YANMAR Technical Review

Fossil fuels have transformed the fabric of society in many ways. With their high energy densities^*1 and ability to deliver a large amount of work in a short time, fossil fuels have freed people from arduous labor and enabled mass production and long-distance transportation. Unfortunately, whereas these fuels took hundreds of millions of years to form, burning huge quantities of them over the past 200 years or so has created a major imbalance in the global carbon cycle, triggering global warming and climate change. As the carbon dioxide (CO₂) released by burning fossil fuels is a major greenhouse gas, it is a focus of reduction efforts.

The need for fossil fuel alternatives that can be consumed without increasing the level of CO₂ in the atmosphere is directing attention toward the carbon-neutral fuels listed below.
[1] Fuels that do not contain carbon (C): Hydrogen (H₂) and ammonia (NH₃)
[2] Fuels synthesized from hydrogen and captured CO₂ (e-fuels): Hydrocarbons (C_mH_n) and alcohols (C_mH_nOH)^*2
[3] Fuels made from biological material (biofuels): Methane (CH₄), ethanol (C₂H₅OH), and biodiesel, etc.

Carbon-free fuels [1] do not release CO₂ when burned and can be produced by using renewable energy to make hydrogen (H₂) by the electrolysis of water or by the reaction of H₂ with nitrogen (N₂) to make ammonia (NH₃).
e-fuels [2] can be synthesized by reacting H₂ with CO₂ captured from factory emissions or from the atmosphere. The CO₂ released when these fuels are burned simply returns to the atmosphere where it originally came from. It can then be captured again and reused in fuel synthesis.
As biofuels [3] derive from living organisms, the CO₂ released when they are burned is balanced by the CO₂ absorbed by the organisms during their growth, resulting in no net gain in the amount of CO₂ in the atmosphere.

The choice of which of these carbon-neutral fuels to use for different machinery (such as maritime, land-based, or stationary generation applications) depends on factors such as energy density^*1 and fuel supply infrastructure. This issue of Yanmar Technical Review includes three articles on the development of engine technologies for carbon-neutral fuels. Topic 1 looks at research into technologies for using ammonia as a fuel, Topic 2 at the research and development of ways of using methane (CH₄) as a biofuel, and Topic 3 at the development of spark-ignition engines that can run on multiple fuels.

• *1Energy density can refer to various different measures, including energy per unit volume (Wh/L) and energy per unit mass (Wh/kg).
• *2CmHn and CmHnOH are the generic formulas for hydrocarbons and alcohols respectively.