The effect of VGT vane control on pumping losses during full-load transient operation of a common-rail diesel engine 2007-24-0063

This study measures transient torque, smoke opacity and pumping-losses derived from in-cylinder pressure, as a function of Variable Geometry Turbocharger (VGT) vane position (derived through Engine Control Unit, ECU). Tests were conducted using a typical passenger car/light duty truck application turbo-charged common-rail diesel engine, of 14 configuration. The aim was to seek potential improvements in engine pumping-losses (and thus fuel economy) during full-load transients and at low engine speeds, due to opening of VGT vanes. The objective was to record engine performance (e.g. engine transient-torque, smoke opacity, fuel-demand, engine pressure-ratio etc.), under full-load operation, and at engine speeds of 900-1600 rpm. The effects of “slow” and “fast” transient manoeuvres were established (in a transient test facility) by performing four different acceleration rates (i.e. 2s, 5s, 10s and 20s). The potential reduction in Pumping Mean Effective Pressure (PMEP), due to reduction in engine pressure-ratio by VGT vane opening, and the corresponding gain in transient torque (and hence improvements in Specific Fuel Consumption, SFC) were to be analysed during full-load transients.

Analysis of the results at four engine speeds during the said transients (1000, 1100, 1200 and 1400 rpm), shows that, by opening VGT vanes from fully-closed position (and therefore reducing exhaust back-pressure), PMEPs are reduced. However, by varying the VGT vanes only, two observations are made, specific to regions where boost-lag is present: 1) the reduction in PMEP causes degradation in boost pressure and therefore results in the production of excessive smoke opacity. Therefore, the benefit of reducing transient PMEP is lost as transient torque demand is only achieved by vane closure (producing more boost-pressure, to restrict excessive smoke opacity); and 2) lower engine pressure-ratio reduces PMEP at no extra penalty of excessive smoke opacity. However, the transient torque produced still necessitates similar fuel demand, as compared with high engine pressure-ratios and therefore the amount of reduction in PMEP was not large enough to produce a significant SFC advantage during a full-load transient.

Furthermore, collated results from all of the above transient tests show an almost linear relationship between PMEP (as a function of engine speed) and engine pressure-ratio. This admittedly empirical relationship might be useful in the Engine Control Unit (ECU) for monitoring high levels of PMEP resulting from additional sources (e.g. activation of Intake Throttle, ITH non-optimal turbocharger match or part-load operating condition with Exhaust Gas Recirculation, EGR control). VGT vane control can then be influenced with Model Based Control (MBC) in order to achieve minimum PMEP and thus SFC.