Seminar on Miller Cycle-Presentation

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 (engineer), an American engineer, in the 1940s.

A Miller-cycle engine is very similar to an Otto-cycle engine. The Miller-cycle uses pistons, valves, a spark plug, etc., just like an Otto-

cycle engine does. There are two big differences:

• A Miller-cycle engine depends on a supercharger.

• A Miller-cycle engine leaves the intake valve open during part of the compression stroke, so that the engine is compressing against the

pressure of the supercharger rather than the pressure of the cylinder walls. The effect is increased efficiency, at a level of about 15 percent.

In the Miller cycle, the intake valve is left open longer than it would be in an Otto cycle engine. In effect, the compression stroke is two discrete cycles: the initial portion when the intake valve is open and final portion when the intake valve is closed.

This two-stage intake stroke creates the so called 'fifth' cycle that the Miller cycle introduces.

As the piston initially moves upwards in what is traditionally the compression stroke, the charge is being pushed back out the still-open valve. Typically this loss of charge air would result in a loss of power. However, in the Miller cycle, the piston is over-fed with charge air from a supercharger, so pushing some of the charge air back out into the intake manifold is entirely planned.

A key aspect of the Miller cycle is that the compression stroke actually starts only after the piston has pushed out the 'extra' charge and the intake valve closes. This happens at around 20% to 30% into the compression stroke. In other words, the actual compression occurs in the latter 70% to 80% of the compression stroke

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. In total, this reduces in the power needed to run the engine by 10% to 15%.

The intake air is first compressed by the supercharger and then cooled by an intercooler. This lower intake charge temperature, combined with the lower compression of the intake stroke, yields a lower final charge temperature than would be obtained by simply increasing the compression of the piston. 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.

Due to the reduced compression stroke of a Miller cycle engine, a higher overall compression ratio (supercharger compression plus piston compression) is possible, and therefore a Miller cycle engine has a better efficiency

Miller-cycle Technical Details There are basically four means that the Miller-cycle uses to

obtain its increased efficiency.

1)Smaller engine (lower displacement)

2)reduced compression stroke and pumping losses

(from late closing of the intake valve)

3)cooler intake charge (intercooled air)

4)combustion improvements

Engine Size vs Frictional Losses When the displacement of an engine is reduced, there is a

substantial reduction in frictional losses

The graph below indicates the fuel efficiency increase as displacement is decreased

normally use an engine over 3.0L, the reduction in size to 2.3L provides an improvement in fuel efficiency of around 13 percent

The lower friction associated with the smaller engine also improves efficiency.

Theoretical vs Actual Compression Ratio The theoretical compression ratio is simply a comparison of the

volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC).

However, in practice, the actual compression ratio is determined by the valve timing,

the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.

in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke.

In miller cycle,the compression ratio is "artificially" reduced down to 8:1.

In miller cycle, compression stroke is reduced but the power or expansion stroke remains the same.

Thermal Efficiency By increasing the compression ratio, the thermal efficiency of

an engine is also increased

Due to the late closing of the intake valves (reduced compression ratio), less heat is added to the intake charge by the piston during this stroke. The loss in thermal efficiency of reduced compression ratio from 10 to 8:1 is only about six percent. This slight loss in thermal efficiency from the decrease in

compression ratio is more than made up for by a much denser charge supplied by the compressor. Cool dense air is pushed through twin intercoolers into the cylinders. This reduces the combustion chamber temperature at TDC of the compression

stroke and so lowers the potential for detonation to occur and also production of NOx.

Pumping Losses This refers to the energy required to rotate an engine during two

of the three non-power producing strokes

The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses), in the process of completing a normal compression stroke.

Volumetric Efficiency The term volumetric efficiency refers to the ability of an engine

to fill its cylinders with a volume of air equal to their displacement (100 percent Ve). The greater the Ve then the

greater will be the output of that engine.

half the intake charge back out the intake valves would be a reduction in volumetric efficiency.

In the Miller-cycle engine, where the compressor comes to the rescue. Any loss of intake charge through "back flow" is more than compensated for by the density of the charge provided by the compressor.

Combustion Improvements

On the Miller engine, the intake port has been shortened to promote smooth but strong intake air flow. A mask is added to the intake side of the combustion chamber to concentrate the air flow to the centre of the cylinder; strengthening the

tumble motion. Tumble promotes more ideal intake dynamics and combustion

events that enhances the anti-knocking performance of the engine.

high performance coupled with between 10 and 15 percent less fuel consumption .

Miller Cycle is widely employed in large-scale applications such as generators and ship motors, the only manufacturer to ever sell a Miller Cycle powered automobile is Mazda with their Mx-6 and Millenia/929 (both of which are out of production in North America, although the Millenia is still being sold in Japan).

This engine utilises well proven conventional technology, but further enhances it to take into account growing international

concerns for the environment and resource preservation. While, in the fullness of time, engines which use alternative

forms of energy may come to pass, Miller-cycle technology will be seen to have advanced the cause of efficiency and

responsibility.