A numerical investigation of thermal engine encapsulation concept for a passenger vehicle and its effect on fuel consumption

Abstract

Increasingly tough regulations for emission levels and a growing demand for an environmentally clean motor industry impose high requirements in modern automotive development. During recent decades, carmakers have been utilizing various strategies to minimize energy losses in the powertrain to meet legislative and market demands. A great part of research efforts has been focused on improving engine performance during cold starts characterized by increased friction losses. Thermal engine encapsulation is an effective design choice to reduce engine friction in applications with frequent cold starts. In the present work, a coupled 1-D–3-D system-level approach is used to investigate the effects of a novel engine-mounted encapsulation concept featuring air shutters on fuel consumption in a Volvo S80 passenger vehicle. Simulations are performed for sequences of the Worldwide harmonized light vehicles test cycle (WLTC) drive cycle, which include different time intervals of engine inactivity when the car is parked in air of an quiescent ambient temperature. The results show that engine encapsulation with high area coverage (97%) can retain engine oil temperature above 19°C for up to 16 h after engine shutdown at an ambient temperature of 5°C, leading to 2.5% fuel saving during engine warm-up when cold starts occur between 2 and 8 h after key-off. Encapsulations with a lower area coverage (90%) have proven to be less effective, with fuel saving of 1.25% as the temperatures of the oil and engine structures decrease more quickly after key-off compared to the fully enclosed encapsulation.

Numerical investigation of buoyancy-driven heat transfer within engine bay environment during thermal soak

“…Upon the studies on the simplified engine bay geometry [4][5][6][7], the transient simulation method for the buoyance-driven convection flow in the engine bay was developed further to apply in passenger vehicles for unsteady thermal management [8][9][10]. Numerical investigations [8] were carried out combining 3D computational fluid dynamics (CFD) simulation from STAR-CCM+ with 1D thermal modelling in GT-SUITE for the heat retention analysis in an engine bay with thermal encapsulation of a Volvo S80 passenger car.…”

“…600 CPU hours was used for most cases of the steady-state 3D CFD [8]. The buoyancy-driven flow and the heat retention in an engine bay with engine thermal encapsulation of a Volvo S80 passenger car was further on numerically investigated with the integrated 1D-3D approach in an iterative process to evaluated the potential benefit on engine friction reduction in applications with frequent cold starts [9]. The 3D CFD was initialised with the temperatures of engine solids calculated at the end of the 1 st WLTP cycle by the 1D engine thermal model in GT-SUITE.…”

“…This adds uncertainty to the prediction of the fuel consumption saving for the second WLTP drive cycle. The computing resource used in this study [9] was 24,000 CPU-hours for a 16 hours simulated drive cycle.…”

Heat retention analysis with thermal encapsulation of powertrain under natural soak environment

“…One of the pioneer works conducted by Chen et al [4] on a simplified under-hood model [4] with open enclosure simulating the vehicle static soak condition, suggested that the surrounding air flow was in the laminar regime for the engine block and exhaust regions. Similarly, Minovski et al [5] conducted heat transfer modelling on a detailed fullgeometry engine model [5] with additional CFD simulation addressed in the buoyancy-driven flow of oil inside the engine oil sump during the vehicle soak period and identified the laminar flow characteristics of the engine oil indicated by the Grashof number, which was calculated at around 1.35 ×10 6 .…”

“…However, from simplified geometries to full designed geometries of the engine bay compartment, to capture the natural convection flow and the convective heat transfer coefficients at around the engine, the computing resources are usually found demanding [4][5][6][7][8][9]. A 384 ~ 2,720 CPU-hours was used for simulating 1 minute transient flow structure in a simplified engine model [4], and 24,000 CPU-hours was used for a 16 hours simulated drive cycle with intermittent steady-state flow analysis of the full-geometry engine CFD model coupled with transient 1D thermal engine model [5]. A coupled transient flow dynamics 3D CFD and 3D vehicle thermal model for the full-size designed geometries of a passenger vehicle and its under-hood region took 258,000 CPU-hours for 30 minutes soak simulation [7].…”