Well-to-Wheel Efficiency and Energy Consumption Analysis for Mild Hybrid Electric Vehicles

Abstract

Over the last few decades, there has been an increased focus on reducing greenhouse gas emissions among all sectors to keep the global temperature increase below 1.5°C above pre-industrial levels. Automotive sector has been at the fore front of this transition as necessitated by the impact that gas powered vehicles play on overall greenhouse gas emissions. Consequently, the past few decades saw rapid increase in the efficiency of gas-powered vehicles as well as introduction of alternative technologies to power ground transportation such as hybrid electric vehicles and battery powered electric vehicles. Hybrid electric vehicles have been prominent in increasing vehicle efficiency owing to the unique architecture of power generation and transmission. Today, most automotive OEMs have a significant hybrid electric vehicle portfolio along with battery electric vehicles and traditional combustion engine portfolio. Although battery electric vehicles have been gaining in popularity, hybrid electric vehicles have been in the market for more than two decades and there are still a considerable number of vehicles being developed and operated as compared the battery electric vehicles running today. Thus, understanding the chain of energy conversions and transmissions in operating a hybrid electric vehicle is crucial to formulate strategies to improve the overall efficiency and reduce greenhouse gas emissions. Although, efficiency is a universally used term that signifies the energy consumed to energy generated, the scope of application is different. Most generally, efficiency of a vehicle refers to the amount of energy consumed (via the fuel) vs amount of useful energy generated at the wheels. This scope is limiting as it does not consider the energy consumed in processes relating to sourcing of the fuel, transportation of fuel, manufacturing and maintenance of the vehicle. Therefore, it becomes essential to compute an efficiency term that involves the impact of all the energy consuming processes in the lifecycle of the vehicle for a more comprehensive understanding of efficiency. This efficiency term that encompasses all the energy consuming processes is known as well-to-wheel (WTW) efficiency. This study examines the energy transfer chain of a hybrid electric vehicle, providing a detailed measurement of the efficiency in the entire process, from the supply of fuel to the energy transmitted to the wheels of the vehicle. Within the scope of the analysis, every stage of the energy transfer process from the well to the vehicle, such as extraction, transportation, processing and storage of the fuel is included. The first part of this study focuses on estimating the energy consumption of processes involving mining, transportation, refining and distribution of the fuel to compute the well-to-tank efficiency (WTT). The next part of the study uses this well-to-tank efficiency along with the conversion efficiency and energy consumed during manufacturing and maintenance of the vehicle to compute the overall well-to-wheel efficiency of hybrid vehicles.