Power-split Hybrid Vehicle Research Articles

Development of Hybrid-Vehicle Energy-Consumption Model for Transportation Applications—Part I: Driving-Power Equation Development and Coefficient Calibration

This study is the first of a two-part paper. The overall study presents a new methodology to improve the accuracy of hybrid vehicles’ energy-consumption model over conventional transportation modeling methods. The first paper attempts to improve an equation for vehicles’ driving-power estimation to be more realistic and specific for a particular vehicle model or fleet. The second paper adopts the driving-power equation to estimate the requested driving power. Then, the data are utilized to construct the hybrid-vehicle energy-consumption model, namely, the traction-force–speed-based energy-consumption model (TFS model). The main concept of the first paper is to utilize the power-split hybrid powertrain’s accessible on-board diagnostics (OBD) dataset, and its dynamic model to estimate the total propulsion power. Then, propulsion power was applied as the main parameter for driving-power equation development and vehicle-specific coefficient calibration. For coefficient calibration, this study implemented the stepwise multiple regression method to select and calibrate an optimal set of coefficients. Results showed that conventional driving-power equations Vehicle-Specific Power (VSP) LDV 1999 and VSP Prius3Spec provide low prediction fidelity, especially under high-speed (>80 km/h) and heavy-load driving (≥50 kW). In contrast, D r v P w P r i u s 3 , proposed in this study, effectively improved prediction to become more accurate and reliable through all driving conditions and speed ranges. It dramatically helped to reduce prediction discrepancy over the conventional equations at heavy-load driving, from an R-square of 0.79 and 0.78 to 0.96. D r v P w P r i u s 3 also the prediction error at high-speed driving from the maximal error of approximately −20 to −5 kW. This study also discovered that aerodynamics and rolling resistance were the primary factors that caused the prediction error of conventional VSP equations. In addition, results in this study showed that both of the approaches used to establish the P P T d r v and D r v P w P r i u s 3 equations were valid for a power-split hybrid vehicle’s driving-power estimation. For the coefficient-calibration part, the stepwise and multiple regression method is low-cost and simple, allowing to calibrate an appropriate set of optimal coefficients for a specific vehicle model or fleet.

Active damping of driveline vibration in power-split hybrid vehicles based on model reference control

The power-split hybrid vehicles are widely used owing to its advantages of high efficiency and low emission. With the increasing of the requirements of the customer’s satisfaction nowadays, vibration control is getting progressively more attention. Since the transmission system components are elastic to some extent, meantime the torque of the motor responds quickly for a sudden acceleration, hence torsional vibration takes place, henceforth the passengers will suffer inconvenience attributable to this vibration. This paper studies the active control algorithm for the torsional vibration of the power-split hybrid powertrains under rapid acceleration and deceleration circumstances. Primarily, a dynamic modeling of the power-split hybrid vehicle is built, in addition the uncertainty caused by the variable transmission ratio is analyzed. Moreover, this paper proposes a method combining the model reference adaptive control and the pole configuration method to solve the torsional vibration active control problem with uncertainties. The reference model is constructed by pole configuration method. Based on the dynamic characteristics of the reference model, the model reference adaptive control is implemented and the torque ripple reduction of output shaft under the impact conditions is achieved. Furthermore, this paper designs a dynamic coordinated control algorithm combined with active vibration control strategy, which ensures the dynamic performance and comfort performance of the vehicle. Finally, the effectiveness of this active vibration control strategy under cyclic conditions is verified using Simulink simulation and hardware-in-loop simulation.

Investigation of mode transition coordination for power-split hybrid vehicles using dynamic surface control

For passenger cars propelled by the dedicated compound power-split hybrid powertrain, driveline oscillations-induced vehicle jerks are often excited during clutch-to-clutch shift operations while drive mode changes. To tackle this issue, a coordinated dynamic surface control is developed by integrating clutch slips and motor torque compensation strategies through trajectories tracking of both clutch slip speed and wheel speed. Uncertainties or disturbances are treated to be additional inputs of the system, and model nonlinearities are considered and implemented in discretized form through lookup tables. A complex simulation model including electro-hydraulic system is proposed and validated via experiments. The coordinated controller is validated by collaborative simulation. Numerical examples are made and simulation results verify that the controller is effective and robust enough against parameters uncertainties.

