. Mazda leases Mazda5s and RX-8s powered by this engine to commercial customers in Japan and Norway.
  • The dead-sexy and amazing-sounding Furai concept, is powered by a version of the 3-rotor 20B**** engine that can be tuned to run on 100% ethanol or ethanol-gasoline blends.

Further Reading

You can learn more about rotary engines and rotary-powered cars form Rotary Speed Magazine (formerly Mazdasport Magazine), Rotary News, and No Pistons forum.

* By similar induction, I mean that both engines are naturally aspirated or both engines are super- or turbocharged to the same level of boost.

** It may seem as if most engine designs were invented by Germans; Certainly Otto, Diesel, and Wankel get most of the press. However, the Stirling and Miller engines were invented by Anglophones.

*** (Later, by the way, the huge financial and critical success of the first-generation, or NA, Miata---known as the MX-5 Miata in North America, the MX-5 in Europe, and the Roadster in Japan---revitalized the carmaker and is significantly responsible for the company's strong financial position and sporty image today.)

**** The 20B is essentially 3/2 of the 13B, and it thus has peripheral ports.

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