Believe it or not, the engine doesn’t directly drive the wheels of a car. That’s the job of the drivetrain. The engine produces the power for the car, but that power is translated through things like the transmission, driveshaft, and flywheel before it actually reaches the tires.
Each car’s drivetrain is set up a little differently. Some cars are front-wheel drive. Others are rear-wheel drive. Many trucks and SUVs are either all-wheel or four-wheel drive. There are even quite a few cars that are two-wheel drive until all-wheel drive is needed, then the vehicle’s computer turns on the all-wheel drive system.
Speaking of all-wheel drive, here’s the difference between four and all-wheel drive. Spoiler alert: there is a difference:
All-Wheel-Drive:
Photo: Courtesy of Brakes Plus
All-wheel drive systems send power to all four wheels in varying amounts when slippage is detected. In other words, if the rear passenger wheel is slipping in an all-wheel drive vehicle, the other wheels will pick up the slack by having more power sent to them.
Most all-wheel drive systems are completely automated and some are only active when needed. Generally speaking, all-wheel drive systems are more road-friendly than four-wheel drive systems which is why they are automated. Subarus are especially well known for having all-wheel drive, but many SUVs such as the Honda CR-V, Mazda CX-5, and Toyota RAV 4 also have all-wheel drive.
Four-Wheel Drive:
Photo: Courtesy of How Stuff Works
The main difference between a four and all-wheel drive system is the amount of transferred power. Rather than sending power to all wheels in variable amounts, the transfer case simply sends power to all four wheels in equal amounts.
Four-wheel drive systems are often turned on manually because off-road transfer cases decrease on-road usability. Four-wheel drive systems are found mostly in pickup trucks and more off-road dedicated SUVs like the Jeep Wrangler, most Land Rovers, and the Toyota 4 Runner.
If some of those other terms are ones you don’t know, don’t worry. We’re going to go over them in a minute. A vehicle’s power has to get to the wheels somehow. Without making things too complicated, here’s what makes up the drivetrain:
Torque Converter:
Photo: Courtesy of Car Throttle
A torque converter couples the engine to an automatic transmission. Because most cars have automatic transmissions anymore, most cars also have torque converters.
Unlike a clutch, a torque converter is a fluid coupling. This means that it takes the rotation produced by the engine and translates that to the transmission of an automatic vehicle through the use of liquid. The spinning of the torque converter and its components produce pressurized flow of liquid that causes the spinning of the transmission.
This is why an automatic transmission vehicle doesn’t need a clutch pedal like a manual transmission vehicle. It’s also why you can stop a car with an automatic transmission without stalling the engine.
Clutch:
Photo: Courtesy of WhoCanFixMyCar.com
The torque converter is to an automatic transmission what the clutch is to a manual transmission. The clutch is engaged when the clutch pedal of a car is not being pressed. When the clutch pedal is pressed, the clutch itself is physically pulled away from the driveshaft or flywheel connected to the engine.
A clutch contains a friction disk which has abrasive material on its surface. The material causes the clutch to “catch” as the clutch pedal is released and eventually fully engaged. Slipping occurs when the friction disk doesn’t have enough friction to allow it to spin at the same speed as the revving of the engine.
When you release the clutch pedal too quickly, the stationary driven components are quickly connected to the rotating components connected to the engine, causing the car to stall.
Transmission:
Whether you prefer an automatic or a manual transmission, each transmission is basically made up of a series of gears that are larger or smaller, depending on the gear, to make the car go faster or slower. The simplest explanation of how a traditional transmission works is seen in a bike.
On a bike with different speeds, you can easily see the driving chain and the gears on which the chain moves. Moving to a higher gear means that the chain will jump to a gear with a smaller diameter on the back wheel. Moving to the next set of gears will make the chain jump to a gear with a larger diameter at the pedals.
Of course a car doesn’t work that way, but the concept is the same. You can drive the bike faster 14th gear than 1st, but it’s also harder to pedal and get moving in 14th gear as opposed to 1st gear. Cars can drive faster in top gear than in 1st, which is primarily used to start from a stopped position.
But this is 2018, and some non-traditional transmissions are being used in modern cars. All are a type of automatic transmission. Here are a few you need to know:
Semi-automatic or Automated Manual Transmission:
Photo: Courtesy of Car from Japan
Most cars with an automatic transmission that aren’t CVT or dual clutch transmissions would fall under this category. A semi-automated transmission is a traditional automatic that allows the driver the opportunity to put the car in “manual” mode and select the gears by using the shift lever or paddle shifters.
Continuously Variable Transmission (CVT):
The CVT has become the go-to transmission choice in many modern cars. Rather than gears, a CVT utilizes a belt and pulleys that change size and allow the car to go faster or slower. Consequently, a vehicle with a CVT won’t change gears and doesn’t have “speeds.”
