Hybrid system benchmarking

Synergetic testing, simulation and design assessment

28. August 2017 | Engineering Service

FEV has developed a system engineering approach focused on benchmarking. In this approach, the synergetic combination of testing, simulation and design assessment provides fundamental information in the concept phase of hybrid vehicle development programs. The proposed method includes a multi-level framework – from overall vehicle architecture down to detailed component level.

As a benchmarking example, a P2 plug-in hybrid vehicle was evaluated with respect to market assessment, drivetrain layout, components, operation strategy and additional starting devices. In the particular case of the starter device, the proposed approach guarantees correct market positioning (in terms e.g. of comfort, NVH, etc.) and virtual confirmation of vehicle targets.

Benchmarking of P2 PHEV SUV

The system level analysis of the P2 PHEV SUV includes on-road and chassis dyno testing, system simulation and final design assessment in terms of drivetrain layout, components and operation strategy. The selected vehicle is a mid-size SUV currently available on the European market with a P2 hybrid layout.

Market Analysis

FEV offers a comprehensive benchmarking database. Depending on the configuration, different parameters can be compared with each other to evaluate technical solutions and evaluate market trends. With respect to P2 PHEV SUVs, it can be observed that while most OEMs offer P2 PHEV solutions featuring a 25–40 km electric range, only few vehicles reach 50 km to target the Chinese subsidy limit. However, an increase in the electric range up to 70–80 km can be expected in the near future due to higher regulatory restrictions and technological battery improvements (energy density and cost per kWh). Concerning installed electric power, most OEMs currently position themselves in the range of 60 kW–100 kW as a function of brand identity (“sporty” vs. “comfortable”), vehicle mass and projected specified electric driving performance. An increase in the installed power to approximately 80 kW–110 kW as a function of the vehicle mass is also foreseen. Similar conclusions can be derived in terms of other vehicle targets (e.g. acceleration, maximum velocity) and reference component specifications (e.g. electric machine torque, battery size, charging system power).

Vehicle Analysis

In FEV Level 1 benchmarking, actual vehicles are analyzed in terms of performance and fuel/energy economy. These properties are examined by means of minimum, non-intrusive testing equipment within standard benchmarking procedures, including the evaluation of 0–100 km/h acceleration, 50–80 km/h elasticity or fuel/electric consumption in driving cycles relevant to both legislation and real-world customers.

In addition to the cross-checking of catalogue values, standard driving cycles are used within repeatable boundary conditions to analyze and compare the effects of different powertrain configurations and the energy management strategies implemented.

Real-world driving also provides information about off-cycle fuel economy, electric energy consumption or electric range on the one hand and the calibration of driving modes depending on battery SOC and routing information on the other.

Powertrain Analysis

In the level 2 assessment, component efficiencies, energy flow and operating strategies are analyzed.

In the first case the transmission clutch changes to slipping mode in the beginning and P2 EM speed is increased slightly. For the subsequent ICE ramp-up, the separation clutch between the ICE and P2 is closed and P2 torque is increased to crank the ICE up to target speed, whereas P2 speed remains constant. After the ICE is fired up, driving torque is transferred from P2 EM to the ICE before the transmission clutch is closed. In the case of an ICE start with a dedicated starter the EM operates at peak torque before, during and after the ICE start event until the ICE can take over the EM torque. The starting device starts the ICE, which is then synchronized and connected to the driveshaft to provide torque to the wheels.

These measurements are used, for example, to analyze and compare system performance in terms of time demand, comfort of ICE start and effect on hybrid operating strategy. The pros and cons for each configuration are documented in the FEV database, which provides important input for the system design.

During the course of the study, FEV investigated repeated accelerations of the vehicle in electric driving and hybrid driving mode. In the first case, the HV system is allowed to work at peak power without any requested torque reserve to start the ICE due to the presence of the starting device. As a consequence, the first electric acceleration is aggressive, but the thermal derating starts soon thereafter in the 2nd acceleration test.

With hybrid accelerations, there is no quantifiable system degradation for 10 repetitive 0-100 km/h accelerations. The EM is operated up to the continuous torque (no thermal derating) and the full load periods during the hybrid acceleration events are much shorter than for pure electric acceleration.

In general, the results of level 2 benchmarking analyses provide important input for the system design in terms of achievable performance depending on component specifications and control strategy.

Drawing - Hybrid vehicle benchmarking

Vehicle topology: P2 PHEV

Simulation

The level 1 simulation approach is typically used by FEV to support the benchmarking of electrified vehicles in terms of energy management, component assessment and analysis of the operation strategy.

The first step in any simulation activity is the collection of the input data and the model validation. When benchmarking electrified vehicles, this phase is performed by FEV within the course of a multi-step, systematic approach. First, energy consumption at the wheels is determined during the propulsion and braking phases. Afterwards the driveline losses are validated in terms of transmission efficiency and oil warm-up. The electric components are then verified based on the cycle time histories of the mechanical and electrical signals measured during electric driving. Finally, the internal combustion engine and the auxiliaries are validated.

A semi-empirical temperature model for the ICE is fitted along the measured cycles and the fuel consumption time history is checked against the measurements. If no component measurements for transmission, electric machine, ICE or HV Battery are available, the initial map is based on the extended FEV database and is reshaped afterwards to match the losses measured during constant-speed operation. The simulation model is controlled by a rule-based operation strategy, parameterized to reflect the operation points of the actual vehicle. The resulting model is validated with 5% of cycle accuracy in terms of fuel economy and performance time.

The simulation model is used to assess the powertrain in terms of performance and energy economy, component sizing, technology packages and operation strategy.
FEV’s system simulation toolchain also includes the mathematical optimization of the operation strategy along the selected driving cycle by means of Discrete Dynamic Programming (DDP).

Impact on System Design

During the concept phase of a hybrid development program, several details need to be fixed in terms of hybrid layout. Depending on the integration effort, cost target, synergies with other platforms and requirements in terms of start comfort, performance, efficiency or cold start capability, the most fitting solution must be defined. The selection criteria are supported by a dedicated measurements campaign and simulation activities.

Specifically, since the P2 EM is capable of starting the engine, no additional starting device is absolutely necessary. However, when using only the P2 for starting the engine, either an acceleration drop during the start event can occur for high-load EV operation or a power reserve for the ICE start must be provided. These drawbacks can be avoided by means of an additional starting device.

The cheapest starting device is a 12V pinion starter, which fulfills all requirements in terms of a reliable engine start and especially cold cranking. Belt-driven devices are the most suitable to fulfill the increased requirements regarding NVH and start comfort as well as change-of-mind scenarios. HV belt starter generators can also be beneficial, depending on the availability of modular components, maximum power output and the energy reserve of the high voltage (HV) battery.

Graphic - Hybrid vehicle benchmarking

In the course of the study, FEV investigated repeated accelerations of the vehicle in electric driving and hybrid driving mode.

Graphic - Hybrid vehicle benchmarking

As the P2 EM is capable of starting the engine, no additional starting device is strictly necessary.

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