Duty Cycle Characterization and Evaluation Towards Heavy Hybrid Vehicle Applications

Four metrics related to vehicle duty cycle are derived from the energy equation of vehicle motion. Three key application areas are introduced. The first is the ability to quantify the sameness between vehicle duty cycles and the ability to asses a duty cycle’s suitability for hybrid vehicle usage. The second area of application allows for the estimation of fuel consumption for a given vehicle over a target duty cycle. The third area of application allows us to predict how non-propulsion fuel use will affect energy use. The paper ends with real-world examples involving actual heavy-duty hybrids. INTRODUCTION Hybrid electric technology is of great interest to users and manufacturers of heavy vehicles due to the technology’s ability to reduce fleet fuel consumption and emissions. Challenges still exist for hybrid technology in terms of purchase and life-cycle cost, testing, packaging, durability, and reliability. However, independent from the challenges of hybrid technology itself is the question of when and where do hybrids make sense and what are the benefits of hybridization for a given application. Because heavy vehicles often operate as critical elements to a business model, the hybrid value equation often depends on reduced in-use fuel consumption in order to justify the added purchase cost over a conventional vehicle. Thus, it is often critical that hybrid vehicles be deployed over duty cycles where they will show a clear benefit. There are a wide variety of heavy vehicle vocations applicable to hybridization including refuse haulers, transit buses, pick-up and delivery vehicles, utility trucks, and military applications. The variation between heavy vehicle vocations is large though the variation within a vocation is typically significant as well. Thus, it is important for those evaluating hybrids as a purchase option and for those manufacturing hybrids to understand the duty cycle of the intended application. A duty cycle provides a concise, repeatable sequence of vehicle input operations over some time period. A typical duty cycle consists of second-by-second values of speed and elevation, though time-based information on the operation of other systems is sometimes included as well. Duty cycles are most valuable when they are representative of how a vehicle will operate in a target application. However, whether or not a cycle is representative can be a challenge to determine. Creation of representative cycles from test data is no easy task. This is especially true for heavy vehicles as the duty cycle must account for more than just vehicle speed and elevation versus time (as in light-duty applications). In heavy-vehicle applications, cargo weight, roadsurface, route type, and vocational loads (such as trash compaction on a refuse hauler) can vary dramatically over time. The choice of the term “duty cycle” as opposed to “drive cycle” is a conscious choice by the authors to emphasize that there is more to heavy-vehicle cycles than just the time-speed operation. In this paper, we examine some physical-based metrics to aid the reader in duty cycle characterization and duty cycle evaluation with hybrid-electric vehicles in mind. We hope to better equip the reader to determine the applications where hybrids will make the most sense. MAIN SECTION It is well known that vehicle energy usage is closely linked with duty cycle [1, 2, 3]. This is especially true for hybrid-electric vehicles. Therefore, insight can be gained from understanding the physical mechanisms of how vehicle energy use relates to the duty cycle. We will therefore begin by examining the energy usage equation as applied to heavy vehicles. The energy equation will form the basis for a discussion of physical-based metrics—metrics derived directly from the energy equation and holding a physical significance. Examples of how the metrics can be used to characterize and evaluate cycles for hybrids will be presented. We will end by 2 discussing some heavy-vehicle applications of the metrics. The metrics themselves are an extension of the work done by [1]. The reader is directed there for a more in-depth background and derivation. UNDERSTANDING HEAVY VEHICLE ENERGY USAGE The tractive power required to move a vehicle over a roadway surface is the summation of the power required to overcome: 1. Aerodynamic drag 2. Rolling resistance 3. Vehicle inertia 4. Gravitational potential energy. This is given by the classic roadload equation for power presented below [4]: Equation 1: dt dh g M dt dv v M v g M RRC v FA C P