Crunching gears: how different types of transmission work
- •
- 389 reads
- •
- 984 views
- •
- 14 min read
For very good reasons, nearly every car in the world has some type of gearbox (also known as a transmission because it transmits power from the engine to the wheels, though technically this term includes other pieces of equipment which are not part of the gearbox itself).
As far as most drivers are concerned, there are only two types of gearbox: manual, which you have to operate yourself, or automatic, which can largely be left to its own devices.
For car manufacturers, it’s a bit more complicated than that. There are many varieties of gearbox, and the choice of which one to use depends on many factors including performance, fuel economy, customer preference and cost.
What follows is a quick run-through of the different types of gearbox you might encounter, and how they work.
Why we need gearboxes
To begin with, what’s the point of having a gearbox at all? Well, it’s all down to the relatively limited range of speeds an engine is capable of. Most engines in normal road cars are near to bursting point when they’re operating about seven times more quickly than when they’re in danger of stalling.
With the gearbox in first gear, the clutch up and the engine at idle, a car can travel at what in human terms would be a brisk walk. Multiply that speed by seven or so and you’re up to not much more than 30mph.
With only a single gear which would allow their drivers to get away from rest without stalling, cars would travel along motorways very slowly with their engines screaming and using an enormous amount of fuel. Other gears are required so that we can drive at higher speeds more peacefully and economically.
In the olden days, when cars were much slower and less refined than they are now, three forward gears were usually considered enough for the job. Nowadays nearly all cars have at least five, and some have as many as nine.
An exception
Top Fuel dragsters and their Funny Car equivalents, the fastest-accelerating cars in the world, can reach well over 300mph from a standing start in under four seconds, and they can do it with only one forward gear.
This is possible partly because their clutches are slipping nearly all the time. Controlling the amount of slip, and how it changes during a run, is one of the most important technical parts of drag racing.
Another factor is that dragster tyres are very flexible, and become much taller as the car gains speed. This means that the effective gear ratio changes considerably even though the mechanical gear remains the same.
Clutch slip and expanding tyres are not things we want to deal with in a road car, so we can leave them to the drag racers.
Electric cars
Like dragsters, road-going electric cars generally have a single forward gear. This is because electric motors do not behave the same way as petrol or diesel engines. They produce power when they’re hardly turning at all and can turn at high speeds, so they have a very wide operating range.
There are exceptions. Formula E cars have five-speed gearboxes, partly because the rules say they have to and partly so that their electric motors can be kept in the most efficient part of their operating range throughout a race.
The manual gearbox
Most of us learned to drive cars with a manual gearbox even if we later decided to buy an automatic. Using it can seem unnatural at first but it becomes instinctive eventually.
The principle of a manual gearbox is that there is are two shafts, each with several gear wheels mounted on it. (There can be other shafts too, but there don’t have to be.) Moving the gearlever changes the positions of the shafts and brings pairs of gear wheels together.
The number of teeth on each gear determines the ratio. For example, if you have a 34-tooth gear pushing a 27-tooth gear, the ratio between them is 1.26:1, which about what you would it expect to be for third in, say, a petrol-engined supermini.
Although they are complicated to use, manual gearboxes are lighter, cheaper and more efficient than automatics. This is why smaller and less expensive cars rarely use them, though manufacturers tend to offer them as an option on some models so that they can be driven by people who either physically can’t operate a manual or simply prefer not to.
The planetary transmission
Manual gearboxes were the norm in the early days of the motor industry, but the Ford Model T didn’t have one. Instead, it had what is known as a planetary transmission, so-called because of three gear wheels which rotate not only individually but also as a set.
The Model T had two speeds, low and high. To select low, you pressed what looked like – but wasn’t – the clutch pedal. To select high, you operated a lever and then released that pedal. You’d want to do this quite soon, because the pedal was heavy and passed all the vibrations of the engine and road surface through your left leg.
This all sounds awkward and complicated, but it didn’t put people off buying the Model T. Although it went out of production in 1927, it held the record for the world’s best-selling car until 1972.
Automatic transmission
The ‘classic’ automatic transmission includes both a gearbox and a torque converter. The gears are selected according to the amount of load on the engine, so, in more basic examples, backing off the throttle makes the car change up, though the driver can usually select their own gears if they prefer.
