Car petrol is usually some form of alkane, which has the general formula CnH2n+2. Let us assume we are using Butane (C4H10). The desired chemical reaction for the complete combustion of butane is 2C4H10 + 13O2 --> 8CO2 + 10H2O.However, if there is an insufficient supply of oxygen, then there is incomplete combustion, 2C4H10 + 9O2 --> 8CO + 10H2O.As you can see, instead of carbon dioxide given off, the products are water and carbon monoxide, which therefore, results from incomplete combustion.