Imagine you are working at GM, etc. First you would gather requirements: power, torque, fuel consumption [Mech Engr]. Then you would look at previous designs. Hopefully you will have taken thermodynamics and combustion courses in college [Mech Engr], so you could calculate or estimate what the new engine ought to look like. A big company should have the right software to model this new engine and predict the performance. If OK, then you would create engineering drawings and get prototypes built [machine shop]. If not, maybe you would need to troubleshoot the design. Was there a temperature problem [Thermal Engr]? Can the electronics support this design [Elec Engr]. Bad air flow? [Fluid Dynamics]
So there is plenty to do to get a new design out, and plenty of design iterations.