Gasdiesel and dualfuel engines

Growing opportunities for dual-fuel and gas-diesel engines in land and marine power markets have stimulated designs from leading medium speed and low speed enginebuilders. Development is driven by the increasing availability of gaseous fuels, the much lower level of noxious exhaust emissions associated with such fuels, reduced maintenance and longer intervals between overhauls for power plant. A healthy market is targeted from floating oil production vessels and storage units, rigs, shuttle tankers, offshore support vessels and LNG carriers; LNG-fuelled RoPax ferries have also been proposed with dual-fuel diesel propulsion.

Valuable breakthroughs in mainstream markets have been made since 2000 with the specification of LNG-burning engines for propelling a small Norwegian double-ended ferry (Mitsubishi high speed engines), offshore supply vessels and a 75 000 cu.m. LNG carrier (Wartsila medium speed engines).

Natural gas is well established as a major contributor to the world's energy needs. It is derived from the raw gas from onshore and offshore fields as the dry, light fraction and mainly comprises methane and some ethane. It is available directly at the gas field itself, in pipeline systems, condensed into liquid as LNG or compressed as CNG.

Operation on natural gas results in very low emissions thanks to the clean-burning properties of the fuel and its low content of pollutants. Methane, the main constituent, is the most efficient hydrocarbon fuel in terms of energy content per amount of carbon; operation on natural gas accordingly reduces emissions of another key pollutant—carbon dioxide—by over 20 per cent compared with operation on diesel fuel. Natural gas has very good combustion characteristics in an engine and, because it is lighter than air and has a high ignition temperature, is also a very safe fuel. The thermal efficiencies of various prime movers as a function of load are illustrated in Figure 2.1.

Wartsila's dual-fuel (DF) four-stroke engines can be run in either gas mode or liquid-fuelled diesel mode. In gas mode the engines work according to the lean-burn Otto principle, with a lean pre-

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Figure 2.1 Thermal efficiency of diesel and gas-diesel engines as a function of load (Wartsila)

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Figure 2.1 Thermal efficiency of diesel and gas-diesel engines as a function of load (Wartsila)

mixed air-gas mixture in the combustion chamber. (Lean burn means the mixture of air and gas in the cylinder has more air than is needed for complete combustion, reducing peak temperatures). Less NOx is produced and efficiency increases during leaner combustion because of the higher compression ratio and optimized injection timing. A lean mixture is also necessary to avoid knocking (self-ignition).

The gas is fed into the cylinder in the air inlet channel during the intake stroke (Figure 2.2). Instead of a spark plug for ignition— normally used in lean-burn gas engines—the lean air-gas mixture is ignited by a small amount of diesel fuel injected into the combustion

Figure 2.2 The lean burn dual-fuel diesel operating system (Wartsila)

chamber. This high energy source ensures reliable and powerful ignition of the mixture, which is needed when running with a high specific cylinder output and lean air-gas mixture. To secure low NOx emissions it is essential that the amount of injected diesel fuel is very small. The Wartsila DF engines therefore use a 'micro-pilot' injection, with less than 1 per cent diesel fuel injected at nominal load, to achieve NOx emissions approximately one-tenth those of a standard diesel engine.

When the DF engine is running in gas mode with a pre-mixed airgas mixture the combustion must be closely controlled to prevent knocking and misfiring. The only reliable way to effect this, says Wartsila, is to use fully electronic control of both the pilot fuel injection and the gas admission on every cylinder head. The global air-fuel ratio is controlled by a wastegate valve, which allows some of the exhaust gases to bypass the turbine of the turbocharger. This ensures that the ratio is of the correct value independent of changing ambient conditions, such as the temperature (Figure 2.3).

Figure 2.3 A special electronic system for the Wartsila DF engine controls combustion in each cylinder, and optimizes efficiency and emissions under all conditions by keeping each cylinder within the operating window

The quantity and timing of the injected pilot fuel are adjusted individually together with the cylinder-specific and global air-fuel ratio to keep every cylinder at the correct operating point and within the operating window between the knock and misfire limits. This is the key factor for securing reliable operation in gas mode, automatically tuning the engine according to varying conditions, Wartsila explains.

In diesel mode the engine works according to the normal diesel concept using a traditional jerk pump fuel injection system: diesel fuel is injected at high pressure into the combustion chamber just before the top dead centre. Gas admission is de-activated but the pilot fuel remains activated to ensure reliable pilot ignition when the engine is transferred to gas operation.

The gas pressure in the engine is less than 4 bar at full load, making a single-wall pipe design acceptable if the engineroom is arranged with proper ventilation and gas detectors. The gas valve on every cylinder head is a simple and robust, electronically-controlled solenoid valve, promising high reliability with long maintenance intervals.

The pilot fuel system is a common rail system with one engine-mounted high pressure pump supplying the pilot fuel to every injection valve at a constant pressure of 900 bar. Due to the high pressure a double-walled supply system is used with leak fuel detection and alarms. The injection valve is of twin-needle design, with the pilot fuel needle electronically controlled by the engine control system. It is important that the injection system can reliably handle the small amount of pilot fuel with minimum cycle-to-cycle variation. The main needle design is a conventional system in which mechanically-controlled pumps control the injection.

Other main components and systems are similar to designs well proven over a decade on Wartsila standard diesel engines, further underwriting high reliability of the dual-fuel engines.

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