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.
Active thermal management systems offer a potential for small improvements in fuel consumption that will contribute to upcoming legislation on carbon dioxide emissions. These systems offer new degrees of freedom for engine calibration; however, their full potential will only be exploited if a systems approach to their calibration is adopted, in conjunction with other engine controls. In this work, a design-of-experiments approach is extended to allow its application to transient drive cycles performed on a dynamic test stand. Experimental precision is of crucial importance in this technique since even small errors would obscure the effects of interest. The dynamic behaviour of the engine was represented mathematically in a manner that enabled conventional steady state modelling approaches to be employed in order to predict the thermal state of critical parts of the engine as a function of the actuator settings. A 17-point test matrix was undertaken, and subsequent modelling and optimisation procedures indicated potential 2-3% fuel consumption benefits under iso-nitrogen oxide conditions. Reductions in the thermal inertia appeared to be the most effective approach for reducing the engine warm-up time, which translated approximately to a 1.3% reduction in the fuel consumption per kilogram of coolant. A novel oil-cooled exhaust gas recirculation system showed the significant benefits of cooling the exhaust gases, thereby reducing the inlet gas temperature by 5°C and subsequently the nitrogen oxide emissions by 6%, in addition to increasing the warm-up rate of the oil. This suggested that optimising the thermal management system for cooling the gases in the exhaust gas recirculation system can offer significant improvements. For the first time this paper presents a technique that allows simple predictive models of the thermal state of the engine to be integrated into the calibration process in order to deliver the optimum benefit. In particular, it is shown how the effect of the thermal management system on the nitrogen oxides can be traded off, by advancing the injection timing, to give significant improvements in the fuel consumption.
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