1. An apparatus for analyzing the operation of a diesel engine, said diesel engine defining a working cycle and having at least one internal chamber and means for passing a vapor through said internal chamber, said internal chamber including a prechamber and cylinder and said vapor having a temperature, said diesel engine further including a glow plug partially within said prechamber, said glow plug having a resistance dependent upon said temperature of said vapor within said prechamber, comprising, in combination: passive temperature sensor means, interconnected to said diesel engine, for sensing said temperature of said vapor within said prechamber of said internal chamber and for providing a temperature signal representative of said temperature, said passive temperature sensor means including impedance sensor means for sensing said resistance of said glow plug, said temperature signal being dependent upon said resistance throughout said working cycle; and display means, interconnected to said passive temperature sensor means, for monitoring said temperature signal throughout said working cycle and responsively providing a display based upon said temperature and representative of a brake mean effective temperature.