This thesis describes an experimental investigation into ignition systems, their effects on the combustion process, and how the discharge is affected by the prevailing pressure, temperature and flow. The work is divided into four main areas, a comprehensive literature review, engine testing for ignition system suitability, non-flow rig testing (including erosion) and flow rig testing. The literature review concluded that the most practical ignition system for lean burn gas engines will continue to be based on the spark plug, but in the medium to long term, laser ignition may become viable. The measurement of the HT voltage and current is not straightforward, and appropriate methods have been identified. Capacitive and inductive ignition system types were compared in lean and diluted conditions on a single cylinder research engine of modern design at different engine loads and speeds. It was found that the most beneficial ignition system was an inductive ignition system, although that for some conditions, capacitive systems induced better engine performance with a fraction of the stored energy of the inductive alternative. Non flow tests showed that the early part of the spark discharge is sensitive to pressure and temperature effects, and as a consequence, the latter stages of the discharge are also affected. A correlation has been developed, for use with conventional nickel electrode spark plugs, to predict breakdown voltage as a function of pressure, temperature and gap. Experiments were carried out at elevated pressures in a stream of flowing air with capacitive and inductive ignition systems. Different electrode designs and orientations were also compared. It was shown that when exposed to a flow field, the discharge can be stretched which results in a shortened spark duration; in some cases the electrode can shield the discharge from flow field effects. This work showed that flow through the spark gap is a hindrance to the spark process, especially for longer duration systems. However for flame kernel growth, the literature review identified that flow is beneficial, serving to convect the kernel away from the electrodes, reducing the heat transfer from the flame. Analysis of the glow voltage history in the pressurised flow rig has been used to develop a correlation relating the voltage, current, flow velocity, pressure and time. This correlation was used to analyse the velocity records from the spark plug in a firing engine. The predicted velocities and turbulence intensity were in agreement with independent measurements.

Direct fuel injection technology is increasingly being applied to the spark ignition internal combustion engine as one of the many actions required to reduce the CO2 emissions from road transport. Whilst the potential for CO2 reductions is compelling, the technology is not without disadvantages. Early examples typically emitted over an order of magnitude more Particulate Matter (PM) than vehicles with conventional spark ignition engines. Consequently, future revisions to European and North American exhaust emissions legislation are likely to regulate the particulate emissions from vehicles with direct injection gasoline engines. This thesis undertakes to investigate a) instrumentation capable of simultaneously resolving the number concentration and size distribution of particles in the 5-1000 nm size range and b) the factors affecting the PM emissions from spark ignition engines with direct fuel injection. The first objective is achieved by evaluation and comparison of a differential mobility spectrometer; photo-acoustic soot sensor; condensation particle counter and electrical low pressure impactor. To address the second question, a differential mobility spectrometer is applied to quantify the PM emissions from a number of direct injection gasoline engines, together with investigation of their dependence on various calibratable parameters, operating temperature and fuel composition. The differential mobility spectrometer showed good agreement with the other more established instruments tested. Moreover, it exhibited a faster time response and finer resolution in particle size. The number weighted size distribution of the PM emitted was typically lognormal with either one or two modes located between 20 and 100 nm. Chemical analysis of PM samples showed the presence of elemental carbon, volatile organic material and sulphates. Transient PM measurements enabled short time-scale events such as mode switching between homogeneous and stratified mixture preparation to be identified. PM number concentrations in stratified mode exceeded those in homogeneous mode by a factor of 10-100. Dynamometer based experiments showed that PM emissions increase for rich air fuel ratios, retarded fuel injection and advanced ignition events. They also demonstrated a strong dependence on fuel composition: the highest PM emissions were measured with an aromatic fuel, whereas blending alcohols such as methanol or ethanol tended to suppress PM emissions, particularly in the accumulation mode size range. These measurements are amongst the first of their kind and demonstrate the applicability of the differential mobility spectrometer to the measurement of ultra-fine particulate emissions from engines with direct fuel injection systems. Numerous explanations are put forward to describe the data obtained, together with suggestions for future work on PM control and abatement.

