|The core of the system: the Hybrid Drive Unit. Click to enlarge.|
Eaton will begin full production this year of its hybrid electric drive system for medium-duty trucks. The Hybrid Electric System (HES), which was seven years in development, is currently deployed in 167 pre-production field units: 93 with FedEx, 50 with UPS, and 24 HTUF utility trucks.
Eaton is primarily targeting the pickup and delivery (P&D) and the utility markets with the HES. In the P&D market, use of the Eaton system provides a 30-50% improvement in fuel economy. In the utility market, the system delivers a 40-60% reduction in fuel consumption, including the fuel that would be otherwise burned to support power generation at the job site.
|System diagram for the Utility HEV system. Click to enlarge.|
The hybrid system uses a parallel, pre-transmission design. Primary components are the Hybrid Drive Unit (HDU), which combines a clutch, a 44 kW/420 Nm motor/generator and automatically controlled manual transmission; the motor inverter/controller; the DC/DC converter; and a 2 kWh li-ion battery pack.
The basic core of the system (HDU, inverter and battery pack) is the same for both target markets. For the utility market, however, Eaton is providing two additional features: auxiliary power generation and the ability to drive PTO (power take off operations) with the electric motor.
Features of the system include:
Electric motor vehicle launch and acceleration assist.
Charge sustaining battery system.
For utility applications, on-board power generation capability.
Fall back to engine-only operation in case of hybrid system failure.
A clutch is the interface between the engine and the motor, which is directly connected to the input shaft of the transmission. After startup, the clutch disengages and the vehicle is propelled by the motor. When the battery depletes to a specified state of charge, or when the power demand by the driver exceeds the specified limit, the hybrid control module engages the clutch and begins to blend engine power with motor power for operation. The blending is extremely precise, according to Eaton, and clutch slippage is not an issue.
The engine during driving operation is always on with the Eaton system. That’s due to the lack of electric accessories, according to Matthew Johnson, a member of the Eaton hybrid team who was explaining the system at the SAE Hybrid Vehicle technology Symposium 2007. The engine stays at low idle even in EV drive mode to support the belt loads.
For the utility application, when the driver requests PTO, the engine switches off and the motor powers the tools and hydraulic lift. When the battery is depleted, the hybrid control module restarts the engine, closes the clutch, and recharges the battery. Recharge time is about 5 minutes, and under constant operation, the battery will then support 45 minutes of auxiliary power operation. Since most PTO applications are not constant, however, actual time between charging can be as much as two hours, according to a PG&E user of the hybrid.
Fuel savings in the utility truck vary with the type of mission, which Eaton reduces to two primary forms: more driving with less time on the job site, and less driving, with more time on the job site.