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No description available.

No description available.

Despite being hailed as a green fuel, emissions from diesel engines including particulate matter (PM10 and PM 2.5) have been implicated in a range of adverse human health effects from lung and bladder cancers to premature mortality. In this study diesel engine exhaust emissions were collected from a light duty direct injection diesel engine on a standard test bed. Engine conditions of speed and load were altered to provide a set of total emission samples from over the engine's operating range. Diesel emission samples collected were fractionated on a silica column into aliphatic, aromatic, and polar groups of compounds, which were tested for their genotoxicity in the chromosome aberration assay in Chinese hamster ovary CHO-KI cells both with and without metabolic activation (rat liver S9 fraction). The aliphatic fractions did not exhibit cytotoxicity up to the maximum concentration assayed, and one emission sample (3000 rpm speed and 5 Nm load) assayed for effect on chromosome aberrations was not clastogenic (up to 600 pg/ml). The aromatic fractions of all engine emission samples assayed and of the fuel were not clastogenic, but did show high levels of cytotoxicity at relatively low doses, raising concern that any genotoxic effect was masked by the toxicity of certain chemicals within the fraction. Further fractionation, using 1 PLC, was therefore performed which separated the aromatics into various ring sizes. Assay of the ring fractions showed evidence of increasing clastogenicity with increasing ring size, with the -1+ -ring fractions of both the fuel and one emission sample clearly clastogenic when assayed with metabolic activation (evidence of the presence of indirect-acting genotoxic compounds within both samples). The final fractions to be assayed, the polar fractions, were clastogenic when assayed both with and without metabolic activation. All seven fractions from emission samples collected over a range of speed and load conditions caused highly significant increases in chromosome aberrations at concentrations as low as 20 μg/ml. An engine running for less than 30 minutes at 1000 rpm speed and 55 Nm load (urban driving conditions for a heavily laden vehicle) would emit 148 mg of polar group compounds for every litre of fuel consumed. Polar compounds have been shown to be a highly mutagenic fraction of air particulate samples, and as diesel emissions contribute up to 80 % of the particulate matter in urban air in some areas, diesel emissions and the polar compounds in particular are of real concern to human health. 3

This thesis considers the effect of compression ratio on the performance of a direct injection diesel engine. One aspect of engine performance is considered in great detail, namely the combustion performance at increased clearance volume. This aspect was of particular interest because variable compression ratio (VCR) systems normally operate by varying the clearance volume. The investigation relied upon results obtained both from experimental and computer simulating models. The experimental tests were carried out using a single-cylinder direct-injection diesel engine, under simulated turbocharged conditions at a reduced compression ratio. A number of one-dimensional computer models were developed; these simulate the induction and compression strokes, and the fuel spray trajectories in the presence of air swirl. The major objectives of the investigation were: to assess the benefits of VCR in terms of improvements in output power and fuel economy; to assess the effects on combustion of increased clearance volume, and investigate methods for ameliorating resulting problems; develop computational models which could aid understanding of the combustion process under varying clearance volume conditions. It was concluded that at the reduced compression ratio of 12.9:1 (compared to the standard value of 17.4:1 for the naturally-aspirated engine), brake mean effective pressure (BMEP) could be increased by more than 50%, and the brake specific fuel consumption (BSFC) could be reduced by more than 20%. These improvements were achieved without the maximum cylinder pressure or engine temperatures exceeding the highest values for the standard engine. Combustion performance deteriorated markedly, but certain modifications to the injection system proved successful in ameliorating the problems. These included: increase in the number of injector nozzle holes from 3 to 4, increase in injection rate by about 28%, advancing injection timing by about 6°CA. In addition, operation with weaker air fuel ratio, in the range of 30 to 40:1 reduced smoke emissions and improved BSFC. Use of intercooling under VCR conditions provided only modest gains in performance. The NO emission was found to be insensitive to engine operating conditions (fixed compression ratio of 12.9:1), as long as the peak cylinder pressure was maintained constant. Engine test results were used in order to assess the accuracy of four published correlations for predicting ignition delay. The best prediction of ignition delay with these correlations deviated by up to 50% from the measured values. The computer simulation models provided useful insights into the fuel distribution within the engine cylinder. It also became possible to quantify the interaction between the swirling air and the fuel sprays, using two parameters: the crosswind and impingement velocities of the fuel spray when it impinges on the piston-bowl walls. Tentative trends were identified which showed that high crosswind velocity coincided with lower smoke emissions and lower BSFC.

Recent imaging studies have provided a new conceptual model of the internal structure of direct injection diesel fuel jets as well as empirical correlations predicting jet development and structure. This information was used to create a diesel cycle simulation model using C language including compression, fuel injection and combustion, and expansion processes. Empirical relationships were used to create a new mixing-limited zero-dimensional model of the diesel combustion process. During fuel injection five zones were created to model the reacting fuel jet: 1) liquid phase fuel 2) vapor phase fuel 3) rich premixed products 4) diffusion flame sheath 5) surrounding bulk gas. Temperature and composition in each zone is calculated. Composition in combusting zones was calculated using an equilibrium model that includes 21 species. Sub models for ignition delay, premixed burn duration, heat release rate, and heat transfer were also included. Apparent heat release rate results of the model were compared with data from a constant volume combustion vessel and two single-cylinder direct injection diesel engines. The modeled heat release results included all basic features of diesel combustion. Expected trends were seen in the ignition delay and premixed burn model studies, but the model is not predictive. The rise in heat release rate due to the diffusion burn is over-predicted in all cases. The shape of the heat release rate for the constant volume chamber is well characterized by the model, as is the peak heat release rate. The shape produced for the diffusion burn in the engine cases is not correct. The injector in the combustion vessel has a single nozzle and greater distance to the wall reducing or eliminating wall effects and jet interaction effects. Interactions between jets and the use of a spray penetration correlation developed for non-reacting jets contribute to inaccuracies in the model.