Modern Engineering Thermodynamics

13.20 Miller Cycle 509
13.19.1 Modern Atkinson Cycle
In 1947, an American engineer named Ralph Miller patented an ingenious variation of the original Atkinson
cycle. Rather than varying the actual length of the intake stroke, he realized that you could simply delay closing
the intake valve past the end of the intake stroke. Then, as the piston traveled back up thecylinder,itsimply
pushed air back out into the intake manifold. The compression began only when the intake valve was finally
closed, and by altering when the intake valve closed, you could effectively change the compression ratio of the
engine.
Today, the term Atkinson cycle is used to describe Miller’s modified four-stroke Otto cycle, in which the intake
valve is held open longer than normal to allow the piston to push some of the intake air back out of the cylinder.
This reduces the compression ratio, but the subsequent expansion ratio is unchanged. This means that the
compression ratio is smaller than the expansion ratio, meeting one of the essential features of the Atkinson
cycle.
The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the
power stroke to be as close to atmospheric pressure as possible. This maximizes the energy obtained from the
combustion process.
BecauseanAtkinsoncycleenginedoes not compress as much air as a similar size Otto cycle engine, it has a
lower power density (power output per unit of engine mass). Four-stroke Atkinson cycle engines with the addition
of a turbocharger or supercharger to make up for the loss of power density are known as Miller cycle
engines.
While an Otto cycle engine modified to run on the Atkinson cycle provides good fuel economy, it has a lower
power output than a traditional Otto cycle engine. However, the power of the engine can be supplemented by
an electric motor during times when more power is needed. This forms the basis of Atkinson cycle hybrid
electric automobiles. Their electric motors can be used independently of, or in combination with, the Atkinson-cycle
engine, to provide the most efficient means of producing the desired power. Toyota, Ford, Chevrolet,
Lexus, and Mercedes have all produced hybrid electric automobiles using Atkinson cycle engines in recent
years. For more information, see the animated Atkinson cycle engine at http://www.animatedengines.com/
atkinson.shtml.
13.20 MILLER CYCLE
By closing the intake valve when the piston is at the bottom of its stroke, the Otto cycle engine begins compressing
the air when the crankshaft has no leverage to push it up. A flywheel is often necessary to keep the engine
running. In the 1940s, the American engineer R. H. Miller (1890–1967) realized that the crankshaft would have
a much easier time pushing the piston up if it did not start the compression stroke until it had rotated part way
up, so he used a longer lever arm (see Figure 13.52).
When a Miller cycle engine has delayed (late) intake valve closure, it reduces the load on the piston as it rises to
begin the compression stroke. This can produce significant horsepower with the addition of a turbocharger or
supercharger, which compresses the air for the engine. A modern-day Atkinson cycle engine, on the other hand,
delays its intake valve closure simply to cause the compression stroke to be shorter than the power stroke, thus
realizing some of the same efficiency benefits of the original Atkinson cycle engine.
A Miller cycle engine is very similar to a modern Atkinson cycle engine. However, there are two big differences:
1. A Miller cycle engine depends on a turbocharger or supercharger.
2. A Miller cycle engine has either an early or late intake valve closing during the compression stroke. When
the intake valve closes late, the piston travels 20 to 30% of the way back up to the top of the cylinder before
the intake valve finally closes, so that the engine compression starts at the pressure of the turbocharger or
supercharger.
The effect is increased efficiency, up to about 15%. This type of engine was first used in ships and stationary
power-generating plants, but in the 1990s, it was adapted by Mazda for use in the Mazda Millennia.
To be effective, the Miller cycle turbocharger or supercharger must be able to compress the air with less energy
than with the engine’s pistons.Thisoccursonlyatlowpressures,so the Miller cycle uses the turbocharger or
supercharger for the first part of the compression process and uses the piston for the remainder. Successful production
versions of this cycle use variable valve timing to control the Miller cycle to maximize the engine’s
efficiency.