Tech Guide – Gearboxes Part 1
So, why do we even need a gearbox? Let’s face it, the engine is spinning and so why can we not just connect it to the rear wheels to produce the required drive through a clutch? Well, unfortunately, unlike a steam engine with unbelievably high torque levels where direct drive is possible, the internal combustion engine, whether it be petrol or diesel, does not produce sufficient torque to allow this, and so we need a system of gears to allow the engine to drive the wheels at different speeds so that the engine operates within an effective speed range. Very high torque engines, like the V12 in the 850CSi, could famously pull away from stationary in top gear, although this was quite slow and you would not want to try it on a T-junction in front of an articulated lorry!
We would wager that pretty much everyone reading this has owned a car with a manual gearbox, or at least driven one, at some time. It is the most basic and simplest of the gearboxes that we’ll look at this month.
As the name implies, a gearbox is, quite literally a box full of gears arranged in a number of different configurations that, when connected by you physically moving the gear lever, offer you the various different gear ratios. BMW currently has up to six forward gears while Porsche offers seven ratios in its manual gearboxes. The gearbox features an input shaft and output shaft, in-line with each other in a rear-wheel drive car, and a countershaft or layshaft with gears that are matched to gears on the output shaft. The ratios are designed to provide maximum torque in the lower gears by reducing the rpm at the gearbox output shaft compared with the engine rpm at any given point, with a 1:1 direct drive gear ratio, usually one down from top, and usually a top gear ratio that essentially overdrives the rear wheels by increasing the rpm of the output shaft over engine speed. (Fun fact: in the E36 and earlier E46s, the five-speed gearboxes did not use an overdrive top gear as BMW felt that it had done enough work on engine efficiency to improve fuel economy to such a degree that it was decided a direct drive top gear was sufficient).
When you move your gear lever around the gate, you’re not actually moving any gears directly, what you’re doing is operating a selector fork that connects a collar with dog teeth that engage with matching notches in the gears themselves. The gears themselves are always meshed and, with the engine running, always turning, simply freewheeling on the output shaft. When you go to select a gear, you’re moving the selector fork to the corresponding collar and then engaging it with a chosen gear, moving that gear into mesh with it’s corresponding one; if you mess up a gear change and hear a grinding sound, it’s the dog teeth on the collar crunching against the faces of the gears themselves. Often, the dog teeth are undercut so that as power is applied after the successful engagement of the gear pair, the gears pull together to eliminate the gear selected ‘sliding out’ of mesh.
For reverse, an idler gear is used to rotate one gear on the output shaft in the opposite direction to the rest of the gears. Generally speaking, when you select reverse you will be physically moving the idler gear in between two other gears to reverse the output of the engine. As you only ever engage reverse when stationary (unless you are some sort of stunt driver), there is no synchromesh on the reverse gear as it’s not normally needed, although some manufacturers now install one for smooth operation of reverse. Speaking of synchromesh, this is a system designed for smooth operation that allows the collar and the gear to make contact and for the speed between them to match before the dog teeth engage. The systems used for this vary but all that’s required is a way for the gear and collar to make physical contact before the dog teeth, normally achieved using a cone clutch and a blocking (synchro) ring.
Automatic & Steptronic
An automatic gearbox is significantly more complex as it uses just one set of gears to produce the necessary different ratios via a planetary gearset. This is the main part of an automatic gearbox and is made up of three separate components: the sun gear; the planet gears and their carrier; and the ring gear. The sun sits in the centre, with the planets around it in the carrier and the ring gear is on the outside and each of these can be the input, output or can be held stationary, and the different combinations result in different gear ratios. In order to produce more ratios, you need compound planetary gearsets, which use multiple sun gears and planet sets to produce the different ratios. The gears are arranged in such a way that only certain planets will engage with the ring gear and their corresponding sun. A series of clutches and bands are used to hold specific components of the gearset stationary whilst allowing others to rotate in order to create the required ratio. These are in turn controlled by a valve body, known as the valve chest, and the sequence of operation is controlled by the transmission’s programming, with the metal bands being physically actuated by pistons that are operated by hydraulic pressure routed into the cylinder through a set of valves. The clutches are also actuated using hydraulic pressure, with fluid entering a piston inside the clutch causing it to engage whilst springs ensure it releases when the pressure drops sufficiently.
In older gearboxes, when it comes to actually changing gear, shift valves are used that operate with pressure from both the throttle and the centrifugal governor, fed by pressure from the output side of the gearbox. When you accelerate hard, the throttle pressure exceeds the pressure generated by the governor, and so the shift valve is prevented from moving and the car won’t change gear, allowing you to accelerate hard in-gear without having the ’box change up halfway through the rev range. Under light acceleration there’s very little pressure from the throttle allowing the higher pressure from the governor side to operate the shift valve, which is why autos will shuffle up through the gears quite quickly when being driven gently.
If you’ve got a vaguely modern car it’s likely to have an electronically controlled gearbox, which still uses clutches and bands but each hydraulic circuit is operated electronically via solenoids, which means less plumbing and the ability to programme in more advanced features and functions, such as downshifting when you’re travelling downhill or staying in a single gear when going through a corner rather than upshifting and downshifting unnecessarily. It also gives you Steptronic and its ilk, where manual control is electronically actuated, often by paddle-shift, and sends a signal to the gearbox to engage the next gear, or not if the electronics decide that it’s the wrong gear for a given situation and engine or transmission damage could occur should your selection be made. Modern gearboxes are much better at obeying your commands and some will not shift for you in manual mode, allowing you to really use the full rev range without worrying about unwelcome upshifts. Often, a ‘sports’ mode is available which accesses different programs in the gearbox controlling system that changes the rpm that the transmission shifts at and, in some cases, the speed of the shift.
In terms of actually transmitting engine power to the gearbox without a clutch, a torque converter is attached to the flywheel and creates a fluid coupling to the gearbox. The internals are complex but essentially use a turbine and a stator to move fluid through the converter similar to a viscous coupling. At idle, when stationary, the internals allow fluid to flow through the turbine in such a way that the engine can still spin but the car does not move forward, though there is usually enough pressure to see the car creep forward with the brakes off. Some manufacturers have now developed a system where this creep has all but been eliminated but have had to add an electronic parking brake system to prevent the vehicle running back on a hill during a hill start. When you accelerate, the increased speed of the flywheel and torque converter forces fluid through the turbine and causes it to spin, transmitting drive through the gearbox to the wheels. Once you reach a certain speed, when the turbine has reached roughly 90 per cent of the speed of the impeller, a lock-up clutch is applied, creating a more conventional fluid coupling, improving fuel efficiency and throttle response.
A few other things that are interesting and worth mentioning are: when you put your car in park, a metal pin mechanism physically locks the output shaft to the gearbox casing, preventing it from moving. It’s not a good idea to just use ‘park’ to hold a car in place as it places the drivetrain components and gearbox internals under stress. Selecting ‘park’ whilst moving can also shear this pin necessitating an expensive transmission repair. Also, modern autos always produce very favourable official economy and emissions figures as manufacturers are able to programme them to stay in a certain gear at certain speeds to make the most of the test parameters.
Words by: Elizabeth de Latour and Gerry Speechley | Photos: BMW, ZF