As an automotive engineer deeply involved in the development of electrified powertrains, I have had the opportunity to analyze and appreciate the groundbreaking design of the HF35 transmission, which is central to the Ford Mondeo hybrid electric vehicle. This hybrid electric vehicle represents a significant leap forward in combining fuel efficiency with dynamic performance, largely due to this innovative electrically controlled continuous variable transmission. In this article, I will provide a comprehensive, first-person perspective on the structure, components, operating principles, and benefits of the HF35 transmission, emphasizing its role in advancing hybrid electric vehicle technology. I will incorporate numerous tables and formulas to summarize key aspects, ensuring a detailed exploration that exceeds 8000 tokens in length.
Hybrid electric vehicles are at the forefront of reducing emissions and improving energy efficiency in the automotive sector. The HF35 transmission, specifically designed for such hybrid electric vehicles, replaces conventional pulley-based CVTs with an electromechanical system that enhances flexibility and efficiency. From my analysis, this transmission is a key enabler for the impressive fuel economy and performance of the Mondeo hybrid electric vehicle. I will begin by examining its structural composition.
The HF35 transmission features a compact and integrated design, weighing approximately 102.5 kg. Unlike traditional automatic transmissions or CVTs, it utilizes a single planetary gear set and two electric motors to achieve continuous variable ratios without clutches or brakes. This design is particularly suited for hybrid electric vehicles, as it allows seamless power splitting between the internal combustion engine and electric motors. The main components can be summarized in the following table:
| Component | Description | Function in Hybrid Electric Vehicle |
|---|---|---|
| Starter/Generator Motor | Permanent magnet synchronous motor with stator coils, rotor, and temperature sensor. | Dual-function: starts the engine and generates electricity to charge the high-voltage battery. |
| Drive Motor | Identical in structure to starter/generator, but connected to the output path. | Propels the vehicle in electric modes and acts as a generator during regenerative braking. |
| Planetary Gear Set | Simple single planetary gear arrangement with sun gear, ring gear, and planetary carrier. | Provides power-splitting mechanism for continuous variable transmission; no clutches required. |
| Intermediate Shaft | High-strength shaft with fixed gears, including connections to ring gear and drive motor. | Transmits torque from planetary set and motors to differential and drive wheels. |
| Oil Pump and Filter | Integrated assembly driven by gears from planetary carrier. | Ensures lubrication and cooling for transmission components. |
| Parking Lock Mechanism | Mechanism engaging with gear on ring gear unit. | Secures the hybrid electric vehicle when parked. |
In my detailed examination, each component plays a critical role. The starter/generator and drive motors are both three-phase AC permanent magnet synchronous motors. Their operation can be described by fundamental electromechanical equations. For instance, the torque \(T_m\) produced by these motors is proportional to the quadrature axis current \(I_q\): $$T_m = k_t \cdot I_q$$ where \(k_t\) is the torque constant. The mechanical power \(P_m\) is given by: $$P_m = T_m \cdot \omega_m$$ where \(\omega_m\) is the angular velocity. These motors are controlled by power inverters that convert DC from the high-voltage battery to AC, using insulated-gate bipolar transistors (IGBTs) for efficient switching. The drive motor has a maximum power of 92 kW, providing high torque at low speeds—a crucial advantage for hybrid electric vehicles during launch.
The planetary gear set is the core of the HF35 transmission’s power-splitting capability. Its kinematic relationship is expressed by: $$\omega_s + k \omega_r = (1 + k) \omega_c$$ where \(\omega_s\), \(\omega_r\), and \(\omega_c\) are the angular velocities of the sun gear, ring gear, and planetary carrier, respectively, and \(k\) is the gear ratio (ring gear teeth divided by sun gear teeth). In this hybrid electric vehicle, the sun gear connects to the starter/generator, the ring gear to the drive motor and output, and the planetary carrier to the engine. This configuration allows infinite variability in speed ratios by controlling the electric motors’ speeds and torques.
The intermediate shaft is vital for torque transmission. It includes gears that mesh with the ring gear and drive motor, forming a reduction gear set. The shaft’s design ensures durability under high loads, which is essential for the hybrid electric vehicle’s diverse driving conditions. The power flow through the transmission can be modeled using energy balance equations. For example, the total output power \(P_{out}\) to the wheels is the sum of mechanical power from the engine and electrical power from the motors, accounting for efficiencies \(\eta\): $$P_{out} = \eta_m P_{eng} + \eta_e P_{elec}$$ where \(P_{eng}\) is engine power and \(P_{elec}\) is net electrical power from the motors and battery.
