A coasting mode is a powertrain technology designed to improve the claimed efficiency of a car. These systems achieve improvements in fuel consumption and emissions by effectively uncoupling the engine from the transmission in appropriation conditions. When the driver lifts off the accelerator in a conventional fuel-injected car, the fuel supply to the engine is typically shut off - or greatly reduced - and engine braking will begin to slow the car. No fuel is used but the car's deceleration will, in many cases, require that the driver soon begin accelerating and burning fuel again.
The use of a dedicated coasting mode can instead prove more efficient; because the engine is disconnected from the transmission, no engine braking takes place and the car decelerates far more slowly. It consequently travels a longer distance before the engine needs to be called upon, consuming less fuel overall. It is also safer than coasting manually (either using the clutch or by engaging neutral) as drive can automatically and immediately be restored should it be required.
The simplest set-ups just disconnect the engine from the transmission, and drop the engine speed to idle, allowing the car to sail along unimpeded. More advanced systems shut the engine off when coasting, which permits further benefits such as a greater reduction in fuel consumption. There are many examples of these systems but manufacturers do brand them differently. Volkswagen, for examples, calls its engine-off coast mode the 'eco-coasting' feature.
How do coasting systems work?
A coasting system is effectively an extension of the stop-start set-up already found in many a car with an automatic transmission. When the vehicle is in motion, the system monitors numerous parameters - including accelerator pedal position, brake pedal actuation and speed. If the driver takes their foot off the accelerator and the vehicle is still rolling, and then doesn't touch the brake, the coasting system uncouples the engine from the transmission - typically by disengaging a clutch within the gearbox - and brings the engine back to idle.
This allows the car to roll along for an extended period, as it is not slowed by engine braking, and reduces its overall fuel consumption and emissions. An idling engine still burns fuel, though, which is why more advanced set-ups will shut the engine off when it is uncoupled from the transmission; this permits greater efficiency and emissions improvements. As soon as the driver touches either the accelerator or the brake, the engine is restarted - if necessary - and the transmission re-engaged.
Coasting systems can be found in cars with dual-clutch transmissions and those featuring torque converter-based automatics, too; the ZF 8HP transmission, for example, permits both engine-on and engine-off neutral coasting at speeds of up to 99mph if the manufacturer requires it. Manufacturers could also employ coasting systems in manual transmission-equipped cars, if so inclined, by employing a clutch-by-wire system. This would grant the automatic operation of the clutch, allowing the engine to be disconnected from the transmission and shut down to permit engine braking-free coasting.
In certain situations, that said, the engagement of a coasting mode may prove a disadvantage. Consequently, some systems - including the one used in seven-speed DCT Minis - make use of both the navigation or forward-looking safety systems to assess whether the driver is likely to require engine braking to ensure better control and safety. If it's deemed that the car is approaching a junction or similar, then the coasting mode may be automatically locked out.
Hybrid vehicles can also benefit from coasting set-ups; Porsche, for one, has a special 'sailing' mode for its hybrids that features a clutch that disconnects the engine from the transmission but leaves the electric motor engaged. The motor can then be used to gently nudge the hybrid along, helping extend its engine-off range. Porsche also makes use of engine-on coasting in several of its other models, including the 911 and Macan.
Are engine-off coasting systems really that beneficial?
During the average trip, according to Bosch, around 30 per cent of the time is spent coasting - but the engine couldn't be shut off for the entirety of that time, as many coasting moments would be very brief. The savings made when the car is coasting for an extended period, however, could deliver a claimed fuel saving of around 10 per cent - and notable reductions in CO2 emissions. Furthermore, the larger the engine then the greater the potential saving made through coasting.
Bosch isn't the only company to claim such improvements in fuel consumption. Transmission manufacturer ZF, for example, states that its coasting function can similarly deliver a 10 per cent improvement in fuel consumption and a 10 per cent reduction in CO2 emissions. The use of a hybridised coasting set-up is claimed to have more significant benefits - with some manufacturers stating an overall fuel saving of up to 25 per cent in real-world conditions.
One further minor benefit to the use of coasting systems is a reduction in both vibration and noise, particularly in engine-off systems. In any case, as emissions and economy targets become increasingly tough to meet, expect to see coasting systems on a more regular basis.