As the ratio of oxygen to fuel (or air to fuel) decreases, the products change from the desired carbon dioxide to the undesired carbon monoxide and then to soot (which is almost pure carbon) or unburned heptane. When an engine is operating on a high value of air/fuel ratio, we say the engine is running lean. Sometimes we may also speak of a lean-burning engine. When the air/fuel ratio is low, we say that the engine is running or burning rich. The carburetor or fuel injector system of an engine can be adjusted, at least to some extent, to change this ratio. It would seem,.
