Academic literature on the topic 'Engine performance and combustion characteristics'

In the first part of studies, Controlled Auto-Ignition (CAI) combustion was investigated in a Ricardo E6 single cylinder, four stroke gasoline engine. CAI combustion is achieved by employing positive valve overlap configuration in combination with various compression ratios and intake air temperature strategies. The CAI operational region is limited by engine load due to knock and partial burned boundaries. The combustion characteristics and emissions are studied in order to understand the major advantages and drawbacks of CAI combustion with positive valve overlap. The enlargement of the CAI operational region is obtained by boosting intake air and external EGR. The lean-boosted operation elevators the range of CAI combustion to the higher load region, and the use of external EGR allows the engine to operation with CAI combustion in the mid range of region between boosted and N/A CAI operational range. The results are analyzed and combustion characteristics, performance and emissions are investigated. A Ricardo Hydra single cylinder, four stroke optical gasoline engine with optical access is then experimented to investigate CAI combustion through negative valve overlap configuration and an intake heater. The effects of direct fuel injection timings spark timings and air/fuel ratio are studied by means of simultaneous incylinder heat release study and direct visualization, chemiluminescence techniques which uses full, OH radical and CHO species. Both heat release analysis and chemiluminescence results have identified the pressure of minor combustion during the NVO period. Both the charge cooling and local air/fuel ratio effects are also investigated by varying the quantity of direct air injection.

The thesis presents an experimental investigation of combustion performance and emissions of waste cooking oil (WCO) based biodiesel. To evaluate the comparative performance of biodiesel and diesel, combustions tests were conducted using Continuous Combustion rig (CCR) and Land Rover VM diesel engine. Firstly, physical properties of WCO biodiesel and diesel samples were measured in the laboratory. Elemental analysis of WCO biodiesel showed that there are differences between the functional groups in diesel and biodiesel which lead to major differences in the combustion characteristics of the two fuel types. It was found that biodiesel had 10% lower carbon content, almost no sulphur content for biodiesel and up to 12% more oxygen content compared with diesel. This explains the lower caloric value for WCO biodiesel (up to l8 %) compared with diesel. However, higher oxygen content and double bounds in WCO biodiesel increase its susceptibility to oxidation. The CCR test results showed an increase in combustion gas temperature with the increases in biodiesel blend ratio in diesel. This was due to a faster reaction rate for biodiesel than that of diesel leading to a faster brakeage of the hydrocarbon chain to release more heat. The engine tests were performed to measure the torque and emissions for different engine speeds and loads. In general a decrease in engine torque with up to 9% for biodiesel was observed, which was due to the lower calorific value of biodiesel compared with that of diesel. The brake specific fuel consumption (BSFC) increased as the biodiesel blend ratio in diesel increases due a greater mass of fuel being injected at a given injection pressure, compared with diesel. Using WCO blends ratio up to 75% in diesel showed a reduction in exhaust emission compared with diesel, however, at the cost of increased fuel consumption. A common conclusion can be drawn in favour of the WCO biodiesel as being a greener alternative to petro-diesel when used in blend with diesel. However, due to large variations in the biomass used for biodiesel production could lead to variations in physical and chemical properties between biodiesel produced from different biomass. Therefore more stringent standards need to be imposed for biodiesel quality in order to diminish the effect of variation in physicochemical properties on engine performance and emissions. The future work in developing standard test procedures for establishing fuel properties and limits/targets would be beneficial in using a large amount of waste cooking oil in the production of biodiesel, thus contributing to reduction in CO2 and waste minimisation.

Internal combustion engines are approaching their theoretical maximum efficiency, which could indicate limited future technological improvements in performance and exhaust emissions with standard fuels. In addition, fossil fuel dependence can only be reduced by implementing appropriate renewable fuel sources. The experimental investigation in this work only concerns the compression-ignition (CI) engine combustion process both in normal operation and “dual-fuel” operation. The dual-fuel mode allows low-cetane number fuel to be used in CI engines, with a “pilot” fuel spray injection of high-cetane number fuel to provide ignition. Initially, rapeseed methyl ester (RME) and two water-in-RME emulsions were compared with normal diesel fuel during normal operation. Neat RME generally performed similarly to diesel fuel, while giving higher specific fuel consumption (SFC) levels. Both water- in-RME emulsions performed fairly similarly to neat RME. This suggests that the cooling effect of water vapourisation was a negligible factor throughout the operating range. Natural gas dual-fuel operation reduced NOx at certain conditions and overall CO2 emissions while thermal efficiencies were maintained compared with normal operation. However, significantly higher unburnt hydrocarbons (HC) and CO emissions were recorded at low and intermediate engine loads. For the emulsified pilot fuels, better fuel-air mixing (possibly as a result of “microexplosions”) increased NOx after an equivalence ratio of about 0.6. Hydrogen dual-fuel operation generally increased NOx emissions while CO2 emissions were reduced compared with normal operation. Thermal efficiencies remained comparable for all pilot fuels. NOx emissions in the emulsified fuel cases were generally comparable to the neat RME pilot. Lower volumetric efficiency was also recorded, while power output was limited to maintain engine stability and avoid abnormal combustion caused by excessively high pressure-rise rates (called “hydrogen knock”). Overall, significant optimisation is needed to improve combustion efficiency at low and intermediate engine loads during dual-fuel CI engine operation. As these engines are designed specifically for liquid fuels, substantial engine customisation or even complete redesign (particularly in the fuel supply system) is needed to improve the combustion quality on a scale larger than that seen in this work.

