Abstract
This study focuses on designing the transmission ratio for a two-speed gearbox in an electric vehicle (EV) and validating its performance via simulation using ADVISOR software. By comparing the vehicle dynamics and energy efficiency of a fixed-ratio reducer with the designed two-speed transmission, the results demonstrate that the two-speed configuration significantly enhances the EV’s power performance and economic efficiency. The findings provide a critical basis for optimizing EV powertrain design.
Keywords: electric vehicle, ADVISOR, transmission ratio, powertrain, simulation
1. Introduction
Electric vehicles (EVs) have gained substantial traction due to their zero emissions, rapid acceleration, low noise, and flexible motor speed regulation capabilities . However, most current EVs employ fixed-ratio transmissions, which impose higher technical requirements on the motor and battery to adapt to diverse driving conditions . This limitation motivates the design of a two-speed transmission to improve drivability and energy efficiency.
Previous research has highlighted the benefits of multi-speed transmissions in EVs. For instance, studies have shown that two-speed gearboxes can enhance vehicle performance by matching motor operation to specific driving scenarios, such as climbing or high-speed cruising . Scholars like Liu et al. (2019) and Huang et al. (2019) have used ADVISOR to simulate and optimize transmission ratios, proving that multi-speed systems outperform fixed ratios in power and efficiency , .
2. Vehicle Specifications
The research focuses on a specific EV model, with its key parameters listed in Table 1. These specifications are crucial for determining the transmission ratio requirements, including dimensions, mass, aerodynamic coefficients, and motor performance –.
Table 1: Basic Parameters of the Electric Vehicle
| Technical Parameter | Value |
|---|---|
| Length × Width × Height (mm) | 4995 × 1910 × 1495 |
| Wheelbase (mm) | 2920 |
| Front/Rear Track (mm) | 1650 / 1630 |
| Curb Mass (kg) | 2100 |
| Gross Mass (kg) | 2475 |
| Drag Coefficient (\(C_D\)) | 0.23 |
| Frontal Area (A, m²) | 2.2 |
| Wheel Rolling Radius (r, mm) | 400.55 |
| Rotational Inertia Conversion Coefficient (\(\delta\)) | 1.1 |
| Rolling Resistance Coefficient (f) | 0.02 |
| Mechanical Transmission Efficiency (\(\eta_T\)) | 0.92 |
| Rated Torque (\(T_{额}\), N·m) | 130 |
| Peak Torque (\(T_{max}\), N·m) | 330 |
| Rated Speed (\(n_{额}\), r/min) | 4400 |
| Maximum Speed (\(n_{max}\), r/min) | 15500 |
| Top Vehicle Speed (\(v_{max}\), km/h) | 100 |
| Maximum Gradeability (%) | 30 |
3. Design of Two-Speed Transmission Ratio
3.1 Design Principles
The two-speed transmission is designed to address the limitations of fixed-ratio systems, especially in scenarios requiring high torque (e.g., starting, climbing) or high speed (e.g., highway cruising). The transmission ratios must satisfy the EV’s power performance requirements, including top speed, maximum gradeability, and acceleration time –.
3.2 Calculation of Gear Ratios
3.2.1 First Gear Ratio (\(i_{g1}\))
- Lower Limit: Determined by the maximum gradeability requirement under the motor’s peak torque:\(i_{g1} \cdot i_0 \geq \frac{r}{T_{max} \cdot \eta_T} \left( mgf \cos\alpha + mg \sin\alpha + \frac{C_D A v^2}{21.15} \right)\) where \(i_0\) is the final drive ratio, g is gravitational acceleration, and \(\alpha\) is the maximum climbing angle .
- Upper Limit: Constrained by the minimum stable speed during climbing:\(i_0 \cdot i_{g1} \leq \frac{0.377 \cdot n_{max} \cdot r}{v}\) where v is the vehicle speed .
3.2.2 Second Gear Ratio (\(i_{g2}\))
- Lower Limit: Calculated based on resistance at top speed:\(i_{g2} \cdot i_0 \geq \frac{r}{T_{max} \cdot \eta_T} \left( mgf + \frac{C_D A v_{max}^2}{21.15} \right)\) .
