One method of using alternativefuels in diesel engines is by adopting amixed combustion process called dualfu elling w h er e alt ernativ e fu eI s uc h as natural gas (Ir{ G) is induc e d into the cylinder as a primary fuel with air and is subsequently isnited with a pilot injection of dieselfuel. The ignition delay in a dualfuel (DF) engine is differentfrom that in a diesel engine because the primaryfuelalters the properties of the charge, r e duc e o xy g e n av ailable and under go e s pr e -ignitio n r e ac tio n s durin g c o mp r e s s io n. V ario u s c o nclu sio n s of DF ignition delay have beenreachedusing different engines. In the presentwork a constantvolume combustionvessel (CVCV) has beenusedto study the ignition delay of aDF combustionpFocess. E xp erim e nt s hav e b e e n p e rform e d to inv e s tigat e th e i gniti o n d e lay p e rio d at dffi r e nt initial t e mp e r atur e s andpressures. The results obtainedwere usedto modify the Hu and Milton'ss DF ignition delay correlation. The proposed coruelation predicts a delay periodfor a wide range of initialtemperatures andpressures. The trends exhibitedby the correlation are consistentwith DF ignition delay engine tests datafrom other researchersl'2. In particular, it explains why some reported tests results show that ignition delay is always rising while others show that it decreases temporarily before rising againto very highvalues. The rising of ignition delay occurs withlow pilot diesel quantities and the latter with high one s.