3.2 Auto Stop-Start Logic, Cranking Control & Restart Sequencing via MG1
Key Takeaways
- Hybrid auto-stop logic evaluates a comprehensive decision matrix including Engine Coolant Temperature (>40°C-60°C), HV battery State of Charge (>40%), vehicle speed, brake pedal line pressure, catalyst temperature (>350°C-400°C), and cabin HVAC thermal demand.
- Engine cranking in power-split hybrids is performed by high-voltage Motor-Generator 1 (MG1), which spins the ICE up to 800-1,200 RPM in under 300 milliseconds before fuel injection and spark are enabled.
- MG1 rapidly sweeps the ICE through its mechanical resonant vibration frequency band (300-500 RPM) to completely eliminate the shudder and jerk characteristic of conventional 12V starter systems.
- Decompression valve timing strategy utilizes the intake cam phaser (VVT-iE) to hold intake valves open during initial spin-up, minimizing cranking torque requirements and HV battery current draw.
- Torsional driveline dampening is achieved through dual-mass drive dampers with tuned internal springs, active counter-phase motor torque pulsation control, and electronically controlled engine mounts.
Auto Stop-Start Logic, Cranking Control & Restart Sequencing via MG1
One of the primary fuel-saving mechanisms of a hybrid electric vehicle is the complete elimination of internal combustion engine (ICE) idling. In urban stop-and-go driving, a conventional vehicle wastes up to 20% to 30% of its total fuel consumption idling at traffic lights, in congestion, and during deceleration. Hybrid powertrains utilize automatic engine stop-start strategies to shut down the ICE whenever vehicle propulsion is unnecessary or can be handled solely by the high-voltage electric drive system.
However, executing seamless engine stops and restarts dozens of times per drive cycle without customer-perceptible shudder, vibration, or driveline hesitation requires intricate coordination between the Hybrid Powertrain Control Module (HV-ECU), the Engine Control Module (ECM), the Inverter, and Motor-Generator 1 (MG1).
1. Engine Auto-Stop Decision Matrix & Inhibit Criteria
The HV-ECU and ECM continuously evaluate multiple vehicle operating parameters to determine whether the ICE is permitted to shut down. Auto-stop is not simply a function of vehicle speed; it is governed by a strict multi-variable decision matrix.
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| HYBRID ENGINE AUTO-STOP DECISION MATRIX |
| |
| [ VEHICLE SENSORS / STATUS ] |
| - Engine Coolant Temp (ECT) ---------> [ ECT > 40°C - 60°C? ] -------\ |
| - Hybrid Battery State of Charge ----> [ HV SOC > 40% - 45%? ] -----\ \ |
| - Vehicle Speed & Deceleration ------> [ Speed <= Threshold? ] -----\ \ \ |
| - Brake Master Cylinder Pressure ----> [ Brake Applied? ] ---------\ \ \ \ |
| - Catalytic Converter Temp ----------> [ Cat Temp > 400°C? ] -----\ \ \ \ \ |
| - Cabin HVAC Heating / Cooling ------> [ HVAC Demand Met? ] -----\ \ \ \ \ \ |
| - Fuel Tank Vapor Purge Status ------> [ EVAP Purge Inactive? ] -\ \ \ \ \ \ \ |
| v v v v v v v |
| +---------------+ |
| | ALL CONDITIONS| |
| | MET? | |
| +---------------+ |
| / \ |
| YES / \ NO|
| v v |
| [AUTO-STOP] [INHIBIT|
| (Fuel/Spark ENGINE |
| Cut Off) RUNS] |
+-----------------------------------------------------------------------------------+
Primary Auto-Stop Inhibit Parameters:
-
Engine Coolant Temperature (ECT):
- During cold start, the engine must reach a calibrated minimum operating threshold (typically 40°C to 60°C / 104°F to 140°F) to ensure proper oil circulation, fuel atomization, and passenger cabin heat.
- Modern hybrids categorize engine warm-up into stages (e.g., Stage 1 Catalyst Warm-Up, Stage 2 Engine Warm-Up). During Stage 1, the ICE will run continuously even if the vehicle is stopped with the brake applied.
-
High-Voltage Battery State of Charge (SOC):
- If the HV battery SOC drops below the calibrated lower operating boundary (typically 40% to 45%), auto-stop is strictly inhibited. The ICE is forced to run—even at a standstill in Park or Drive—to drive MG1 and charge the battery pack back into its nominal 50%–65% target operating window.
