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In engineering, the Miller cycle is a
combustion process used in a type of four-stroke internal
combustion engine. The Miller cycle was patented by Ralph Miller,
an American engineer, in the 1940s. This type of engine was first
used in ships and stationary power-generating plant, but has
recently (late 1990s) been adapted by Mazda for use in the Mazda
Millennia which is also known as a Eunos 800 in some countries.
The traditional Otto Cycle used four "strokes", of which two can
be considered "high power" – the compression and power strokes.
Much of the power lost in an engine is due to the energy needed to
compress the charge during the compression stroke, so systems to
reduce this can lead to greater efficiency.
In the Miller cycle the intake valve is left open longer than it
normally would be. This is the "fifth" cycle that the Miller cycle
introduces. As the piston moves back up in what is normally the
compression stroke, the charge is being pushed back out the
normally closed valve. Typically this would lead to losing some of
the needed charge, but in the Miller cycle the piston in fact is
over-fed with charge from a supercharger, so blowing a bit back
out is entirely planned. The supercharger typically will need to
be of the positive displacement kind (due its ability to produce
boost at relatively low RPM) otherwise low-rpm torque will suffer.
The key is that the valve only closes, and compression stroke
actually starts, only when the piston has pushed out this "extra"
charge, say 20 to 30% of the overall motion of the piston. In
other words the compression stroke is only 70 to 80% as long as
the physical motion of the piston. The piston gets all the
compression for 70% of the work.
The Miller cycle "works" as long as the supercharger can compress
the charge for less energy than the piston. In general this is not
the case, at higher compressions the piston is much better at it.
The key, however, is that at low compressions the supercharger is
better than the piston. Thus the Miller cycle uses the
supercharger for the portion of the compression where it is best,
and the piston for the portion where it is best. All in all this
leads to a reduction in the power needed to run the engine by 10
to 15%. To this end successful production versions of this cycle
have typically used variable valve timing to "switch on&off" the
miller cycle when efficiency does not meet expectation.
In a typical Spark Ignition Engine however the miller cycle yields
another benefit. Compression of air by the supercharger and cooled
by an intercooler will yield a lower intake charge temperature
than that obtained by a higher compression. This allows ignition
timing to be altered to beyond what is normally allowed before the
onset of detonation, thus increasing the overall efficiency still
further. The Miller Cycle engine joins Mazda's
Rotary engine as one of only a few unconventional engine designs used in
modern cars. The Wankel Designed Rotary engine was first put into Mazda
products in the sixty's and has found a great market with the high
performance cars like the RX-7 and Rx-8.
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