Wankel diesel engine

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The Wankel diesel engine was the idea of ​​realizing the diesel cycle in a rotary piston engine . The experiments started in the 1960s and lasted until 1974. Among other things, operates Rolls-Royce , the "Diesel-Ring" ( MAN , KHD , Krupp and Daimler-Benz ) and Felix Wankel their own developments. Ultimately, however, due to technical inadequacies, the Wankel diesel engine never got beyond an early test stage and is therefore not considered to be functional.

functionality

Test engine 2-R6 from Rolls-Royce

A Wankel diesel engine is structurally similar to a conventional four-stroke Wankel engine , but differs from it fundamentally in that the features of the diesel engine are implemented and therefore a supercharger is required. An essential characteristic of the diesel engine is the auto-ignition of the fuel due to high compression. This is achieved in the diesel engine through a high compression ratio. Due to the design of the combustion chamber, it is fundamentally not possible in a rotary engine to achieve a sufficiently high compression ratio without a pre-compressor to reliably initiate auto-ignition of the fuel. There are several approaches to the design of the pre-compressor. Felix Wankel designed it as a separate compressor, while Rolls-Royce built an 8-shaped rotary piston engine with two rotary pistons, the large rotary piston of which served as a pre-compressor.

Reasons for the development of the Wankel diesel engine

The disadvantage of gasoline engines compared to diesel engines is their lower efficiency. In the late 1960s, however, the advantages still predominated: Otto engines were lighter, quieter and less vibrated than diesel engines, which was particularly important for driving passenger cars. Rotary engines are characterized by a low mass and smooth running of the engine, so that there were considerations to implement the diesel cycle process in a rotary engine in order to construct a compact, light, low-vibration, yet economical engine. Another approach was the development of a classic Wankel engine that can be operated with inexpensive diesel fuels, but does not work according to the diesel process. The EPA had investigated whether rotary engines with a diesel process might have better emission behavior than conventional gasoline engines.

Various developments

Diesel ring

The joint coordination of the research programs of the German engine industry, namely Klöckner-Humboldt-Deutz , Daimler-Benz , MAN and Krupp , is referred to as the “diesel ring” . The developments were stopped in 1969 because the shape of the combustion chamber of the Wankel engine was classified as unsuitable. However, the decisive factor for the hiring was possibly also that the companies mentioned were building reciprocating diesel engines that were well established on the market and that there was little interest on the part of management in changing the functioning reciprocating piston principle.

Rolls Royce

Rolls-Royce began developing a rotary diesel engine in 1965 for the British Department of Defense. Fritz Feller was the lead developer. It was recognized early on that a sufficiently high compression ratio for compression ignition could not be achieved in a rotary engine without auxiliary equipment and therefore favored an engine with a circular piston-shaped supercharger, but first attempts were made with a modified NSU rotary engine that was operated temporarily with heated compressed air. It was found that the seals on the housing were insufficient, so new seals had to be developed. The shape of the combustion chamber also turned out to be problematic, which is why a large part of the development work concentrated on the construction of a suitable combustion chamber that would allow fuel and air to mix sufficiently well. At first they experimented with pre-chamber injection , but this was rejected because the unfavorable housing design of the Wankel engine made the accommodation of the pre-chamber appear problematic. The first 8-shaped prototypes with two rotary pistons were the R4 and the R5 . On the R5, the lower rotary piston should only be used for pre-compression; the exhaust gas should not flow back through the lower rotary piston, but rather drive a separate exhaust gas turbine . It is not known whether these engines have been run on a test bench.

At the beginning of 1971 the prototype 2-R6 was completed. This engine had an 8-shaped structure with the small main circular piston on top and the large supercharger circular piston below. The fuel was injected directly into the upper combustion chamber. The chambers of the two rotary pistons were connected to each other, so that the pre-compressed air flowed into the inlet of the main chamber and the exhaust gas again hit the rotary piston of the pre-compression chamber before it flowed into the exhaust. Both rotary pistons were connected to one another at a ratio of 1: 1 and turned in the same direction. The power was taken from the precompressor rotary piston. The motor should a displacement volume of 396 in 3 (6489 cm 3 ) have, 350 hp (261 kW) at 4500 min -1 afford and 929 lb (421 kg), weigh which is about half of the mass met in 1971 of a conventional Hubkolbendieselmotors same power would have. Other plans included the development of a 700 hp (522 kW) engine. In fact, the engine with 350 hp projected power could not be realized. Despite the light alloy, the engine weighed around 1150 lb (522 kg) and the power was only 180 hp (134 kW). It is doubtful whether this engine ever ran on its own. In the described test runs of this Wankel diesel engine, externally supplied, preheated compressed air was always necessary. To preheat and pre-compress the intake air, more power was required than the engine itself could generate. The further development of the 2-R6 was stopped in 1974, possibly for financial reasons. It is believed that the Yom Kippur War was decisive, which caused the British military to lose interest in a compact drive for tanks, so that payments to Rolls-Royce for the development of the engine were stopped and the company went bankrupt. The decisive factor for discontinuing the project, however, was that it did not appear technically possible to build a permanently functional Wankel diesel engine.

Felix Wankel

The Wankel diesel engine designed by Felix Wankel has two cloverleaf-shaped housings with a supercharger housing in between, which lie on a longitudinal axis. In contrast to the conventional Wankel engine, the rotary pistons are not triangular but square, while the supercharger has the shape of an ellipse and two corners. Despite this unusual design, the engine was able to run under test conditions on the test bench. Another prototype for basic test experiments had a housing and a rotary piston with a displacement of 700 cm 3 . On the dynamometer, this engine ran for around 20 hours and reached a speed of 5500 min −1 , at 3500 min −1 a torque of 60 Nm was delivered.

Causes for the lack of functionality

One reason for the inadequate functionality is the convex, elongated, unfavorable combustion chamber shape of the rotary piston, which does not allow a sufficiently high compression despite the pre-compressor. The use of externally supplied, pre-compressed air is always necessary. Furthermore, the combustion chamber is shaped unfavorably, so that the injection jet cannot be optimally directed towards the combustion chamber. The consequence of this is that the fuel does not burn completely; In a prototype of the Rolls-Royce type examined on behalf of the EPA , excessive amounts of hydrocarbons and carbon monoxide were found in the exhaust gas.

swell

  • Fritz Feller: The 2-stage rotary engine - A new concept in diesel power , Proceedings of the Institution of Mechanical Engineers, 1970-71, Vol. 185 13/71, pp. 139-158
  • Popular Science, February 1971
  1. ↑ a b c d David Scott: NOW! A Diesel Wankel from Rolls-Royce , p. 80
  • Popular Science, March 1973
  1. ^ A b Jan P. Norbye: The View Down the Road , p. 62
  • Ulrich Christoph Knapp, Gunter Bayerl: Wankel on the test stand , Waxmann, ISBN 978-3-8309-6637-1 , pp. 120-122
  1. ↑ a b c p. 120
  2. ↑ p. 121
  1. ↑ p. 100
  2. ↑ p. 101
  3. ↑ a b p. 102
  4. ↑ a b c p. 103
  1. ↑ a b Sections 1–15
  • Road Test, Volume 9, Quinn Publications, 1973
  1. ↑ p. 10
  2. ↑ a b c p. 11
  3. ↑ p. 92