User talk:Aij
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I have attempted to rewrite this article. I think it was, and the articles from Toyota and mazda are, misleading. I have not taken the time to determine how to add appropriate references nor am I sure of the process for that (or to get this posted). Overview ==
The Miller cycle is a version of Atkinson cycle. This type of engine was first used in ships and stationary power-generating plants, but has been adapted by Mazda for their KJ-ZEM V6, Subaru, Toyota ], General Motors (called LIVC, 'late intake valve closing' ) and Ford. It is basically a super expansion cycle which reduces the energy lost when the exhaust valve opens by making the expansion (power) stroke larger than the compression stroke. This is done in the Miller cycle by reducing the compression stroke by closing the intake valve late.
The original Atkinson cycle engine offered the ability make the compression stroke mechanically shorter than the expansion stroke by lowering the lower right hand pivot point in the diagram in the animation on Matt Keveney's Web site. The original Atkinson engine also completed four strokes in one crankshaft revolution. Today’s engines typically achieve the Miller/Atkinson cycle using only late intake valve closing. A traditional four-stroke cycle (Otto cycle) engine uses four "strokes", the intake, compression, expansion and exhaust strokes (in two crankshaft revolutions). Much of the internal power loss of an Otto cycle engine is due to the energy lost when the exhaust valve opens and releases the hot, high pressure exhaust gas. About 10% more net cycle work could be produced if the power stroke could continue until the pressure inside the cylinder is equal to atmospheric pressure. That is where the Miller/Atkinson cycle comes in.
In the Miller cycle, the intake valve is left open longer (through about 20% of the compression stroke) than it would be in an Otto cycle engine. In effect, the compression stroke is shortened. The full compression stroke is two discrete cycles: the initial portion when the intake valve is open and final (effective) 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 partially expelled back out the still-open intake valve. This loss of charge air results in an effective loss of displacement which requires the engine to be larger for the same power. However, in the Miller/Atkinson cycle, this is partially compensated for by the increased expansion ratio. If converted to a Miller cycle, a 3.3 liter engine might draw only as much air as a 2.7 liter Otto cycle engine (the intake volume is reduced to 2.7 liters by the late closing intake valve) but produce the power of a 3 liter Otto cycle engine due to the fact that the expansion volume is still 3.3 liters. Since the compression ratio is usually maintained in the Miller cycle to maximize efficiency and power, a more useful description of the Miller cycle might be that if converted to a Miller cycle (by increasing its expansion stroke), a 2.7 liter Otto cycle engine would still draw as much air as a 2.7 liter Otto cycle engine (the intake volume remains at 2.7 liters with a longer stroke by the late closing intake valve) but produce the power of a 3 liter Otto cycle engine due to the fact that the expansion volume is now 3.3 liters – but the weight and size would be similar to a 3.3 liter engine.
The term “Miller cycle” is sometimes used to imply that turbocharging or supercharging is used to regain the lost power to weight ratio of the Miller cycle. Ralph Miller employed the miller cycle to obtain more power from engines, that were already supercharged, without increasing combustion pressures beyond safe limits. The Mazda KJ Miller Cycle V6 engine uses a supercharger, the Subaru B5-TPH uses a turbo charger while the Miller-cycle engine in the Mazda Demio is naturally aspirated. The advantages of supercharging a Miller/Atkinson cycle engine, in terms of power density; and the disadvantages, in terms of complexity and need for low internal compression ratio (and/or the need for high octane fuel) are much the same as for supercharging the usual Otto cycle. A key consideration of the Miller cycle is the compression ratio. Because the combustor chamber volume is not easily varied, engines with non-variable, late intake valve closing will have a constant compression ratio that can be set to the maximum practical value. For variable or optional Miller cycle engines (where the intake valve delay can be reduced to provide increased power), the compression ratio is set to the maximum value that is practical for when the intake delay is least (for maximum power) and is then unavoidably reduced by the reduced effective compression stroke when the intake valves are closed later. The reduced compression ratio for these engines during Miller cycle operation (late intake valve closing) is akin to the reduced effective compression ratio when the throttle partly closed on IC engines.
A Miller cycle engine is well suited to hybrid vehicles where the electric motors can provide short term increased power to compensate for the lower power of the Miller cycle. For hybrid vehicles that have a single cruise horsepower requirement (e.g., the Prius), the fixed miller cycle is attractive. For hybrid vehicles that have a multiple cruise horsepower ranges (Hybrid Tahoe with and without a towing load) the variable delay Miller cycle is attractive. In either case, for acceleration and up short hills, the electric motor supplements. During cruise, deceleration and down hill, the batteries are recharged. --ToppaTom (talk) 07:36, 31 March 2008 (UTC)
