Stop-Start Primer: Not All Systems Are Created Equal

There is some ambiguity out there regarding “stop-start” systems as found in all full hybrids, mild hybrids, and now even trickling into some non-hybrid combustion-engine vehicles being offered with basic, less-expensive examples of this fuel- and emissions-saving technology.

It’s this latter application – stop-start systems used on less than mild and full hybrids – that we’d like to focus on here.

For conventional, non-hybrid vehicles, stop-start is a relatively low-tech solution that moderately improves fuel economy and tailpipe emissions. These systems are also known as “start-stop,” “idle-stop,” “idle-elimination,” “micro hybrid,” and a variety of names branded by automakers such as Auto Start-Stop (Ford) and Eco Start/Stop (Mercedes-Benz).

In simplest terms, a stop-start system automatically shuts the engine off every time the vehicle stops, such as at a traffic signal, and restarts it instantly when it’s time to go.

The idea of stop-start dates back to the 1930s, and its use can be traced back to the 1980s by European automakers Volkswagen and Fiat. Gasoline-electric hybrids from Honda and Toyota introduced Americans more than a decade ago to stop-start systems.

An example of a "heavy" stop-start system as found in the GM eAssist cars.

An example of a “heavy” stop-start system as found in the GM eAssist cars.

Depending on the system design and driving environment, stop-start by itself can add 3-10 percent to mpg numbers. Combined with other fuel efficiency technologies such as regenerative braking, direct fuel injection, electric powered steering, electric-powered air conditioning compressor, as well as regenerative braking, fuel economy gains can reach 15-25 percent improvement.

Stop-Start System Types

There is quite a variety in degree of complexity and cost for automakers to manufacture – and for consumers to purchase – stop-start systems.

Lux Research, a global automotive market research firm that has analyzed start-stop technology for several years, classifies different stop-start versions as light, medium and heavy.

Light Stop-Start

The most simple and least expensive, light stop-start systems employ a higher durability starter and a more powerful and longer lasting battery. Also, the engine controller requires reprogramming to pre-position the fuel injection system, starter and transmission to provide instant engine restart when the driver either releases the brake pedal or depresses the accelerator pedal. Or, with a manual transmission, depress the clutch pedal.

The 2014 Ram 1500 HFE uses a light stop-start system.

The 2014 Ram 1500 HFE uses a light stop-start system.

Most light stop-start systems include a driver-selectable on-off switch, and some add a small auxiliary battery to eliminate a momentary dimming of lights or slowing of the air-conditioning fan when the engine stops and starts. Extreme hot or cold weather can prevent systems from activating.

The light stop-start feature doesn’t use an electric motor and batteries to move the car down the road and is not considered a hybrid by many standards.

Because of their mechanical simplicity, a stand-alone light stop-start system only costs $300-$400 more than a conventional vehicle. Fuel economy improvement is 3-5 percent, slightly more if most of the driving occurs on city streets and stop-and-go traffic.

Medium Stop-Start

Like a light stop-start system, a medium stop-start version uses a beefed-up starter, more powerful battery and usually an auxiliary battery. In many medium systems, an enhanced alternator is used that allows regenerative braking to recharge the auxiliary battery, usually a small lithium-ion.

The Mazda6 with i-ELOOP slated as a 2014 model is perhaps the only example in the U.S. of a medium stop-start system.

The Mazda6 with i-ELOOP slated as a 2014 model is perhaps the only example in the U.S. of a medium stop-start system.

Medium start-stop systems add $500 to $700 to the price of a conventional vehicle. Fuel economy improvement is 7-12 percent, and again, slightly more if most of the driving occurs on city streets and stop-and-go traffic.

Heavy Start-Stop

Heavy stop-start systems offer the highest level of functionally. They are most often a design called Belt Alternator Starter (BAS) or, in Europe, Belt-Driven Starter-Generator (BSG). (See illustration)

A BAS is integrated into the belt drive system of a conventional combustion engine. It replaces the belt-driven alternator with an electric motor that serves as a generator and a motor. Thus when the engine is running the motor, and acting as a generator, will charge a separate battery. When the engine needs to be started, the motor then applies its torque via the accessory belt, and cranks the engine instead of using the starter motor. The separate battery is also recharged via a regenerative braking system.

GM's eAssist as found in the Malibu Eco is an example of a heavy stop-start system.

GM’s eAssist as found in the Malibu Eco is an example of a heavy stop-start system.

In this scheme, the motor/generator is made larger than a standard starter motor so more torque can be generated when in the motoring mode. This allows for quicker starts of the engine, and makes the stop-start operation possible.

A BAS system is fairly sophisticated and, in addition to the stop-start function, can enhance fuel economy even during highway driving by cutting off the fuel supply when cruising or decelerating. Some systems can also provide some electric assist to the engine during acceleration, but not all-electric operation – thus the term “micro-hybrid.”

When an automatic transmission is part of the drivetrain equipped with BAS, an auxiliary electric-driven oil pump is added to the transmission. This keeps it primed and the fluid flowing when the engine shuts down at a stop. That sustains the transmission’s readiness to perform when the driver accelerates.

A BAS heavy stop-start system can add $1,000 to $2,000 to the price of a car, but fuel economy gains are 15-25 percent or more.