Manual & automatic transmissions

The purpose of the transmission is to transmit the power and torque from the engine to the wheels.The engine is limited to the amount of power it can produce because it can only work within a certain number of RPM (revolutions per minute). 

There are different types of transmissions used in cars today. See below to find out more about three; manual, automatic and CVT transmission.

Interactive car

Automatic transmission

CVT

Manual transmission

Manual transmission provides a means by which the driver can vary the speed of the vehicle beyond the limits of the engine, by manually selecting different gears.

• The input shaft of the transmission is connected to the engine crankshaft; it transmits the power through the gears to the output shaft, which eventually connects to the wheels through the driveshaft.
• The input shaft is connected to the crankshaft via the clutch.
• The clutch is a means of disconnecting the drive between the engine and transmission to allow gear changes and a gradual movement from a standing position.
• The input shaft turns at the same speed as the engine and the output shaft's speed is determined by the RPM of the engine and the gear selected.
• Gear ratios:
  • The number of teeth on the gears determines the relative speed of one gear to another.
  • If gear "A" and gear "B" have the same number of teeth then both will rotate at the same speed.
  • If gear "A" is twice the size of gear "B" then it will rotate at half the speed.
  • If the gear on the input shaft has 20 teeth (this is being driven by the engine) and the gear on the output shaft has 40 teeth (this is driving the wheels) then the output shaft will turn at half the speed of the input shaft.
  • By varying the size (number of teeth) of the gears, the speed of the car will be varied. 
• Most manual transmissions have either four or five forward gears and a reverse gear.
• Reverse gear:
  • An extra gear called an idler is placed between the gears to reverse the direction of the output shaft.
• Gear clash:
  • Trying to change gear without depressing the clutch will result in a grinding noise.
  • This noise is called gear clash and it is caused by the rotating gears trying to engage with each other.
  • Manufacturers incorporate synchronisers in the transmission to prevent gear clash.
  • These slow down the rotating gears and help them to engage.

Automatic transmission

By selecting 'drive' with the gear lever and gently pressing the accelerator the car advances smoothly, selecting the correct gear itself, with no effort from the driver. To achieve this the automatic transmission combines three different systems:

• A torque converter.
• One or two sets of planetary gears.
• A hydraulic control system.

The torque converter is a fluid filled coupling and is made up of three parts:

• The impellor is bolted to the engine crankshaft via the flywheel. It looks like an inverted saucer with vanes attached.
• The turbine is attached to the transmission input shaft, and is shaped like the impellor.
• The stator sits between the impellor and the turbine. It has vanes and also a one-way clutch, which means that the stator can only turn in one direction.
• The torque converter does the same job as the clutch in a manual transmission, but there is one vital difference, the torque converter can multiply the engine power.
• The transmission input shaft can be turned faster than the engine speed.
• It does this by using the stator to deflect oil from the impellor into the turbine vanes and back against the impellor vanes.
• When the engine turns, the vanes on the impellor pick up the oil in the torque converter and the oil is flung outwards.
• This oil is deflected through the stator, which rotates, throwing the oil against the vanes on the turbine.
• The force of the oil causes the turbine to rotate and this drives the input shaft of the transmission and eventually the wheels of the vehicle.

The automatic transmission uses planetary gear sets to vary the gear ratios.

• The planetary gear set has a sun gear at the centre, a number of gears rotating on a carrier around the sun gear called planet gears and an outer ring gear called the annulus.
• By holding some gears stationary and rotating others the transmission achieves different ratios.
• The gears are held stationary by using hydraulically operated brake bands and internal friction plates (clutches).
• The planetary gears will rotate around the sun gear.
• The rotation of the planetary gears will cause the annulus to rotate.
• The annulus will drive the transmission output shaft at a higher speed than that of the input shaft, which is driving the planetary gear carrier.
• To reduce speed the annulus is driven by the input shaft and the planetary gear carrier drives the output shaft.
• The sun gear is held stationary.
• In this situation the driven gear (annulus) is rotating faster than the output shaft (planetary gear carrier) and the speed is reduced.

The hydraulic control system is responsible for applying fluid pressure to components in the transmission, which drive or hold the different gears.

• A pump in the transmission generates fluid pressure.
• A complex system of valves and pumps then operate the friction plates and bands to stop parts of the gear set from moving and allow others to turn freely.
• When the gear selector is in the 'drive' position and the engine is idling there is not enough force from the fluid being thrown by the impellor to rotate the turbine and turn the input shaft.
• When the accelerator is pressed the impellor speeds up and the force of the fluid increases until it starts moving the turbine and input shaft gradually and the vehicle moves slowly because the transmission is in the lowest gear ratio.
• Once a certain speed is reached the flow of fluid changes direction and pressure is applied in a different area.
• This changes how the gear set operates and a higher gear ratio is achieved.
• The gear ratios will change as required until the highest ratio is achieved.
• Overtaking or accerlating quickly:
  • There is a cable (called the kick-down cable) connected between the transmission and the accelerator, which senses when the accelerator is pressed to the floor.
  • This immediately selects a lower gear ratio in the automatic transmission.
• The bump:
  • Can be felt in the car when the gear ratios change either up or down.
  • This has been a sticking point for manufacturers for a long time and it's been only in the past few decades that this problem has been overcome with the advent of the CVT (continuously variable transmission) in production cars.

CVT (continuously variable transmission)

Its operation is quite simple.

• There are two pulleys, which are V-shaped.
• One pulley is fixed and the other pulley is in two sections.
• One section is fixed and the other is moved on the shaft by hydraulic pressure.
• A V-belt or in some applications a chain sits between the pulleys.
• Moving one side of the pulley and allowing the belt or chain to move up and down in the groove provides the varying gear ratios.
• Speed increases progressively; there are no apparent gear changes, just a smooth transition from stopped to full speed.
• The benefits of CVT's are:
  • Lower fuel consumption figures due to the more efficient use of engine power.
  • Smoother driving, as there are no apparent gear changes.
  • Simplicity of design.