Exhaust Heat Recovery System for Modern Cars 2001-01-1020

The fuel consumption and the emissions of modern passenger cars are highly affected by the fluid and material temperatures of the engine. Unfortunately, the high thermal efficiencies of Direct Injection (DI) Diesel and Spark Ignition (SI) engines cause in many driving situations low heat transfer to the engine components and especially to the oil and the coolant. In these conditions the normal operating temperatures are not achieved. Especially at low ambient temperatures and low engine loads the requirement of a comfortable cabin heating and a fast warm-up of engine oil and coolant cannot be satisfied simultaneously.

To reach the required warm-up performance, an Exhaust Heat Recovery System (EHRS) will be demonstrated.

Further design and optimization processes for modern cooling systems in fuel-efficient engines require numerical and experimental investigations of supplemental heater systems to meet all requirements under all circumstances. According to the car manufacturers' definition of thermal objectives for the engine and for the passenger cabin any heat generated or recovered can be distributed to the customers' needs. Modeling the fluid flows (oil, coolant) and material surface temperatures is a prerequisite tool to analyze the heat fluxes to the various components inside an engine. The coolant temperatures at the engine outlet can then be determined by using an energy balance. The maximum potential to improve fuel efficiency is demonstrated based on a cycle simulation of a typical SI engine.

The EHRS system is described with special attention paid to the cabin heating of passenger cars with modern DI-Diesel engines. The available heat in the exhaust gas behind the catalyst is sufficient to support the warm-up adequately for most of the ambient conditions at no fuel consumption penalty. For very cold temperatures of -18 °C (0 °F) sufficient additional energy can be generated and recovered by throttling the exhaust flow additionally.