In Figures 4.1 and 4.2 we have written two chemical equations with the same reactants but different products. How do the molecules know which reaction to undergo? Under most conditions, both reactions will ocuur to some extent, but the relative inportance of each reaction depends on the reaction conditions. The ratio of fuel to oxygen is of particular and is closely monotored by the fuel injection system in modern car engines. Other factors, including engine temperature, are also important. A properly tuned engine ensures that the extent of complete combustion is maximized, limits hazardous CO emissions, and also improves gas mileage.