In urban situations especially, stop-start should be making a real-world difference, but will the durability of engines be affected in the long term?
What is stop-start technology?
Stop-start is a system on most modern cars that cuts the engine when the car is stationary in order to reduce fuel consumption and emissions. In hybrid cars, the same system may also shut down the combustion engine when the car is cruising at low throttle loads, when descending gradients and when decelerating from higher speeds. The engine starts again when the clutch is engaged or the brake is released, or when the driver is ready to move or accelerate again.
How does stop-start work?
The system uses a computer to detect when the car is stationary or out of gear, or when it's running in low-load conditions; at which point it halts fuel delivery and spark to the engine. In the case of hybrid cars, a limited amount of torque can be supplied by the car's electric drive motor while the engine is off, although it's usually only enough to maintain speed on a level grade or around town. The ignition starts again when the car begins moving, the clutch is pressed or more power is needed.
The process happens automatically, but drivers can choose whether the system is active or disabled by pushing their car’s stop-start button; a capital A with an arrow circling clockwise.
A conventional electric starter motor works by engaging a small pinion gear with a large ‘ring’ gear fitted around the outside of the engine flywheel.
The latest stop-start technology looks much the same but the motors are more powerful, faster acting and more robust, and often they're necessarily backed by higher-voltage electrical systems with bigger storage batteries. Some are designated ‘TS’ for ‘tandem solenoid’ and designed to cope more smoothly with scenarios where the engine is about to stop and then the driver accelerates again.
Such a moment may come when the driver has decided to stop, but for whatever reason has a change of mind, such as when the traffic moves off unexpectedly.
At that moment the engine might be ‘committed’ to stopping but is still spinning, so to avoid crunching, one solenoid fires up the starter motor to synchronise its speed with the engine before the second smoothly engages the gear.