Understanding stop/start automobile-engine design, Part 5: Additional considerations

Thus far, we have looked at mostly non-electronic and mechanical implications of stop/start, such as requirements on the battery technology, the crankshaft, and the starter motor itself. There is also an impact on the vehicle’s power electronics and battery management system (BMS).

Q: How does stop/start begin to affect the “electronics”?

A: It begins with a neutral-gear sensor, wheel-speed sensor, and a crankshaft sensor, all providing information about whether the car is moving or stationary. The engine control unit (ECU) coordinates the start-stop processes and harmonizes them with the engine management system. The electronic battery sensor (EBS), which is part of the BMS, communicates data about the state of charge, voltage, and battery temperature. These parameters help determine if stop/start should even be attempted or if it should be terminated early once it has been engaged.

Q: Is there another example?

A: The electrical system’s architecture in modern vehicles requires compatible battery technology to function properly and reliably. For this reason, factory-installed and replacement batteries must be “registered” – the BMS must know what type of battery is installed in the vehicle to exploit its full potential. If an incorrect battery is installed in the vehicle or is not correctly registered, this can result in premature battery deterioration and another breakdown. For this reason, only EFB or AGM batteries should be installed in vehicles with automatic stop-start systems. If an AGM battery is already installed in the vehicle, it must be replaced with another AGM battery.

Q: What is the impact on basic electronic circuitry and components?

It encompasses both “smarter” use of the existing drivers and additional circuitry (Figure 1). The circle on the right, labeled “M/G” (motor/generator), now has two power inputs and associated MOSFETs: one for the excitation stage, which moves and engages the starter gearing; the other to operate the starter motor itself. In contrast, a non-stop/start M/G has a single power input, which energizes the engagement solenoid and also applies power to the starter.

This figure does not show the many sensors that are simultaneously providing data to the engine control unit or module (ECU or ECM) via the CAN bus, such as battery information from the BMS, wheel-speed information, the status of brake and accelerator pedal, the status of the various accessories such as infotainment, ADAS, and A/C – they all have some impact on the algorithms which manage stop/start operation.

Q: What happens when coming to a stop?

A: When the vehicle is coming to a stop, just before the engine stops rotating during stop/start operation (at approximately 50 RPM), the ECM energizes the starter-pinion solenoid-actuator relay to push the pinion gear into the flywheel gear without gear clash. When the engine stops rotating during stop/start operation, the starter pinion gear is fully engaged. The starter motor is ready to become energized to start the engine again quickly.

Q: What about when slowing down but maybe not stopping (the keyword is “maybe”)?

A: There’s another requirement with stop/start operation: the starter pinion must be driven into the flywheel gear before the engine stops rotating to respond to changing demands on the engine quickly. For example, this occurs when a driver is slowing nearly to a stop — and the stop/start system is preparing for its normal autostop mode but suddenly decides to release the brake and accelerate.

Here, the engine has already stopped rotating, or nearly so. A conventional starter cannot restart the engine until the engine has completely stopped so the starter motor, pinion, gearing, and engine can engage without clashing. However, with stop/start, the starter pinion gear needs to be fully engaged and ready to begin rotating the engine even before it fully stops turning. Otherwise, the engine would actually have to stop rotating before the pinion can engage smoothly to begin a restart.

Q: How is this problem resolved?

A: To prevent a lag in engine operation, the car’s engine control module uses predictive speed to match the rotational speed of the flywheel gear and the pinion gear to engage the pinion gear into the flywheel gear without gear clash before the engine fully stops (here, speed does not mean RPM, it means their circumferential speeds). By predicting via an algorithm how long it takes the starter motor to spin up, the pinion gear speed can be matched to the flywheel gear speed. The result is an “almost” instant restart at extremely low engine speeds.

Q: What are some of the steps in the algorithm “flow chart” for stop/start?

A: It’s a long list of conditions. For example, the auto-stop function will be implemented only if these conditions are met (this is a greatly reduced list):

• The engine must reach its various critical operating points for many parameters, such as the minimum coolant temperature.
• Conditions relating to the vehicle must be met, including sufficient voltage in the on-board electrical system. The interior climate has been regulated following the car starting, and the vehicle must be stationary (obvious but important).
• Conditions associated with the car’s driver must also be met: the transmission’s selector lever must be set to Drive or Neutral; there should be no movement of the accelerator or the steering wheel; the driver’s foot must be on the brake, or the HOLD function must be active; the doors must be closed, the driver’s seat belt must be fastened, and the hood must be closed.
• The car speed must be greater than a pre-defined minimum speed after the car was started by the driver or during maneuvering such as parking.

Q: OK, the engine is stopped, so what about the conditions for restarting?

