Overdrive: Keep it off when overtaking or lugging loads

If there is one thing any columnist tries to avoid, it is repeating oneself. Unfortunately, I will have to do just that this week.

I had talked about overdrive earlier, but reader feedback suggests I left a lot of unsatisfied curiosities out there, so we will have to put aside the happy-go-lucky merry-making of test drives and racing this week and step back into the lecture theatre.

Class is now in session, and the topic today is automotive transmission characteristics in general and two things in particular: the overdrive unit and Continuously Variable Transmissions — close relatives of the typical automatic gearbox.

The Overdrive Unit

This is one of the most misunderstood aspects of automotive transmission systems, especially when coupled to an auto-box.

Let us start with the overdrive unit in a manual transmission car as it is easier to explain away. In some instances, it is used as a standalone gear, just after top (1-2-3-4-O/D).

In such a case, the top gear of a car channels power directly from the clutch past the gearbox to the differential unit, with no gear reduction whatsoever.

With overdrive, what you have is a gear taller than top, in a reverse situation where, instead of gearing down, now the unit gears up the entire powertrain.

The overdrive unit/gear gives the gearbox a higher output speed than input speed (in top gear, both input and output speeds are the same).

It is usually used for cruising in low load situations because it keeps engine speeds low thus saves fuel and reduces wear and tear on the engine.

The other type, used in old British sports cars was the type you engaged or disengaged at will. It provided intermediate gears for the manual transmission, such as third-and-a-half (taller than third but just below fourth).

In those days three-speed and four-speed boxes were all the rage, so the gearing was interstellar at best to cover the high-torque, low-speed demands met by the lower gears and still provide power-sensitive top end zoom in the higher gears.

For the sake of example, let us use third and fourth. Shifting up and down between third and fourth is not only annoying for the driver, but it also impedes smooth progress and affects fuel economy.

An intermediate gear becomes necessary, let us call it third-and-a-half. The only way of getting this gear 3½ without changing your entire gearbox is to use an overdrive for the third gear, giving the intermediate ratio.

This overdrive could also be used for the other gears, even reverse. Nowadays most manual transmissions are six-speed, so the overdrive gear has been rendered unnecessary.

In automatic powertrains, the overdrive unit is a bit more complex. Long ago, it was a selectable position in the auto-stick, P-R-N-D-O, but nowadays, it is electrically activated by a push-button, commonly found on the gear lever itself. For practical purposes, we will look at the overdrive unit in a Volvo car:

The overdrive unit uses an epicyclic gear set, which is in essence a set of gears, one nestled inside the other, almost concentrically, if you will.

It is not entirely dissimilar to the planetary gear set used in most auto-box transmissions, except that it is not so far-reaching and versatile.

When engaged, the driveshaft connects to the carrier gear, the outermost gear set of the epicyclic arrangement.

When the carrier gear turns, the internal gears rotate slightly faster. The innermost gear set is called the sun gear, and it rotates much faster than the outer planetary gears courtesy of the diverse ratios.

The sun gear is the one connected to the drive axles, which turn the wheels of the car. In a nutshell, the sun gear (output) rotates much faster, at higher rpm, than the carrier gear (input).

Pressing the O/D button on the gear lever (turning it on, in this case) sends an electronic signal to a switch located within the transmission that engages the overdrive gear.

The end result is reduced engine speed for a given road speed, which in turn means improved fuel economy and less engine strain. For cars with high torque outputs, this could also mean a higher top speed.

So when to use it? I’ll tell you when NOT to use it. Leave it off when lugging heavy payloads, when going up steep hills, when overtaking or accelerating hard and when off-roading.

In other words, where high torque application is necessary, using overdrive is self-defeating. Also, do not use overdrive when going downhill and depending on engine braking to keep your speed in check.

Engaging it will allow the car to “run away”, seeing that the rev range necessary to provide sufficient compression resistance to slow the car down might correspond to much higher road speeds than anticipated.

Leave it on during ordinary driving, though. The benefits are enormous. However, some people claim that using overdrive when passing slower traffic may boost your speed, but this is only applicable in cars with high torque outputs.

Try that in a Vitz, on a small hill, and you will see dust.

Continuously Variable Transmissions

This is an adaptation of a typical automatic gearbox, and some of you may have across it. Have you ever driven a car with what looks like an auto-box, but vehicular acceleration is not at par with engine revs?

