Modeling investigation of thermal insulation approaches for low heat rejection Diesel engines using a conjugate heat transfer model

Abstract

Heat losses through combustion chamber walls are a well-known limiting factor for the overall efficiency of internal combustion engines. Thermal insulation of the walls has the potential to decrease substantially these heat losses. However, evaluating numerically the effect of coating and of its location in the combustion chamber and then design an optimized combustion system require the use of high-fidelity engine models. The objective of this article is to present the whole workflow implying the use of three-dimensional computational fluid dynamics techniques with conjugate heat transfer (CHT) models to investigate the potential benefits of a coating on a passenger car Diesel engine. First, the baseline combustion system is modeled, using CHT models to solve in a coupled simulation the heat transfers between the fluid in the intake and exhaust lines and in the combustion chamber, on one hand, and the solid piston, head and valves, on the other hand. Based on this setup, a second simulation is performed, modeling a thermo-swing insulation on all combustion chamber walls by a contact resistance, neglecting its thermal inertia to keep a manageable computational cost. Results show a decrease of 3.3% in fuel consumption with an increase in volumetric efficiency. However, decoupled one-dimensional/three-dimensional simulations highlight the inaccuracy of these results and the necessity to model the coating thermal inertia, as they show an overestimation of the heat insulation rate and, consequently, of the gain in fuel consumption (−2.1% instead of −1.6%), for a coating on the piston with no thermal inertia.

A study of suspension plasma-sprayed insulated pistons evaluated in a heavy-duty diesel engine

“…One factor is poor air and fuel mixing due to a change in the turbulence because of the higher surface roughness and higher temperature of the coated surface. 10,11 Higher surface roughness also means a larger surface area for heat transfer to the piston and potentially a higher heat transfer coefficient, 12 which can counteract the purpose of the coating. It has also been proposed that the porous TBC may trap fuel and gas during the early stages of combustion, reducing the maximum heat release rate and shifting the combustion to a later stage and thereby having a negative effect on the fuel efficiency.…”

Editorial — ‘Thermo- and Fluid-Dynamic Processes in Direct Injection Engines’: THIESEL 2018 Special Issue by Xandra Margot

“…The paper selection covers all topics of the conference, reflecting current research trends, from the most traditional ones such as injection, sprays and combustion [1][2][3][4][5] to more recent pre-occupations such as studies to improve engine thermal management, [6][7][8] understanding the source of various emissions such as unburned hydrocarbons (UHC) and nanoparticles 9,10 or determining adequate gas exchange strategies for an efficient operation of exhaust after-treatment devices and for emission reduction in cold-start conditions. 11,12 Research on injection and sprays is still a hot topic, for diesel as well as gasoline direct injection (GDI) engines, as it is very relevant to understand the combustion process and the emissions.…”

“…Improving the thermal efficiency of engines is also one of the important topics that has become very relevant in recent years and is the main concern in three of the selected papers presented here. [6][7][8] Lee et al 6 present another collaborative work between university and industry (Seoul National University and Hyundai Motor Group), this time more focused on the experimental investigation of dual-fuel combustion at various operating loads aimed at improving the gross indicated thermal efficiency of diesel and gasoline engines. Inaba et al 7 also seek to understand how to improve the thermal efficiency and the exhaust emissions of a premixed combustion by controlling the intake oxygen concentration and gas pressure in two geometrically different combustion chambers.…”

“…Improving the thermal efficiency of engines is also one of the important topics that has become very relevant in recent years and is the main concern in three of the selected papers presented here. 6–8…”

“…Inaba et al 7 also seek to understand how to improve the thermal efficiency and the exhaust emissions of a premixed combustion by controlling the intake oxygen concentration and gas pressure in two geometrically different combustion chambers. Finally, Poubeau et al 8 use CFD combined with one-dimensional (1D) and conjugate heat transfer (CHT) simulations to analyse whether heat losses through the walls of a diesel engine may be reduced by adding some coating material on the walls of the combustion chamber.…”