Engine Encapsulation for Increased Fuel Efficiency of Road Vehicles

Doktorsavhandling, 2017

Thermal engine encapsulation is an increasingly popular design choice, which insulates the engine from the external environment and retains heat in the engine after it is turned off. This decelerates motor cool-down and increases the probability for high initial temperature at a subsequent engine start, resulting in shorter warm-up and reduced friction between engine parts.  This work investigates thermal engine encapsulation (TEE) as a means to reduce engine friction and fuel consumption during engine warm-up. In order to predict the effects of TEE on the fuel consumption, it is necessary to model a wide range of thermal phenomena in different subsystems of the powertrain. The presented work proposes an integrated simulation methodology that enables efficient numerical simulations of heat transfer in the powertrain cooling systems and the engine structures not only during dynamic driving but also during the process of engine cool-down when the vehicle is parked. The integrated simulation includes a number sub-models that capture relevant phenomena in the vehicle powertrain and underhood. Presented in detail is the simulation procedure, which ultimately predicts the continuous development of the temperatures of the engine oil and the coolant as well as the temperatures of the different engine parts and components from the powertrain cooling system. An automated coupling between the one-dimensional (1D) thermal representation of the engine and powertrain cooling systems with a three-dimensional (3D) CFD model of buoyancy-driven flow in the engine bay computes the heat rejection during engine cool-down. By use of an integrated friction correction map the engine model computes the variation of friction losses at different engine temperatures. The integrated simulation model makes possible to predict the temperature of engine structures after a long period of engine inactivity preceded by dynamic driving, the exact temperature development of engine structures and cooling fluids after a recurring engine start, as well as the variations in the instantaneous fuel consumption.  Furthermore, a TEE concept for a passenger vehicle has been designed. The presented simulation method is applied to evaluate the effect of the proposed encapsulation on the development of engine temperatures during cool-down and their effect on the fuel consumption during a sequence of two Worldwide harmonized light vehicle test cycles (WLTC) separated with a period of inactivity. The results indicate a significant capability of encapsulations with high degree of coverage to retain engine heat for long time periods after key-off. The obtained results show that the encapsulated engine setup has potential for up to 3.1% savings of the fuel burned during a WLTC by a non-encapsulated engine in a cold environment. The amount of fuel saved depends primarily on the specific engine (mass, size and geometry), encapsulation design (geometry, thickness and degree of coverage), ambient temperature, and time of inactivity between engine shut-down and start-up. For periods of inactivity between 2 to 8 hours the potential for fuel saving is at least 2.5% of the total fuel burned during WLTC at ambient temperature of 5°C for encapsulation with 97% coverage.