Fuel economy can be improved with water injection. Depending on the engine, the effect of water injection, with no other modification, like leaning out the mixture, may be quite significant.[1] In some cases water may also reduce CO emissions. This may be attributable to the water gas shift reaction, in which CO and H2O shift to form CO2 and H2.[1] However, water may also increase hydrocarbon emissions, possibly due to an increased quenching layer thickness.