LPG options for two-stroke engines
All electronically-controlled ME engines in MAN’s two-stroke range are available as ME-GI dual-fuel versions, incorporating high pressure gas injection arrangements, writes David Tinsley.
The main focus has been on using natural gas (NG) in such dual-fuel installations in LNG carriers and other vessels. However, MAN Diesel & Turbo has now widened the options for the future by preparing an LPG-based propulsion solution.
In the case of an ME-GI engine designed to run on LPG, the dual-fuel capability offers the owner or operator the opportunity to shift between HFO and LPG in accordance with price changes and varying emission controls and regulations, depending on where the ship is trading.
LNG has been widely advocated as a more environmentally compatible, ‘greener’ fuel than HFO or other conventional marine fuels. However, the environmental argument for LPG can be as strong as that for natural gas, since it offers the same emission benefits relative to conventional marine fuels. In addition, the wider availability of LPG offers logistical benefits to shipowners and operators. In a global context, the LNG bunkering infrastructure is very limited.
Other factors in favour of the MAN proposal is the company’s experience in supplying and servicing ships’ engines run on cargo-derived LPG and volatile organic compounds (VOCs), and the suitability of the new LPG variant of the ME-GI engine for other fuel types such as dimethyl ether (DME).
One of the market segments targeted for such plant is that constituted by small tankers engaged in coastal and waterway transportation. The initiative has already been endorsed by an unnamed shipowner, with whom MAN has signed a letter of intent for an engine of the new version to be installed in a general cargo carrier newbuild, designed by the Swedish consultancy FKAB.
LPG normally consists of higher hydrocarbons, notably propane and butane. It does not change the engine’s performance as regards speed, thermal efficiency and power output, for the same rating as that of a fuel oil burning engine. Whereas an ME-GI engine using natural gas has the methane injected into the combustion spaces in gaseous form, the ME-GI/LPG option uses liquefied gas for injection.
The ME and ME-B series provide the design basis for the ME-GI dual-fuel engine, in both its LNG and LPG variants. However, as the density of LPG in liquid form is higher than the density of natural gas in its gaseous state, some changes to components and auxiliaries are necessary in the ME-GI/LPG version.
GI/LPG engine components can be smaller, but the LPG has to be pressurised to 550bar compared with 250bar to 300bar for natural gas. The higher pressure is necessary to achieve complete atomisation of the liquid fuel at the injector nozzles.
Dual-fuel operation requires the injection of both pilot fuel oil and LPG fuel into the combustion chamber via different types of valves arranged in the cylinder head. The ME-GI engine head is fitted with two valves for LPG injection and two for pilot fuel.
MAN already has examples of small medium-speed engines being run on LPG. One of four auxiliary engines (MAN 16/24-type) in a Danish-owned ethylene/LPG/VCM carrier built in 2007 was modified as a dual-fuel engine. This is able to burn evaporated LPG from the cargo. Similar ‘gas packages’, as they were called, were ordered for subsequent vessels of the class.
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