Diesel hybrid powertrain for passenger and light commercial vehicles

Abstract

The powertrain architecture was initially defined as a diesel series-parallel split hybrid with a dual clutch transmission, comprising of the four cylinder 1.6 l Ford Duratorque engine, the Ricardo eDCT, a seven speed dual clutch transmission with electromagnetic actuation, the Eldor AC permanent magnet electric machines as traction motor(s), an Eldor AC permanent magnet belt-driven starter/generator (BSG), Eldor dual inverters and Ford Escape Ni-MH standard production Batteries. The inclusion of a BSG in the architecture is intended to satisfy the requirements fast engine start while in electric-only mode to respond to increased load demand, additional generation capacity to charge the high voltage traction battery and the possibility for limited series hybrid mode (primarily for development purposes). The disadvantages of the BSG include added drag on the engine (increased Friction Mean Effective Pressure, FMEP) and increased cost and complexity. Since the cold-cranking torque of the diesel engine increases significantly below 0 °C this, combined with reduced battery performance at sub-zero ambient conditions, meaning that it is possible that the BSG may not be able to start the engine. This work will therefore explore the feasibility of deleting the BSG from the architecture. The use of a dual clutch transmission in a hybrid powertrain architecture presents a number of opportunities. Acceleration performance and driveability are superior compared to standard manual transmissions whilst retaining comparable efficiency. The objective of the initial work was to explore how the Ricardo eDCT could be incorporated into the hybrid powertrain to achieve even greater benefit, such as positioning the traction motors on each of the transmission shafts (i.e. one on the even gear set, the other on the odd gear set), as shown in ❶ (left). The benefits of this, compared with the traction motor positioned between engine and transmission, ①right, needed to be understood and considered against other factors such as package, complexity, cost and control overhead. Open image in new window ❶ Vehicle and powertrain architecture – dual traction motors mounted in eDCT (left); vehicle and powertrain architecture – single traction motor between engine and transmission (right) These two concepts, amongst others, were explored using Ricardo V-Sim, a simulation tool developed in-house based on Matlab-Simulink to carry-out such investigations on different hybrid architectures. The value of hybrid architecture depends on a vehicle usage pattern specified by a family of possible drive cycles. For this analysis, the New European Driving Cycle (NEDC) and the Ford Parcel Delivery Cycle (FPDC), ❷, were used to explore the architecture options. Open image in new window ❷ New European Driving Cycle (NEDC) (above); Ford Parcel Delivery Cycle (FPDC) (below) The V-Sim model was used to estimate the driveline power requirements over each of the two drive cycles. It was found that for the target application the power requirements are mostly within the ± 20 kW range where transitions outside this range are very limited, apart from the highway section of the NEDC, ❸. A total electric-only driveline power of 20 to 25 kW, with a typical permanent magnet AC motor power and torque characteristic was therefore selected for the ongoing analysis. Open image in new window ❸ Driveline power requirements for each drive cycle The V-Sim model was then configured into the range of powertrain architectures being considered; a selection of these is shown in ❹. Dynamic Programming techniques were applied to each V-Sim model and drive cycle instance to ensure a optimal strategy was being considered in the comparisons between architectures. It is beyond the scope of this article to provide details of each optimisation; however, the outline control strategy was based on the following key principles: : Electric-only mode is used without engine support as long as the battery state of charge (SOC) exceeds a certain level or torque demanded is achievable from the electric machine only. : In hybrid mode, generation from fuel is performed when engine is on with sufficient speed and battery SOC is sufficiently low. The engine has a minimum “on” time of 15 s. By this rapid on/off transits of the engine with the corresponding negative effect on driveability are avoided. Also, there are possible durability concerns owed to repeated stop/start events behind this strategy. Positive results achieved through control system development on other Ricardo hybrid programmes also suggested this “on” time. Open image in new window ❹ Ricardo eDCT with various traction motor configurations (above: twin motors, one on each transmission shaft; middle: single traction motor switched between each transmission shaft; below: single traction motor located between engine and transmission) The electric machine is used to boost performance based on accelerator pedal position. The magnitude of regenerative braking is a function of brake pedal position, vehicle speed and SOC. Primary gear selection is based on accelerator pedal position and vehicle speed. Secondary gear selection (for eDCT dual motor solution) is either one gear up or down from primary. This approach has allowed relative rather than absolute comparisons to be made between the different architecture configurations being considered. This was sufficient to allow key decisions to be made prior to concept selection for more detailed analyses. The V-Sim models, with Dynamic Programming methodologies, were used to compare overall driveline efficiency over the NEDC and FPDC drive cycles. By way of comparison, results achieved on NEDC for a selection of the configurations are provided below: : single traction motor located between engine and transmission: baseline : single traction motor located between engine and transmission with BSG: 1 % improvement : twin motors, one on each transmission shaft: 4.4 % improvement : single traction motor switched between each transmission shaft: 2 % improvement. These results clearly show that an architecture configured with the traction motors integrated into the eDCT tended towards a more efficient solution.