Heavy-Duty PHEV Yard Tractor: Controlled Testing and Field Results

Diesel powered tractors are used to shuttle cargo trailers from point to point within the confines of a port facility, terminal or warehouse yard. Such operations are similar to those in ground support applications at airports and in industrial warehouses with lift trucks, in that the vehicles are used as tools to move goods in a semi-regular pattern. Southern California Edison Company (SCE) and the Electric Power Research Institute (EPRI) have partnered to help electrify vehicle operations in both of those venues with great success and see good prospects for the same at port operations. However, current port operations might require large investments in infrastructure and operational changes to implement electric drive all at once. To help demonstrate the benefits of electric drive without requiring large-scale changes, a plug-in hybrid electric vehicle (PHEV) yard tractor design was proposed by EPRI and member utilities as a means to reduce operational emissions and diesel fuel use. Four member utility companies with large port customers in their service area (SCE, Southern Company, CenterPoint Energy, and New York Power Authority) agreed to work with EPRI to study the benefits and impacts of a PHEV yard tractor. In 2007 the Electric Power Research Institute (EPRI) contracted US Hybrid Corporation (USH) to design and construct a unique PHEV yard tractor. SCE agreed to test and evaluate the PHEV yard tractor for EPRI. To properly evaluate the benefits realized by the yard tractor in comparison to unmodified conventional yard tractors as well as other alternative fueled tractors, SCE had to test the tractor in controlled conditions with realistic loads in addition to field testing. SCE developed test procedures for controlled testing and for field evaluation. The field testing was conducted in four ports across the United States, each with different operating conditions and climate: Long Beach, California; Houston, Texas; Savannah, Georgia; and New

Institute (EPRI) have partnered to help electrify vehicle operations in both of those venues with great success and see good prospects for the same at port operations. However, current port operations might require large investments in infrastructure and operational changes to implement electric drive all at once.
To help demonstrate the benefits of electric drive without requiring large-scale changes, a plug-in hybrid electric vehicle (PHEV) yard tractor design was proposed by EPRI and member utilities as a means to reduce operational emissions and diesel fuel use. Four member utility companies with large port customers in their service area (SCE, Southern Company, CenterPoint Energy, and New York Power Authority) agreed to work with EPRI to study the benefits and impacts of a PHEV yard tractor. In 2007 the Electric Power Research Institute (EPRI) contracted US Hybrid Corporation (USH) to design and construct a unique PHEV yard tractor. SCE agreed to test and evaluate the PHEV yard tractor for EPRI.
To properly evaluate the benefits realized by the yard tractor in comparison to unmodified conventional yard tractors as well as other alternative fueled tractors, SCE had to test the tractor in controlled conditions with realistic loads in addition to field testing. SCE developed test procedures for controlled testing and for field evaluation. The field testing was conducted in four ports across the United States, each with different operating conditions and climate: Long Beach, California; Houston, Texas; Savannah, Georgia; and New York City.
SCE designed a test procedure that simulates an accelerated duty cycle of cargo operations. The accelerated duty cycle has multiple starts and stops and little idle time. SCE measured the idling fuel consumption separately so it can be inserted to match the duty cycle of any particular port. The test cycle was performed with the vehicle both unloaded and loaded to profile the effects of load on system 1 Introduction

Background
The diesel-powered tractors that pull containers and cargo are used extensively in ports, warehouses, and other applications where it is necessary to shuttle cargo trailers from point to point within the confines of a specific facility, terminal or yard. Often called yard tractors, yard hostlers, or terminal tractors, this equipment is of a specific design with a single driver compartment and a fifth wheel that has the ability to be raised and lowered. These widely used yard tractors are unique to the cargo industry.
Seaports are under increasing pressure to reduce operational emissions. Some ports and their tenants are beginning to take significant steps in reducing emissions from various aspects of their operations, from lower sulfur fuels in vehicles and equipment to electric power for ships docked in port. Many emission reduction efforts focus on cargo handling equipment such as the yard tractor due to this sector's share in port emissions and concerns over oxides of nitrogen (NOx) and particulate matter (PM) emissions that result from this equipment. Electric technology can play an important role in this target sector by significantly reducing equipment emissions. This project, managed by the Electric Power Research Institute (EPRI), with participation by Southern California Edison (SCE), CenterPoint Energy, New York Power Authority, and Southern Company, began in earnest in 2007 when EPRI contracted for the design and construction of a first of a kind PHEV yard tractor. SCE provided oversight of the development of the prototype vehicle, developed vehicle evaluation procedures, conducted tests, and collected and analyzed the field data under a subcontract to EPRI.