With the depletion of hydrocarbon fuels, the hydrogen-hydrocarbon combustion system provides a good solution for the transition period from a hydrocarbon-based energy sys- tem to a hydrogen-based energy system because of its desirable combustion characteristics and the low level of modification to current combustion systems. Though extensive re- search has been carried out to investigate the combustion process of hydrogen-hydrocarbon fuels, no experiments have been reported to study the Particulate Matter (PM) formations in hydrogen-hydrocarbon combustion systems. To measure the PM concentrations in a laminar diffusion flame, a new optical diagnostic technique, called Cone-Beam Tomographic Three Colour Spectrometry (CBT-TCS) has been developed to measure the spatially distributed temperature, soot diameter and soot volume fraction. This technique is based on the principle of three colour pyrometry, but uses a more rigorous light scattering model to calculate the soot diameter and soot volume fraction. The cone beam tomography technique has also been used to reconstruct the 3D property fields from the 2D flame images. The detailed theoretical principles, the exper- imental setup, the optical considerations, the reconstruction algorithm and the sensitivity analysis are all introduced. The CBT-TCS technique has been successfully applied to several laminar diffusion flames to study the PM formation. The temperature and soot volume fracction profiles measured by CBT-TCS for a ethylene laminar diffusion flame are consistent with the data reported by Snelling et al. [77]. The helium-ethylene-air flame tests show that adding helium reduces the PM formation (due to the dilution effect). The hydrogen-ethylene-air flame tests show that adding hydrogen is more effective in reducing the PM formation due to the combined effect of dilution and direct chemical reaction. A PM sampling system has also been de- veloped to verify the PM size distributions measured by CBT-TCS. The comparison results show that the CBT-TCS tends to overestimate the particle size. Several optical engine experiments have also been undertaken to investigate the effect of adding hydrogen on the PM emissions from a Gasoline Direct Injection (GDI) engine. The hydrogen-ethylene engine tests show that adding hydrogen can reduce the PM emissions without sacrificing the power output. The hydrogen-base fuel (65% isooctane and 35% toluene) tests show that adding hydrogen can improve the combustion stability and reduce the PM emissions, especially at low load. Adding 5% stoichiometric of hydrogen can reduce the total PM number concentration by 90% for a stoichiometric mixture and 97% for richer mixture at low load. At high load, adding 10% stoichiometric of hydrogen can also reduce the total PM number concentration by 85% for richer mixture but has little effect upon the stoichiometric mixture.

This thesis is concerned with the development and application of laser induced thermal grating spectroscopy (LITGS) as a tool for thermometry in reacting and non-reacting flows. LITGS signals, which require resonant excitation of an absorbing species in the measurement region to produce a thermal grating, are acquired for systematic measurements of temperature in high pressure flames using OH and NO as target absorbing species in the burned gas. The signal obtained in LITGS measurements appears in the form of a time-based signal with a characteristic frequency proportional to the value or the sound speed of the local medium. With knowledge of the gas composition, the temperature can be derived from the speed of sound measurement. LITGS thermometry using resonant excitation of OH in the burned gas region of in oxygen enriched CH4/O2/N2 and CH4/air laminar flames was performed at elevated pressure (0.5 MPa) for a range of conditions. Measurements were acquired in oxygen enriched flames to provide an environment in which to demonstrate LITGS thermometry under high temperature conditions (up to 2900 K). The primary parameters that influence the quality of LITGS signal were also investigated. The signal contrast, which acts as a marker for the strength of the frequency oscillations, is shown to increase with an increase in the burnt gas density at the measurement point. LITGS employing resonant excitation of NO is also demonstrated for quantitative measurements of temperature in three environments – a static pressure cell at ambient temperature, a non-reacting heated jet at ambient pressure and a laminar premixed CH4/NH3/air flame operating at 0.5 MPa. Flame temperature measurements were acquired at various locations in the burned gas close to a water-cooled stagnation plate, demonstrating the capability of NO-LITGS thermometry for measuring the spatial distribution of temperature in combustion environments. In addition, the parameters that in influence the local temperature rise due to LITGS were also investigated in continuous vapour flows of acetone/air and toluene/air mixtures at atmospheric conditions. Acetone and toluene are commonly targeted species in previous LITGS measurements due to their favourable absorption characteristics. Results indicate that LITGS has the potential to produce accurate and precise measurements of temperature in non-reacting flows, but that the product of the pump intensity at the probe volume and the absorber concentration must remain relatively low to avoid significant localised heating of the measurement region.