Fuel economy optimization of power split hybrid vehicles: A rapid dynamic programming approach

Fuel economy of hybrid vehicles is affected by their powertrain configurations, powertrain parameters, and energy management strategies. It is most beneficial to optimizing all the three factors simultaneously. However, when the design search space is large, an exhaustive, optimal control strategy, such as dynamic programming (DP), is too computationally expensive. Hence, a faster optimization method with higher computational efficiency and acceptable accuracy is required. Based on the DP approach, an approximate optimization method, called rapid dynamic programming (Rapid-DP), is developed and discussed in this paper. This method effectively reduces the decision-making time (the time can be reduced by a factor of 700, compared to the DP approach) for optimal control. The optimization processes and results are described and then compared with the original DP and PEARS + methods under two different driving cycles: FTP72 and HWFET. In conjunction with particle swarm optimization (PSO), the rapid-DP is leveraged, for the first time, to optimize key powertrain parameters for power split hybrid electric vehicles. Based on two power-split hybrids: Toyota Prius and Prius++, the joint optimization approach is exploited to examine vehicular fuel savings attributed to synergistic parameters optimization and operating-mode increase. The multi-mode configuration with optimal component parameters is demonstrated to be most fuel-efficient, with 6.56% and 3.15% fuel reductions under FTP72 and HWFET cycles, respectively, with respect to the original Prius 2010.

Configuration optimization for improving fuel efficiency of power split hybrid powertrains with a single planetary gear

In order to significantly reduce vehicular fuel consumption and emission pollutants, power-split hybrid electric vehicles are increasingly deployed to ensure that internal combustion engines work at their high-efficiency regions. Because of multiple power components (internal combustion engine, two electric machines, and a driveshaft to the wheels), configurations of such vehicular powertrains are typically very complicated. In order to systematically analyze and design a fuel-optimal powertrain, an innovative hierarchical topological graph approach is proposed. This method comprises four major design processes: (1) modeling of hybrid vehicle powertrain systems, (2) generation of a configuration pool, (3) identification of isomorphism, and (4) classification of configuration modes. Potential power-split hybrid vehicle designs are rigorously examined via a dynamic programming algorithm to estimate their acceleration performance (0–100 km/h) and fuel economy in various driving cycles.

Virtual Vehicle Control Concept for Hydrostatic Dynamometer Control1

An approach for controlling a hydrostatic dynamometer for the hardware-in-the-loop (HIL) testing of hybrid vehicles is proposed and experimentally evaluated. The hydrostatic dynamometer, which is capable of absorbing and regenerating power, was specifically designed and built in-house to evaluate the fuel economy and control strategy of a hydraulic hybrid vehicle being developed. Unlike a chassis dynamometer whose inertia is similar to the inertia of the vehicle being tested, the inertia of this hydrostatic dynamometer is only 3% of the actual vehicle. While this makes the system low cost, compact, and flexible for testing vehicles with different weights and drag characteristics, control challenges result. In particular, the dynamometer must apply, in addition to the torques to mimic the wind and road drag, also the torques to mimic the acceleration and deceleration of the missing inertia. To avoid estimating the acceleration and deceleration, which would be a noncausal operation, a virtual vehicle concept is introduced. The virtual vehicle model generates, in response to the applied vehicle torque, a reference speed profile which represents the behavior of the actual vehicle if driven on the road. This reformulates the dynamometer control problem into one of enabling the actual vehicle dynamometer shaft to track the speed of the virtual vehicle, instead of directly applying a desired torque. To track the virtual vehicle speed, a controller with feedforward and feedback components is designed using an experimentally validated dynamic model of the dynamometer. The approach has been successfully tested on a power-split hydraulic hybrid vehicle with acceptable virtual vehicle speed and dynamometer torque tracking performance.