Now, let’s delve into the operating modes of the HF35 transmission in the hybrid electric vehicle. These modes are optimized for different driving scenarios, contributing to the vehicle’s overall efficiency. Based on my analysis, there are five primary modes, each with distinct power flows and control strategies. The following table summarizes these modes:
| Mode | Engine State | Starter/Generator | Drive Motor | Battery Involvement | Primary Power Flow |
|---|---|---|---|---|---|
| Pure Electric Drive | Off | Off or idle | Motoring | Discharging | Battery → Drive Motor → Wheels |
| Hybrid Drive | On | Generating | Motoring | Minimal or none | Engine → Planetary Gear → (Mechanical path to wheels) and (Electrical path via Generator → Drive Motor → Wheels) |
| Electric Assist | On | Generating | Motoring | Discharging | Engine + Battery → Drive Motor → Wheels |
| Parking Charge | On | Generating | Off | Charging | Engine → Generator → Battery |
| Regenerative Braking | Off (fuel cut) | Off | Generating | Charging | Wheels → Drive Motor → Battery |
In pure electric drive mode, the hybrid electric vehicle operates solely on electric power, ideal for low-speed urban driving. The drive motor is powered by the high-voltage battery, with the engine off. The maximum speed in this mode is limited to 137 km/h to protect components. The energy conversion efficiency \(\eta_{elec}\) can be expressed as: $$\eta_{elec} = \frac{P_{wheels}}{P_{batt}}$$ where \(P_{batt}\) is battery output power. This mode highlights the zero-emission capability of hybrid electric vehicles when the battery is sufficiently charged.
Hybrid drive mode engages both the engine and electric motors. Here, the engine power is split through the planetary gear set: part goes mechanically to the wheels via the ring gear, and part is converted to electricity by the starter/generator to power the drive motor. This mode is used during moderate acceleration or high power demands. The overall system efficiency \(\eta_{hybrid}\) can be derived from: $$\eta_{hybrid} = \frac{P_{wheels}}{P_{fuel} + P_{batt,net}}$$ where \(P_{fuel}\) is fuel power input and \(P_{batt,net}\) is net battery power (positive if discharging). In this hybrid electric vehicle, the transmission enables a total system power of 136 kW, achieving 0-100 km/h in 9.7 seconds.
Electric assist mode is similar to hybrid drive but with additional power from the battery to boost performance. This mode is activated during hard acceleration, where both the engine and battery supply energy to the drive motor. The power balance is: $$P_{drive motor} = P_{generator} + P_{battery}$$ This enhances torque output without sacrificing efficiency, a key advantage for hybrid electric vehicles in dynamic driving conditions.
Parking charge mode allows the hybrid electric vehicle to charge its high-voltage battery while stationary. The engine runs to drive the starter/generator, which produces electricity for the battery. This mode is useful when the battery is low and external charging is unavailable. The charging efficiency \(\eta_{charge}\) depends on generator and battery characteristics: $$\eta_{charge} = \frac{P_{batt,charge}}{P_{eng} \cdot \eta_{gen}}$$ where \(P_{batt,charge}\) is power delivered to the battery and \(\eta_{gen}\) is generator efficiency.
Regenerative braking mode recovers kinetic energy during deceleration. The drive motor acts as a generator, converting vehicle motion into electrical energy stored in the battery. The regenerative braking force \(F_{reg}\) is given by: $$F_{reg} = \frac{T_m \cdot \eta_{gen}}{r_w}$$ where \(T_m\) is motor torque, \(\eta_{gen}\) is generation efficiency, and \(r_w\) is wheel radius. In this hybrid electric vehicle, regenerative braking is managed in three sub-modes based on braking demand and battery state, as shown in the table below:
| Sub-mode | Condition | Braking Method | Energy Recovery Efficiency |
|---|---|---|---|
| Mode 1 | Low braking demand | Regenerative braking only | High, typically over 70% |
| Mode 2 | High braking demand | Regenerative braking plus friction braking | Moderate, depends on blend |
| Mode 3 | Battery full or ABS active | Friction braking only | Zero recovery |
The control strategy for these modes is orchestrated by the engine control unit (ECU) and transmission control unit (TCU). They use algorithms, such as equivalent consumption minimization strategy (ECMS), to optimize power distribution. The cost function \(J\) minimized over time \(T\) is: $$J = \int_{0}^{T} (\dot{m}_{fuel} + s \cdot P_{batt}) dt$$ where \(\dot{m}_{fuel}\) is fuel flow rate, \(P_{batt}\) is battery power, and \(s\) is an equivalence factor. This ensures that the hybrid electric vehicle operates at peak efficiency across diverse scenarios.
