Gasdiesel Engines

Wartsila gas-diesel (GD) technology differs from the dual-fuel diesel concept in using direct, high pressure injection of gas and 3 per cent pilot fuel for its ignition. GD medium speed engines can be operated on heavy fuel and diesel oils, crude oil directly from the well or natural gas; supply can be switched instantly and automatically from one fuel to another without shutdown. Conversions of existing engines from normal heavy fuel mode to natural gas/diesel oil operation can be executed with small modifications. Containerized engineroom packages have been delivered along with the associated gas compressors and other ancillaries.

The gas-diesel engine was developed in the late 1980s mainly as a prime mover for the offshore industry. High pressure direct injection for the gaseous fuel maintains the diesel process, making the concept insensitive to the methane content of the fuel. Such a feature makes the GD engine especially suitable for mobile oil production processing plants, where the gas composition may change with the location of the vessel and the stage in the production process.

Initially, the gas injection was controlled by a mechanical/hydraulic system using a cam-actuated jerk pump to generate the control pressure for the gas needle. Although reliable, Wartsila explains, this system did not offer the required flexibility and was later replaced by an electro-hydraulic system based on a constant rail pressure to control the gas injection via solenoid valves. Gas-specific monitoring and safety functions were all integrated, and the system allowed adjustment of individual cylinders. This represented the first step towards fully computerized systems for engine control, and valuable experience was gained in both software and hardware requirements.

Rolls-Royce is another specialist in the technology, its Norwegian subsidiary Bergen Diesel having started development of a lean-burn gas-fuelled version of the 250 mm bore K-series medium speed diesel engine in the mid-1980s. Numerous engines of this type have been sold for land power and co-generation installations, reportedly demonstrating high efficiency and very low emissions. The company has also been keen to demonstrate the potential in gas-fuelled ferries in an era of concern over atmospheric coastal pollution and the increasing availability of gas from domestic pipeline terminals.

Rolls-Royce Bergen spark-ignition lean burn gas engine technology has matured through four generations of engines and thousands of operating hours in shoreside power installations, running on various gas fuels including natural gas and biogas derived from landfill. The 250 mm bore KV-G4 engine features variable geometry turbocharging and individual electronic gas and air controls for each cylinder to yield a shaft efficiency of 44 per cent on a brake mean effective pressure of 16 to 18 bar. Very low NOx emissions (1.1 g/kWh) are reported from the engine, which is available with ratings up to 3600 kW. A gas engine version of the 320 mm bore Bergen B32:40 medium speed diesel, exploiting the proven features of the KV-G4 series, was planned to offer outputs up to around 6000 kW.

Medium and low speed gas-burning engine designs are offered by the MAN B&W Diesel group, citing successful experience with a Holeby 16V28/32-GI (gas injection) four-stroke model (Figure 2.5),

Atomizer Fuel Engine Man
Figure 2.5 Fuel injection system of the MAN B&W Diesel 16 V 28/32-GI high pressure gas-injection engine

and a 6L35MC two-stroke model. A high pressure gas-injection system for MC low speed engines was jointly developed by the group and its key Japanese licensee Mitsui, which delivered a 41 000 kW 12-cylinder K80MC-GI engine for power generation in an environmentally sensitive region (Figures 2.6 and 2.7).

Figure 2.6 MAN B&W Diesel's MC-GI low speed dual fuel engine, indicating new or modified components compared with the standard diesel version

The prime marine target for large gas-fuelled engines is LNG carrier propulsion, which allows the cargo boil-off gas to be tapped, but a modified high pressure gas-injection version of the MC low

Atomizer Fuel Engine Man

Pilot oil pump Control oil pump

Figure 2.7 Gas injection is stopped immediately on the first failure to inject the pilot oil by the patented safety valve incorporated in the control oil pump of the MAN B&W MC-GI engine

Pilot oil pump Control oil pump

Figure 2.7 Gas injection is stopped immediately on the first failure to inject the pilot oil by the patented safety valve incorporated in the control oil pump of the MAN B&W MC-GI engine speed engine is at the heart of MAN B&W Diesel's system for burning volatile organic compound (VOC) discharges from offshore shuttle tankers. Oil vapours or VOCs are light components of crude oil evaporated mainly during tanker loading and unloading but also during a voyage when cargo splashes around the tanks.

VOC discharge represents a significant loss of energy as well as an environmental problem (the non-methane part of the VOC released to the atmosphere reacts in sunlight with nitrogen oxide and may create a toxic ground-level ozone and smog layer). The VOC-burning system was first applied at sea to the twin 6L55GUCA main engines of the 125 000 dwt shuttle tanker Navion Viking in 2000, an onboard recovery system collecting the discharges and liquefying the product by compressing and cooling. The resulting condensate is stored and burned as fuel for propulsion, yielding substantial savings in heavy fuel oil consumption (Figure 2.8).

