Ongoing efforts to reduce CO2 and other pollutant tail pipe emissions have led to escalated demand for diesel-electric hybrid bus powertrains in Europe, similar to the trend in passenger car markets. This is fuelled by public expectations and initiatives by various European governments to reward bus fleet operators for reduced in-city emissions and noise thus improving air quality and wellbeing of the general population.

This paper describes the engineering efforts that developed a Euro VI certified diesel engine system, catering for series hybrids operating under ‘charge-depleting’ as well as ‘load following’ battery management strategies.

The development team delivered improved fuel economy whilst dealing with requirements around legislation, unique customer duty cycles and engine mechanical robustness. Focus was placed on capturing requirements from a diverse range of sources and harmonising them to develop a technical solution fit for purpose in day to day operation that differs from validation cycles and standard drivetrain operation.

In order to deliver a field-ready solution, application specific tuning and validation processes had to be defined and developed. This was achieved through close coordination with the European bus OEMs and their chosen hybrid system suppliers. Six-sigma tools were used to highlight key expectations and drive technical solutions. At a system level the focus was on OBD reliability, exhaust after-treatment management, controls functionality, hardware durability and tail pipe emissions. Performance targets including the number of start-stops per hour, idle management and engine speed-torque ramp rates were defined.

Drive cycle simulations helped define optimal engine and hybrid system operating strategies followed by physical testing to further optimise these running points. Vehicle-level validation was completed through field testing, specific European bus test cycles, as well as under exceptional scenarios encountered in real world use. This exercise was designed to find and solve interface and OBD issues. Integration challenges in the areas of engine speed-torque control, diesel particulate filter management and HVAC control were addressed.

The outcome is the release of a bespoke Euro VI diesel engine package, which enabled the hybrid bus system to exceed customer expectations. This integrated system operates on a set of optimised parameters delivering efficient sub system behaviour including aftertreatment management, engine protection and operating state control. It handles the full range of real-world vehicle operation with improved fuel economy, frequent start/stop operation and enhanced driveability.

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