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Power into Propulsion: How the Transmission & Driveline Work

Text by Dmitriy Orlov

The powertrain, also commonly called the drivetrain, is the sum of all components in a vehicle, from those that generate power to those that send power to the wheels to move a vehicle forward. The engine, transmission, axles, hubs, and driveshafts all fall under the umbrella term "powertrain.”

This article series covers the powertrain extensively — defining, disassembling, and explaining the processes involved in power generation, power transmission, and power application (think tires and suspension). Once again, we will "zoom in” on various aspects and details and then "zoom out” to show the big picture and emphasize how the various parts are connected.

We have already mentioned how the engine converts chemical energy into rotation and the components that allow it to turn over. The next step in understanding the powertrain is to discuss the driveline.



The driveline, for the focus of this article, specifically pertains to the hardware that converts the output of the engine into movement — basically, everything that happens after the engine. As the engine rotates, the components in the driveline interface with the motor and help use its power efficiently.

Shifting into Gear

In explaining the driveline, we will use a manual transmission, the most common and oldest setup, but we will also explain any variations or deviations.

The primary function of a transmission assembly is to interface with the engine. If the motor were always directly connected to the driveline, the whole vehicle would want to jolt forward as soon as the motor starts. This might happen if a beginning driver forgets to push in the clutch pedal or disengage the gears in the transmission when starting the car.



The clutch provides that interface — the ability to disconnect and reconnect the driveline from a rotating engine. The flywheel, a large disc bolted directly to the crank, is one-half of the clutch assembly. The other half is the clutch itself, which consists of a special disc and spring-loaded mechanism. The disc, which is composed of high-friction materials, gets pressed against the flywheel and begins to rotate and "sync up” with the rotation of the engine, eventually locking into place as the friction increases. The synchronization process is controlled by the driver, by letting out or pressing in the clutch pedal. Letting out the clutch pedal too quickly will cause the clutch disc to engage the flywheel too abruptly, stalling the car. If the clutch pedal is let out too slowly, while potentially achieving motion, one risks generating too much frictional heat and burning the clutch disc, which shortens its life span. Regardless, clutches do wear out over time and must be replaced as part of the regular maintenance cycle of a car.



Automatic transmissions have an alternative to the clutch — the torque converter. In the most common designs, there is never a physical connection. Fluid and pressure are used to transfer motor rotation to the transmission. Two turbine wheels are inside a torque converter, each linked to either the crankshaft in the motor or the transmission input shaft. As the engine is revved up, fluid is pressurized by one turbine and at a preset threshold starts to rotate the other turbine. At low RPMs, such as idle speed, the motor can spin freely. Think of stirring molasses with a spoon — you must first work against the inertia of the thick fluid until enough momentum builds up to rotate the rest of the liquid.



Inside the transmission are sets of gears between the input and output shafts. These gears scale the power output of the engine to best move a vehicle’s weight. Different gear ratios allow for better hauling of weight or optimize acceleration and performance. Shifting gears is done with a lever that moves selector rings inside and clicks them into place, depending on your selection. In an automatic transmission, this process is accomplished with fluid pressure, electronics, and small clutch packs, which help change gears.

Other transmission systems and configurations have been developed that are sometimes better suited for specific applications. Dual-clutch setups have been implemented in mass-produced cars and utilize a fully automated, electronic clutch setup. Response time and precision are much greater than what a human can achieve. This technology trickled down from race cars and is becoming quite common. Other transmissions, such as sequential and CVT (Constant Velocity), have also been used and have their own benefits. We will explain the intricacies of these setups in later articles.

Driveshafts 




The transmission transfers power to a driveshaft, which is the next step in the driveline.
Driveshafts have U-joints that allow for some flexibility, and these help "snake” power through the vehicle. They also provide a margin of error for the chassis and components in the driveline, giving the whole car added flexibility in delivering power to the wheels.

The classic setup in a front-engine vehicle has the driveshaft sending power to the rear wheels. Sometimes, there are multiple driveshafts if power is needed for both the front and rear wheels, as found in All-Wheel-Drive (AWD) setups. Front-wheel drive cars typically have no need for a driveshaft, as the wheels are located at the output of the differential, which is driven directly by the transmission.

Differentials



While the driveshaft transfers rotation from the transmission, the differential is responsible for splitting that rotation to either the driver or passenger side of the vehicle. The differential serves as a device that allows the wheels on either side to have slight variations in speed. For example, when cornering, the inside wheel will spin more slowly than the outside wheel, and the differential compensates for this, distributing the power evenly to both wheels. The differential also contains the final drive — the final gear of the driveline. This gear determines whether or not the car has better low-end grunt or a higher top speed. Changing the ratio between the final drive and transmission dictates how the car will accelerate and its top speed.

Everything Else



Out of the differential come the drive axles, or CV shafts. The older design had the axles coming straight out at a fixed angle, with the hubs and wheels mounted to the whole assembly. This made for one large, solid unit in the back, which hung from a suspension assembly. The solid rear axle is great for delivering power in a straight line and handling large loads but maneuverability, weight, and agility are sacrificed compared with newer setups having independent rear suspensions.

With independent suspension, you get smaller, more flexible axles — this resembles the function of the driveshaft, adding flexibility to the driveline. Hubs are at the very end of axles and are the interface between the driveline and wheels, which ultimately turn the rotation of the engine and driveline into the motion of the vehicle as a whole.

Wrapping up

The vehicle now has a way to transfer rotation of the engine into rotation of the wheels. The driveline is contained within the chassis of the vehicle and connected to the body via suspension technology, which has many functions to ensure consistent power delivery and comfort for passengers inside the car. In the next article, we will discuss shocks, springs, linkages, brake systems, wheels, and tires and how these keep the car grounded, safe, and comfortable.

Other articles in the Building Blocks Series - The Basics:




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