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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.

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Gasoline Direct Injection (GDI) engines work with stratified charge at part load and burn with lean mixtures in order to save fuel, whilst at full load, the fuel and air mix homogeneously for maximum power output. The higher compression ratio and the absence of throttling are two of the most significant benefits of GDI engines. The key issues facing GDI combustion include in-cylinder mixture preparation and post-combustion soot formation. This work was intended to investigate these aspects and was undertaken on a dedicated Jaguar single-cylinder optical GDI engine with a spray-guided combustion system. The spray-guided concept does not rely as much on charge motion or piston design, and can avoid wall-wetting effects so as to reduce engine emissions. Relevant engine control hardware and data acquisition equipment were commissioned. Data/image processing software was also developed to suit the measurements. A data-processing case study with data from a small two-stroke glow ignition engine has been conducted to develop a method to combine the burn rate and heat release analyses in the study of engines with premixed charge but compression ignition. Difficulties such as unknown ignition timing and polytropic index have been addressed. Results for all operating conditions have shown good correlations between the two methods. The technique of quantitative planar laser-induced fluorescence is useful for measuring 2-D fuel distribution in GDI engines. The relevant physics and literature were reviewed in depth. A multi-component fuel was designed to give reasonable co-evaporation characteristics with tracers matching different fuel fractions. The absorption and fluorescence features of each fuel component and tracer were characterised. Optimisation of hardware and signal-to-noise ratio was performed. A recirculating loop was set up for the calibration of the technique. The technique of colour-ratio pyrometry (CRP) for estimating the temperature and loading of soot was applied on the GDI engine. Critical features of the candidate CCD colour camera including its spectral response and noise behaviours were fully studied. Validation tests with reference sources together with an error analysis suggested an accuracy of ±50K within the combustion temperature range. Engine combustion images were then taken under various operating conditions. Temperature estimates were shown to be insensitive to the concentration of soot. Simulation with a thermodynamic modelling package, ISIS, was introduced for comparison with the experimental data. With careful tuning, ISIS gave outputs comparable to the CRP and proved to be a cost-effective tool to study GDI engines. High-speed combustion imaging was carried out using a CMOS camera, allowing the study of flame properties as well as crank-angle resolved CRP. By using a lens in the piston crown to give full bore optical access and appropriate image processing, the flame front could be detected reliably throughout the main combustion process.

The combustion of clouds of fuel droplets is of great importance in many industrial applications, such as gasoline and diesel engines, gas turbines and furnaces. Here, efficient combustion has to be combined with minimum noxious emissions. Aerosols also might produce a particularly hazardous explosion risk. To optimise their performance a fundamental understanding of the complex processes in aerosol combustion systems is necessary. A fundamental study of aerosol combustion has been conducted to quantify the parameters of importance. For this, a novel aerosol combustion apparatus was developed, that offers a well controlled environment with respect to aerosol properties, temperature, pressure and turbulence. Aerosols were generated using the Wilson cloud chamber principle of expansion cooling, which produces a homogeneously distributed, near monodisperse droplets cloud. Drop sizes of 10 to 30μm, pressures between 100 and 360kPa and temperatures of 263 to 292K were used. Laminar mixtures between the overall equivalence ratios of 0.8 and 1.2 were studied. A considerable burning velocity enhancement of up to 420% was observed. This enhancement was shown to be a function of drop size and liquid fraction. From the study, it was concluded that burning velocity enhancement probably is caused by the increase in surface area due to wrinkling, caused by the development of instabilities. At low temperature (<275K) the formation and destruction of wrinkles and cells was random. At higher temperatures (>290K) cell formation and division was progressive and traceable, like that observed in gaseous flames. Cellular acceleration at these temperatures was similar to that of gaseous flames. Stretch appeared to have a damping effect on the instabilities, caused by the aerosol. Oscillating flames were observed for some experimental conditions and these also showed enhanced flame speeds. These oscillations were possibly caused by aerodynamic interaction between droplets and gas motion ahead of the flame. Also Stretch and radiation probably influenced these oscillations. Inert glass particles in a gaseous fuel-air mixture had no effect on flame speed or structure. However, water aerosols caused significant burning velocity enhancement (50%). These findings contradict the hypotheses that fuel rich pockets, flame propagation through "easy-toburn" regions or a "grid-effect" trigger instabilities in aerosols. Comparison with a linear stability analysis of heat loss from the flame (Greenberg et al.,1998), yielded good qualitative agreement with the data of the present work.

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