Miller cycle

In engineering, the Miller cycle is a thermodynamic cycle used in a type of internal combustion engine. The Miller cycle was patented by Ralph Miller, an American engineer, US patent 2817322 dated Dec 24, 1957. The engine may be two-stroke or four stroke and may be run on diesel fuel, gas fuel or dual fuel.

This type of engine was first used in ships and stationary power-generating plants, and is now used for some railway locomotives such as the GE PowerHaul. It was adapted by Mazda for their KJ-ZEM V6, used in the Millenia sedan, and in their Eunos 800 sedan (Australia) luxury cars. More recently, Subaru has combined a Miller cycle flat-4 with a hybrid driveline for their concept "Turbo Parallel Hybrid" car, known as the Subaru B5-TPH.

Overview

A traditional reciprocating internal combustion engine uses four strokes, of which two can be considered high-power: the compression stroke (high power flow from crankshaft to the charge) and power stroke (high power flow from the combustion gases to crankshaft).

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" stroke 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 through the still-open intake valve. Typically this loss of charge air would result in a loss of power. However, in the Miller cycle, this is compensated for by the use of a supercharger. The supercharger typically will need to be of the positive displacement (Roots or Screw) type due to its ability to produce boost at relatively low engine speeds. Otherwise, low-rpm power will suffer.

In the Miller cycle engine, the piston begins to compress the fuel-air mixture only after the intake valve closes; and the intake valve closes after the piston has traveled a certain distance above its bottom-most position: at around 20% to 30% of the total piston travel of this upward stroke. So in the Miller cycle engine, the piston actually compresses the fuel-air mixture only during the latter 70% to 80% of the compression stroke. During the initial part of the compression stroke, the piston pushes part of the fuel-air mixture through the still-open intake valve, and back into the intake manifold.

Charge temperature

In a typical spark ignition engine, the Miller cycle yields an additional benefit. 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 advanced beyond what is normally allowed before the onset of detonation, thus increasing the overall efficiency still further. An additional advantage of the lower final charge temperature is that the emission of NOx in diesel engines is decreased, which is an important design parameter in large diesel engines on board ships and power plants.

Compression ratio

Efficiency is increased by having for the same effective compression ratio a larger expansion ratio. This allows more work to be extracted from the expanding gases as they are expanded to almost atmospheric pressure. In a ordinary spark ignition engine at the end of the expansion stroke of a wide open throttle cycle, the gases are at approximately 5 atmospheres when the exhaust valve opens. Because the stroke is limited to that of the compression there is still some work that could be extracted from the gas. Delaying the closing of the intake valve in the Miller cycle in effect shortens the compression stroke compared to the expansion stroke. This allows the gases to be expanded to atmospheric pressure. Increasing the efficiency of the cycle.

Supercharger losses

The benefits of utilising positive displacement superchargers come with a cost. 15% to 20% of the power generated by a supercharged engine is usually required to do the work of driving the supercharger, which compresses the intake charge (also known as boost).

Major advantage/drawback

The major advantage of the cycle is also its major drawback. In a Miller cycle at part load when the intake has been throttled, the pressure at the end of expansion will be lower than atmospheric. To achieve this part vacuum in the cylinder, work must be supplied by the crankshaft to the gases, greatly reducing the cycle efficiency. So at part load the Miller cycle is less efficient than a conventional spark ignition engine.

Summary of the patent

The overview given above may describe a modern version of the Miller cycle but it differs in some respects from the 1957 patent. The patent describes "a new and improved method of operating a supercharged intercooled engine". The engine may be two-cycle or four-cycle and the fuel may be diesel, dual fuel or gas. It is clear from the context that "gas" means gaseous fuel and not gasoline. The pressure-charger shown in the diagrams is a turbocharger, not a positive-displacement supercharger. The engine (whether four-stroke or two-stroke) has a conventional valve or port layout but there is an additional "compression control valve" (CCV) in the cylinder head. There is a servo mechanism, operated by inlet manifold pressure, which controls the lift of the CCV during part of the compression stroke and releases air from the cylinder to the exhaust manifold. The CCV would have maximum lift at full load and minimum lift at no load. The effect is to produce an engine with a variable compression ratio. As inlet manifold pressure goes up (because of the action of the turbocharger) the effective compression ratio in the cylinder goes down (because of the increased lift of the CCV) and vice versa. This "will insure proper starting and ignition of the fuel at light loads".

Atkinson cycle engine

A similar delayed-valve closing method is used in some modern versions of Atkinson cycle engines, but without the supercharging. These engines are generally found on hybrid electric vehicles, where efficiency is the goal, and the power lost compared to the Miller cycle is made up through the use of electric motors.

Sources

• Miller cycle