The torque convertor has a lot of slippage, so the engine revs do not necessarily relate directly to road speed.
Automatics of this type became popular – particularly in America, where people have gone through their whole lives without ever using a ‘stick shift’ – because there is no clutch pedal and you don’t have to worry about selecting gears if you don’t want to. Their ease of operation has made them almost universal on luxury cars.
The downside of automatics
Compared with manual gearboxes, conventional automatics are heavier and less efficient, so the cars fitted with them are slower and use more fuel.
This was certainly the case when automatics had only three gears. More recently, though, eight- and even nine-speed automatics (in, for example, the Range Rover Evoque) have become common.
The weight and inefficiency remain, but with so many gears the engine can easily be made to run at the optimum speed for any driving condition. Performance, economy and CO2 emissions have all improved greatly as a result of this.
The 'clutchless' manual
There have been several examples of cars with manual gearboxes but no clutch pedal. One was the NSU Ro80, produced between 1967 and 1977.
Among other radical features, it had a clutch which was operated by a sensor which could tell when the driver took hold of the gearlever. The C-Matic system on the Citroen CX, on sale in the same period as the NSU, worked in a similar way.
The CVT
Better known now as a truck manufacturer, Daf once built small passenger cars which all used a type of continuously variable transmission (CVT) called Variomatic.
In this, a belt ran around two pulleys, each of them made of two cones. The gear ratio depended on how far apart the cones were, which in turn determined how near the rims the belt ran.
Between two extremes, there was essentially an infinite number of gear ratios chosen entirely by the car. Driver selection was limited to a choice of forward and reverse. Dafs could go equally quickly in either direction.
At the time, only Daf (and later Volvo, which bought the Dutch company in the mid 1970s) used this system, though other manufacturers have done so since then. The main objection to it is that the engine speed has very little to do with the speed of the car, which results in quite alarming sound effects.
The return of the clutchless manual
Clutchless manuals of the type mentioned earlier made a brief comeback in the 1990s through the efforts of Renault, Saab and Volkswagen.
An amputee model called Heather Mills (better later known as the wife of Sir Paul McCartney) was employed by Saab to promote its car, and said that it allowed her to drive sportily without having to buy an automatic.
None of these cars sold well and they were quickly discontinued. Volkswagen, which shifted only about 110 examples of its Golf Ecomatic in 1994 and 1995, realised too late that it was the cheapest two-pedal diesel car on the UK market, and that a marketing opportunity had been missed.
The stepped CVT
A later development of the CVT, used occasionally by Fiat and Alfa Romeo and much more often by Audi, was a ‘stepped version’ with ‘ratio holds’. These limited the enormous number of possible gear ratios to a specific number – frequently six or seven – and made the CVT feel like a conventional automatic.
The philosophical objection to this was that these manufacturers were using continuously variable transmission which didn’t continuously vary.
There was a good reason for it, though. CVTs are more efficient than conventional autos, so the usual performance and economy penalties are significantly smaller.
The automated manual
This is broadly the same thing as a clutchless manual, in that neither of them requires a clutch pedal. Automated manuals go a step further because they don’t require a gearlever either.
Cars from manufacturers as far apart as Citroen and Maserati have used this system, but it has come in for a lot of criticism. Although the gearchanges are no slower than a gentle one with a normal manual gearbox, they seem to take ages because the driver isn’t doing anything.
To add to the problem, all the gearchanges take the same amount of time as each other, which makes the process feel particularly laborious if you’re trying to accelerate quickly.
The twin-clutch semi-automatic
This is used by many manufacturers but is often referred to as DSG because that’s the name given to the one developed by Volkswagen. (Audi calls it S tronic, but this is the same as the system used by all the other VW Group brands.)
A better technical term would be semi-manual, though as far as the driver is concerned it feels pretty much like a conventional automatic.
The gearbox is divided into two. Each contains half the total number of gears, and has its own clutch. Thanks to the operation of the clutches, only one half is engaged at any one time. The car predicts what the next required gear will be and already has it engaged before it’s needed, though it can change this if the driver unexpectedly shifts down rather than up.
Because what feels like a gearchange is actually just two clutches swapping duties, the shifts are extremely fast. Efficiency is greater than in a conventional automatic because there is no power-sapping torque converter in the system.