Start-Stop System

A start-stop system, sometimes called automatic engine stop-start, is a fuel-saving feature that shuts the engine down when the car is brought to a halt and restarts it the instant the driver prepares to move off. It exists because an idling engine consumes fuel and emits carbon dioxide while producing no useful work, and in congested urban driving a vehicle can spend a remarkable proportion of its journey stationary at junctions, traffic lights and in queues. By eliminating that wasted idling, the system delivers measurable savings precisely where conventional engines are least efficient.

The sequence is managed automatically by the engine control unit, which monitors a range of conditions before deciding to stop the engine. In a manual car, the engine typically switches off when the vehicle is stationary, the gearbox is in neutral and the clutch is released; in an automatic, it usually stops when the brake is held with the car at rest. Restarting is triggered by the driver depressing the clutch or releasing the brake, and the engine fires again within a fraction of a second so the delay is barely perceptible. The system will also keep the engine running, or restart it unprompted, if it detects that battery charge is low, the cabin needs heating or cooling, the engine is not yet warm, or the car is on a steep gradient.

The benefit to the driver is reduced fuel consumption and lower emissions, most pronounced in the stop-start traffic for which the feature is named, together with the elimination of needless noise and exhaust while waiting. Official fuel economy and CO2 figures reflect these savings, which is part of why the technology became near-universal as manufacturers worked to meet fleet emissions targets.

Frequent restarting places far greater demands on a car's hardware than ordinary use. Engines so equipped are fitted with a reinforced, higher-durability starter motor or an integrated starter-generator, and an upgraded battery, commonly an absorbent glass mat or enhanced flooded type, able to withstand many tens of thousands of cycles and to power ancillaries during the stop. Sensors track battery state of charge, oil and coolant temperature, and brake vacuum to ensure a restart is always available.

Increasingly, basic start-stop has been absorbed into mild-hybrid systems, in which a small electric motor-generator running on a 48-volt network restarts the engine almost silently, smooths the transition, and can even allow the engine to be switched off while coasting. This addresses the chief criticism of early systems, the slight shudder felt on each restart. Drivers who find the behaviour intrusive can usually disable it with a dashboard button, though it generally re-enables itself at the next start, reflecting its role in the car's official emissions compliance.