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Engine types

Engine types

Basically, three engine types are available. The gas engine and dual-fuel engine both utilize a “lean-burn” Otto combustion process when operating on gas. The gas-diesel engine, which dates back to the 1970s, uses a diesel process. All three engine types are installed on ships on order today.

 

Gas engines

Lean-burn gas-fuelled engines run only on gas, with “lean” signifying a high air-fuel ratio. The extremely lean air-fuel mixtures lead to lower combustion temperatures and therefore reduced NOX formation. The engine operates according to the Otto cycle, with combustion triggered by spark-plug ignition. The gas is injected at low pressure. Rolls-Royce started developing its lean-burn gas-fuelled engines in the 1980s for land power and cogeneration. They are now also used for propulsion of some of the LNG-fuelled ships operating in Norway.

 

Dual-fuel engines

Dual-fuel engines can run in either gas mode or liquid-fuelled diesel mode. The engine works according to the lean-burn Otto principle in gas mode, but the lean air mixture is ignited by injection of a small amount of diesel fuel into the combustion chamber instead of by a spark plug. The injected diesel fuel is normally less than 1 percent of total fuel. In diesel mode, the engine works according to the normal diesel cycle with diesel fuel injected at high pressure just before top dead centre.

 

Work on development of new low-speed 2-stroke dual-fuel gas engine technology was recently initiated by Wärtsilä. With time, the benefits of dual-fuel technology will thus be available for larger engines and the broader marine market.

 

Gas-diesel engines

Gas-diesel engines run on various gas and diesel mixtures or alternatively on diesel alone. The engines use the diesel cycle, burning a mixture of gas, diesel and air, with the gas being injected at high pressure. Conversion of existing engines to natural gas operation involves only limited modifications. Emissions of NOX are higher from these engines compared with lean-burn and dual-fuel engines. The gas-diesel engine does not therefore comply with IMO Tier III regulations. The MAN ME-GI engine is of the gas-diesel engine type.

 

Main characteristics of different engine types

Engine type Characteristics

Spark-ignited lean-burn gas engine

- spark ignition

- meets IMO Tier III

- sensitive to methane slip; minimized by design and combustion control

- sensitive to gas quality

Low-pressure dual-fuel engine

- pilot fuel injection (0.5-1%)

- meets IMO Tier III

- sensitive to methane slip; limited scope for process control

- flexible: can also use HFO/MGO

- sensitive to gas quality

Gas-diesel

- pilot fuel injection (5%)

- does not meet IMO Tier III

- flexible: can also use HFO/MGO

- no methane slip

- simpler conversion of existing engines

- not sensitive to gas quality

Environmental impacts are illustrated here.

 

Further reading

Three LNG engine types

There are basically three types of engines available. Gas engines and dual-fuel engines utilize a “lean-burn” Otto combustion process when operating on gas, while gas-diesel engines use a Diesel process. All three engine types are being installed on LNG-fuelled ships on order today. The gas engine and dual-fuel engine can meet the IMO Tier III emission standard.