Currently, 80 per cent of European diesel passenger cars are turbocharged and, as emission standards become more stringent, this figure is expected to approach 100 per cent in the near future. One major focus that has emerged for the high-speed diesel engine is the application of variable geometry turbocharging (VGT). An extensive steady state experimental investigation has been undertaken on a prototype 1.8 L direct injection (DI) diesel engine to compare the potential benefits of VGT relative to the standard build of the engine with a wastegated fixed geometry turbocharger (FGT). Under part load operation, where emission production is significant in the European drive cycle, independent control of both VGT vane position and exhaust gas recirculation (EGR) valve position was used to optimize emission levels. A reduction in the levels of nitrogen oxides (NO x) of up to 45 per cent was observed at discrete operating points without compromising FGT levels of fuel consumption or smoke. Under limiting torque conditions a 10 per cent improvement was achieved with the VGT over and above the figures of the baseline FGT build within the limiting criteria set for maximum cylinder pressure, smoke level and pre-turbine temperature.
Modifications to the coolant and oil circuits of a modern production 2.4L diesel engine have been made in an attempt to promote oil warm up to reduce fuel consumption. The new system used oil to cool EGR gases and incorporates a number of coolant flow control valves to reduce heat loss during warm up. The engine was run over cold start NEDC cycles with various flow strategies as a screening exercise to understand the behaviour of the system. Fuel consumption benefits of up to 4% were observed, but these were accompanied with 3% increases in NO x emissions. Detailed analysis of the coolant flows and temperatures showed that when throttling the flow, the mass of coolant in the degas bottle and radiator could be isolated from the system during warm up, essentially reducing the thermal inertia. Heat transfer directly to the oil from EGR gases rather than via the coolant allowed more heat to be put into the oil, with engine oil supply temperatures up to 6 o C hotter, however it was not possible to verify that the oil was hotter at the bearings, valve train and cylinder liner. The engine strategy was seen to react to the faster warm up and retard injection timing, reducing NOx but also compromising overall fuel consumption benefits. Further tests were conducted with varying injection timing to establish a NOx/fuel consumption trade off to demonstrate further benefits when the engine strategy is included in the operation of novel thermal management systems.
The measurement of vehicle modal emissions is technically challenging owing to the major issue of determining exhaust-gas mass flowrate and ensuring that it is synchronous with the corresponding ‘slug’ of gas to be measured. This is also extended to the simultaneous measurement of pre- and post-catalyst emissions to determine small passive NOx conversion efficiencies. Although only really evident for passive NOx conversion efficiencies where the magnitude of catalyst performance is low in comparison to HC and CO, a misalignment between these measuring points of between will cause the resulting NOx conversion efficiency to lie anywhere between 0 per cent and 20 per cent. Further alignment issues arise when the CO2 tracer method is used for determining exhaust-gas volume flowrates. The sensitivity of time-alignment along with techniques and associated issues concerned with modal gas-flow measurement is presented in this paper.
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