5.3 Continuously Variable Transmissions (CVT) & Hybrid Drivetrains

Continuously Variable Transmissions (CVT) & Hybrid Drivetrains

A continuously variable transmission (CVT) provides an infinite number of gear ratios within its design range, maximizing engine efficiency. In hybrid vehicles, this continuous speed variation is often achieved electromechanically via a power-split planetary system integrating high-voltage motor-generators.

Continuously Variable Transmission (CVT) Principles

Unlike geared transmissions, a steel-belt or chain-drive CVT utilizes two variable-diameter pulleys connected by a metal push belt or a heavy-duty chain. The pulleys are designated as the drive (primary) pulley (connected to the engine) and the driven (secondary) pulley (connected to the final drive).

Pulley and Belt Mechanics

Each pulley consists of two V-shaped sheaves: one fixed sheave and one sliding sheave that moves axially. Hydraulic pressure, controlled by electronic solenoids and a transmission controller, moves the sliding sheave to change the pulley width.

• Low Ratio (Underdrive/Starting): The drive pulley sheaves are spread wide apart, forcing the belt to ride low in the groove (small diameter). The driven pulley sheaves are clamped tight, forcing the belt to ride high (large diameter).
• High Ratio (Overdrive/Cruising): The drive pulley sheaves are clamped tight, forcing the belt to ride high (large diameter). The driven pulley sheaves are spread apart, allowing the belt to sink low in the groove (small diameter).

Lubrication Requirements

CVTs require specialized CVT fluid that differs chemically from conventional automatic transmission fluid (ATF). While ATF is formulated to allow controlled clutch slip, CVT fluid must provide a high coefficient of friction (boundary lubrication) between the steel belt and the steel pulley faces to prevent metal-on-metal slippage under extreme clamping pressures (which can exceed 800 psi / 5,500 kPa). Using incorrect fluid results in rapid belt slippage, pulley scoring, and catastrophic transmission failure.

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Hybrid Drivetrain Architectures and eCVT Power-Split Systems

Hybrid electric vehicles combine an internal combustion engine (ICE) with high-voltage electric motor-generators. Architectures are classified into three types:

1. Series Hybrid: The engine drives a generator to produce electricity, which powers the electric traction motor that drives the wheels. The engine has no mechanical connection to the drivetrain.
2. Parallel Hybrid: Both the engine and the electric motor are mechanically connected to the drive wheels. They can drive the vehicle individually or together.
3. Series-Parallel (Power-Split/eCVT): Can operate in series or parallel modes. This layout replaces the conventional transmission with a Power-Split Device (PSD), which is a single planetary gearset.

Power-Split eCVT Power Flow

In a power-split system (such as the Toyota Hybrid System), the mechanical components are connected to the planetary gearset as follows:

• Planet Carrier: Connected directly to the output shaft of the internal combustion engine (ICE).
• Sun Gear: Connected to Motor-Generator 1 (MG1). MG1 acts as the engine starter, controls engine speed, and acts as a generator to charge the battery.
• Ring Gear: Connected to Motor-Generator 2 (MG2) and the final drive assembly. MG2 is the main traction motor that drives the wheels and performs regenerative braking.

By dynamically controlling the rotational speed and torque of MG1, the hybrid control module can continuously vary the engine's speed relative to the vehicle's road speed, creating an electromechanical continuously variable transmission (eCVT).

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High-Voltage Safety, PPE, and Isolation Procedures

High-voltage (HV) hybrid and electric vehicle systems operate at lethal voltages, typically between 200V DC and 650V DC (or higher). All high-voltage cables and conduits are colored orange for easy identification.

Personal Protective Equipment (PPE)

When working on or near high-voltage systems, technicians must wear:

• Class 0 Insulated Rubber Gloves: Rated for 1,000V AC / 1,500V DC.
• Leather Outer Protectors: Worn over the rubber gloves to protect them from physical punctures, cuts, or abrasions.
• Air Roll Test: Before each use, the technician must inspect the rubber gloves by rolling the glove cuff toward the fingers to trap air. Squeeze the inflated glove and listen/feel for leaks. If any leakage, cracking, or damage is detected, the gloves must be discarded.

De-Energization & Capacitor Discharge

Before servicing any high-voltage component:

1. Turn the ignition off, remove the key/fob, and store it at least 5 meters away.
2. Disconnect the auxiliary 12-volt battery negative terminal. This disables the low-voltage control circuit that powers the high-voltage System Main Relays (SMRs).
3. Put on Class 0 insulated gloves.
4. Remove the high-voltage safety service plug (or safety disconnect). Secure the plug in a lockout/tagout box.
5. Wait 5 to 10 minutes to allow the high-voltage capacitors inside the inverter/converter assembly to discharge through internal bleed resistors.

Zero-Voltage Verification (Live-Dead-Live Test)

To verify that the system is de-energized, use a Category III (1,000V) or Category IV (600V) digital multimeter (DMM) with insulated leads:

1. Verify the DMM is functioning by testing it on a known live low-voltage source (like the 12V battery).
2. Measure the voltage between the high-voltage positive terminal and chassis ground.
3. Measure the voltage between the high-voltage negative terminal and chassis ground.
4. Measure the voltage between the high-voltage positive and negative terminals.
5. Verify all readings are less than 12V DC (ideally 0.0V).
6. Re-test the DMM on the same known live low-voltage source to ensure the meter did not fail in a closed or non-reading state.

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Hybrid Diagnostics and Insulation Testing

A common hybrid system fault is a loss of isolation (ground fault), which occurs when the high-voltage insulation on a cable, inverter, or motor-generator winding breaks down, allowing voltage to leak to the vehicle chassis.

To diagnose isolation faults:

• Insulation Resistance Tester (Megohmmeter): Traditional multimeters only output 9V and cannot detect insulation breakdown under operating loads. A megohmmeter (megger) applies a high voltage (typically 500V or 1,000V DC) to test the insulation resistance of the motor-generator stator windings and high-voltage cables. The resistance must typically be greater than 10 Megohms (or manufacturer spec) to be considered safe.