This thesis presents a laser diagnostic investigation into the combustion, NOx formation and mixing processes occurring within the optically assessed combustion chamber of a methane-fired (10kW), fully premixed, self-aspirating, Helmholtz pulse combustor. The inlet geometry of the combustion chamber consisted of a step expansion and a bluff body obstacle formed by a stagnation plate. The focus of the investigation was the effects of the stream-wise position of the stagnation plate on the pulse combustion processes. A comprehensive parametric study of the performance of the pulse combustor is presented with stagnation plate position, air/fuel ratio and tailpipe length as the variables. The operating frequency and peak pressure amplitude trends were found to vary in accordance with the Rayleigh criterion. The operation of the combustor was more stable with the effective heat-release point preceding the resonant acoustic peak. Operation outside of this regime produce increased levels of CO. Time-resolved, laser-sheet flow visualisation images are presented of the flow structures within the combustion chamber. The inlet mixing - between the reactants and residual gases - was dominated by the formation of two counter-rotating toroidal vortices. In general, the inlet mixing was found to decrease as the stagnation plate was moved further into the combustion chamber. However, other mechanisms that tended to counter this trend were observed. Under certain conditions, significant flow reversals were imaged with gases penetrating the combustion chamber from the tailpipe. The combustion event was investigated using cycle-resolved chemiluminescence and laser induced fluorescence imaging of OH* radicals. Ignition of the fresh reactants by residual combustion/radical activity was found to occur along the interface between reactants and residual gases. The increase in reaction zone area generated by the action of the toroidal vortices provided the necessary mechanism for the rapid combustion of the reactants. The reduced mixing associated with moving the stagnation plate further into the combustion chamber produced a more compact combustion zone with less interaction between combusting reactants and cooler residual gases. This modification to the combustion zone was consistent with the measured trends of rising NOx tailpipe emissions and decreasing N02/NOx ratio. Under certain conditions, a reversal in the NOx and N02/NO, ratio trends was observed. This was explained by an augmentation of heat transfer rate out of the combustion chamber, characterised by increased flow reversal strength, which lead to cooler residual gases. Additional mechanisms, which modified the inlet mixing process, were also identified as contributing to the reversal of the NOx trends.

The automotive industry is becoming more global and cosmopolitan, while the markets are becoming fragmented and differentiated because of the sophistication of the customer. This requires the automotive manufacturers to have a product well suited to these factors more quickly and cheaply, which necessitates the accurate definition of the product. Two European automotive manufacturers NPI processes were studied and this showed that poor performance in the product definition phase was deemed to be a major reason for sub-optimal performance in both companies, even though the approaches were very different. It was therefore decided to develop and apply tools to assist in the development of petrol engines to overcome these deficiencies. When these shortcomings are considered in the context of petrol engine development it can be seen through examination of the literature and industry that: • There is no method for the translation of the values of the company or product into tangible engineering terms and in the context of this project, with focus on vehicle and engine performance. This can result in the poor positioning of a vehicle in the market due to inappropriate characteristics • Having defined the required vehicle performance, it is not possible to demonstrate the feel of the vehicle until the hardware is physically available. This means that often there is a long lead-time between the setting of a target and the concept ratification through driving the new vehicle. This often results in a point of no (or very costly) return very early during the programme. • Fuel economy is becoming an increasingly significant issue with the introduction of fiscal penalties for poor fuel consumption vehicles. There are currently no processes available for the calculation of steady state or drive cycle fuel economy which allow for the accurate modelling to include combustion, pumping and friction losses, and the control of the engine with the engine management system To overcome these limitations three main groups of innovative tools/techniques have been developed and applied on new engine and vehicle programmes. • Marque engineering: a proposal to translate the brand objectives into engineering terms has been developed to a level where the engine torque output can be linked to the product position • Engine and vehicle performance simulation: a vehicle has been built to demonstrate how a new vehicle/engine will feel to drive by controlling the original engine performance to allow the simulation of the concept engine performance in the concept vehicle. This means it is possible to demonstrate and ratify a given engine/vehicle performance based on experience of the simulated product and to conduct sensitivity studies to discreet aspects of the performance feel of the vehicle • Steady state and drive cycle fuel economy: simulation programs have been written that take the basic engine efficiency relationships and through manipulation it is possible to determine the exact operating point of the engine, steady state or transient, and then determine the fuel economy. In order to determine the exact operating point it is necessary to consider detailed component data and key calibration data. Therefore it is possible to understand the effects of small changes to engine geometry, components or calibration on the fuel used. These developed techniques have been compared with traditional methods to determine the benefits in the concept confirmation phase of a new programme. This investigation showed that the reductions in phase duration, resource requirements and cost could be achieved in the order of 49, 27 and 17% respectively. This is coupled with the ability to obtain a more accurate product positioning through the capacity precisely to predict the product attributes. To conclude, the ability to define the product well is paramount to the success of an automotive manufacturer, and in support of this my project has developed tools and techniques that will greatly assist the development of petrol engines.

