Thermal efficiency of a Diesel engine

[xJJx]

Hi, i’m learning about the thermal efficiency of a diesel cycle at university and I'm trying to understand the equation. The first law of thermodynamics states Wnet + Qnet = 0, but one of the attachments below says Qnet - Wnet = 0 and I don't understand how this is the case. Also, for process 4-1 (isochoric heat rejection), it says that Qout = -mcv(T1-T4); I don't get why the minus is there. Can anyone explain why? Thank you.

 

[DrClaude]

The first law of thermodynamics states Wnet + Qnet = 0

That's not what the first law says. It says that ΔU = Q - W: any change in the internal energy of a system must come from heat entering/leaving the system or work done on/by the system. The convention used here is that positive W is the work done by the system, hence the negative sign.