-
Catalytic Converter Temperature & Emissions Compliance:
- To meet ultra-low emissions standards (SULEV30 / Tier 3 Bin 30), the three-way catalytic converter must maintain its light-off temperature (350°C to 450°C / 662°F to 842°F). If frequent auto-stops cause catalyst core temperature to drop toward the threshold where conversion efficiency falls, the ECM commands a brief engine run cycle with retarded ignition timing to heat the catalyst substrate.
-
Cabin HVAC Climate Demand:
- Heating: In cold weather, if the driver selects a high cabin temperature, the engine must run to circulate hot coolant through the heater core unless the vehicle is equipped with an auxiliary high-voltage PTC coolant heater or heat pump.
- Defrost / Defog: Selecting Windshield Defrost on the climate control panel overrides auto-stop on many platforms to ensure immediate defogging capacity.
- Air Conditioning: If the high-voltage A/C compressor load draws significant battery power, causing SOC to trend downward rapidly, the ICE will restart to supply electrical generation.
-
Brake Booster Vacuum & Hydraulic Pressure:
- In vehicles with electro-hydraulic brakes (EHB), if the brake master cylinder pressure accumulator pressure drops below threshold, or if conventional manifold vacuum for a vacuum booster is inadequate, the engine runs to replenish vacuum/pressure reserves.
2. High-Voltage Motor-Generator (MG1) Cranking Mechanics
In conventional vehicles, engine cranking is handled by a 12-volt brushed series-wound starter motor engaging a flywheel ring gear via a Bendix drive pinion. In a Toyota/Lexus/Ford power-split hybrid architecture, there is no 12V starter motor, no Bendix drive, and no starter ring gear. Engine cranking is executed directly by Motor-Generator 1 (MG1) through the planetary Power-Split Device (PSD).
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| CONVENTIONAL 12V STARTER VS. HYBRID MG1 CRANKING |
| |
| CONVENTIONAL 12V STARTER: HYBRID HIGH-VOLTAGE MG1: |
| - Voltage: 12V DC - Voltage: 200V - 650V DC (Boosted|
| - Current: 150 - 250 Amps - Power: 15 - 40 kW (20 - 55 hp) |
| - Cranking Speed: 150 - 250 RPM - Cranking Speed: 800 - 1200 RPM |
| - Time to Fire: 800 - 1500 ms - Time to Fire: 150 - 300 ms |
| - Start Shudder: SEVERE (Cranking through - Start Shudder: ZERO (Passes |
| resonance at 300-500 RPM with fuel firing) resonance BEFORE fuel/spark) |
| |
| Engine RPM |
| 1200 | +== MG1 Spun to 1000 RPM |
| 1000 | / (Fuel/Spark Enabled) |
| 800 | / |
| 600 | / |
| 400 | - - - - - - - - - - - - - - - - - - - -/ - Resonant Band (300-500 RPM) |
| 200 | 12V Starter (180 RPM) / |
| 0 +======================================+===========================> Time |
+-----------------------------------------------------------------------------------+
The Problem of Resonant Frequency (300–500 RPM)
Every automotive internal combustion engine mounted on rubber/hydraulic isolators has a natural torsional resonant frequency, typically situated between 300 and 500 RPM. When an engine passes slowly through this speed range while cylinders are firing unevenly, torsional resonance causes violent engine shudder, cabin vibration, and driveline clatter.
The MG1 Rapid Spin-Up Solution
When an engine start is requested, the hybrid control system executes a precisely timed 4-phase sequence:
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| MG1 RESTART SEQUENCING CHRONOLOGY |
| |
| Phase 1: Motoring & Decompression (0 - 100 ms) |
| - Inverter supplies 3-phase AC power to MG1 from HV battery. |
| - MG1 applies rapid torque to the sun gear, spinning the planetary carrier/ICE. |
| - VVT-iE cam phaser holds intake valves open (decompression mode). |
| |
| Phase 2: Resonance Sweep (100 - 200 ms) |
| - MG1 accelerates ICE through 300-500 RPM in under 80 milliseconds unfueled. |
| - Zero fuel injection, zero spark -> zero combustion shudder during resonance. |
| |
| Phase 3: Compression Restoration & Cranking Target (200 - 250 ms) |
| - Engine reaches stable cranking speed of 900 - 1,100 RPM. |
| - VVT-iE advances camshaft to standard Atkinson combustion timing. |
| |
| Phase 4: Fuel, Ignition & Catch (250 - 350 ms) |
| - ECM enables sequential fuel injection and commanded spark advance. |
| - Combustion catches cleanly at 1,000 RPM; MG1 immediately shifts from motoring |
| mode to speed control / generation mode, absorbing excess combustion torque. |
+-----------------------------------------------------------------------------------+
3. Decompression Valve Timing Strategy During Cranking
To further minimize the electric power required to spin the engine from a dead stop, hybrid engines utilize an active decompression strategy executed by the variable valve timing system.
