Our mechanical engineer explains how auto stop-start squeezes a little more fuel economy out of your car’s engine.
THE internal combustion engine is inherently ineffi cient, which is why automotive manufacturers and engineers have always had fuel effi ciency as one of their priorities. Electronic fuel injection, forced induction, lean-burn combustion, low-friction bearings and variable valve control are some of the essential technical ingredients that can improve fuel consumption. It is also aff ected by drivetrain designs and aerodynamic considerations. Auto stop-start switches off the engine when the car comes to a standstill, say, at a traffi c light junction. Some drivers find this feature annoying, but its fuel-saving eff ect is indisputable. Think about it for a moment: When stationary, any motor vehicle with an internal combustion engine that’s running does exactly zero kilometre (or metre, for that matter) per litre of precious fuel.
On the instantaneous readout in certain on-board computers, the displayed value changes from litres/100km (or km/ litre) to litres/hour when the engine is idling, just to let the driver know that the fuel is still flowing/ combusting even though the vehicle isn’t moving. Auto stop-start shuts down the engine when the car comes to a complete stop. As soon as the brake pedal is released (or in the case of a manual transmission, the clutch pedal is depressed), the vehicle’s “sleeping” engine is restarted immediately. The system includes a dedicated control unit that monitors a variety of parameters, including the battery’s state, airconditioning demand and even the position of the crank. If the combination of factors is not ideal for autostop operation, the function will not be activated.
For example, when the battery is not fully charged up yet, or if the air-con needs to stay on, the controller will not shut down the engine. The starter motor, alternator and battery in a car with auto stopstart have been specified/ modified accordingly, so that the high frequency of starts/ restarts won’t reduce the life cycle or require shorter service intervals compared to a passenger car without auto stop-start. Do note that the fully sealed lead-acid battery in a car with auto stop-start is a special one, which cannot be substituted with a normal 12-volt unit you can find at every tyre and battery shop. An important element in every auto stop-start system is the automatic/automated transmission’s ability to allow the engine to start and engage drive in less than half a second. Auto stop-start alone has been proven to reduce average fuel consumption by around 8 percent, although this varies depending on the extent of urban driving.