Cylinder pressure analysis is widely used in the experimental investigation of combustion processes within gasoline engines. A pressure record can be processed to reveal detail of charge burning, which is a good indicator of combustion quality. The thesis describes the evaluation of an approximate technique for calculating the mass fraction of the charge that has burnt; a novel approach for determining heat loss to the block; the development of a powerful system for combustion analysis; and the investigation of the correlation between the crank angle location of the 50% mass burnt and minimum timing advance necessary to obtain the maximum engine torque. A detailed examination has been carried out into the uncertainties in the determination of the mass fraction burnt as suggested by Rassweiler and Withrow. A revised procedure has been developed which does not require a priori identification of the combustion end point, and a new approach is suggested to calculate the polytropic indices necessary for the pressure processing. This particular implementation of the analysis is able to identify late burning and misfiring cycles, and then take appropriate steps to ensure their proper analysis. The problems associated with the assumption of uniform pressure; alignment of the pressure changes to the volume changes; pressure sampling rate; clearance volume estimation; and calibrating the acquired pressure to absolute are also evaluated. A novel method is developed to ascertain, directly from the pressure history, the heat loss to the cylinder block. Both experimental and simulated data are used to support the accuracy of the suggested heat loss evaluation, and the sensitivity of the method to its inputs is examined. The conversion of procedures for combustion analysis into a format suitable for undertaking high speed analysis is described. The analysis techniques were implemented so that the engine can be considered to be on-line to the analysis system. The system was entitled Quikburn. This system can process an unlimited number of cycles at a particular running condition, updating the screen every 1.5 seconds. The analysis system has been used to study the potentially beneficial correlation between the location of the 50% mass burnt and MBT. The correlation is examined in detail, and found to be valid except under lean fueling conditions, which is seen to be caused by slow flame initiation. It is suggested that the optimum location of the 50% mass burnt can be used as a reference setting for the ignition timing, and as an indicator of combustion chamber performance. An engine simulation was employed to verify that changes in bum shape account for the small variation seen in the optimum 50% bum locations at different operating conditions of the engine. The bum shape changes also account for the range of optimum locations of the 50% mass burnt encountered in different engines.

The cold start performance of automotive diesel engines is currently poor when compared to similar gasoline units. This thesis describes an experimental and theoretical investigation into the factors limiting diesel cold start behaviour. Studies have been carried out on IDI and DI designs of engine. Start behaviour has been characterised in terms of times taken to complete various stages of startup, engine speed variations and processes which affect these. Combustion and friction behaviour have been investigated in detail. Engine friction losses are dependent on temperature. During start-up these losses are relatively high compared to those when the engine is fully-warm. The work output from combustion is low at low speeds, and prone to a further deterioration at lower temperatures. Consequently, combustion output during cold cranking is initially insufficient to overcome frictional losses. The start times are extended by the need to keep the starter motor engaged until heat generated in the engine causes frictional losses to fall. Eventually, when combustion output is able to overcome friction without the aid of starter motor work, idle speed is reached. Changes to fuel injection and glowplug parameters have been used to achieve a limited improvement in low-temperature starting. Measurements of engine friction have been carried out to determine the influence of temperature and speed, and the relative contributions from each of the main component assemblies. At low temperatures, much of the friction originates in hydrodynamically lubricated components such as journal bearings, due to high oil viscosity at low temperature. Additionally, engine friction as rotation begins has been shown to be far higher than measured by conventional "steady-state" motoring tests (over twice the quasi-steady state friction at -200 e). This initially high friction transient decays towards the quasi-steady values throughout the start. For crankshaft bearings, a friction model has been developed for cold start-up through to fully warm engine conditions. The friction behaviour in the bearings is dependent on thermal conditions around the friction surfaces. Models for the starter system and blowby processes are also presented as part of a broader theoretical investigation to assess the impact of design changes on start quality.