Multibody dynamics analysis for the coupled vibrations of a power split hybrid electric vehicle during the engine start transition

Minimizing working time of the internal combustion engine is an important method for hybrid electric vehicles to save energy and reduce emissions. However, vibrations caused by frequent engine starts and stops deteriorate driveability and ride comfort. Especially in power split hybrid vehicles, there is no kinematically decoupled device between the engine and transmission, so that vibrations transfer from the powertrain to the car body through not only engine mounts but also the driveline system. Hardware test results show that the longitudinal vibration of seat track is the most severe with a maximum peak-to-peak acceleration of 1.8 m/s2. In this study, dynamic models of the driveline system and the powertrain mounting system are respectively established in detail. The natural characteristic analyses of the two dynamic models reveal that engine order vibrations can stimulate a series of low-frequency resonances during the engine starts, which is finally embodied in the car body vibrations. Then, two dynamic models are coupled together to analyze the transient dynamic characteristics associated with engine starts. It is evident by simulations that the longitudinal vibration of car body is mainly transferred by the driveline, whereas the vertical vibration of car body is mainly transferred by the powertrain mounting system. Besides, the initial crankshaft position of the gasoline engine has an obvious effect on the engine order vibration, and further results in different transient responses of the full vehicle system.

Design of Multimode Power-Split Hybrid Vehicles—A Case Study on the Voltec Powertrain System

Planetary gears (PGs) have been used in power-split hybrid and plug-in hybrid vehicles, sometimes with clutches to enable multimode operations. Chevrolet Volt is a well-known production vehicle using such technologies; it achieves excellent performance in both charge-depleting (CD) and charge-sustaining modes. In this paper, a detailed analysis of the General Motors (GM) Voltec powertrain used in the 2016 Chevrolet Volt (Volt Gen 2) is presented. We search through a large topological space, which includes a total of around 1 million design candidates to find different powertrain layouts that achieve the same or better acceleration and fuel economy in comparison with the benchmark: a simulated Volt Gen 2. The key technologies enabling such large-scale design exercise are discussed, and preliminary findings are reported. The case study results show that 49 designs achieve better charge-sustaining, CD, and launching performances than the benchmark, which clearly demonstrated the value of the proposed design methodology.

A Near-Optimal Power Management Strategy for Rapid Component Sizing of Multimode Power Split Hybrid Vehicles

In the design of hybrid vehicles, it is important to identify proper component sizes along with the optimal control. When the design search space is large, exhaustive optimal control strategies, such as dynamic programming (DP) is too time consuming to be used. Instead, a near-optimal method that is orders of magnitude faster than DP is needed. One such near-optimal method is developed and presented in this paper. This method is applied to identify the optimal powertrain parameters of all power-split hybrid configurations utilizing a single planetary gear. There are 12 possible configurations, six input and output splits, and each configuration has up to four modes. Based on the analysis of the efficiency of powertrain components of the four modes, and the power-weighted efficiency concept, we show that the computation time can be reduced by a factor of 10 000 without consequential performance compromise, compared with the DP approach. The optimal design of each configuration is analyzed and presented.

Not Just for Fuel Economy: A Systematic Design and Exhaustive Search for Hybrid Multimode Light Trucks

The Planetary Gear (PG) system has been widely employed by power-split hybrid and plug-in hybrid vehicles. The use of a PG system can enable multiple operating modes through the addition of clutches, as in the Chevrolet Volt. Besides passenger vehicles, the multi-mode hybrid powertrain can also be beneficial to heavy duty vehicles like pick-up trucks, since the benefits of each individual mode can be combined to meet complex driving requirement while maintaining desirable fuel economy. In this paper, a systematic design methodology is adopted for a hybrid pickup design problem using a PG system. This method involves automated modeling, a drivability screening procedure and the development of optimal energy management strategy. The design candidates are compared with the conventional F150 MY2012 and its conceptual parallel hybrid version using the same transmission and engine but an additional 60kW electric motor. The results show that after a search through over one million design candidates, 464 designs can achieve better performance on both fuel economy and launching, than not only the conventional F150, but also the parallel benchmark.

