Concept design of electric cruise and startability axles for long haul heavy vehicles to maximise driving range

Electrification of commercial heavy vehicles has led to the offer of different electric drive-based powertrain architectures, most of them with the adaptation of the diesel engine based architecture. Due to the limited driving range possible, optimal configuration of the powertrain becomes a vital task with the battery as the energy source, which also takes a significant portion of payload capacity. A modular and distributed powertrain architecture using separate cruise and startability axles is introduced in this paper with the aim to maximise driving range. The concept is developed and evaluated using a joint hardware and control design framework for a target vehicle combination of 36 tonnes over a typical regional long haul drive cycle using simulations. Sensitivity study of gear ratios and motor sizes with vehicle target requirements is performed to identify the optimal configuration for the powertrain concept. The simulation results show that by configuring around 50% of the total electric machine power to the cruise axle and choosing a gear ratio between 12–13, the range capability is maximised, while satisfying startability and cruise mode operations.

I. Introduction

Since 1923, commercial heavy-duty vehicles, have largely been propelled by diesel fuel based internal combustion engines (ICE) [1]. Today in class 8¹

US vehicle's gross vehicle weight rating (GVWR) class8heavy truck.

, the diesel engine is used in the majority of vehicles in the running fleet. The diesel engine is a powerful converter of diesel energy to mechanical energy and power, but it needs to be run on a minimum idle speed, around 300–500 revolutions per minute (rpm) and generally has a maximum speed about 2000–2500 rpm [2]. To get this energy converter to function well, the powertrain needs a clutch for handling take off from standstill. In order to manage hill-starts with a gross combination weight about 30–40 metric tonnes, a large gear ratio is needed between the engine and the wheel that can amplify the engine torque to the output wheel torque. This will generate the necessary propulsion force to overcome the motion resistance, and in particular, the contribution from large road gradients. Additionally to allow for a speed range from 0 to 100 km/h, multiple rotational gear ratios between the engine and wheel are needed to overcome the road resistances at efficient operating points of the engine [3]. In today's transmissions, the powertrain design usually contains around 8–16 gears depending on the vehicle type and application, usually complemented by a rear axle differential, with further speed reduction [4]. The overall conversion efficiency of the diesel energy by an ICE to kinetic energy of the vehicle is still less than 50% and efficiency improvement possibilities are limited [5] [6]. In contrast to an ICE, the electric machine offers high zero speed torque, higher conversion efficiency and wide operating range (up to 15000 rpm) leading to the need for fewer number of gears [7].

This work was supported by Energy Agency in Sweden, part of Vinnova/FFI.