A CVT doesn’t contain perceived transmissions shifts as a belt can be adjusted to any size while a geared transmission has to switch between two gears with a set diameter. The belt of a CVT allows the ECU of the car to always allow the car to function within optimal efficiency by constantly adjusting the belt size.
Companies like Subaru, Toyota, and Honda have adopted CVT transmissions in place of traditional automatic transmissions. CVTs are often used in cars with low to moderate horsepower numbers as they can’t reliably handle high power or high torque forces.
Dual Clutch Transmission:
Photo: Courtesy of Gear Patrol
A dual clutch transmission utilizes a clutch like a manual transmission, but is automated like a fully or semi-automatic transmission. Odd and even gears have their own clutch, meaning that 1st, 3rd, and 5th gear will have one, while 2nd, 4th, and 6th will have another.
Both operate as a singular unit, but because there are two clutches doing the work of one, shifts can be made quickly. Performance cars and some sports car are moving to dual clutch setups, because their quicker shift times translate to faster acceleration.
The Porsche 911, BMW M4, and Nissan GT-R all use dual clutch transmission setups. A dual clutch can handle the power output of their larger engines while increasing performance numbers and maintaining a certain level of efficiency.
Driveshaft:
A driveshaft simply connects a power source or source of rotation to what it will be powering. In other words, the driveshaft takes the rotation produced by the engine through the transmission and ultimately sends it to the wheels.
You can actually see a driveshaft working if you see a box truck or flatbed driving down the road. If you look underneath the truck, you will see a long shaft connected to the rear axle of the truck. As the truck moves, you will see the driveshaft rotate.
This is how a rear-wheel drive car sends power to the rear wheels. In a front-wheel drive car, there are one or two driveshafts that power the front wheels. Of course, they are going to be shorter than in rear-wheel drive car, and they do differ slightly, but the concept is still the same.
Front-wheel drive vehicles have a driveshaft that is connected to a constant velocity joint or CV joint, because the driveshafts are not perfectly aligned with the center of the front wheels. This allows for the angled driveshaft to power the wheels that need to drive flat on the road.
Differential:
Most front wheel drive cars have a transaxle rather than a differential like in rear wheel drive cars. A transaxle does the same thing as a differential, but it functions as more of a whole unit with the transmission than in a rear-wheel drive car. Because of that, we are just going to cover the function of a differential.
A differential takes the rotational movement of the engine, sent through the driveshaft, and translates it to horizontal rotational movement sent to each wheel. Again, to put this simply, it takes the centrally-located rotational movement of the driveshaft and sends it out to the wheels.
A differential also does one very important task. It allows one wheel to spin faster than the opposite wheel during a turn. A quick look at physics will show that the outer wheel of a car moves faster and covers more distance than the inside wheel during a turn.
The differential will allow this to happen without damage being done to the drivetrain. There are also several types of differentials you need to understand:
Open Differential:
The open differential is the typical type of differential described above. An open differential will send more power to the wheel with the least resistance.
Because of this, it doesn’t help in low traction situations as the slipping wheel will receive most of the engine’s power and the car won’t be able to move as quickly around a turn.
Limited Slip Differential:
Many performance and sports cars are equipped with a limited slip differential. Functioning like an open differential under normal driving situations, the limited slip differential also allows for increased cornering speeds when necessary.
It does this by using something similar to a clutch. When one tire slips, pressure is applied to a plate which locks and causes power to be sent to the wheel that does have traction. The wheel with traction powers the car through the corner, preventing slippage and loss of traction.
Locking Differential:
A locking differential is similar to a limited slip, however it is all or nothing. A locking differential will lock causing equal power to be distributed to both tires without taking the amount of slippage into account. With a locked differential, all tires will be spinning at the exact same speed.
This type of differential is favored in many off-road vehicles. It’s often manually operated which is also different than a limited slip differential which is operated automatically.
Torque Vectoring Differential:
Torque vectoring differentials are electronically controlled. A computer dictates wheel speed when cornering and accelerating. Some cars even use slight application of the brakes to control wheel speed during cornering.
Transfer Case:
Photo: Courtesy of Four Wheeler Network
All and four-wheel drive vehicles use a transfer case. A transfer case simply takes the power from the engine and transmission and sends it to all four wheels by locking and unlocking a driven driveshaft. A transfer case can be hydraulic, gear driven, or chain driven.
All-wheel drive vehicles have either permanent or conditionally functioning transfer cases. Those vehicles that have part-time all-wheel drive are electronically controlled. Four-wheel drive vehicles usually have a transfer case that is controlled by a mechanical or electrical switch.
That’s the basic makeup of a vehicle’s drivetrain. There are books that could be written explaining every single component of the drivetrain we’ve already seen here. There are even more components that we haven’t touched as well. Those are all for another time and place.
It’s really fascinating to see just how many things go into making a car move forward. It’s also more complicated than it seems at face value. At least now you have a basic understanding of how your car takes power from the engine and translates it to the tires of your car.
