Brouzos N.P. Experimental Studies of CAI combustion in a Four-stroke GDI Engine with an Air-assisted Injector

Thesis submitted to University of Brunel United Kingdom for the degree of Doctor of Philosophy. 2007. 199 p.Abstract
CAI combustion and the factors affecting it were intensively investigated in a single
cylinder, air-assisted gasoline direct injection engine. CAI was achieved by means of
residual gas trapping by utilising low-lift short duration camshafts and early closing of the
exhaust valves. The effects of EVC (Exhaust Valve Closure) and IVO (Inlet Valve
opening) timings, spark timing, single and split injection timings, coolant temperature,
compression ratio, cam lift and duration on exhaust emissions and CAI operation were
investigated experimentally. Engine speed throughout the course of the experiments, was
varied from 1200rpm to 2400rpm and the air/fuel ratio was altered from stoichiometric to
the misfire limit.
The results show that the EVC timing, compression ratio, cam lift and duration had
significant influences on CAI combustion and emissions. Early EVC when combined with
higher compression ratio and higher cam lift, enhance CAI combustion operation and
stability. IVO timing had minor effect on CAI combustion while spark timing hardly
affects CAI operation as soon as fully-developed CAI conditions were established. Coolant
temperature was revealed to have substantial impact on CAI combustion when the coolant
temperature was below 65 C.
The results also show the importance of injection timing. Early injection gave faster and
more stable combustion, less HC and CO emissions, but more prone to knocking
combustion and higher NOx emissions. Furthermore, CAI operation range could
considerably be extended with injection during the recompression process. Late injection
led to slower and unstable combustion, higher HC and CO emissions but lower combustion
noise and NOx emissions. Split injection gave even further extension of CAI range in both stoichiometric and lean mixture operations. All the above clearly suggest, that optimising
injection timing and using split injection is an effective way to control and extend CAI
operation in a direct injection gasoline engine.