Potential of Exhaust Energy Use for Charge Air Cooling in Supercharged Diesel Engines

A rough estimate of the energy components in internal combustion engines for motor vehicles indicates that the total fuel energy is converted to one-third each into mechanical energy, engine coolant heat and exhaust energy. This large share of waste heat in the exhaust motivates various attempts to recover the remaining exergy in the exhaust. Intensifying charge air cooling by an exhaust-heat-driven cooling system provides a promising approach to engine waste heat recovery. The exhaust energy is most suitable for this recovery effort due to its higher temperature level in comparison to engine coolant.

A further decreasing of charge air temperature provides an additional degree of freedom, which expands the boundaries concerning engine application. In diesel engines, this intensified charge air cooling decreases cylinder temperature level, which can be used to reduce nitrogen oxide emissions in the raw exhaust at constant brake efficiency or vice versa to improve brake efficiency at constant nitrogen oxide emissions. Simulation results from the diesel engine described in this paper show a decrease of nitrogen oxide emission up to 37% at constant brake efficiency. Leaving the level of nitrogen oxide emissions unchanged led to an increase of the brake efficiency up to 4.7%.