Advanced Medium Speed Gas Engine for Land and Sea

Using diesel engines of proven reliability as a basis, Niigata Power Systems has continuously developed gas engines to meet contemporary market needs. The most recent development is a spark-ignition, lean-burn engine running on 100 % fuel gas and a dual-fuel engine capable of running on both fuel oil and fuel gas. As well as land-based and marine power generation, the engine has also been developed for propeller curve applications in tugs.

Gas engine development

Using diesel engine designs of proven reliability as a base, Niigata Power Systems has continuously developed gas engines to meet contemporary market needs. The most recent development is a spark-ignition, lean-burn engine running on 100 % fuel gas and a dual-fuel engine capable of running on both fuel oil and fuel gas. As well as land-based and marine power generation, the engine has also been developed for propeller curve applications in tugs.

Among the gas engines derived from proven diesel designs by Niigata Power Systems (NPS) is the micro-pilot ignition series, 22AG and 28AG. First developed ten years ago, it achieved brake mean effective pressures (BMEP) as high as 2.0??MPa, and in the 3 MW class of power generation systems this engine series attains electrical efficiency exceeding 43??%. Engines are available in two types, one covering outputs from 1 to 3 MW (22AG) and one for 6 MW (28AG). To date, over 100 of these engines have been delivered with a total output of 226 MW and are currently operating successfully in Japan. Building on the reputation acquired by the 22AG and 28AG engines for efficiency, environmental-friendliness and long maintenance intervals (4000 hours), stable operating performance, and high economic efficiency, NPS has now developed a new gas engine that provides higher economic and environmental performance, including a marine gas engine featuring superior load-following capability and enhanced environmental design.

Gas engine trends in power generation

Electricity shortages have become a major concern in Japan due to the calls for denuclearisation after the devastating earthquake and tsunami disasters of 2011. Since power supply is critical to productivity for manufacturers, especially in factories, many Japanese manufacturers are considering introducing cogeneration systems using gas engines to assure electrical power supply.

Another reason for the increased attention on gas engines is the projected price gap between natural gas and oil, which is expected to widen in the long term.

Gas engines in marine applications

Environmental protection has long been a major concern throughout the world and regulatory organisations are tightening standards. Designing a diesel engine that satisfies IMO Tier III standards for NO_x in Emission Control Areas (ECAs) requires additional equipment such as a Exhaust Gas Recirculation (EGR) or selective catalytic reduction (SCR), which occupy additional space aboard ship.

On the other hand, the fuel burning process in gas engines produces lower emissions than diesel engines; by nature, exhaust gases from gas engines contain less fuel-derived emissions, as shown in Figure 1. However, the transient characteristics of gas-fuelled engines are inferior to existing marine diesel engines and thus they are rarely used for marine applications. Hence, improving the transient operating characteristics of marine gas engines has been an NPS priority.

FIGURE 1

Exhaust emissions of different fuel types (© Niigata)

Spark-ignition gas engine 28AGS

Spark-ignited gas engines in the NPS 28AGS series are equipped with a pre-combustion chamber. The line-up consists of six and eight cylinder inline and twelve, 16 and 18 cylinder vee configuration engines, covering an output range from 1 to 6 MW. The lean-mixture combustion and pre-combustion chamber shapes are optimised for efficiency in power generation applications while maintaining minimised NO_x emissions. The specifications of the NPS spark-ignition gas engine 28AGS is shown in Table 1.

TABLE 1 Specifications of NPS spark-ignition gas engine

In addition to the optimised design of the combustion and pre-combustion chambers, the series has higher thermal efficiency than conventional spark-ignition engines thanks to Miller Cycle technology which is also employed in the 22AG and 28AG series. In particular, the performance of the in-line engines for power generating systems in the 2 MW output class have been closely matched to Japanese market demands and have attained highest world class thermal efficiency.

Load following and load acceptance

As shown in Figure 2, the air-fuel ratio control system adopted in this series provides the necessary load-following capabilities and performance under load required by engines in power generation applications on land. These engines also have a function called “important load survival operation”, which has proved useful in the Japanese market. In addition, the 28AGS series engines have a function that enables the power generator to establish the rated output voltage within 40 seconds. This feature attracted considerable attention after the earthquake and tsunami in Japan due to its effectiveness in cases where a power generation system is also used as an emergency power system.

FIGURE 2

Start to load application characteristics (© Niigata)

With the characteristics described, the 28AGS series engines are compatible with a radiator cooling system and can be used under various operating conditions around the world. In addition, they can also be used in high temperature areas without de-rating, thanks to an intake air temperature setting 10 °C higher than in conventional NPS engines. Likewise, to ensure that the engines can run on internationally available fuel gases with various properties without affecting rated output, NPS conducts operation tests on its products using a fuel gas with methane number (MN) 65.

