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Idle STOP-START Systems

by Larry Carley copyright

Idle Stop-start systems (also called "Idle Stop") that automatically shut the engine off when the vehicle stops moving to save fuel, and then automatically restarts the engine when the driver presses down on the accelerator pedal are becoming more and more common on late model vehicles. For city driving, Stop-Start systems can improve average fuel economy from 8 to 15 percent or more! The actual savings will vary depending on the drive cycle and how much of that time is spent idling or not moving.

Why the Use of Idle Stop-Start Systems is Growing

Stop-Start technology has been around for nearly a decade on hybrid vehicles such as the Toyota Prius. In fact, Stop-Start technology is one of the features of all hybrid vehicles. This includes "full hybrid" vehicles like the Prius and Ford Escape hybrid that can use their high voltage battery and electric motor to drive the vehicle in electric mode only under certain conditions, and "mild hybrid" vehicles that only use their high voltage battery for automatic engine Stop-Start and to boost engine performance when accelerating.

Some examples of the latter approach include the Honda Insight, Accord and Civic hybrids, Saturn VUE hybrid, Saturn Aura Green Line, Chevy Malibu ECO hybrid, Chevy Tahoe and GMC Yukon hybrids, and the 2013 Buick LaCrosse with eAssist.

Idle Stop-Start is not required by law. But as time goes on, more and more auto makers are incorporating this feature into new models to improve their overall Corporate Average Fuel Economy (CAFE) numbers. The goal is to raise CAFE standards nationwide to 52.5 mpg by 2025.

How Idle STOP-START Works

The engine management system monitors engine speed, temperature and load as well as vehicle speed, the positions of the brake and accelerator pedals, and the position of the transmission gear selector so it knows when the vehicle is moving and when it is stopped. It may also look at accessory electrical loads on the engine (headlights, wipers, state of battery charge, etc.), and A/C cooling requirements so decide if and when to shut the engine off when the vehicle stops moving.

With most Stop-Start systems, the engine will shut off after the vehicle has been motionless for a few seconds if the transmission is in drive and the driver is holding their foot on the brake pedal and the accessory loads on the engine (such as A/C) are minimal. The Stop-Start system will then automatically restart the engine when the driver lifts their foot off the brake pedal and/or presses down on the accelerator. The restart is supposed to be instantaneous and seamless, though there may be a slight lag in some situations.

With full hybrid vehicles like the Toyota Prius, the engine does not restart immediately. The electric motors inside the transmission (there are two) start the vehicle moving. When a certain combination of speed and load are reached, one of the electric motors shifts its output torque back to the flywheel to crank and start the engine. The engine then takes over the job of driving the vehicle (and recharging the high voltage hybrid battery).

 Saturn VUE start-Stop BAS system

GM Belt-Alternator_Starter BAS and eAssist Systems

General Motors currently uses a couple of Idle Start-Stop systems on certain vehicles. On the Saturn VUE hybrid, the system is called BAS (Belt-Alternator-Starter). On the Buick LaCrosse, it is the eAssist system. In both applications, the alternator serves a dual purpose as both a generator and starter.

When the engine is running, the alternator produces voltage the same as any other alternator to meet the vehicle's electrical requirements and to keep the 12V and high voltage hybrid batteries charged. When the Stop-Start mode kicks in, the engine shuts off when the vehicle stops to save fuel. The engine is then restarted by routing current back to the alternator from the high voltage hybrid battery. The alternator now spins like a starter and cranks the engine via its drive belt to restart it. On these applications, the conventional starter motor is NOT used for Stop-Start operation, and is only used for cranking a cold engine (or to restart the engine if the hybrid battery is low).

With BAS, eAssist and similar Idle Stop-Start systems, the alternator/starter can also provide extra torque when the vehicle is accelerating. The extra boost usually isn't a lot (typically about 15 to 20 horsepower) in most cases.

 Chevy Malibou eAssist start-Stop system

The eAssist system on the 2013 Chvy Malibou and Buick LaCrosse provides up to 79 lb./ft. of additional torque to assist the engine's power output. The ability to add extra torque on demand via the Start-Stop system allows the use of smaller displacement engines to improve overall fuel economy. A V6 can replace a V8, and a four cylinder can replace a V6 in many instances.

GM's eAssist Stop-Start system is only be active when the transmission is in Drive and the brake pedal is being depressed. It may not activate if the engine is cold, the outside temperature is below 5 degrees F., the A/C or defrost is on, or the hybrid battery charge is low.

The eAssist system uses a 125 amp fuse in the battery energy control module wiring junction block assembly to protect the system against overloads. Disconnect relays inside the controlled will also disconnect and isolate the hybrid battery if the system senses any voltage loss to vehicle ground.

