Most trans-oceanic cargo ships are powered by 2-stroke diesel engines. These can be modified to accommodate methanol by adjusting fuel injectors, installing double-walled fuel distribution systems, increasing the size of fuel storage containers, and implementing methanol purging systems, amongst other additional safety considerations and worker training [1] “The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.. To understand how to modify these engines to run on a combination of diesel and methanol, one must first understand the basic setup of a marine diesel engine.
Marine diesel engines are internal combustion engines. They can be powered by a variety of fuels, including heavy fuel oil (HFO), marine fuel oil (MFO, a low sulfur fuel), and diesel fuel; these fuels are made by refining crude oil [2]“Diesel Fuel Explained – U.S. Energy Information Administration (EIA).” Accessed November 20, 2021. https://www.eia.gov/energyexplained/diesel-fuel/.. Unlike the spark-ignition gasoline engines that power most cars, diesel engines use compression to ignite the fuel.
Generally, compression-ignition engines function in a series of steps. First, as the crankshaft turns, raising the piston, air inside the cylinder is compressed. Then, fuel is injected into this compressed air, which is at high pressure and temperature due to compression. These conditions ignite the fuel, which combusts. The force of this controlled explosion pushes the piston back down, turning the crank, and converting linear momentum into rotational motion that turns the propellers and drives the ship forward. As the piston is pushed down, exhaust escapes through the exhaust valve, and an air intake valve opens to recharge the cylinder with air. This air is again compressed, fuel is injected and combusted, and the cycle repeats. One “stroke” is when the piston moves from the bottom of the cylinder to the top; therefore, in a 2-stroke engine, the piston travels across the cylinder twice in one complete combustion cycle.
These compression-ignition engines rely on a chemical property of their fuels called the cetane rating. This property indicates how easily a substance can ignite when compressed. Compression-ignition engines work well for high-cetane fuels like diesel fuel; unfortunately, methanol has low levels of cetane and therefore requires a pilot fuel to initiate combustion. This means that, along with the methanol, another fuel like diesel must be injected into the cylinder (though only about 5% of the injected fuel must be pilot fuel to initiate ignition [5]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.).
Achieving this injection ration means modifying the fuel injection system. The primary modification will be a hydraulic pump, which increases the pressure at which methanol can be injected into the cylinder [6]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.. As mentioned above, the chemical properties of methanol differ from those of traditional diesel fuel; in the case of the fuel injection system, an increase in injection pressure is necessary to optimize the ignition of methanol fuel [7]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.. Fuel injection systems must also be adjusted to accept both methanol and diesel fuel in the proper proportions from two different fuel delivery systems.
These modified fuel injection systems are the first of a few necessary changes to existing marine diesel engines to allow them to run on methanol. Methanol is liquid at room temperature, meaning no cryogenics or high-pressure chambers are required for methanol storage. This is a benefit of methanol as opposed to other alternate fuels, such as ammonia or hydrogen. Unfortunately, methanol’s volumetric energy density is lower than traditional fuels: about twice as much container space is required to allow a ship to travel the same distance on methanol as traditional diesel fuels [8]Andersson, Karin, and Carlos Márquez Salazar. Methanol as a Marine Fuel Report. Methanol Institute, October 2015. … Continue reading [9]“Comparison of Alternative Marine Fuels.” Environment Advisory, July 5, 2019. https://safety4sea.com/wp-content/uploads/2019/09/SEA-LNG-DNV-GL-Comparison-of-Alternative-Marine-Fuels-2019_09.pdf.. However, since methanol requires no additional pressurization or temperature decreases, the costs of upgrading these storage tanks are minimized. For an overall cost analysis of retrofitting engines from diesel to dual-fuel diesel/methanol, click below.
Two other chemical properties of methanol include its low viscosity (it flows more readily than other fuels) and the toxicity of its vapors [10] Marine & Offshore. “An inside Look at Methanol as Fuel.” Accessed November 20, 2021. https://marine-offshore.bureauveritas.com/inside-look-methanol-fuel.. To ensure the safety of a ship and its crew, fuel delivery systems (namely, the pipes used to transport methanol from the storage tank to the engine) must be double-walled to prevent methanol leakages [11]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.. Additionally, to ensure the engine can be inspected and repaired safely as necessary, as well as for certain operational measures, methanol engines will require purging systems to flush the engine of methanol fuel before these operations can take place. Installing additional safety measures (such as hydrocarbon sensors in the space between double-walled pipes) can also help reduce risk [12]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20..
It is important to note that methanol-powered ships will require workers who have undergone methanol-specific training. A crew that knows how to safely handle methanol as a fuel is essential to minimize risk.
A final modification for marine diesel engines that is unnecessary for successful operation but can limit nitrogen oxide (NOx) emissions is the installation of a water injection unit[13]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20.. NOx gasses are known to contribute to the creation of smog and acid rain, so reducing such emissions is a necessary step in increasing the sustainability of transoceanic freight transport. Consisting mainly of a pump, filters, and pressure transmitters, a water injection unit delivers pressurized water to mix with methanol and reduce NOx emissions [14]“The Methanol-Fuelled MAN B&W LGIM Engine.” MAN Energy Solutions, n.d. https://www.man-es.com/docs/default-source/marine/tools/5510-0172_online-1.pdf?sfvrsn=212d3731_20..
In conclusion, existing 2-stroke marine diesel engines can be modified to run on methanol (plus a small amount of a pilot fuel, such as diesel) by implementing a few relatively simple changes. These modifications include changes to fuel injectors, the installation of double-walled fuel distribution systems, increasing the size of fuel storage containers, and implementing methanol purging systems. Ship employees will also need to undergo training in order to safely handle methanol.
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