GEAR MECHANISM

Manual Transmissions

As the name suggests, the selection of gear ratios on manual transmissions is accomplished by manually shifting a gear selector mechanism that disengages one gear and selects another. Most modern types of this transmissions have five or six forward (and one reverse) gears.

This type of transmission is connected to the engine via a clutch, which if disengaged (by pressing the foot pedal or hand lever), removes torque from the system. With the clutch engaged, power is delivered to the transmission through an input shaft that is separated from the primary cluster, and which can rotate at a different speed to the cluster. The cluster is comprised of gears that can be moved to either engage, or disengage from fixed gears on the secondary shaft, which is normally located below the cluster gears, and supported by roller bearings in its casing.

Gear Selection

The input shaft has only one gear that engages with a gear on the secondary shaft. When it is running and the clutch engaged, the transmission is said to be in “Neutral”. No gears on the primary are engaged with gears on the secondary shaft.

When shifting into first gear (for example), the gear on the primary shaft is slowed down by a synchronisation mechanism, and aligned with a gear on the secondary shaft. The current gear is always disengaged before the next one is engaged.  When the clutch is re-engaged, rotational energy is transferred from the transmission to the axle(s) by the drivetrain.

                           
By using progressively smaller ratios, the speed of the vehicle can be maintained or increased using less power from the engine.

Advantages

• Extremely robust, and can handle high torque loads.
• Very reliable, and relatively easy to service, maintain, and repair.
• The solid link between driving wheels and the engine provided by this transmissions provide a valuable driving aid to drivers in the off-road environment, by allowing the use of engine braking while descending slippery slopes where the use of ABS brakes could be dangerous.
• Cheapest to repair

Disadvantages

• Learning curve – can be difficult to operate for drivers who are accustomed to automatic

Fully Automatic Transmissions

In an automatic transmission, the hydraulically operated control systems are managed electronically by the vehicle’s computer instead of the clutch and gear stick. All the driver has to do is shift the selector from Park (P) or Neutral (N), into Drive (D), and the gear shifting will take place automatically and smoothly, without any additional input from the driver under normal driving conditions.

Gear Selection

Gear selection in automatic transmissions depends on many operational conditions, such as vehicle speed, engine speed, performance mode (where fitted) selected as well as driver assist systems such as traction control, stability control, automatic/autonomous braking and cruise control.

Provided all necessary conditions are met, and there is agreement between the Engine Management and Transmission Control systems, pressurized transmission fluid is automatically channelled to mechanisms that drive sets of planetary gears and clutches, which are roughly analogous to the gear ratios found in in this type of transmission.

Rotational energy is provided by a torque converter, a mechanism consisting of two freely rotating parts. One half is attached to the engine and the other to the transmission input shaft. The two halves of the converter are positioned very closely together. The fluid that circulates between them is influenced by the rotational energy of the engine which in turn impacts the transmission side of the converter. Torque is transferred from the engine to the transmission thanks to the shearing strength of fluid.

In older automatic transmissions, the shifting action was accomplished by internal fluid pressures overcoming spring tension – closing one circuit before opening another. This control system produced harsh, jerky shifting. Modern systems moderate the action of the controlling valve shuttles, providing almost seamless shifting.

Advantages

• Very easy to use
• Provides a comfortable driving experience
• Modern automatic transmissions match manual transmissions in terms of performance and fuel economy

Disadvantages

• Complex and prone to failures, malfunctions, and unsatisfactory performance due to a variety of possible issues, some of which are unrelated to it itself
• More expensive than a manual transmission to maintain over a vehicle’s lifetime
• Expensive to repair. Replacement is often the more cost effective option
• Unsuitable for use in off-road environments because it cannot provide engine braking

Semi-Automatic Transmissions

Also known as an “automatic manual” or “clutchless manual” transmission, the simplest way to describe this type is to call it a hybrid between a fully automatic and manual transmission.

Similar to a manual transmission, gears are changed via a simple shifter or paddles located behind the steering wheel. However, there is no need to operate a clutch pedal. Processors, sensors, pneumatics and actuators are all used to “automatically” shift the gears once the drive has signalled the change.

The basic principles of shifting mechanical gears in a semi-automatic fashion have been in use on heavy commercial vehicles for many years. The latest and best designs provide lightning-fast, almost undetectable gearshifts.

The design of these systems varies, but all semi-automatic transmissions rely on microprocessors to control the changing of mechanical gear ratios with the help of electrically operated actuators and servos. These transmissions were limited to high-end supercars at first due to their high cost, but an increasing number of manufacturers are fitting them to mid-range cars.

Dual clutch systems employ two clutches, one controlling gearshifts in the even numbered gears and, anther that controls the odd numbered gears and reverse. This arrangement does not interrupt the power flow from the engine. The driver still has to initiate a gearshift via a shifter or paddles located behind the steering wheel, but there is no need for the driver to operate a clutch.

Advantages

• Smoother shifting/driving experience
• No energy losses due to slippage in torque converters, or during the time lag of manual shifts

Disadvantages

• Complex and prone to failures, malfunctions, and unsatisfactory performance due to a variety of possible issues, some of which are unrelated to the transmission itself
• More expensive to maintain than a manual transmission
• Very expensive to repair. Replacement is often the more cost effective option

Continuously Variable Transmissions (CVT)

Unlike the others on this page, this transmission doesn’t use gears as its means of producing various vehicle speeds at different engine speeds. Instead of gears, the system relies on a rubber or metal belt running over pulleys that can vary their effective diameters. To keep the belt at its optimum tension, one pulley will increase its effective diameter, while the other decreases its effective diameter by exactly the same amount. This action is exactly analogous to the effect produced when gears of different diameters are engaged.

“Gear” Selection

Since one pulley is driven by the engine and the other is connected to the drive shaft, an infinite number of ratios can be produced. This enables it to always run at the most efficient speed, regardless of the load placed on it. Microprocessor-controlled sensors quantify load variations and by adjusting both pulleys, the optimum operating speed for the engine can be maintained without any input from the driver.

Advantages

• Constant, stepless acceleration throughout the engine’s optimum operating range
• Provides a comfortable ride by eliminating “shift shock”
• Better fuel efficiency
• Faster response to changing driving conditions such as variations in throttle and engine speed
• Eliminates energy losses associated with torque converters. Bottom of Form

Disadvantages

• Unsuitable for use in off-road environments because of limited torque-handling ability.
• Cannot provide engine braking