Marine Dual-fuel Engines 28AHX-DF

NPS continues to develop gas-fuelled engines for marine applications based on dual-fuel technology, with its in-built redundancy for the ship’s propulsion system (switching to 100 % diesel mode). The specifications of the NPS dual-fuel engine 28AHX-DF are shown in Table 2. The design concept of this marine gas engine specifies two engine operation modes. The diesel mode is used for start-up, emergency running and shutdown while the gas mode is used for all other operating phases, typically cruising. By switching the operation mode according to this scheme, it is possible to maximise the operating time in the gas mode and thus satisfy the design concept of a clean emissions ship. A time-series typical operational cycle of the engine is shown in Figure 3 (time on the horizontal axis). Potential emergencies include abnormal combustion inherent to the gas engine (e.g. knocking), trouble with the electronic fuel control system and fuel shortage.

FIGURE 3

Schematic of mode shifts of a marine gas engine (GE = gas engine, DE = Diesel engine) (© Niigata)

TABLE 2 Specifications of NPS marine dual-fuel engine

The target set for NO_x emissions for this engine is to meet IMO Tier III in ECAs in gas fuel mode and IMO Tier II in diesel fuel mode. A comparison of emissions between gas and diesel operation is shown in Figure 4. As additional technical information, the NPS 22AG and 28AG series micro-pilot ignited engines and the 28AGS series spark-ignition engines easily satisfy the requirements of the IMO Tier III ECA standard.

FIGURE 4

Emissions comparison, diesel and dual-fuel engines (© Niigata)

Rapid load change

As mentioned earlier, additional development is needed before gas engines can accommodate rapid changes in load. When a ship’s main propulsion engine drives via a propeller in a mechanical system, its speed is changed according to the change in load. During lower loads, the engine speed is slow, and a small amount of low-temperature exhaust gas is released. This leads to sluggish operation of the turbocharger since its turbine is driven by the energy in the exhaust gas flow. Under these operating conditions the turbocharger cannot supply sufficient charge air to keep the air-fuel ratio in the appropriate zone. As a result, the engine may fail to continue running due to knocking or other abnormal combustion phenomena.

When a marine gas engine is operated with a rapidly changing load, the turbocharger’s speed may fail to rise following a load increase, creating load zones where air supply is inadequate. This can potentially cause the engine to knock. To overcome these shortcomings, NPS’s newly developed gas engine employs common rail injection for the diesel fuel micro-pilot and turbochargers with Variable Turbine Geometry (VTG), which enable fuel and air management according to changing load conditions. To control these mechanisms, NPS has developed a high performance control system and succeeded in achieving stable engine operation under low loads and proper load-following capability under rapid load changes. An example of the results, showing that a rise in load factor from 15 to 100 % on a propeller curve can be completed in approximately 15 s, is shown in Figure 5.

FIGURE 5

Improved characteristics at rapid load changes (© Niigata)

In addition, the control system is optimised for instant switching of engine operation from gas to diesel mode and vice versa, to be able to deal with emergency situations. Figure 6 shows an example of an operating mode switch. The control system is adjusted so that switching from diesel to gas is possible at over 15 % load and the operation load in the gas mode is between 0 and 100 % load.

FIGURE 6

Schematic of engine operation mode switching (© Niigata)

Tugboat applications

For many years NPS has produced medium speed diesel engines combined with the Z-peller steerable propulsion system for tugboat applications and has delivered many of these units. As IMO Tier III standards come into force in 2016 in ECAs, many tugboat operators are contemplating the use of gas engines that can satisfy the emissions standard without additional equipment. Applying gas engines to tugboats has so far been considered impracticable because, as already seen, it is difficult for gas engines to follow the frequent and rapid load changes which are unavoidable in tugboat operation. NPS has solved this problem by taking the technical approaches discussed above and intends to meet customers’ demands by supplying a gas-engine-powered version of the mechanical propulsion unit with Z-peller and is developing a gas-fuelled engine optimised for tugboat applications.

A typical operation pattern of an ordinary diesel-engine-powered tugboat is shown in Figure 7. It shows frequent changes in speed (load) including a rapid change from the idling speed (400 rpm) to the rated speed (750 rpm) in just over 10 seconds. To ensure that the gas engine properly follows the rapid load changes experienced in tugboats, NPS has employed various types of control systems and optimised them. The result of the development is shown in Figure 8; the engine is well adapted to rapid load changes as seen in the operation pattern of ordinary tugboats, even with a fixed pitch propeller (FPP). The first 28AHX-DF, employing these technologies, was delivered at the end of 2014 for a tugboat which is the first gas-fuelled ship in Japan other than an LNG carrier.

