Coast Mode can be manually triggered on cars with dual-clutch gearboxes, such as the Volkswagen Golf.
An incredible amount of engineering goes into the development of fuel-saving measures in cars.
Auto stop-start systems, for one, have become quite common, and are now found in many automobiles.
Then there is Coast Mode.
WHAT IS IT?
In layman’s terms, Coast Mode disengages the gearbox/transmission from the engine. This lets the car freewheel.
It is like driving a manual car and shifting into Neutral to let it roll along using its momentum.
When this happens, a bit of fuel is saved because the engine is not driving a gearbox. It is, in effect, idling.
Coast Mode can reduce the car’s fuel consumption by a few percentage points. This depends on the number of coasting opportunities and distances “coasted”.
HOW DOES IT WORK?
Certain gearboxes, such as the ZF 8-speed automatic, will have a special hydraulic pressure accumulator.
This component makes it possible to disengage Drive when the driver lifts his foot from the accelerator. Drive is re-engaged when the driver applies pressure to the throttle pedal.
CAN I DO THIS WITH ANY AUTO GEARBOX?
Any automatic transmission without Coast Mode can be manually shifted to Neutral to simulate Coast Mode.
However, without the benefit of backup oil pressure, the result is premature wear on the gear-ratio selector clutches.
So, if you don’t have Coast Mode, do not do this.
But in certain automated dual-clutch gearboxes, such as Volkswagen’s DSG or Porsche’s PDK, you can. These transmissions have an electric oil pump that provides hydraulic pressure to engage Drive on engine restart at a traffic light.
Said pump also enables the switch between Drive and Neutral for Coast Mode.
In fact, in any dual-clutch gearbox, there is no adverse effect when shifting manually to Neutral while coasting. It does not matter whether the car comes with auto stop-start and/or Coast Mode.
In such gearboxes, power transmission doesn’t rely on positive hydraulic pressure, as is the case with the automatic transmission.
Instead, the hydraulics are for the purpose of mechanically releasing the clutches or shifting gears.
There is no risk of damage if you choose to select N when “freewheeling” downhill. You can do the same with any manual gearbox.
SAFETY REMINDER
That said, if your car doesn’t have Coast Mode, don’t try to effect it manually.
If you’re not an attentive driver, you might forget that you shifted into Neutral. When you suddenly need power to avoid an accident, that delay could mean the difference between a fender-bender and a serious mishap.
