Waste Heat Recovery in Hydrogen Fueled Internal Combustion Engines

Waste Heat Recovery is one of the most investigated and promising technologies for energy efficiency in the transportation sector. It consents to maintain the high-level technology of the present propulsion systems, based on Internal Combustion Engines, while increasing the overall engine and vehicle system efficiency. At the same time, the use of alternative fuels, like hydrogen, has the same crucial role to reduce harmful and greenhouse emissions, without overturn the existing mature technology. A hydrogen-fueled Internal Combustion Engine is proposed in this paper, equipped with waste heat recovery consisting in an additional radial turbine downstream the turbocharger of the engine (Turbo-Compound). The aim is to have a reduction of the specific consumption in most of the operating points of the engine, considering the effect of the recovery and the engine equilibrium rearrangement. The use of hydrogen increases recoverable enthalpy at the engine exhaust, which is intended to be recovered through an expansion of the gases inside the additional turbine. When this secondary turbine is installed downstream of the turbocharger, the overall engine backpressure increases. This alters the turbocharger's operating point, which in turn shifts the engine’s operating conditions. Ultimately, this has a counterproductive effect on the engine efficiency: it faces higher backpressure at exhaust valve opening, leading to an increase in specific fuel consumption. This paper examines the modifications to the engine’s exhaust line resulting from the conversion of waste heat into mechanical energy via a Turbo-Compound system. It discusses the conditions under which the system yields a net positive effect, primarily by compensating for the increased back pressure it introduces. Changes in key engine parameters—such as Variable Geometry Turbocharger control, boost pressure, air/fuel mass flow rate, and equivalence ratio—are analyzed, along with their influence on in-cylinder pressure. Furthermore, the paper identifies the operating range in which the Turbo-Compound system provides a net performance benefit.