During the compression stroke,before combustion, it seems that the mass of the burned gas and unburned gas are equal.(-so my book ICEngines by Heywood states). Why is this? By unburned gas here, one means the nascent injected fuel-air mixture during the intake stroke. By burned gas here, one means the product mixture left over from the previous combustion stroke+the EGR intake gas. I can't see why the author would cite such a statement. By law of conservation of mass,where mass of products=mass of reactants, this would not hold for a chemical reaction taking place in different strokes, would it? (the statement is also given in the last page of the attached pdf file).