An air-standard Diesel cycle has a compression ratio of 16 and cutoff ratio of 2. At the beginning of the compression process, air is at 95kPa and 27°C. Accounting for the variation of specific heats with temperature, determine: a) The temperature after the heat-addition process

b) The thermal efficiency

c) The mean effective pressure

d) The pressure at the beginning of the constant-volume heat rejection process

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.