Economy runs dictate that hard braking and hard acceleration should be avoided completely, yet most drivers, in order to acquire the most momentum before coasting, push the pedal to floor
You have most likely witnessed one of the most overrated driving techniques ever.
You probably even practise it yourself. In these harsh economic times, one would be forgiven for trying to save a penny here and a penny there by eking out the most from whatever fate has to offer — in this case, let’s say half a tank of petrol.
To go the farthest on that half-tank, most of us engage in this habit, but are the benefits scientifically sound… or are they just psychological?
I am talking about coasting or dieseling, the act of “rolling” a vehicle in neutral, and thus using its momentum (and the earth’s gravity) to move rather than its engine power.
It mostly happens in downhill sections.
The approaching flat is used to accelerate as hard as one can, and then the transmission is thrown into neutral once the downhill section is reached so as to coast as fast as possible over the farthest possible distance before the power train is put back into gear for the next flat or uphill section.
But there are several things wrong with this scenario; the first being that it is in direct contravention of the basic rules of economy driving.
Economy runs dictate that hard braking and hard acceleration should be avoided completely, yet most drivers, in order to acquire the most momentum before coasting, push the pedal to the floor to accelerate rally fast.
This is self-defeating because you end up using large amounts of fuel to attain the desired momentum, and the savings on the coasting don’t compensate for it.
Again, on economy runs, the rule of thumb is to accelerate gently downhill (low engine load, low fuel consumption, maximum momentum gained since engine power is supplemented by gravity), get off the power on the next incline and slowly feed the power in as you gradually lose momentum, all while in gear.
Nowhere is it recommended that one coasts.
We tend to do the exact opposite.
Safety is another key issue.
The welfare of both the motorcar and the motorist are compromised whenever a vehicle is driven in neutral at high speeds.
Typically, most drivers re-engage gears (top or second to top) on the next uphill section to power on or to overtake, but for a car at speed, this is risky.
There is a danger of ruining the entire transmission (clutch system, drive-shafts and gearbox) if you accidentally get the gear change wrong, such as engaging third or first instead of fifth, or engaging R (Reverse) or 2/L (Second/Low) instead of D (Drive) when sliding from N (Neutral) in an automatic.
Over-revving the engine is another risk, forgivable in most petrols but fatal for any diesel engine. Diesel engines cannot over-rev by simply applying the throttle — and should not, hence they have a rev-limiter fitted.
But slip the transmission into a lower gear at high speed in a diesel-powered car, and you will be party to the rare phenomenon of “hydraulicking”, a situation where the engine feeds on itself and thus cannot be switched off.
Hydraulicking occurs when the diesel engine over-revs, usually caused by downshifting inappropriately.
The heavy components that make up the diesel engine have high moments of inertia, so high rpm engine speeds will cause the entire valve train to shatter and oil to start seeping into the combustion chamber past the damaged valve seals, drawn in by the negative pressure of each induction stroke.
Most people don’t know this, but a diesel engine can actually run on atomised engine oil, so the leaking oil now becomes the fuel.
Deactivating the fuel pump (if electronic) by turning or removing the ignition key is futile because the engine will not stop running until it burns up all the oil or seizes, hence “feeds on itself”.
Either way, you will not be having an engine any more by the end of it.
Downshifting wrongly is also a way of inducing “drift” or oversteer in a car; it is actually a drifting technique used by professional drifters, and when used intentionally is referred to as “shift-lock drifting”.
The surfeit of torque channelled to the driving tyres when downshifting early causes them to lose traction and slide.
When done on a skid-pad at low speed, it is considered expert driving; when it occurs on a highway at high speed, it will only result in tears and a painful, expensive and possibly fatal end of one’s trip.
Again, for diesels, is the application of the exhaust brake.
This is meant for larger vehicles to assist in hilly descents without using the actual brakes to minimise fade on the brake pads.
It is also meant as a safeguard against brake failure. Engine compression is used to slow down the vehicle whenever the exhaust brake is used, due to the back pressure induced by closing off the exhaust manifold.
Applying the exhaust brake continuously in neutral will cut out the engine, and I don’t need to explain the risks of steering systems locking up or brake boosters losing functionality whenever the engine goes off.
The same compression braking applies for petrol engines, only they aren’t usually equipped with an exhaust brake.
That is the downside of driving in neutral. So, what is the upside of driving in gear?
Automotive technology is so advanced nowadays that almost every system in the car has sensors transmitting signals to the ECU in real time. There are sensors closely monitoring engine load, so let us consider a circumstance where one is several metres from coming to a dead stop say, approaching a roundabout.
Situation 1: Throw the transmission into neutral. The car coasts slowly up to the roundabout with the engine idling.
Situation 2: Get off the power and slow down while still in gear.
Modern technology allows the engine to use zero fuel should the momentum exceed the engine power available (reverse load), then provides fuel the moment the situation shifts (courtesy of those real-time sensors), which means the momentum of the car rolling in gear will keep the engine turning without need for fuel.
Situation 1 demands some fuel to keep the engine from stalling, while situation 2 demands no fuel whatsoever.
Which has saved more fuel than the other?
It’s all here. Don’t drive in neutral, there are no real benefits.
To find out how to drive economically, we have an upcoming article on hyper miling, getting the most distance on the least amount of fuel.