Other Articles - April - 2021

Start Stop Systems – To Assist or Not to Assist? That is the Question

It seems as time marches on, more and more of what we do is driven by government rules and regulations. The government requires and monitors automotive and truck manufacturers to improve fuel economy and reduce emissions. Manufactures that fall short in either of those requirements may face fines. Consequently, manufactures are always looking to technology to help them meet their goals. Hybrid and electric drive vehicles are an attempt by the manufacturers to satisfy the friendly folks’ requirements in Washington DC.

Hybrid systems are commonplace worldwide. Different levels of hybrid systems are used, from a mild hybrid that uses “start-stop” technology to the full-blown hybrid, which uses the hybrid system to supplement the internal combustion engine during acceleration, typically referred to as “E Assist.” According to studies, the auto/start-stop accounts for a 4-5% increase in fuel economy depending on the driving conditions.

figure 1The vast majority of the manufacturers use a start-stop system, and most of them are similar in operation. Start-Stop systems shut the engine off when the vehicle comes to a stop. When the customer accelerates, the engine starts automatically. The vehicle operates normally until you make another stop. Many customers, including myself, are annoyed by the operation of most systems and would gladly pay to have someone disable it. But since the system is classified as part of the vehicle emission controls, your local dealer is unable to install a calibration to disable its operation.

figure 2The transmission is one of the main components in all hybrid systems, so even though you may prefer not to work on the system, you will be required to. The start-stop system consists of two main systems: a system to restart the engine and a system to keep the transmission engaged when the engine is off. Other systems may include a coolant pump to circulate coolant through your heater core when the engine is off, and a control, such as a button on the dash (figure 1) to turn the system on and off for the duration of the key cycle and indicator in the dash telling the customer when the auto stop feature is active. Many vehicles also incorporate a hill-hold, or “auto stop,” to keep the car stationary while on a grade (figure 2). It uses the ABS for this feature.

Two different systems are used to restart the engine, depending on the vehicle’s year and make. Some vehicles use a heavy-duty generator/alternator that acts to spin the engine over for auto start. Other vehicles utilize a heavy-duty starter that engages to provide the auto-start feature.

figure 3To keep the transmission engaged when auto stop is active, two different systems are used. One system uses an electrically-operated auxiliary transmission fluid pump (Figure 3) to keep the clutches engaged, while the other system uses a hydraulic accumulator (Figure 4). In either case, it is imperative that the transmission retain some pressure on the forward clutch when the engine is in auto-stop mode, or a flare will occur when the customer accelerates the vehicle.

To start the engine, a heavy-duty 3 phase AC generator/alternator system was very prevalent in earlier vehicles. GM named their system “BAS” for belt, alternator, starter and was used as far back as the 4T40E days. Using the generator to also act as a method of restarting the engine required the manufactures to design an updated generator/alternator and belt system as the generator not only needed to charge the vehicle’s battery, but it now needed to act as an electric motor to crank the engine for auto start.

figure 4The GM system used a heavy-duty generator that was connected to three 12V NiMH battery cassettes wired in series, providing up to 42 volts. The battery cassette was located in the trunk with three large blue-colored medium-voltage cables connecting the battery cassette and its electronics to the generator. Later model systems moved to a liquid-cooled heavy-duty generator operating on either 86 or 115 volts. The system is now connected using three orange high-voltage cables. The higher voltage system also provides a power boost during acceleration, thus the name “E assist.” Like any other hybrid, precautions must be taken when working with the system to assure your safety. The use of high-voltage gloves and powering down the system is essential to personal safety.

The transmission system uses an auxiliary electric pump and solenoid to keep the forward clutch engaged when the engine is off during the auto-stop mode. When the customer releases the brake and accelerates, the pump is shut off, and the transmission resumes normal operation. In some applications, the pump is mounted externally on the transmission, while others are mounted internally.

Later systems eliminated the electric pump and transitioned to an accumulator system. The transmission hydraulic pump charges the accumulator, and a solenoid controls the flow into and out of the accumulator. Later auto-stop systems eliminated the generator/starter system and began to use the engine starter for auto start. Since the starter is used much more frequently, a heavy-duty starter and flexplate are used. This system will typically use two 12 volt batteries, one for starting and one for other electrical loads. Some systems use a capacitor to provide a boost of current to the starter for restarts.

One question I am sure you have is how the starter drive and the flexplate survive the constant restarts? The new design starter does not work like a conventional starter. When the engine stops rotating, the starter drive is reengaged with the flexplate teeth preparing for the next restart. That means only the motor drive needs to be energized for the starter to rotate. Your next question might be what happens if the customer thinks he/she is going to stop, and just before the engine comes to a complete stop, they release the brake and reapply the accelerator? Well, they thought of that too. The ECM knows the engine rpm and will drive the starter to match the engine rpm making the drive engagement with the flex plate transparent. They do this by using software known as ‘predictive drive speed matching,” using the same ideas we see in many new transmission designs.

Like other vehicle systems, the start-stop has preset criteria that must be met for the system to operate. The criteria vary with the vehicle, but you will get the idea.

Based on the criteria needed for my Chevrolet Equinox, it includes:

Auto-stop ON

  • VSS during the initial drive cycle with a minimum of 12 mph (19 km/h). Subsequent stops with a minimum 1-6 mph (2-10 km/h).
  • M/T in neutral and clutch fully depressed. A/T in Drive
  • Ambient and engine coolant temperature correlation: ECT less than 248F (120C)
  • The hood is closed.
  • Brake pedal depressed, 27% or greater.
  • APP indicates the throttle is closed.
  • Brake booster vacuum is greater than 45 kPa (7 PSI).
  • Engine speed is below 1500 RPM.
  • No A/C compressor request from the HVAC system (A/C or Defrost modes).
  • Battery voltage greater than 12 V. Battery state of charge is greater than 75%

The engine will not auto-start if:

  • The transmission gear selector is moved from the Drive position.
  • The driver depresses the clutch pedal.
  • The brake is released, or the accelerator is pressed
  • Brake booster vacuum is less than 40 kPa (6 PSI).
  • A/C or Defrost modes are activated
  • Battery voltage less than 11 V. Battery state of charge is less than 73%
  • The hood switch indicates the hood is open.
  • Auto Stop time exceeds 2 minutes.

figure 5Well, you now have a good idea of how these systems are supposed to work. Many customers, including myself, find the system to be the most annoying thing ever developed by the car industry. A recent study of customers found that several things annoyed them regarding the start/stop system (figure 5).

So, what do you do with an annoyed customer such as myself? The aftermarket has some solutions that eliminate start/stop system operation. The one I chose was from “Smart Start-Stop,” (Figure 6).

figure 6It connects in series with the hood switch. Devices like this are definitely an upgrade that a lot of customers will be willing the pay for.

Until next time remember: “My mind is like a web browser, 20 tabs are open, five are frozen, and I have no idea where the music is coming from.”


EDITOR’S NOTE: ATRA and Gears Magazine do not recommend or endorse bypassing any emissions-related device without first contacting your State EPA for compliance. ATRA and Gears Magazine are not responsible for legal or financial penalties as a result of bypassing the auto-start feature of any vehicle, based on the information from this article.