To further illustrate the performance benefits, I have compiled key parameters of the Ford Mondeo hybrid electric vehicle with HF35 transmission in the table below. These metrics underscore the synergy between the transmission and the Atkinson-cycle engine, which has a thermal efficiency of 40%.
| Parameter | Value | Unit | Formula/Notes |
|---|---|---|---|
| Engine Thermal Efficiency | 40% | – | \(\eta_{eng} = 1 – \frac{1}{r^{\gamma-1}} \frac{\alpha^{\gamma} – 1}{\gamma(\alpha – 1)}\) for ideal Atkinson cycle |
| Drive Motor Max Power | 92 | kW | \(P_{motor,max} = T_{max} \cdot \omega_{max}\) |
| Total System Power | 136 | kW | \(P_{total} = P_{eng} + P_{motor}\) at optimal point |
| 0-100 km/h Acceleration | 9.7 | s | Derived from torque and mass: \(a = \frac{T_{net}}{m \cdot r_w}\) |
| Combined Fuel Consumption | 4.2 | L/100 km | Calculated over standard driving cycles |
| Electric-Only Top Speed | 137 | km/h | Limited by motor and battery capabilities |
| Transmission Weight | 102.5 | kg | Contributes to overall vehicle mass |
| Battery Voltage Range | 300-600 | V | High voltage for efficient power transfer |
The Atkinson-cycle engine, with its high expansion ratio, is well-suited for hybrid electric vehicles because it prioritizes efficiency over low-speed torque. The electric motors compensate for the engine’s torque deficit at low speeds, a synergy that enhances overall powertrain efficiency. The combined efficiency \(\eta_{combined}\) over a driving cycle can be estimated as: $$\eta_{combined} = \sum_{i} t_i \eta_i$$ where \(t_i\) is the time fraction in mode \(i\) and \(\eta_i\) is the efficiency of that mode. This hybrid electric vehicle achieves a combined fuel economy of 4.2 L/100 km, showcasing the effectiveness of the HF35 transmission.
From a comparative perspective, the HF35 transmission offers distinct advantages over traditional CVTs and other hybrid systems. Traditional CVTs use pulleys and belts, which incur friction losses and limit torque capacity. In contrast, the HF35’s electromechanical design reduces losses and enables seamless mode transitions. The table below highlights these differences:
| Feature | HF35 Transmission | Traditional CVT | Typical Automatic Transmission |
|---|---|---|---|
| Mechanism | Planetary gear set + electric motors | Pulleys and belt/chain | Planetary gears with clutches/brakes |
| Efficiency at Low Load | High due to electric path | Moderate due to belt slip | Lower due to hydraulic losses |
| Torque Capacity | High, supported by motors | Limited by belt strength | High but with shift interruptions |
| Suitability for Hybrid Electric Vehicles | Excellent, inherent power-split | Poor, requires add-on motors | Fair, often used in mild hybrids |
| Maintenance Requirements | Low, no clutches to wear | High, belt replacement needed | Moderate, clutch and fluid changes |
In my analysis, the HF35 transmission also excels in packaging and integration. Its compact design allows for efficient use of space in the hybrid electric vehicle, accommodating the high-voltage battery and auxiliary systems. The absence of clutches simplifies the control algorithm and reduces potential failure points. However, challenges include managing thermal loads on the electric motors and ensuring durability of the planetary gear set under continuous variable loading. Future developments may involve integrating the HF35 with plug-in hybrid systems to extend electric-only range, further enhancing the appeal of hybrid electric vehicles.
The dynamic modeling of the HF35 transmission involves solving equations of motion for the planetary gear set. For instance, the torques and inertias can be related through: $$J_s \dot{\omega}_s = T_s – r_s F$$ $$J_r \dot{\omega}_r = T_r – r_r F$$ $$J_c \dot{\omega}_c = T_c – r_c F$$ where \(J\) are moments of inertia, \(T\) are applied torques, \(r\) are pitch radii, and \(F\) is the internal mesh force. These equations, combined with motor models, enable simulation of the hybrid electric vehicle’s response under various driving conditions.
Energy management is crucial for maximizing the benefits of the HF35 transmission. Besides ECMS, other strategies like rule-based control or model predictive control can be employed. The goal is to minimize fuel consumption while maintaining battery state of charge within optimal bounds. For a hybrid electric vehicle, the overall energy efficiency \(\eta_{overall}\) over a trip can be defined as: $$\eta_{overall} = \frac{E_{wheels}}{E_{fuel} + E_{batt,initial}}$$ where \(E_{wheels}\) is energy delivered to wheels, \(E_{fuel}\) is energy from fuel, and \(E_{batt,initial}\) is initial battery energy (assuming grid charging is considered for plug-in variants).
In conclusion, the HF35 transmission is a transformative technology for hybrid electric vehicles. Its innovative use of a planetary gear set and dual electric motors enables efficient power splitting, seamless mode transitions, and impressive fuel economy. From my engineering perspective, this transmission exemplifies how mechanical and electrical systems can be harmonized to advance hybrid electric vehicle performance. The Ford Mondeo hybrid electric vehicle, with its HF35 transmission, sets a benchmark for future developments in the automotive industry. As hybrid electric vehicles continue to evolve, transmissions like the HF35 will play a pivotal role in achieving sustainability goals without compromising driving pleasure.