- Upper Limit: Determined by the top speed constraint:\(i_{g2} \cdot i_0 \leq \frac{0.377 \cdot n_{max} \cdot r}{v_{max}}\) .
3.2.3 Additional Constraints
- Traction Limit: To prevent wheel slip, the transmission ratio must satisfy:\(i_g \cdot i_0 \leq \frac{mg r \varphi}{T_{max} \cdot \eta_T}\) where \(\varphi = 0.7\) is the road adhesion coefficient .
- Smooth Shifting: The ratio distribution must ensure continuous power transfer:\(\frac{i_{g1}}{i_{g2}} \leq \frac{n_{max}}{n_{额}}\) .
3.3 Final Ratio Selection
By substituting the vehicle parameters from Table 1 into the above equations, the designed transmission ratios are:
- Final drive ratio (\(i_0\)): 3.9
- First gear ratio (\(i_{g1}\)): 2.4
- Second gear ratio (\(i_{g2}\)): 1.6 .
4. ADVISOR-Based Simulation Model
4.1 Model Setup
The EV simulation model is established in ADVISOR by selecting the EV template in the GUI interface (Metric units). The model components include the vehicle, energy storage system, motor, wheel/axle, powertrain controller, and transmission, as listed in Table 2 , –.
Table 2: EV Model Components in ADVISOR
| Component | Parameter |
|---|---|
| Vehicle | VEH_SMCAR |
| Energy Storage | ESS_LI7_temp |
| Motor | MC_AC83 |
| Wheel/Axle | WH_SMCAR |
| Powertrain Control | PTC_EV |
| Transmission | TX1_sd (fixed ratio) and custom two-speed |
4.2 Simulation Parameters
For the two-speed transmission, the ratios (2.4 and 1.6) and final drive ratio (3.9) are input. The fixed-ratio model uses \(i_0 = 3.9\) alone. The urban driving cycle (CYC_UDDS) is selected for simulation, with one cycle period –.
5. Simulation Results and Analysis
5.1 Vehicle Speed Performance
The top speed simulation results (Figure 1) show:
- Fixed ratio: 97 km/h
- Two-speed: 99 km/h (2.0% increase) , .
This improvement indicates that the two-speed transmission better utilizes the motor’s high-speed range, enhancing highway drivability.
5.2 Battery Energy Efficiency
The battery state of charge (SOC) results (Figure 1) show:
- Fixed ratio: Final SOC = 0.85
- Two-speed: Final SOC = 0.88 (3.5% higher) , .
The two-speed system reduces energy consumption by optimizing motor operation in efficient zones, extending driving range.
Table 3: Comparison of Top Speed and Battery SOC
| Performance Indicator | Fixed Ratio | Two-Speed | Improvement |
|---|---|---|---|
| Top Speed (km/h) | 97 | 99 | 2.0% |
| Final Battery SOC | 0.85 | 0.88 | 3.5% |
5.3 Transmission and System Efficiency
- Transmission Efficiency: Fixed ratio = 0.95; two-speed = 0.97 (2% higher) .
- System Efficiency: Fixed ratio = 0.424; two-speed = 0.446 (5% higher) .
The two-speed transmission enhances efficiency by allowing the motor to operate in high-efficiency zones more frequently, as shown in Figure 2 and Figure 3.
Table 4: Efficiency Comparison
| Efficiency Type | Fixed Ratio | Two-Speed | Improvement |
|---|---|---|---|
| Transmission Efficiency | 0.95 | 0.97 | 2% |
| System Efficiency | 0.424 | 0.446 | 5% |
6. Conclusion
This study demonstrates that a two-speed transmission significantly improves an electric vehicle’s power performance and energy efficiency compared to a fixed-ratio system. Key findings include:
- The two-speed design increases top speed by 2.0% and extends battery life by 3.5% , .
- System efficiency is enhanced by 5%, primarily due to optimized motor operation in high-efficiency regions .
- The designed ratios (2.4 and 1.6) with a final drive ratio of 3.9 effectively balance torque and speed requirements for various driving conditions .
Future work will focus on refining the shift control strategy to further optimize energy management and driving comfort in electric vehicles.