One-dimensional modelling of pulse separation strategy, waste-gated turbines and electric turbocharger systems for downsized turbocharged gasoline engines

The demand for CO2 emission reduction for modern road vehicles has seen engine downsizing become a key trend in internal combustion engine design: a smaller engine has reduced pumping, frictional and heat losses, and therefore better fuel economy. Turbocharger technology is one of the enabling technologies, offering lower specific fuel consumption and producing more power for a given engine capacity. The turbocharger matching process, which specifies an appropriate turbocharger design for a particular engine, is crucial in obtaining optimum engine performance. In order to achieve a high level of accuracy in the system-level prediction, high fidelity turbocharger models are required; but such models have not yet reached fruition. The present study has assessed the effect of preserving the exhaust pulse energy from an engine right through to the turbine on the steady and transient engine performance. A combination of appropriate turbine sizing and pulse-divided exhaust manifold was applied, and as a consequence, lower back pressure and improved engine scavenging reduced residual content by 28%, while the brake specific fuel consumption (BSFC) improves by approx. 1.2% on average over speed range. Furthermore, the implementation of electric turbo assist (ETA) system on the engine results in better fuel economy by 2.4%. The present work has also assessed the overall engine performance using a commercial 1-D gas dynamics simulation tool by modelling the waste-gated turbines in a novel manner. This approach has been validated experimentally. The study also examined the benefits of electric turbocharger systems for a highly-downsized engine, a modified version of the baseline engine. Some potential multi-boosting systems were applied, and the overall benefits in terms of engine performance were assessed. An integration of an electric turbocharger and a low-pressure turbine with electric turbo compounding gives the best advantages particularly in pumping loss, residual and transient performance while improving fuel economy in comparison with other systems.Open Acces

Turbocharger matching method for reducing residual concentration in a turbocharged gasoline engine

In a turbocharged engine, preserving the maximum amount of exhaust pulse energy for turbine operation will result in improved low end torque and engine transient response. However, the exhaust flow entering the turbine is highly unsteady, and the presence of the turbine as a restriction in the exhaust flow results in a higher pressure at the cylinder exhaust ports and consequently poor scavenging. This leads to an increase in the amount of residual gas in the combustion chamber, compared to the naturally-aspirated equivalent, thereby increasing the tendency for engine knock. If the level of residual gas can be reduced and controlled, it should enable the engine to operate at a higher compression ratio, improving its thermal efficiency. This paper presents a method of turbocharger matching for reducing residual gas content in a turbocharged engine. The turbine is first scaled to a larger size as a preliminary step towards reducing back pressure and thus the residual gas concentration in-cylinder. However a larger turbine causes a torque deficit at low engine speeds. So in a following step, pulse separation is used. In optimal pulse separation, the gas exchange process in one cylinder is completely unimpeded by pressure pulses emanating from other cylinders, thereby preserving the exhaust pulse energy entering the turbine. A pulse-divided exhaust manifold enables this by isolating the manifold runners emanating from certain cylinder groups, even as far as the junction with the turbine housing. This combination of appropriate turbine sizing and pulse-divided exhaust manifold design is applied to a Proton 1.6-litre CamPro CFE turbocharged gasoline engine model. The use of a pulse-divided exhaust manifold allows the turbine to be increased in size by 2.5 times (on a mass flow rate basis) while maintaining the same torque and power performance. As a consequence, lower back pressure and improved scavenging reduces the residual concentration by up to 43%, while the brake specific fuel consumption improves by approx. 1%, before any modification to the compression ratio is made