Miller Cycle

Introduction

In the early 1890s, Rudolf Diesel presented his idea of an economic combustion engine with the idea of completely replacing the steam engine. The year 1903 marked the beginning of the success story of the diesel marine propulsion engine.

 Ever since its introduction the manufacturers have been striving towards improvement in the operational reliability and thermal efficiency of the diesel engine. Improved reliability has been achieved by the application of improved material, fuel injection, lubrication and cooling methods and improved design of components,

Improvement of thermal efficiency or reduction of specific fuel oil consumption (SFOC) has been made possible by supercharging and increased compression ratio with consequent increase in maximum firing pressure (pmax) and temperature

Variable Injection Timer (VIT)

Following oil crisis of 1973 which almost overnight saw nearly fourfold rise in bunker price. Many ships – particularly bulk carriers and tankers - began to slow-steam in order to reduce oil consumption for the voyages. Apart from technical difficulties, running the engine at reduced power reduces thermal efficiency and increases the SFOC. To counter these effects to some extent, variable injection timer (VIT) was introduced by some makers whereby the commencement of fuel injection was advanced automatically as the engine load increased.

Advancing the beginning of fuel injection results in increasing the maximum firing pressure and combustion temperature. This in turn improves thermal efficiency of the cycle and reduces the SFOC.

 

NO_X emission

The MARPOL Convention was adopted on 2 November 1973 at IMO. The Protocol of 1978 was adopted in response to a spate of tanker accidents in 1976-1977. As the 1973 MARPOL Convention had not yet entered into force, the 1978 MARPOL Protocol absorbed the parent Convention.

Annex I (pollution by oil) and II (pollution by noxious liquid substances) entered into force on 2 October 1983. Other Annexes – III (pollution by harmful packaged substances), IV (sewage) and V (garbage) came into force on different dates thereafter.

The original MARPOL covered pollution of the sea only. Air pollution was not covered. In 1997, a Protocol was adopted to amend the Convention and a new Annex VI was added which entered into force on 19 May 2005 dealing with the pollution of the atmosphere by ships.

The regulations in this annex, among other things, set limits on emission from ship exhausts of nitrogen oxide (Regulation 13) and sulfur oxide (Regulation 14). 

Ships can meet the new limit requirements for SO_x gas emission by using low sulfur fuel oil such as Marine Gas Oil (sometimes called distillates). Ships may also meet the SO_x requirements by using approved equivalent methods, such as an, Exhaust Gas Cleaning Systems or “scrubbers”, which “clean” the emissions before they are released into the atmosphere.

 

As regards the emission of NOx, following are the specified limits:

 

Tier	Ship construction date on or after	Total weighted cycle emission limit of NOx (g/kWh) n = engine’s rated speed (rpm)
		n < 130	n = 130 - 1999	n ≥ 2000
I	1 January 2000	17.0	45·n(-0.2) e.g., 720 rpm – 12.1	9.8
II	1 January 2011	14.4	44·n(-0.23) e.g., 720 rpm – 9.7	7.7
III	1 January 2016*	3.4	9·n(-0.2) e.g., 720 rpm – 2.4	2.0

 

Tier III applies only on new ships when in designated Emission Control Areas (ECA)

When the global Tier II limits came into force in 2011, engine-makers were able to tune the engines to comply with these new emissions limits. Tier III poses a challenge to engine designers, as tuning is not an option anymore and they need to apply NOx reduction measures using other engine technologies.

 

The available technologies for compliance with the IMO NOx Tier III limits currently include:

 

• ·       Selective catalytic reduction (SCR) systems: This is the most widely used method for purification of NOx from an engine’s exhaust gas.

• ·         Exhaust gas re-circulation (EGR): Re-circulation of the exhaust gas back to the engine’s combustion process. This is still a relatively new technology for maritime applications, but is developing into a competitive option for NOx compliance.

• ·         Alternative fuels such as liquefied natural gas (LNG) 

For the 4-stroke medium speed propulsion engine early closing of air inlet valve according to Miller cycle presents another option.

 

Miller cycle

High temperature of combustion increases the amount of NOx formation. In most part, the amount of NOx generated is through the reaction of N₂ and O₂ in the atmosphere. For instance, if the duration of combustion. If the duration of combustion is approximately 10 micro seconds, the generated quantity of NOx at flame temperature 2 400 K (2130⁰ C) is 10 times as much as that generated at 2200 K (1930⁰ C). Thus, any measures to improve the SFOC by raising the temperature of combustion, e.g. by the VIT, act against NOx emission reduction measures. This inevitably led to some “trade-off” i.e. some sacrifice had to be made reducing the combustion temperature and accepting somewhat higher SFOC in order to comply the NOx emission limits.