Vehicle Characteristics
The goal of the EPRI project was to develop a vehicle that could demonstrate the fuel economy benefits and emissions reductions of electrifying a major source of off-road emissions in port areas, without compromising vehicle functionality in those areas that had not installed charging infrastructure to a large extent. The US Hybrid prototype PHEV yard tractor is a posttransmission parallel hybrid system installed on a Kalmar Ottawa yard tractor. The original engine and transmission are unmodified with the US Hybrid 125 kW, 1000 newton-meter electric motor installed between the transmission and the axle. The PHEV yard tractor has a nominal 32 kWh LTC/Gaia lithium-ion battery pack. The motor controller, charger, and battery are integrated on the right-hand side of the vehicle, outboard of the frame rails, and aft of the cab and forward of the wheels. The added weight of the hybrid system is approximately 2,100 pounds, for a total weight of 17,300 pounds,. The gross combined vehicle weight rating of the PHEV yard tractor is unchanged at 96,000 pounds. The maximum speed of the vehicles is just under 20 mph.

Controlled testing procedures
SCE decided that a controlled test similar to real world conditions could simulate the port environment better than a dynamometer test. To test on a dynamometer, two different coast down profiles would have been needed, loaded and unloaded. Then when testing, the operator would have to alter the weight and horsepower profiles between cycles, and it would be difficult to simulate accurately the trailer pickup and drop-off maneuvers.

Vehicle Drive Loop Efficiency Tests
SCE designed a test loop that was flexible enough to simulate all sorts of duty cycles. The test cycle used was designed to simulate a near 100% duty cycle with short pauses between cycles. With this method, which is efficient in terms of test time, other duty cycles could be simulated by measuring the idling and creeping fuel consumption and interjecting various amounts of idle time between the actual test loops.
In dynamometer testing, a driver typically follows a trace screen to recreate speeds in a given sequence. Since this is difficult to perform in use, with the complex tasks of picking up and dropping off containers, SCE decided to use a space-oriented approach rather than a speed oriented approach. The drive loop tests are designed to test the yard tractor under conditions similar to actual yard tractor operations. SCE initially looked at dynamometer cycles developed by CALSTART [1] but decided that these wold be too difficult to implement on a repetitive course. Instead, SCE used a simplified cycle similar to the duty cycles the PHEV manufacturer used in the original proposal for modelling. The test course was designed such that the distances matched a speed profile used by the manufacturer in simulations.

Vehicle Drive Loop Course
The test course was designed with two sections to simulate two types of operation. The inner loop was designed to emulate stop-and-go movement and queuing, while the outer loop was designed to give the operator a chance to develop speed. The course is laid out as shown in Figure 1 The course has an inner loop (A-B-C-D-E-A) and an outer loop (A-F-C-D-G-E-A). The points labeled A through G are marked with traffic cones. The cones denote the inner portion of the drive path.

Driving the Course
The driver starts at position A without trailer, drives to B and comes to a complete stop and pauses 5 seconds. The driver then proceeds to C, D, and then E, turning at each, and comes to a full stop at A and pauses 5 seconds. Repeat three more times, for a total of four loops for this sequence. The driver then drives the outer loop from A to F to C to D to G to E to A non-stop for a total of three times and then comes to a complete stop at A. The driver then connects a trailer (if applicable) and starts the second cycle of seven loops with the trailer, completed by unhooking the trailer (if applicable) at A. In brief, the sequence is: -4 times: AB(stop) CDEA(stop) -3 times: AFCDGEA (stop) -Stop, Hook/Unhook trailer -Repeat The driver records the number of loops driven. After each set of seven loops the driver records fuel consumption, state of charge, and time. This sequence of seven loops constitutes one complete test cycle, covering an overall distance of 8800 ft, or 1.7 miles (2700 m), with 45 seconds idling for unloaded cycles, and approximately 90 seconds idling for each trailer change, for a total of three minutes, forty-five seconds idling for loaded cycles. In EV mode, the test was done without a trailer and then again with a loaded trailer to obtain clear metrics on the electrical energy consumption (AC kWh/mi). In the hybrid and diesel modes, the testing was performed as described above.

Controlled Test Results
The first test conducted by SCE was the electric vehicle (EV) range test. This was performed in the EV-only mode with no engine operation until the minimum SOC level was reached, and the engine started. The EV1 test (unloaded) was done with no trailer or payload and the EV2 test (loaded) was done with the trailer and payload included. The PHEV yard tractor completed an average of nine drive cycles (with one cycle being seven loops, two miles total) during EV1 testing before the system turned the engine on to recharge the battery (Table 1). SCE found the SOC data unreliable and it is not included in this report. The USH-reported SOC difference was not correlated well with recharge energy. Reported SOC sometimes dropped from 20% to 4% between key off and back on. After charging, a difference in ending SOC from 10 to 12 % was not uncommon. This indicates recalculation of SOC by the LTC battery management system during key-on. For further information about this issue see [2]. During EV2 testing, with loaded trailer, the vehicle completed an average of 3.2 drive cycles (Table 2) before engine start -a 64% reduction in EV range. The data shows a 159% increase in energy use per cycle for the loaded test.

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