Mechanical Mechanism of Decompression
- When the engine shuts down during an auto-stop, the electric camshaft phaser (VVT-iE) rotates the intake camshaft to its maximum retarded position (or holds the intake valves open deep into the compression stroke).
- When MG1 begins motoring the crankshaft, the intake valves remain open for up to 70° ABDC.
- As the pistons move upward on the compression stroke, air flows freely back into the intake manifold without being compressed against closed valves.
- Diagnostic & Electrical Impact:
- Peak cylinder compression pressure drops from ~180 psi down to under 50 psi during the initial spin-up.
- The cranking torque required from MG1 is reduced by over 50%.
- High-voltage battery discharge current spike is minimized, protecting battery state of health (SOH) and preventing DC bus voltage dips.
- Once the crankshaft exceeds 700–800 RPM, the VVT-iE actuator rapidly advances the intake camshaft into normal Atkinson-cycle timing, restoring cylinder compression just milliseconds before the first injector pulse fires.
4. Driveline Vibration Attenuation & Torsional Dampening
Because the ICE connects mechanically to the planetary gearset without a torque converter's hydraulic fluid cushion, any engine torque pulses would transfer directly into the gear teeth and driveshafts. Hybrid powertrains incorporate three layers of mechanical and electronic vibration suppression:
+-----------------------------------------------------------------------------------+
| THREE-LAYER TORSIONAL DAMPENING SYSTEM |
| |
| [ LAYER 1: MECHANICAL ] |
| - Dual-Mass Drive Damper assembly between Crankshaft and Transaxle Input Shaft. |
| - Dual-stage coil springs & friction hysteresis plates absorb combustion pulses.|
| |
| [ LAYER 2: ELECTRONIC / INVERTER ] |
| - Inverter Active Damping Control: MG1 and MG2 inject high-frequency |
| micro-torque counter-pulses (180° out of phase with ICE cylinder firing). |
| |
| [ LAYER 3: STRUCTURAL / MOUNTING ] |
| - Active Control Engine Mounts (ACM) / Vacuum-Switched Fluid Mounts |
| counteract low-frequency engine rocking during start/stop transitions. |
+-----------------------------------------------------------------------------------+
1. The Dual-Mass Drive Damper
Unlike a conventional vehicle with a manual clutch disc or torque converter flexplate, the hybrid ICE utilizes a dedicated Drive Damper Assembly bolted to the crankshaft flange. This assembly contains:
- An outer inertia mass and an inner hub splined directly to the transaxle input shaft (planetary carrier).
- Circumferentially arranged two-stage damper springs with progressive spring rates: soft low-rate springs absorb minor idling/cranking fluctuations, while stiffer high-rate springs handle maximum engine torque transfer.
- Dry friction washers providing controlled rotational hysteresis damping.
2. Inverter Micro-Torque Active Damping
The HV-ECU samples high-resolution resolver position sensors on MG1 and MG2 at kilohertz frequencies. By calculating the instantaneous rotational acceleration of the planetary carrier caused by individual cylinder combustion events, the inverter commands MG1 to apply an opposing micro-torque ripple (anti-phase torque). This active electronic cancellation neutralizes torsional gear rattle in the transaxle.
3. Active Control Engine Mounts (ACM)
Many hybrid vehicles utilize electronically controlled or vacuum-actuated hydraulic engine mounts. During an engine auto-start or stop transition, the ECM energizes a solenoid valve (or electromagnetic linear actuator inside the mount) to change the hydraulic fluid orifice stiffness, providing maximum compliance during cranking and high rigidity during driving.
A hybrid vehicle with a warm engine stops at a red light. The driver observes that the internal combustion engine continues to run and will not auto-stop. A scan tool displays: ECT = 88°C, Cabin Setpoint = 70°F (Ambient = 68°F), Vehicle Speed = 0 mph, Brake Line Pressure = 3.2 MPa, HV Battery SOC = 36%. What is the primary cause preventing engine auto-stop?
How does the hybrid powertrain control module prevent driver-perceptible shudder and cabin vibration during an engine restart from an auto-stop condition?
What is the primary function of the decompression valve timing strategy executed by the electric camshaft phaser (VVT-iE) during initial engine cranking by MG1?