Effects of Fuel-Water Emulsion on Diesel Combustion

Currently, positive effects of water to diesel engine combustion are discussed in the industry. Basically three systems are in consideration:

  • Humid Air System (HAM), humidifying of cylinder charge air.
  • Water direct injection (WDI), direct injection of water into the cylinder using a second injection nozzle.
  •  Fuel Water Emulsion (FWE), adding water directly to the fuel as emulsion.

While the first two systems rely on humidification of the cylinder gas, therefore adding water equally distributed to the cylinder charge, FWE-Systems add water directly to the fuel and concentrate water to the flame front.

Humidification of the cylinder gas  reduces temperature rise during compression and combustion phase due to higher specific heat capacity of the cylinder charge. This results in decreased NO-formation, but also in inferior ignition and combustion conditions, leading to increased soot and particulate matter emissions. To achieve significant reductions in NO-formation relatively high amounts of water are necessary (70-80% of fuel flow are reported).

On the opposite, FWE-injection doesn´t influence cylinder charge air and compression phase, providing optimized ignition conditions. Furthermore, the water, incorporated into the fuel, will evaporate explosion like due to the high gas temperature and injection energy, leading to an improved droplet formation (micro explosion) and directly reduce the temperature inside the flame front due to (water-)evaporation and increased heat capacity.

  • Both effects lead to:
  • Improved droplet formation,
  • evenly distributed flame and temperature profile,
  •  reduced soot-formation (PM) and
  • reduced NOx-formation.

While HAM and WDI-Systems require large modifications to the engine installation (Humidifier for HAM, Second injection system for WDI) FWE-Systems require non or only minor changes.

The FWE-System can be incorporated into the fuel system without any changes to the engine itself. Eventually minor changes on the injection system (eg. increased injection nozzle spray holes) are necessary to compensate the higher volumetric flow at full load.


While HAM-Systems can be operated with sea water (no supply problem), both WDI- and FWE need to be operated with desalinated or drinking water (FWE). Both are subject to a supply problem for offshore operation, but due to the reduced water consumption this problem is reduced for FWE-operation.