In the internal combustion engine, a large amount of energy is rejected in the form of exhaust heat without being converted into brake work.

Additionally, in gasoline engines, throttle losses are also a considerable disadvantage limiting the capability to achieve higher thermal efficiency. Under part load conditions, both the power demand and engine speed are much lower than the maximum achievable. The throttle is partially closed to restrict inlet air mass flow to regulate the brake power production. To overcome the friction and turbulence losses at the small throttle opening, negative pressure is produced in the manifold at the cost of engine power.

This paper explores the effectiveness of an expander installed in the inlet duct of the engine to lessen, even eliminate, the throttle losses by allowing power to be reclaimed from the pressure drop across the expander, which will otherwise be wastefully dissipated across the throttle. In this way the pumping losses are reduced.

The engine system was modelled in GT-Power which is a 1-dimensional engine simulation code. The limits in decreasing in pressure drop through the throttle and the power generation from the expander were explored. Together with a turbo machine recovering energy from the exhaust flow, this system was able to enhance the fuel economy by about 5% when operating at 1.75 bar BMEP from 500–3000rpm compared with a conventional turbocharged engine. The influence of the expander machinery on the combustion and the turbocharger performance was also discussed. To achieve the highest performance level, careful optimization of the expander size and control strategy and proper matching with engine system are critical.

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