Wednesday, December 25, 2019

The Miller Cycle and Emissions Reduction

The Miller cycle was developed by Ralph Miller in the 1940s. With the introduction of turbocharging to the 4 stroke diesel engines, the Mean Effective Pressure and thus the power output of 4 stroke engines rose by 50 - 60%. However this was about the limit; If the  inlet air pressure was increased further, then the charge air reached excessive pressures and temperatures on compression causing burning of the LO film and thermal stressing.

Miller challenged the thinking of the day by closing the inlet valve before the piston reached bottom dead centre. This had the effect of lowering the cylinder pressure as the piston continued downwards, as well as dropping the temperature of the air (Boyles and Charles' Law). Although the engine is still doing work as the piston is descending on the inlet stroke, there is a saving in work during the compression stroke, and the maximum air temperature and pressure is reduced on compression. The timing of the inlet valve of  Miller's engine was governed by a mechanical link arrangement, and varied automatically with engine load. Miller's engine doubled the MEP of the engine when compared with a naturally aspirated engine.

Advances in design and materials led to more efficient turbochargers, higher compression ratios and more efficient cooling of marine diesel engines. However, with the introduction of MARPOL VI, manufacturers had to look more closely at lowering NOx and smoke emissions.

One of the methods used is to reintroduce the Miller cycle using variable inlet closing, so that at full load, the maximum cylinder temperature is reduced. (NOx formation occurs at temperatures in excess of 1200°C). This is combined with higher compression ratios and slightly later fuel injection timing.

Miller relied on mechanical methods to vary the timing. Modern methods linked to a computer controlled engine management system use a hydraulic push rod.

 Low load operation: The throttle valve opens against a spring as the follower moves up the cam and oil is displaced under the push rod piston, opening the valve. When the follower comes off the cam, the throttle valve is closed and oil can only flow through the throttle orifice, delaying the closing ofthe inlet valves.
At full load operation, an air signal opens the throttle valve. This means that as soon as the follower descends from the cam peak, the pushrod piston moves downwards, allowing the inlet valves toclose
In this second method of control, the Variable Inlet Closing consists of two hydraulic cylinders connected by two passages, the flow through one of these passages being controlled by a valve, and the other by the position of the hydraulic piston driven by the cam follower.

When the follower moves up the slope of the cam the oil in the lower cylinder moves to the upper cylinder displacing the push rod piston and opening the valves. When the follower is on the peak of the cam, the hydraulic piston is covering the passage between the  cylinders.

When the VIC control valve is open, the pushrod follows the follower immediately, which results in early valve closure. When the control valve is closed, the downward movement of the pushrod is delayed until the piston actuated by the tappet reveals the passage between the two cylinders.

Make up of oil is from the main engine Lub Oil supply via a non return valve. Build up of air is prevented by an air release in the push rod hydraulic cylinder.

By increasing the compression ratio, giving a higher air temperature the ignition delay is reduced. Later injection over a shorter period combined with improved fuel atomisation and combustion space design result in lower NOx formation.

The two stroke engine cannot utilise the Miller cycle. However they can use variable exhaust valve closing; easily achieved with an electronically controlled camshaftless engine or by involving hydraulic valves as in the case of the modified Sulzer RTA

Supercharged petrol engines also make use of a form of Miller cycle in which the inlet valve is left open during the first part of the compression stroke, so that compression only occurs during the last 70% of the compression stroke. Over the entire compression range required by the engine, the supercharger is used to generate low levels of compression, where it is most efficient. The air is then cooled in the air cooler. Then, the piston is used to generate the remaining higher levels compression, operating in the range where it is more efficient than a supercharger. Thus the Miller cycle when used in a petrol engine 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 the power needed to run the engine by 10% to 15%.

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