If the eAssist Stop-Start function is not working, and no fault codes are found, the problem may be a hood that is not completely latched shut or a misadjusted hood switch. The system's operating logic disables Stop-Start if it senses the hood is open.

Next Generation Idle Start-Stop Systems

In Europe, the approach has been to use beefed up starters and batteries for Start-Stop systems. This same approach will be used on upcoming U.S. vehicles too as it is less expensive than the current mild hybrid approach. On these systems, the starter is typically rated to handle up to 300,000 or more starting cycles, making it much more durable than a standard starter motor. The standard lead-acid battery is also replaced with a more durable Absorbant Glass Mat (AGM) battery that can handle deeper discharge cycles.

Direct Injection Idle Stop-Start

Another approach to Stop-Start uses no battery power at all. Instead of cranking the engine with an electric motor or a combination alternator/starter to restart it, the Mazda "i-Stop" system uses Direct Injection to kick start the engine. When the engine shuts off, the engine management system keeps track of the exact position of each piston. It can then restart the engine by spraying fuel directly into a cylinder that is just at the start of its power stroke and firing the spark plug to ignite the mixture. The resulting combustion starts the crankshaft turning and starts the engine in about 1/3 second. This trick only works if the engine is warm, so a conventional starter is required for cold starting. But the Direct Injection I-Stop approach approach works just as well for idle stop as a high voltage hybrid system, and it can be used on any gasoline or diesel engine with Direct Injection. BMW uses a similar system on its 2012 model 328i.

Idle Start-Stop System Parts

Because of the increased load and wear on the belt drive system such as GM's BAS and eAssist systems, regular belt inspections and tension checks are recommended to make sure the belt drive system is maintained in good operating condition. If a belt is showing signs of wear and needs to be replaced, make sure you buy a top quality name brand belt from your auto parts store. A cheap quality replacement belt won�t stand up in this kind of application (or ANY application for that matter!).

Alternator failures can occur on any type of vehicle (often as a result of overloading and overheating). But on vehicles equipped with Stop-Start, the incidence of failure may be somewhat higher due to the greater loads that are placed on the integrated generator/starter (alternator). Like the drive belt, these units also perform double duty by generating electricity for both the 12V and high voltage hybrid batteries, and by cranking the engine when restarts are required.

Other critical parts in mild hybrid Stop-Start systems include the high voltage hybrid battery (which is covered by an extended warranty but is only available from a new car dealer in most cases), the Stop-Start control module (which is usually mounted on the battery pack in the back of the vehicle) and the high voltage battery cables (usually color coded orange).

Most vehicle manufacturers provide an extended warranty of 8 years or 100,000 miles (or longer) on the main hybrid components (battery, controllers and integrated generator/starter), but not the belt or other powertrain components.

Idle Stop-Start Diagnostics

What happens if a Stop-Start system quits working? In most cases, the system self-diagnostics will detect the fault, set one or more trouble codes and disable the Stop-Start system. It may also disconnect and isolate the hybrid battery if there is any indication of a short circuit.

If a fault occurs in the alternator itself or its control module that prevents it from charging normally, the 12V battery as well as the hybrid battery may run down causing the vehicle to stall or not start. These kinds of problems would be diagnosed in a conventional matter as with any other charging problem.

If the alternator tests okay, the fault is not the alternator but something else in the charging system control circuit. Likewise, the 12V battery should be tested to determine whether or not is can accept and hold a charge.

A key requirement in diagnosing these vehicles is a professional grade scan tool that can fully access not only all system related codes, but also all of the onboard PID data (sensor values, switch positions, autostart status, voltages, etc.). Without this access to this level of information, diagnostics is little more than guesswork.

typical push start circuit

Safety Precautions

CAUTION! Many hybrid vehicles use a high voltage battery (120v up to 330v, although some may be as low as only 40 volts), so special precautions must be followed when servicing or replacing certain components on these vehicles. Even on a lower voltage hybrid system such as a Saturn VUE hybrid (36/42 volts), the increased voltage poses a shock hazard. The high voltage battery cables are usually color coded orange as a warning, and should NEVER be touched unless the high voltage battery has been disconnected or electrically isolated by opening its relays or flipping its power disconnect safety switch. Refer to the factory service literature for the disconnect procedure and safety precautions for your vehicle.

CAUTION: When servicing ANY type of vehicle with an Idle Stop-Start system, make sure the transmission is in Park before exiting the vehicle, and make sure the key (or smart fob) has been removed and is placed at least 10 feet away from the vehicle to prevent the engine from suddenly starting. This could also cause the vehicle to lurch forward.

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