Hybrid drives have been gaining ground in passenger cars for years. That trend is starting to emerge in off-highway equipment. Hybrid drives are particularly advantageous in vehicles with frequent acceleration and braking, such as excavators, loaders, and forklifts. All three of those examples not only brake and accelerate frequently, they travel very short distances and have high load peaks—ideal conditions for using hybrid drives. They also have idling phases in which the motor can be switched automatically off to save even more fuel.
Most current hybrid drives are a combination of an engine and an electric motor, which plays a dual role. On the one hand, it is a generator that stores the energy that would otherwise be dissipated as heat when the vehicle brakes. On the other, it supports the engine at inefficient operating points, for example, when the vehicle starts up or when engine speeds are low. Unlike combustion engines, electric motors can reach high torques in these situations. These advantages contribute to hybrid drives having greater system efficiency, which in turn means lower CO2 emissions and fuel consumption.
In a joint project with Atlas Weyhausen, Deutz used dSpace tools to develop a mild hybrid system for Atlas' AR-65 wheel loader. In the system, the electric motor is rigidly coupled to the diesel engine and supports prolonged acceleration. That feature distinguishes mild hybrids from micro hybrids, which have only an automatic start/stop function, and from full hybrids, which allow pure electric driving.
The wheel loader is driven by a Deutz three-cylinder diesel engine that delivers 36.9 kW (49 hp) at 2100 rpm. The engine is equipped with a permanently excited synchronous motor with a rated and a peak output of 15 and 30 kW (20 and 40 hp), respectively, and is integrated into the flywheel bell housing of the diesel engine. The rotor is directly coupled to the crankshaft, allowing easy installation of the hybrid drive as the electric motor required hardly any additional space.
A lithium-ion battery rated at 400 V is connected to the electric motor via an inverter. The inverter is responsible for commutating the synchronous motor and regulating torque by means of field-oriented control.
The hybrid drive is connected to two hydraulic pumps, the traction pump and the work pump. Like the hydraulic traction motor, the traction pump is designed as an axial piston machine. It is responsible for generating the hydraulic volume flow for the traction motor, or hydrostatic traction drive. The work pump sends the hydraulic oil through proportional valves to the hydraulic cylinders used to raise the bucket and steer the wheel loader.
Several dSpace tools were used to develop the software functions for the hybrid system’s ECU, including Real-Time Interface (RTI), for setting up the I/O interfaces for the MicroAutoBox; RTI CAN MultiMessage Blockset, for setting up CAN communication; and ControlDesk and CalDesk for calibrating the hybrid functions. Designers were able to implement fully functioning system software on the MicroAutoBox in only three months.
Three CAN channels were set up in the wheel loader: engine CAN, hybrid CAN, and vehicle CAN. Because the system software was programmed directly in Simulink, it was possible to try out the software functions immediately on a plant model containing the engine, electric motor, inverter, battery, work hydraulics, and traction hydraulics components. Software functions were able to be tested long before the first prototype components became available, essential in view of the very short development time assigned to this project.
Using the pretested software functions and the I/Os configured with RTI (digital, analog, PWM, CAN), a software version was produced to run on the MicroAutoBox and be tested on the test bench. Functions such as start/stop were tested and calibrated via ControlDesk and CalDesk. Finally, the wheel loader was put into operation with the MicroAutoBox as a superordinate hybrid system ECU, and designers implemented the functions for boosting power and raising/shifting the load point.
By reducing fuel consumption 20-30%, Deutz calculated—with the assumption that fuel consumption was previously 6.5 L/h and that diesel costs €1.30—that €2 per hour would be saved on a typical working day, or €1500 in a year. The total savings for the entire working life of a wheel loader would more than offset the higher price of acquisition for the hybrid drive.
With these obvious benefits in terms of emission and cost reduction, Deutz sees great potential for hybrid drives in off-highway equipment. The company will be deploying several hybrid wheel loaders—each equipped with a dSpace MicroAutoBox—to various customers to gather experience in everyday use. Its goal is to bring the hybrid system up to production status by mid-2012.
Marco Brun, Deutz AG, wrote this article SAE Off-Highway Engineering.