Compared to a conventional diesel bus, hybrid and electric buses are fundamentally different when it comes to producing the compressed air needed to provide the main energy source for the air braking, pneumatic suspension and door opening systems. While diesel-powered buses use engine-driven air compressors, hybrids and EVs typically feature standalone electrically driven screw or vane compressors.
Known as ‘e-compressors’, these sophisticated systems are produced by OEMs including Knorr-Bremse, Gardner Denver, Hydrovane and Mattei. Designed to provide constant and stable compressed air, e-compressor assemblies include compressor, electric drive motor and inverter. Unlike diesel-driven compressors that are lubricated and cooled by the same circuits as the engine, e-compressors feature a standalone lubrication system. As a result, these systems require regular oil and filter changes, in line with the compressor or vehicle manufacturer’s recommendations, to ensure the intended service life of the system.
THERMAL MANAGEMENT
While conventional diesel-powered buses employ engine-driven water pumps driven via the timing gears or belt, and rely on the engine speed to govern the flow rate of the water pump, hybrid and electric buses use electric water pumps to provide thermal management.
Electric water pumps from OEMs such as AVID/Turntide use a CAN-Bus-enabled electronic control unit (ECU) to manage the pump’s speed. This provides the unique ability to precisely control the power consumption and performance of the pump, making electric water pumps considerably more efficient than engine-mounted systems.
Electric water pumps are used to boost the circulation of the existing cooling systems, helping to optimise diesel engine heat-up time on hybrid buses, as well as keeping the engine running at a more consistent temperature. They are also used in separate thermal management systems for the battery assemblies on hybrid and electric buses to maintain optimum battery cell temperature.
Electric water pumps typically use brushless DC motors to ensure performance and reliability. Both 12V and 24V DC electric water pumps are available to ensure compatibility with the electrical architecture of specific vehicles. In addition, some electric water pumps can provide a flow rate in excess of 110 litres per minute, as well as speed control via a CAN-Bus connection, which produces diagnostic information via the vehicle manufacturer’s software.
POWER STEERING
Unlike conventional diesel buses, which use engine-driven pumps to provide the hydraulic pressure for the power steering system, hybrid and EV buses feature a standalone electrohydraulic power steering pump (EHPS) that uses electric motors to power the hydraulic system.
The EHPS provides the demand-oriented amount of oil and pressure that is needed to operate the hydraulic steering system at any given time. EHPS systems from OEMs such as Bosch typically consist of two electric motors, each equipped with power electronics equipment, a vane pump and vehicle-specific mounting bracket.
The EHPS communicates with the vehicle via the CAN-Bus network, which can also be used for diagnostics.