Water Injection

Water has been added to the high performance, reciprocating aero SI engines during Second World War to suppress engine knock. Water addition to intake charge has been investigated by many researchers to reduce NOx formation. Water addition to intake charge is another form of charge dilution to reduce combustion temperatures.

Water has been directly injected into intake manifold or used as water-fuel emulsion. Emulsifying chemicals in about 2 percent by volume are added to form water-gasoline emulsions. The stability of emulsion may be around a few days. The addition of emulsifiers usually reduces the fuel octane number. With water addition ranging from 10 to 30% by volume of gasoline, large reductions in NO_x are possible However,  high increase in HC  is observed although  only a  slight increase in CO occurs.    Some times a small improvement in BSFC with small addition of water is observed but the BSFC increases with higher amounts of water addition. This approach has not been found practical due to harmful effects of water addition as HC and BSFC increase, and corrosion of engine components is also encountered.