Miller engine

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The Miller engine (also Miller engine or Miller-cycle ) is a by Ralph Miller named cycle of an internal combustion engine . As with a gasoline engine , the fuel is introduced into the intake air during the intake process, which results in an ignitable mixture in the cylinder . In contrast to the Otto engine, the compression ratio of the Miller engine is smaller than the expansion during the working cycle. A distinction is made here between the Miller and Atkinson cycles .

history

In 1882 James Atkinson developed an internal combustion engine that could execute the four-stroke process in one revolution of the crankshaft in order to circumvent Nikolaus Otto's patent . With the construction used for this (crank arm), the compression of the mixture could be smaller than the expansion. Such an engine has the low tendency to knock of an engine with low compression and the greater thermal efficiency and the low exhaust gas temperature of a high-compression engine. A similar effect can be achieved with the simple crank mechanism common in engine construction if the intake valve remains open at the beginning of the compression stroke and the piston blows part of the sucked-in mixture back into the intake pipe.

In 1947, the American Ralph Miller registered a patent for a similar engine concept. Here, the inlet valve was closed extremely early - while it was still being sucked in.

But it was not until the Mazda Xedos 9 , built from 1995 to 2001, that the Mazda-Atkinson engine was used, but it worked with the Miller cycle. On the Xedos it ran with compressor support.

The Toyota Prius , built since 1997 , the first series-produced hybrid vehicle, has an engine based on the Atkinson principle, which is implemented here by the valve control. Toyota calls this process the Atkinson cycle.

In 2006 Mazda again presented a model with the Miller engine, the Mazda Demio , later Mazda2 . Subaru used such a boxer engine in a hybrid vehicle that same year.

The Demio (Mazda2), a new production model with this engine, has been available in Japan since July 2007. A 1.3-liter engine with Atkinson valve control works in this compact car. To make it easier, Mazda calls this principle the Miller cycle or the engine as the Miller engine.

Since 2016 and from 2017, the VW Group has offered Miller engines within the TSI engine family with 1.5 liters and 96 kW (EA211 evo) as well as 2.0 liters and 147 kW (EA888 Gen.3B), each with a higher Compression ratio and a larger displacement than their direct predecessors.

The Miller or Atkinson principle

In the simulated Atkinson process, in this new work step, the intake valve is kept open longer during the compression cycle after intake, and the compression is started later. Part of the gasoline-air mixture that has flowed in escapes from the combustion chamber. According to Miller, the intake valve is closed during the intake stroke, which also reduces the filling. The longer open but also the previously closed inlet valve reduces the fill quantity in the cylinder on the one hand and the temperature and pressure at the end of the compression stroke on the other, which reduces the risk of knocking .

In both cases, changes to the valve control reduce the effective compression ratio, while the expansion ratio, which is decisive for efficiency, continues to approximately correspond to the geometric compression ratio, so a conventional crankshaft drive can be used.

A compressor or a turbocharger, which supplies mixture or air at increased pressure, compensates for losses and performance deficits compared to conventional engines. The consequences of these modifications are lower exhaust gas temperatures, fewer pollutants, a low tendency to knock and, as a result, high engine efficiency, achieved by increasing the geometric compression ratio or earlier ignition times. The latter is particularly effective in the area of ​​high torques, which are often associated with a low level of efficiency in turbocharged engines, while the increase in the (geometric) compression ratio in the entire map contributes to an increase in efficiency.

Further advantages are the shorter compression path, which means that less heat energy is given off and nitrogen oxide emissions are reduced. In addition, this gas exchange process is suitable for achieving greater performance and lower consumption with a comparatively small displacement by compensating for the negative properties of supercharging when downsizing.

It is disadvantageous that charging is necessary to compensate for the poor filling of the cylinder, since parts of the gasoline-air mixture penetrate back into the supply line. The charger ensures the introduction of a larger mass of the explosive mixture than the cylinder itself could suck in. In comparison to conventionally controlled turbo engines, the main disadvantages in terms of efficiency can be compensated for. This further development is ultimately the step that is necessary to compensate for all the disadvantages of highly charged engines - the performance characteristics of the machine are better when consistently applied in every map area than with a non-charged engine of the same power, while with normal control times always an advantage in the area of ​​lower loads (Objective of downsizing as part-load consumption measure) as a result of dethrottling, a disadvantage at high loads due to the lower compression ratio and / or later ignition times.

The use of the Miller cycle in the nominal power range can also contribute to a reduction in the exhaust gas temperature due to the earlier combustion and further expansion of the working gas and thereby avoid the need for mixture enrichment or shift it to a higher specific power.

The main disadvantages here are the increased complexity of the internal combustion engine, since a significantly higher boost pressure is required for the same full load torque curve than in the case of conventionally controlled downsizing engines.

A reduction in the specific power and the specific torque - i.e. an increase in the displacement compared to a pure downsizing concept of the same power - can also be successfully implemented through the dethrottling effect of the lower effective intake volume, since the expansion ratio of the working gas increases with the same effective intake volume can be. Depending on the objective, the use of a conventional charging system is sufficient.

The Miller or Atkinson processes compensate for the disadvantages of downsizing, which means that the necessary reduction in stroke volume can be less drastic. At the same time, however, the maximum reduction in displacement is increased, since the operating limits of the work process are expanded. The latter, however, requires a more complex motor, which is also correspondingly more expensive if z. B. replacing a simple turbocharger with a complex charging system.

See also

Individual evidence

  1. ↑ http://www.priuswiki.de/index.php?title=Atkinson-Zyklus Description of the Atkinson cycle on the Toyota Prius Wiki website
  2. ↑ The new 1.5-liter four-cylinder TSI engine from Volkswagen. Retrieved February 21, 2017 .
  3. ↑ MTZ - Motortechnische Zeitschrift 5/2016. Retrieved February 21, 2017 .

Web links / sources