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The effect of combustion chamber design on combustion rate has been investigated experimentally and theoretically. The experimental work concentrated on the measurement of cylinder pressure and flame speed using a piezo-electric pressure transducer and multiple ionisation probes together with a data acquisition/processing system. A total of twenty one chamber designs of varying shape, compression ratio and spark plug arrangement were tested over a range of operating conditions on a single cylinder S.I. engine. The pressure data were analysed to obtain values of pressure rise rate, cyclic dispersion and combustion (mass burn) rate whilst the ionisation data were processed to yield flame travel angles and flame dispersion. The results obtained show that for a given compression ratio, the flame speed is not significantly affected by chamber design. In contrast, the combustion rate and pressure parameters are highly dependent on the chamber design; more compact arrangements giving higher combustion rates and reduced cyclic dispersion. A computer simulation model of the compression, combustion and expansion phases of the engine cycle was developed to predict the effects of the combustion chamber design parameters. Based on the experimental results, the model assumes that the ratio of laminar to turbulent burning velocity is independent of chamber design. The influence of chamber shape on the burnt volume, flame front surface area and heat transfer surface areas is modelled using a simple but effective geometric integration technique. This technique allows an infinite variation of the design parameters to be specified for a large range of chamber shapes with a minimum of input data being required. The model predicts that chamber design does have a major effect on combustion rate and cylinder pressure but shows that the influence of individual design is highly dependent on the setting of all other parameters. The effect of squish area is shown to be due to it changing the compactness of the chamber, optimum squish area being about 50% for conventional engines with higher areas being suited to higher compression ratio designs. Spark plug arrangement is predicted to be the most effective way of controlling the combustion rate with a single centrally located spark plug or alternatively, dual spark plugs, giving large increases in combustion rate. Computer model predictions have been compared directly with experimental results obtained in this study and with experimental results reported by two other independent workers. Good agreement was obtained thereby giving support to the assumption of the flame speed being unaffected 'by chamber design. The model was also used to predict squish velocities in fired engines. The results show that the velocities and, in particular the reverse squish, can be significantly modified by the combustion process with a strong dependence on ignition timing being evident. The predictive model has been modified to yield a heat release program capable of analysing experimental pressure time data to predict combustion rate, flame speed, turbulent burning velocity and many other variables. The predicted flame speeds were in good agreement with corresponding experimental values obtained from ionisation probes. In conclusion, the study has confirmed the importance of combustion chamber design as a means of improving the combustion rate but has shown that the flame speed is not affected by chamber shape (i.e. squish). The semi-empirical simulation model has been shown to predict the effects of the chamber design parameters to an acceptable degree of accuracy.

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Despite challenges with poor emissions and fuel economy, gasoline two stroke engines continue to be developed for a number of applications. The primary reasons for the choice of a gasoline two stroke engine includes its low cost, mechanical simplicity and high specific power output. Some applications for the gasoline two stroke engine include small capacity motorcycles and scooters, off road recreational vehicles, hand held power tools and unmanned aerial vehicles. New technologies, which are already mature in four stroke engines, are now being applied to two stroke engines. Such technologies include direct fuel injection, electronic engine management and exhaust gas after treatment. To implement these new technologies computation models are being continuously developed to improve the design process of engines. Multi-dimensional computational fluid dynamics modelling is now commonly applied to engine research and development, it is a powerful tool that can give great insight into the thermofluid working of an engine. Multi-dimensional tools are however computationally expensive and quasi-dimensional modelling methods are often better suited for the analysis of an engine, for example in transient engine simulation. This thesis reports the development of a new quasi-dimensional combustion model for a loop scavenged two stroke engine. The model differs from other quasi-dimensional models available in the literature as it accounts for a bulk motion of the flame front due to the tumble motion created by the loop scavenge process. In this study the tumble motion is modelled as an ellipsoid vortex and the size of the vortex is defined by the combustion chamber height and a limiting elliptical aspect ratio. The limiting aspect ratio has been observed in experimental square piston compression machines and optical engines. The new model also accounts for a wrinkled flame brush thickness and its effects on the interaction between flame front and combustion chamber. The new combustion model has been validated against experimental engine tests in which the flame front propagation was measured using ionization probes. The probes were able determine the flame front shape, the bulk movement of the flame front due to tumble and also the wrinkled flame brush thickness.