Mercedes-Benz CLS 350 and 500 Under the microscope: ECO start stop
Sophisticated technology gives the highest levels of efficiency
The engine goes off when the vehicle is stationary and back on again when the brake is released – in practice the ECO start/stop system operates virtually unnoticed by the driver. This is all thanks to complex control functions which run in the background to ensure efficiency, comfort and safety.
How the ECO start/stop system works in detail: as a general principle, the engine is switched off every time the vehicle comes to a standstill (auto-stop function. Restarting the engine (auto-starting function) takes place almost imperceptibly; and moving off without any noticeable time lag compared to a stationary vehicle with its engine running.
This rapid action is attributable to two special features: a crankshaft Hall sensor which is able to identify the rotational direction of the crankshaft enables the engine control unit to identify the cylinder in which the piston is ideally positioned for starting. The fuel is injected into this cylinder first, thereby speeding up the starting process.
An additional electric transmission oil pump supplies the clutches of the automatic transmission with oil pressure prior to starting, so as to enable a swift resumption of the journey after direct starting of the combustion engine via the ECO start/stop function.
The starter motor (starter) has also undergone thorough modification: it is now designed to cope with eight times as many starting procedures, ensuring that it will last a car's lifetime in continuous urban driving involving frequent auto-starting. In addition, the on-board electrical system is supported by a second battery.
Preconditions for the ECO start/stop function
It is not always expedient for the engine to be shut down automatically when the vehicle comes to a standstill. The auto-stop function will therefore only be initiated if a number of conditions are met:
- The combustion engine must have attained the necessary operating parameters (e.g. the minimum coolant temperature).
- The relevant conditions relating to the vehicle must be met (e.g. sufficient voltage in the on-board electrical system, the interior climate has been regulated following the key start, the accumulator for the air suspension or brake system is sufficiently full). And the vehicle must be stationary, of course.
- The relevant conditions relating to the driver must be met: the transmission selector lever must be set to D or N; no movement of the accelerator or the steering wheel; the driver's foot must be on the brake or the HOLD function must be active; the doors must be closed, the driver's seat belt must be fastened and the bonnet must be closed.
- The ECO start/stop must not have been switched off via the ECO button.
- Relevant speeds must have been exceeded after starting with the ignition key or during manoeuvring, for example.
Auto-starting takes place when the engine is in auto-stop mode, i.e. when the engine has been shut down via the engine-stop function and the ignition remains switched on. One of the following conditions must also be met:
- The accelerator pedal is pressed.
- Position "R" is selected on the transmission.
- The transmission selector lever is moved out of position "P".
- The brake pedal is released and Hold is not activated and the parking brake is released and the transmission selector lever is not set to "P".
- The ECO start/stop function is switched off via the ECO switch.
- The vehicle begins to roll.
- A function linked to running of the engine, e.g. raising of the ride height, is activated by the driver.
But the start/stop function can think beyond that: it is also able to start the engine automatically as an intelligent comfort and safety function. The engine control unit starts the engine automatically, without any intervention on the part of the driver, when one of the following conditions is met:
- A vehicle-related precondition for the auto-stop function, such as the conditions pertaining to air conditioning, the on-board electrical system, the brake system, the chassis and other vehicle-related influences, is no longer met.
- The driver releases his seat belt or opens the driver's door. Automatic starting takes place in order to prompt the driver to actively switch off the engine by turning the ignition key to position 0, prior to leaving the vehicle. This ensures that the ECO start/stop system is safely deactivated when the vehicle is parked.
To provide the driver of the CLS with the necessary assurance that the ECO start/stop function is working at all times, the availability of the auto-stop function is indicated by the ECO symbol on the multifunction display of the instrument cluster:
- Green = all conditions are met; engine will be shut down when the vehicle stops.
- Yellow = ECO is active, but conditions are not met.
No ECO symbol displayed = ECO is switched off or has been deactivated due to an error.
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