Application of Miller cycle can, to a large extent, reduce or eliminated the need for “trade-off”.

The basic principle underlying the Miller process is that the effective compression stroke can be made shorter than the expansion stroke by suitable shifting of the inlet valve timing. When both the engine output and boost pressure are kept constant, this will reduce the cylinder filling and the pressure and temperature in the cylinders will be lower.

Miller cycle (patented by Ralph Miller in 1957) involves early closing of the air inlet valve, even before the piston reaches the bottom dead centre. After closure of the inlet the entrapped air is expanded and thereby cooled as the piston continues its stroke till the bottom end. This cooling effect helps to reduce the peak temperatures reducing the NO_x formation.

However due to early closing of the inlet the amount of air trapped in the cylinder is reduced and hence less fuel can be burnt and less power developed. In order to compensate this the charge air pressure is increased ensuring that sufficient quantity of air is available to burn more fuel.

The traditional single stage turbocharger the pressure ratio is limited to about 4 in order to cope up with high temperature rise due to compression. By employing two-stage turbocharging with inter-cooling and after-cooling a pressure ratio of up to or more than 8 can be achieved. Leading engine manufacturers are now offering four-stroke medium speed engines employing Miller cycle inlet valve timing in combination with two-stage turbocharging.

During trials using an intensive Miller Cycle under full load conditions and turbocharger pressure ratios of 6.5 to 7, MAN Diesel has recorded reductions in NOx of over 30%, reductions in fuel consumption as great as 8% and a 15% increase in specific power output ^(c)

The original purpose of the Miller process was to increase the power density of engines without exceeding their mechanical and thermal limits. In the 1990s, attention turned to how it could be used to reduce the temperature in the cylinders for a constant engine output, and to using this positive effect to minimize NOx formation. In the case of gas engines, an additional benefit is that the operating range can be increased since there is less tendency for the engines to “knock”

The Miller process is one of the few options that engine builders have for simultaneously reducing emissions and improving engine efficiency. Since all engine builders strive to meet engine emission limits without any loss of efficiency, practically every modern engine is operated today with at least moderate Miller timing. 

 

Variable valve timing (VVT)

Variable valve timing enables variations in the opening and closing of the inlet valves. It can be used to compensate the increase in SFOC associated with lower NOx emissions. VVT is an enabling technology of variable Miller valve timing.

At high load, a strong Miller effect results in an improvement in the NOx-SFOC trade-off. At low load, the Miller valve timings are reduced to attain higher combustion temperatures and thus lower soot emissions.

 

Types of Earlier-Closing Miller Cycle

The figure be below illustrates the various types of earlier-closing Miller cycle with reference to the diagram of air inlet valve open and close operation. While different companies use different terminology, Miller cycle of which inlet valve is closed earlier progressively from the inlet valve closing timing near the bottom dead centre (“Tier II”) are called “strong Miller” and “extreme Miller” respectively. 

 

For the Tier II specification engine, which aim is to reduce NOx by 20% relative to Tier I specification, single-stage turbocharging (a single turbocharger) is used to achieve the higher pressure ratio. The pressure ratio for Tier II specification is as high as 5.5 in some cases, if overload is included, and while it depends on the turbocharger manufacturer settings, this is the limit of what is possible with single-stage turbocharging. Accordingly, if a strong Miller or extreme Miller timing is used to reduce NOx by 30 or 40% relative to Tier I, two-stage turbocharging (dual turbochargers) is needed because the pressure ratios of 6 or higher is required. The terminology used by Yanmar for this is “two-stage turbocharging system”. ^(f)

 

Two Stroke engines

In two stroke engines there are no inlet valves – the air for combustion is supplied through scavenge ports on the liner near the bottom that are opened or closed by the piston movement. Miller cycle cannot be applied for such engines.

In the X-series 2-stroke engines of Wartsila (formerly Sulzer) with uniflow scavenging, a somewhat comparable process has been applied by later closing of the exhaust valve. This reduces the length of compression stroke and thereby lowering the air pressure and temperature

 

Compliance with Tier III NOx emission standard

A combination of two-stage turbocharging, improvement in the fuel injection system and variation of timings of air inlet valve (Miller process) or exhaust valve (two-stroke engine) can ensure compliance with Tier I and Tier II standards for NOx emission. By switching the engine to LNG fuel Tier III standard can be met. However, when using traditional heavy or diesel fuel oil, additional measures such as, selective catalytic reactor (SCR) or exhaust gas recirculation (EGR) are necessary to comply with Tier III. These are briefly described below.