The Tale of Rotary (Wankel) Engines

The Tale of Rotary (Wankel) Engines

Somewhere in our time span as homo-sapiens, we all have been part of a machine-cult or loyal to a certain brand. But being in India you do not get your hands on a lot of great machines. So not only are we loyal to a certain brand or machine-cult, but many of us also fancy certain types, like Rear-wheel drives or Rotary Engines or Midships or a certain vehicle which never gets launched (cough *ZX-6r* cough) and so on. The addenda is long but our focus for now is, Rotary Engines.

The Rotary Engine is a type of internal combustion engine, just like the other petrol and diesel engines. The difference is, that Rotary Engines do not have pistons but instead they use rotors which revolve in a epitrochroid (read oval-like) combustion chamber. It works on the same cycle of Intake-Compression-Ignition-Exhaust. While in four-stroke engines the four stages occur one after another in four steps, here it happens simultaneously in a single stroke. Though the stroke length is 50% longer in Rotary Engines and combustion rate is slower.

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The idea of Rotary Engines was envisaged by Felix Wankel which he patented in 1929 but did not have a working prototype until 1957. Though Felix Wankel had the first engine prototype, the second model developed by Haans Dieter Paschke had been taken as the basis on which the modern developments on Rotary Engines took place. Later on, many manufacturers signed license agreements for developing Rotary Engines. All of them, clearly intrigued by the advantages Rotary Engines had over its reciprocating piston peers. The list of manufacturers who were involved with the development of Rotary Engines was a long one, which included Alfa Romeo, Citroen, Ford, General Motors, Mazda, Mercedes-Benz, Nissan, Porsche, Rolls Royce, Suzuki and Toyota. But out of all, only one manufacturer stands out. That is, Mazda. Mazda was the one manufacturer out of all who could successfully mass produce cars having Rotary Engines and their cars were successful too.

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So why were these manufacturers so keen on developing Rotary Engines and why only one of them succeeded? Well, read on.

The main advantages that attracted so many manufacturers were :

  • Simpler design
  • Less weight
  • Fewer moving parts
  • Compact size

But these are just the tip of the iceberg. The design principle encompasses the combustion cycle in a way that there is a power stroke for every revolution as compared to one power stroke for every two revolutions in reciprocating type engines. This results in higher revolutions per minute and because of fewer moving parts the power delivery was beautifully balanced and smooth. The Rotary Engines were not prone to engine knocking and operated almost without vibrations. Also with fewer parts it was easier and cheaper to mass produce and because of the biggest advantage of lesser weight and lesser space it took, the manufacturers could easily fiddle with the car’s size, also improving safety based construction, more features and extra components such as an electric motor or turbocharger could easily be added. Not to forget the higher power-to-weight ratio. If we had to compare a Reciprocating Type Engine with a Rotary Engine of the same cubic capacity, the table would roughly look like this. The data of Reciprocating Type Engine has been taken just as an example to give an easy and approximate comparison.

  Reciprocating Engine Rotary Engine
Cubic Capacity  2000 cc  2000 cc
Power  200 BHP  300 BHP
Weight  200 kg  70 kg
Redline  6000 RPM  9000 RPM
Volume of Engine Body  3 cubic feet  1 cubic foot
Efficiency  10 km/l  6 km/l

So it is clear that minus the setback of fuel efficiency, the Rotary Engines were far more superior than their counterparts. Then why are we all not using them and why could not any manufacturer but one could mass produce Rotary-Engined Vehicles? Well, there is no hero without a tragedy and the tragedy, or let us say tragedies of Rotary Engines were:

  • High Maintenance
  • High Emissions
  • Low Fuel Efficiency
  • Uneven Heating and Thermal Expansion
  • Sealing of the Combustion Chamber

The design of Rotary Engines was not only its strength but also its weakness. Unlike the Reciprocating Type Engines the combustion chamber experienced unequal heating which led to the materials expanding in disorderly manner. Many exotic materials were tried and tested to counter this complication, but all they could do was to delay the inevitable. All Rotary Engines needed consistent change of parts and re-building after a certain distance (approx. 65,000-70,000 km). The sealing of Combustion Chamber was also a major headache as carbon particles used to get trapped in the space created by unequal thermal expansion and leading to engine being jammed. Though Mazda worked hard to fix these issues and provided around 100 sealing related parts. Also by putting in coolant jackets. But what could not be fixed was high emissions and low fuel efficiency. Due to its shape, the combustion was slow which led to having unburnt fuel and thus resulting in lower engine efficiency. Though two spark plugs were added to somehow manage the combustion but it did not improve the situation much. Often when the unburnt fuel left from the exhaust one could see the exhaust backfire and no matter how amazing it is to look at but from emissions’ point of view it was not a pretty sight at all. Lower engine temperatures, unburnt fuel and lower engine efficiency made it very tough to match the emission norms.

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Though Mazda is working hard to develop Laser-Ignition which would replace the conventional spark plugs and provide deeper ignition and would require only a fraction of gap. That could well solve the sealing issues of the combustion chamber, but before all could come into play the Rotary Engines had to be discontinued due to rising fuel prices and mainly because it could not meet the Euro 5 emission norms. 2012 was the last of Rotary Engines everyone saw and as it is said, all great things must come to an end. Or do they?

Behold the MAZDA RX-VISION, the company had unwrapped its concept car in 2015 which runs on SKYACTIV-R Rotary Engine and it is an FR (Front engine, Rear-wheel drive). The company claims that they have fixed all the issues and have made it a Hybrid vehicle by installing an electric motor. Though still a concept and almost two years since its unwrapping, it has surely got us excited because if Mazda has really fixed the issues then we all are in for a treat. But till it happens, all we can do is wait.

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