Quote:
Originally Posted by racprops
I have read adding a little water injection can help lower the burn temp and lower NOX.
Rich
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And the reasons are quite complex. But, with the chance of producing errors in simplifying . . .
It is not just simply the quenching ability of the water's heat capacity, but also the fact that at 1200 deg C, thermolysis of water results in H and OH molecules who's reaction it is to broaden the flame front and reduce the unit heat release (flux), ie. cooling.
As early as WW2, engine designers looking into water injection saw a measurable amount of reduced BSFC (brake-specific-fuel-consumption) using seemingly small amounts of water at full throttle. The gain was small (2%) but measurable.
NOx can also be controlled through time - if the combustion reaction is rapid enough, the relatively lazy Nitrogen atom loses out in combining with Oxygen before temperatures are diminished. A paper by College of the Desert, showed minimal NOx production in the lean running of their hydrogen fueled engine. This was due in large part to the rapid combustion rate of hydrogen (roughly an order of magnitude greater than hydrocarbons ).
EGR is a more useful system since the exhaust stream is largely H2O and incombustible CO2. The problem is precision of control.