Controlled Auto-Ignition (CAI) also known as Homogeneous Charge Compression Ignition (HCCI) is increasingly seen as a very effective way of lowering both fuel consumption and emissions. Hence, it is regarded as one of the best ways to meet stringent future emissions legislation. It has however, still many problems to overcome, such as limited operating range. This combustion concept was achieved in a production type, 4-cylinder gasoline engine, in two separated tests: naturally aspirated and turbocharged. Very few modifications to the original engine were needed. These consisted basically of a new set of camshafts for the naturally aspirated test and new camshafts plus turbocharger for the boosted test. The first part of investigation shows that naturally aspirated CAI could be readily achieved from 1000 to 3500rpm. The load range, however, decreased noticeably with engine speed due to flow restrictions imposed by the low lift camshafts. Ultra-low levels of NOx emissions and reduced fuel consumption were observed. After baseline experiments with naturally aspirated operation, the capability of turbocharging for extended CAI operation was investigated. The results show that the CAI range could achieve higher load and speed with the addition of the turbocharger. The engine showed increased fuel consumption due to excessive pumping losses. Emissions, however, have been reduced substantially in comparison to the original engine. NOx levels could be reduced by up to 98% when compared to a standard SI production engine.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.<br>Includes bibliographical references (p. 139-143).<br>The performance of the piston ring-pack is directly associated with the friction, oil consumption, wear, and blow-by in internal combustion engines. Because of non-axisymmetric characteristics of the power cylinder system, the performance of a ring varies along its circumference. Investigating these variations is of great interest for developing advanced ring-packs, but is out of the capabilities of the existing two-dimensional models. In this work, three separate but closely related numerical models were developed to study the performance of the piston ring-pack. The model for static analysis was developed to facilitate the design of piston rings. In this model, a finite beam element model is adopted with incorporation of a physics-based sub-model describing the interaction between the ring and the bore as well as the ring and the groove. A step-by-step approach is adopted to calculate the ring/bore and ring/groove conformability if the free shape of the ring is given. A method that can be used to determine the free shape as to achieve a specific tension distribution is also developed. Model results revealed the complex ring/bore and ring/groove interaction. A three-dimensional model for ring dynamics and blow-by gas flow was developed to address non-axisymmetric characteristics of the power cylinder system. In this model, the rings are discretized into straight beam elements. 3-D finite element analysis is employed to address the structural response of each ring to external loads. Physics-based sub-models are developed to simulate each ring's interactions with the piston groove and the liner. The gas flows driven by the pressure difference along both the axial and circumferential directions are modeled as well.<br>(cont.) This model predicts the inter-ring gas pressure and 3-D displacements of the three rings at various circumferential locations. Model results show significant variations of the dynamic behavior along ring circumference. In the ring-pack lubrication model, an improved flow continuity algorithm is implemented in the ring/liner hydrodynamic lubrication, and proves to be very practicable. By coupling the ring/liner lubrication with the in-plane structural response of the ring, the lubrication along the entire ring circumference can be calculated. Model results show significant variations of lubrication along the circumference due to the non-axisymmetric characteristics of the power cylinder system. Bore distortion was found to have profound effects on oil transport along the liner. Particularly, it stimulates the occurrence of oil up-scraping by the top ring during compression stroke. Because the oil evaporation on the liner affects the liner oil film thickness, a sub-model for liner evaporation with consideration of multi-species oil is incorporated with the lubrication model. With consideration of oil transport along the liner, the prediction of evaporation is more precise. The combination of these models is a complete package for piston ring-pack analysis. It is computationally robust and efficient, and thus has appreciable practical value.<br>by Liang Liu.<br>Ph.D.

Growing concerns about interruption to oil supply and oil shortages have led to escalating global oil prices. In addition, increased public acceptance of the global warming problem has prompted car manufacturers to agree to carbon emission targets in many regions including most recently, the Californian standards. Other legislating bodies are sure to follow this lead with increasingly stringent targets. As a result of these issues, spark ignition engines in their current form will need significant improvements to meet future requirements. One technically feasible option is smaller capacity downsized engines with enhanced power that could be used in the near term to reduce both carbon emissions and fuel consumption in passenger vehicles.This research focuses on exploring the performance limits of a 0.43 liter spark ignited engine and defining its operating boundaries. Limiting factors such as combustion, gas exchange and component design are investigated to determine if they restrict small engine performance. The research gives direction to the development of smaller gasoline engines and establishes the extent to which they can contribute to future powertrain fuel consumption reduction whilst maintaining engine power at European intermediate class requirements.