Engine Control of Power-Split Hybrid Vehicle

Planetary power-split hybrid vehicle is a very common structure of hybrid electric vehicle (HEV), which achieves speed and torque double decoupled between engine and wheels. Engine always working on optimal curve is an important reason for low fuel-efficient. Engine speed is controlled by PID, and the optimal loading rate of engine is solved from engine characteristic curves; finally realization of engine working on optimal curve. Offline simulation is used for this algorithm, and comparing with many experimental results demonstrates engine works on operating point; fuel consumption is close to the official fuel consumption of Prius.

Failure Mechanism of the Transmission Shaft of a New Power Split Hybrid Vehicle

Power split driveline ensures a hybrid vehicle good performance in fuel economy and emissions, but might bring about serious torsional vibrations at city traffic operation. A power split driveline is addressed in this paper to understand the mechanism behind the failure of its transmission shaft. Lumped-parameter models were established for the system at various engine and motor configurations corresponding to typical working conditions. Natural frequencies and mode shapes were calculated for each configuration. Sensitivity analysis reveals the variation of the natural frequencies with respect to the inertia or stiffness of the system. It is revealed that there exists unavoidable resonance with the system during vehicle operation, which should be the main reason of the shaft failure.

Minimizing Seat Track Vibration That is Caused by the Automatic Start/Stop of an Engine in a Power-Split Hybrid Electric Vehicle

The use of hybrid electric vehicles is an effective means of reducing pollution and improving fuel economy. Certain vehicle control strategies commonly automatically shut down or restart the internal combustion engines of hybrid vehicles to improve their fuel consumption. Such an engine autostart/stop is not engaged or controlled by the driver. Drivers often do not expect or prepare for noticeable vibrations, noise, or an unsmooth transition when the engine is autostarted/stopped. Unsmooth engine autostart/stop transitions can cause driveline vibrations, making the ride uncomfortable and the customer dissatisfied with the vehicle. This research simulates the dynamic behaviors associated with the neutral starting and stopping of a power-split hybrid vehicle. The seat track vibration results of analysis and hardware tests of the baseline control strategy are correlated. Several antivibration control strategies are studied. The results reveal that pulse cancellation and the use of a damper bypass clutch can effectively reduce the fluctuation of the engine block reaction torque and the vibration of the seat track by more than 70% during the autostarting and stopping of the engine. The initial crank angle can have an effect on the seat track vibration as well.

$\hbox{Prius}^{+}$ and $\hbox{Volt}^{-}$: Configuration Analysis of Power-Split Hybrid Vehicles With a Single Planetary Gear

The majority of hybrid electric vehicles (HEVs) available on the market are power-split hybrid vehicles with a single planetary gear (PG), including the popular Toyota Prius and Chevy Volt. Although both vehicles use a single PG, they have different configurations with different numbers of operating modes. The Prius has no clutch and has a single operating mode, whereas the Chevy Volt uses three clutches and has four modes. The goal of this paper is to present a thorough analysis of all possible configurations of power-split hybrid powertrain using a single PG. The analysis includes the following steps: 1) search for all possible ways to connect powertrain elements to the PG; 2) identify all potential locations for clutch installations around the PG and examine the feasibility of additional operating modes introduced by the clutch installation; and 3) optimize fuel economy for performance comparison. The proposed analysis shows that a single PG can produce 12 different configurations, each of which can have four feasible operating modes by adding three clutches to the PG. In case studies, we focus on the two configurations that are used in the Prius and Volt to find the impact of adding (or removing) clutches and modes on their fuel economy performance. Our results show that adding one clutch to the Prius transmission (which is named “Prius+”) can significantly improve fuel economy in urban driving, whereas removing two clutches from the Volt transmission (“Volt-”) will not significantly affect fuel economy in both urban and highway driving. This multimode configuration analysis can be used to systematically design future power-split HEVs.

