Perhaps you are unfamiliar with how a diesel engine works; the fuel (a liquid) is contained in a cylinder that reduces in volume, compressing the liquid to the point it reaches its ignition point and burns, generating a larger volume of gas, depressing the compression device (the piston, in this case). Fountain solution can be heated by the same process, I have seen many installations where the fountain solution is pumped into the press refrigerated and returns heated (on one press I observed, it was actually hot). This is usually due to a powerful return pump forcing the liquid through a restriction.