Ab isobaric process: Q=Cpm* temperature difference, internal energy increment =Cvm* temperature difference, work =R* temperature difference. As can be seen from the figure, the internal energy increment =3000J, so the heat absorption =3000*Cpm/Cvm=5000J, and the work done =2000J.
There is no work done in bc and other bulk processes, so the internal energy increment and heat absorption are -3000J, Tb=2Tc=2Ta=4000/R, and Pb=2Pc.
The ca process is isothermal, the internal energy is constant, and the work done by heat absorption (all less than 0)=-2000ln2.
Engine efficiency = 2000 (1-LN2)/5000 = 2 * (1-LN2)/5.