Optimal Control of Hybrid Electric Vehicles Based on Pontryagin's Minimum Principle

A number of strategies for the power management of hybrid electric vehicles (HEVs) are proposed in the literature. A key challenge is to achieve near-optimality while keeping the methodology simple. The Pontryagin's minimum principle (PMP) is suggested as a viable real-time strategy. In this brief, the global optimality of the principle under reasonable assumptions is described from a mathematical viewpoint. Instantaneous optimal control with an appropriate equivalent parameter for battery usage is shown to be possibly a global optimal solution under the assumption that the internal resistance and open-circuit voltage of a battery are independent of the state-of-charge (SOC). This brief also demonstrates that the optimality of the equivalent consumption minimization strategy (ECMS) results from the close relation of ECMS to the optimal-control-theoretic concept of PMP. In static simulation for a power-split hybrid vehicle, the fuel economy of the vehicle using the control algorithm proposed in this brief is found to be very close-typically within 1%-to the fuel economy through global optimal control that is based on dynamic programming (DP).

A systematic design approach for two planetary gear split hybrid vehicles

Multiple power sources in a hybrid vehicle allow for flexible vehicle power-train operations, but also impose kinematic constraints due to component characteristics. This paper presents a design process that enables systematic search and screening through all three major dimensions of hybrid vehicle designs – system configuration, component sizing and control, to achieve optimal performance while satisfying the imposed constraints. An automated dynamic modelling method is first developed which enables the construction of hybrid vehicle model efficiently. A screening process then narrows down to configurations that satisfy drivability and operation constraints. Finally, a design and control optimisation strategy is carried out to obtain the best execution of each configuration. A case study for the design of a power-split hybrid vehicle with optimal fuel economy is used to demonstrate this overall hybrid vehicle design process.

Modelling Hybrid Automotive Systems with the POG Technique

In this paper the Power-Oriented Graphs (POG) technique is used for modelling planetary gears and hybrid automotive systems. The POG technique allows to graphically describe the dynamic model of any type of physical system (in different energetic domains) putting in evidence the power flows within the modelled systems. The POG schemes are easy to use, easy to understand even by a neophyte and can be directly implemented in Simulink. Because of the POG modular structure, complex physical systems can be modelled by composing subsystems models. Following this approach a whole power-split hybrid vehicle is modelled, starting from modelling its main subsystems: planetary gear, multi-phase synchronous motor and vehicle dynamics. Simulation results of the modelled hybrid system are reported at the end of the paper.

Modeling and Control of a Power-Split Hybrid Vehicle

Toyota hybrid system (THS) is used in the current best selling hybrid vehicle on the market-the Toyota Prius. This hybrid system contains a power-split planetary gear system which combines the benefits of series and parallel hybrid vehicles. In this paper, we developed a dynamic model of the THS powertrain and then apply it for model-based control development. Two control algorithms are introduced: one based on the stochastic dynamic programming method, and the other based on the equivalent consumption minimization strategy. Both approaches determine the engine power based on the overall vehicle efficiency and apply the electrical machines to optimize the engine operation. The performance of these two algorithms is assessed by comparing against the dynamic programming results, which are non-causal but provide theoretical benchmarks for other implementable control algorithms.

Automated Modelling of Power-Split Hybrid Vehicles

Hybrid electric vehicles (HEV) represent a promising technology to improve the fuel economy of ground vehicles in the near-term. Among the HEV configurations, the power-split configuration offers superior design and control flexibility and achieves highest overall efficiency. In this paper, a methodology to generate dynamic equations automatically for the power-split hybrid power-train is proposed. The designer only needs to specify the topology of the transmission: how the power sources, vehicle, the planetary gears and clutches are connected to each other. The dynamic model can then be generated automatically through a set of rules. This automated generation process makes it possible for a designer to explore different split hybrid configurations quickly and efficiently.