Any one of the following conditions must also be met to engage the auto-restart:

• The accelerator pedal is pressed.
• The transmission is in the reverse R position.
• The transmission is moved out of park P position.
• The brake pedal is released, the parking brake is released, and the transmission selector lever is not set to “P”.
• The vehicle begins to roll.
• A function linked to the running of the engine, such as raising the ride height, is activated by the driver.

Q: Could there possibly be more?

Yes, there is. The stop/start system must start the engine automatically as an intelligent comfort and safety function. The engine control unit starts the engine automatically, without any intervention on the part of the driver when one of the following conditions is met:

• A vehicle-related condition for the auto-stop function is no longer met, such as the conditions related to air conditioning, the on-board electrical system, the brake system, the chassis, and other vehicle-related influences.
• The driver releases the seat belt or opens the driver’s door. Here, automatic starting takes place in order to prompt the driver to actively switch off the engine by turning the ignition key (or button) to the off position before leaving the vehicle. This ensures that the stop/start system is safely deactivated when the vehicle is parked (“we had to start the car in order to stop it”).

In summary, the simple idea of turning the engine off when the car is stopped for more than a few seconds has many rules and exceptions.

Q: Are there cases where stop/start should not engage?

Absolutely: for example, as noted earlier, if the engine is cold, the car engine must be allowed to come up to temperature up, since in “cold-engine”  starts, the lubricants are not flowing, and engine wear will increase. Other examples are if the battery is below a certain level where it may not restart the engine, or if the drive has unfastened his/her seat belt.

Q: What’s another subtle scenario for not engaging stop/start?

A: Consider the air conditioner, which imposes a large electrical load on the car for its compressor. The car driver/passenger turns it on while the car is in the auto-stop” mode. The car should be restarted if the battery charge state is moderate to low because the A/C load could deplete the battery. (This can be unsettling — you turn the A/C, and the car’s engine also starts!)

Also, if the A/C is in use and the car is in stop mode for more than about a minute (a long traffic light or stopped traffic), there’s the same concern, so the car must be restarted even though the driver has not stepped on the accelerator to start the car moving. Having the car shut the AC off is not an option, as the car cabin temperature may increase unacceptably (“greenhouse effect” and all that glass), and occupants will get uncomfortable.

Clearly, stop/start has many subtle issues. The next and final part of this article looks at other issues and even responses to stop/start.

Related EE World Content

Are power systems up to the task of running self-driving cars?
Power modules lessen the load on internal combustion engines
Today in Engineering History: Internal Combustion Engine Patented
Hybrid EVs come to touring car racing
How bidirectional converters speed the design of dual-battery automotive systems

External References

There’s a lot of information available about stop/start, as there is for almost anything related to cars: it’s a big industry that affects just about everyone in one way or another. Perhaps because stop/start is relatively new, contrary to tradition, and has many technical and driver-related issues, the content of these sites ranges from personal opinion to technical discussion, to in-depth technical analysis. These are the useful sites and sources I reviewed for this article:

• Green Car Reports, “Don’t start-stop systems wear out your car’s starter?”
• Autocar, “Stop-start systems: is there a long-term impact on my car’s engine?”
• CarPro USA, “Understanding Stop/start Systems”
• Driving, “How It Works: Starters and automatic start-stop technology”
• Varta (Clarios), “Reasons why the start-stop system does not work”
• Varta (Clarios), “What is automatic start-stop and how does it work?”
• Varta (Clarios), “Why do I need a special battery for the automatic start-stop system?”
• Varta (Clarios), “Start-Stop”
• The Jag Wrangler, “SmartStartStop”
• The Truth About Cars, “The Problem With Start-Stop Systems”
• Best Ride, “Does Auto-Stop/Start Technology Wear Out Engine Components?”
• Practical Motoring, “Myth-busting: Does stop-start damage your engine?”
• NAPA, “Stop-Start Technology — Pros and Cons”
• Idle Smart, “Pros and Cons of Automatic Engine Stop/start Solutions”
• How Stuff Works, “Love It or Hate It: Stop-start Technology Is Here to Stay”
• Wikipedia, “Start-stop system”
• STMicroelectronics, “48V Start-Stop System”
• Daimler, “Under the microscope: ECO stop/start: Sophisticated technology gives the highest levels of efficiency”
• Argonne National Laboratory, “Start and Restart Effects on Modern Vehicle Starting Components”
• Bosch, “Full power for stop/start”
• How a Car Works, “How the starting system works”
• Your Mechanic, “How Does a Starter Motor Work?”
• General Motors, “Cadillac’s Electric Self Starter Turns 100”
• Elreg Distributors, “The Evolution of the Starter Motor”
• PR Newswire, “ZMJ and CRCI to Acquire Bosch’s Starters and Generators Business”
• TechLink, “Enhanced Starter Motor Operation in Engine Stop/Start Systems”
• JeepFan, “How Electronic Start Stop ESS Works on the Jeep Wrangler JL”
• Autoblog/Verizon Media, “Mazda introduces clever start-stop system with direct injection”