The car may accelerate rapidly but the engine revs stay constant, and those who are keen may have asked: what the…?

No need to curse, it is called a continuously variable transmission, and is the only gearbox you will ever find anywhere with an infinite number of gears.

Such are common in Euro-spec and JDM Nissan road cars: it debuted in the second-generation Primera saloon, and has seen action in the minuscule Micra and of late, the second-generation X-Trail crossover.

Even some Toyota Opa cars have this type of gearbox, and most interestingly, those silly go-karts that scared me half to death in South Africa’s Cape Province depend on this type of transmission too.

This is how it works: Unlike your typical gearbox which sports distinct toothed wheels (cogs or, better yet, actual gears) the CVT setup uses belts and pulleys that vary ratios infinitely between low (first gear, for maximum torque) and high (top gear, for maximum speed) and everything in between, all steplessly, hence the claim of having an infinite number of gears.

The most common type of CVT (and the one we will dwell on today) is the belt-and pulley system. This setup uses two opposing cone-shaped variable-diameter pulleys connected by a chain or metal belt.

One pulley is mated to the engine (input shaft) while the other is attached to the wheels via the driveshaft. Each pulley is made of movable halves.

When the halves move apart, the pulley diameter reduces as the belt slides down the cone faces, and the belt is forced to ride lower.

When the halves move closer, the belt slides up the tapered cones and the pulley diameter increases.

Changing the diameter of the pulleys can be done in indistinct steps, and this varies the transmission’s ratios, i.e. the ratio of the rpm of the input shaft to that of the output shaft, which in essence is what a typical gearbox does.

The only difference is in the other transmission types, this is done in distinct steps: the gears themselves. Think of the CVT the same way as a 10-speed bicycle directs the chain over a number of smaller gears to multiply torque.

To maintain the tension in the belt, as one pulley reduces its diameter, the other increases its own, and all this juggling is what creates the infinite gear ratios.

Making the input shaft pulley diameter as small as possible and the driveshaft pulley as big as possible gives “first gear”: maximum engine revs giving minimum road speed.

With acceleration, the pulleys vary their diameters to optimise the engine speed/road speed relationship, up to a point where the input pulley is big and the output pulley small for lower engine speeds and higher road speeds: that is “top gear plus overdrive”.

All this is made possible through sensors and microprocessors. The CVT, however, sounds odd; if anything, the noises coming from under the hood would suggest a transmission failure of some sort in other powertrain configurations, but it is perfectly normal for a CVT.

Also, the seamless power delivery would give a feeling of lethargy from behind the wheel when in actual fact the CVT can outperform other conventional transmission types. As such, CVT cars are still struggling to find acceptance in society.

To counter this, car manufacturers have had to inculcate some features that are in direct contrast to CVT characteristics, such as the creep feature like you would find in a normal automatic, and “gear simulation”, distinct steps in the transmission progression.

Driving a car with a CVT is a bit disconcerting. Even before the unusual acceleration at constant engine speed, stomping the throttle at take off makes the car sound as though the clutch is slipping or the automatic gearbox is failing: there is more noise than movement as the car adjusts the engine speed and road speed for the most appropriate relationship.

The reliability of CVTs has also been brought into question as they are delicate by nature. However, more robust construction has made them able to handle more powerful engines.

Initially, the CVTs used earlier could not handle more than 100 hp, but the current ones are capable of channelling up to 290 hp (Nissan Altima) to the tyres form the engine.

Just how good is this type of transmission? For starters, it was banned in Formula 1 because it was making the cars too fast(!).

It is also used widely in farm machinery, from tiny garden John Deere tractors to full-scale combine harvesters.

The benefits of a CVT are more usable power, a smoother drive and better fuel economy; though how this works out I don’t know.

The Gen-II Nissan Primera with a CVT returned a mere 23 mpg (7 kpl) from a 2.0 litre 4-cylinder engine. Maybe the economy figures have improved since then.

There are other forms of CVT, such as toroidal, hydrostatic, ratcheting and infinitely variable transmissions, but I doubt if I want to get into all that here and now. Maybe later.

Fun fact: The great scientist Leonardo Da Vinci actually invented the CVT back in 1490. Daf (from the Netherlands) put the first CVT into automobile application in 1958, and only in 1989 did the first US-sold production car have a CVT: the Subaru Justy GL hatchback.



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