FIGURE 7

Typical operatin operating profile of a diesel tugboat (© Niigata)

FIGURE 8

Outcome of devel come developmen opment targeting rapid load change response for NPS gas engines in tug applications (© Niigata)

Future gas engines

Gas engines are inherently environmentally-friendly and can be improved in terms of operational economy. Manufacturers are working to increase the operational efficiency of their engines; for manufacturers of power generation systems, competition for higher electrical generating efficiency will become intense. In the case of cogeneration applications with gas engines as key hardware, an increase in waste heat recovery is also expected in order to improve the overall efficiency of the cogeneration system.

Some of these systems must also have functions to deal with emergencies and ensure continued working in the event of disasters. Gas engine manufacturers must thus offer a range of products to accommodate diverse customer needs. Some customers put higher priority on thermal efficiency, others require high total efficiency of the whole system, and others need functions for dealing with emergencies.

Combustion System Variants

The combustion systems used by NPS for its gas-fuelled engines are broadly classified into the micro-pilot ignition system, the spark-ignition system and the dual-fuel engine system. According to customers’ needs, NPS will provide gas engines with one or other of these systems, making the most of their different features and benefits, as indicated in Table 3.

TABLE 3 Characteristics and intended applications of different combustion systems

Methane Number and calorific value in LNG

The properties of LNG differ according to their origin. The calorific value of the LNG imported into Japan and supplied to consumers through pipelines (town gas) is kept constant by adding the necessary quantity of propane gas (C3H8) before distribution. On the other hand, in areas without pipeline networks, LNG is supplied as delivered from satellite supply bases, and the calorific value of such LNG in the gaseous state varies with time. Likewise, the properties of LNG for marine engines differ from area to area. If a gas engine fuelled in one area is refuelled at a bunkering station in other areas, the engine may show a change in performance or have abnormal combustion due to differing properties of the gaseous LNG. Related problems (e.g. misfire and knocking) may be caused in the engine start-up phase and especially at high load operation.

Fuel gas is mostly composed of methane (CH₄), but if it contains high amounts of heavy hydrocarbon compounds with lower self-ignition temperatures, such as C3H8 and butane (C4H10) the engine will be more prone to knocking.

The methane number (MN) is often used as an indicator to express the tendency of a fuel gas to cause knocking. The larger the proportion of heavy hydrocarbon compounds other than CH₄ in the gaseous fuel, the lower the MN becomes and the more likely the engine is to knock. When discussing general gas fuels, an engine using fuel with a higher calorific value, namely a smaller MN, is more prone to knocking.

A case of knocking due to a rise in the calorific value of the fuel gas is shown in Figure 9. Here, the knocking index rises as the calorific value increases. In this example, an anti-knocking control device was activated at the knocking threshold. As a result, in gas-fuelled engines, a change in the fuel’s calorific value significantly affects operational stability.

FIGURE 9

Knocking resulting from changes in calorific values of fuel gas (© Niigata)

In Japanese gas-fuelled engines for power generation applications on land, most use town gases. The calorific value of town gases is stable due to the propane adjustment mentioned above. There are, therefore, usually no problems arising from changes in calorific value in engines fuelled by town gas. In contrast, liquefied natural gas (LNG) is most often used to fuel engines installed in areas without town gas on land and engines in ships.

The individual components constituting LNG vaporise at different temperatures, which may cause their proportions to change from time to time. This change also occurs when the gas flow is changed and leads to variations in the fuel’s calorific value. Changes in the fuel’s calorific value resulting from changes in the gas flow are shown in Figure 10. Technology is necessary to minimise this undesirable change in the calorific value of gaseous LNG or to prevent the engine from knocking when calorific value changes.

FIGURE 10

Change in fuel gas flow and resulting change in calorific value (© Niigata)

Conclusion

Targeting high levels of economic efficiency, environmental friendliness and safety, NPS has completed the development of the 28AGS series of spark ignition gas engines for power generation applications on land. For marine gas engines which must follow frequent changes in load, NPS has developed a new control system consisting of a multiple number of variable control mechanisms and a high-performance controller for controlling these mechanisms. The system ensures that the engine operates stably under low loads and follows rapid load changes.

Through tests conducted by simulating the operational pattern of a diesel engine powered tugboat, the new NPS marine dual-fuel engine 28AHX-DF has demonstrated favourable operating characteristics, comparable to those of a diesel engine. A 28AHX-DF engine has been delivered for the world’s first gas-fuelled tugboat with a mechanically driven FPP.

Based on these results, NPS will continue to offer engines with the most suitable combustion systems for individual land and marine applications, in response to each customer’s requirements, while seeking continuously higher performance from reciprocating engines.