Robert Dodd

The influence of beam energy, mode and focal length on the control of laser ignition in an internal combustion engine

This work involves a study on laser ignition (LI) in an internal combustion (IC) engine and investigates the effects on control of engine combustion performance and stability of varying specific laser parameters (beam energy, beam quality, minimum beam waist size, focal point volume and focal length). A Q-switched Nd?:?YAG laser operating at the fundamental wavelength 1064?nm was successfully used to ignite homogeneous stoichiometric gasoline and air mixtures in one cylinder of a 1.6?litre IC test engine, where the remaining three cylinders used conventional electrical spark ignition (SI). A direct comparison between LI and conventional SI is presented in terms of changes in coefficient of variability in indicated mean effective pressure (COVIMEP) and the variance in the peak cylinder pressure position (VarPPP). The laser was individually operated in three different modes by changing the diameter of the cavity aperture, where the results show that for specific parameters, LI performed better than SI in terms of combustion performance and stability. Minimum ignition energies for misfire free combustion ranging from 4 to 28?mJ were obtained for various optical and laser configurations and were compared with the equivalent minimum optical breakdown energies in air.

A comparative study of optical fibre types for application in a laser-induced ignition system

Various types of optical fibres have been investigated and compared for delivering high power laser beams to an optical plug (comprising of lenses and an optical window) for the application of laser-induced ignition of gasoline and air mixtures in an automotive internal combustion engine. Three main types of optical fibre were examined: multi-mode step index silica, sapphire and photonic crystal. The fibres had various core sizes ranging from 35 to 600 μ ma nd numerical apertures between 0.046 and 0.64. A Q-switched Nd:YAG laser operating at the fundamental wavelength 1064 nm with a pulse length of 15 ns was used for the testing. Fibre output beam properties, including beam mode quality, output divergence, transmission losses, beam energy thresholds and effects of engine vibration were investigated. These fibre beam properties were compared with known beam parameters for laser ignition to assess the suitability of such fibres for a laser ignition system. Online fibre delivery laser ignition engine tests were performed with the most suitable fibres, which showed that combustion could be achieved with this system despite a relatively large percentage of misfires.

Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O2/Ar gas mixtures (0?100%), applied powers (50?600?W) and total discharge pressures (2?25?mTorr). The measurements reveal that the O? ion dominates over with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200?W) and oxygen partial pressures (10% O2). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O? was determined to be ion?ion recombination with O+, and Ar+.In this study, the maximum negative ion density obtained was found to be 7.7 ? 1015?m?3 at 200?W applied power, 25?mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, ? = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.

A study of the plasma electronegativity in an argon–oxygen pulsed-dc sputter magnetron

Using Langmuir probe-assisted laser photodetachment, the temporal evolution of the O− density was determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon–oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5% to 50% and oxygen partial pressures of 10% and 50% of the total pressure.In the plasma on-time, for all duty cycles the negative ion density (n−) rises marginally reaching values typically less than 2 × 1015 m−3 with negative ion-to-electron density ratios, α < 1. However, immediately after the transition from pulse on-to-off, n− falls by about 20–30% as fast O− species created at the cathode exit the system. This is followed by a rapid rise in n− to values at least 2 or 3 times that in the on-time. The rate of rise of n− and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several tens of μs), and therefore the afterglow plasma becomes highly electronegative, with α reaching 4.6 and 14.4 for 10% and 50% oxygen partial pressure, respectively. The rapid rise in n− in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly excited oxygen metastables, which themselves are created in the pulse on-time. At the lowest duty of 5%, the long-term O− decay times are several hundred μs.Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, α has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method.

Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

The time-resolved negative oxygen ion density n− close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O− density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m−3; however, in the off-time, the O− density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n−/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative elec...