VOC treatment and collection \ system on deck

Crude oil supply

Vent to atmosphere (Mostly nitrogen)

2. VOC gas condensation system 1. VOC gas cleaning system

VOC gas

Tanker

Exhaust gas low on SOx, NOx, and particulates

Fuel oil

3. VOC storage tank (Condensed VOC gas)

Fuel oil

3. VOC storage tank (Condensed VOC gas)

4. High pressure VOC supply pump

Figure 2.8 Schematic representation of system for burning volatile organic compound (VOC) discharges in the main engines of a shuttle tanker (MAN B&W Diesel)

Engines for such applications must be able to operate on ordinary heavy fuel oil if VOC fuel is unavailable, and they must also accept any possible composition of VOC condensate since it is not possible to apply any fuel specification: whatever is collected must be burned. Shoreside tests by MAN B&W Diesel confirmed that its dual-fuel MC engines could use any VOC/HFO ratio between 92/8 per cent and 0/100 per cent, as well as any relevant VOC quality.

REVIEWING THE DESIGN FEATURES OF GAS-BURNING ENGINES, MAN B&W DIESEL NOTES:

The fuel type will depend on the demands of the plant and on fuel availability. For large engines, the dual fuel system with a liquid pilot fuel is necessarily applied, providing the plant with redundancy to operate on liquid fuel alone in the event of shortage or failure of the gas supply. In smaller plants, the engine may exploit the same principles or be optimized for using a single gaseous fuel or even a number of different gases.

The combustion chamber design may be the single chamber configuration used on all large engines or it may be a divided chamber design featuring a pre-chamber. In the latter case the pre-chamber normally operates with an over-rich fuel/air mixture, provided either by a separate gas supply to the chamber or by pilot fuel injection. The aim in both cases is to secure stable ignition in the pre-chamber and to minimize nitrogen oxides (NOx) formation (due to the lack of oxygen). Combustion in the main chamber may then use a lean mixture effectively ignited by the hot combustion products emerging from the pre-chamber. The lean mixture burns at a lower temperature which, in turn, minimizes the formation of NOx in the main chamber.

Charge exchange may be with or without supercharging and the medium may be pure air or a gas/air mixture. Small, simple plants are normally naturally aspirated with a gas/air mixture while larger plants, for cost reasons, are turbocharged. Four-stroke engines may use air or a gas/air mixture for the charge exchange but two-stroke engines are restricted to air.

The air/fuel ratio may be selected according to engine type and application. A rich mixture can be exploited to secure a high output and stable running from small engines without emission restraints. Such applications will become scarce in the future: a stoichiometric air/fuel ratio controlled by a sensor and combined with a three-way catalyst to comply with emission regulations is considered a more suitable choice.

Except for such small engines, lean burn or stratified charge is the principle being applied in most new developments, and this may be used without a pre-chamber. The reasons are the favourable emission characteristics and high engine efficiency. Mixture formation, defined as the mixing process of air and gas, may take place in the cylinder by direct gas injection at low pressure (after valve closure but before compression start) or by high pressure gas injection near TDC. Two-stroke engines need to apply one of these alternatives to avoid excessive gas losses which would promote high HC-emissions and reduced efficiency.

With low pressure direct injection, the gas/air mixture is exposed to the compression process and hence increasing pressure and temperature. This may lead to self-ignition and severe knocking, excluding its use in new engine developments.

In the case of late-cycle high pressure gas injection the gas is only admitted to the combustion chamber at the desired time for combustion. This is initiated either by a pilot fuel injection or by directing the gas jets to a glow plug.

The high pressure gas injection system is relatively costly but it renders the engine practically insensitive to fuel gas self-ignition properties. For this reason, MAN B&W Diesel suggests, the system can be considered as underwriting true multi-fuel engines, provided that the auxiliary systems for fuel supply have been designed accordingly.

For most four-stroke gas-burning engines the mixture formation occurs outside the cylinder (an exception is the high pressure gas-injection engine). The gas may be mixed with air before the turbocharger, yielding a very uniform mixture but also a rather large volume of ignitable gas in the intake system of the engine. Alternatively, it may be added to the intake air in the cylinder head or the intake air manifold, close to the intake valve, by a combined intake valve/ gas valve or by a separate gas valve.

Ignition sources take various forms but are all, basically, just high temperature sources. The highest ignition energy, MAN B&W Diesel explains, is provided by pilot injection which also makes it possible to locate multiple ignition points deep inside the gas/air mixture: beneficial for the fast and efficient combustion of lean mixtures. This solution is reliable and yields extended times-between-overhaul but it is more costly than a spark plug or glow plug arrangement. Pilot injection, by its nature, dictates the presence of a liquid fuel which, in some cases, is considered a disadvantage. In other cases, this is viewed positively as a means of fuel redundancy.

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