Understanding Your Car’s Systems: The Powertrain

 Vintage car with hood open, showing engine
The powertrain is responsible for converted a vehicle’s energy input, usually stored as fuel or a charge in a battery, into motion. Obviously, it’s the most important system in your car, since motion is the entire point.

Power Source

The power source is where the energy to turn the wheels comes from. There are three power sources in common use in consumer automobiles; internal combustion, electric, and hybrid. 

Internal Combustion (IC)

Internal combustion engines use the expansive force from burning fuel in a confined space to turn a shaft that feeds into the drivetrain. By far the most common IC engine in road vehicles is the four stroke reciprocating engine. 

As the name implies, a four stroke reciprocating engine work through a cycle of four linear movements. First, the piston moves down as the intake valve opens and the fuel/air mixture is drawn. Most modern cars create the fuel/air mixture by spraying a fine mist of fuel into the air flow immediately before the intake, a process called fuel injection. Next, the intake is closed and before the piston moves up. The fuel/air mixture is ignited and the expansive force moves the piston, and thus the crank shaft and the other piston. Finally, the exhaust valve opens and the piston moves up, forcing the exhaust gases out and starting the cycle again. The piston movements are typically called the intake, compression, power, and exhaust strokes, respectively. 

Typically, one end of the crankshaft is connected to a pulley that’s used to power various components, and the other feeds into the transmission


Electric cars store energy in a battery, which is used to power electric drive motors and all the other electrical systems. The batteries are charged from standard household power. 


Hybrids are a mix of electric and internal combustion. They run as electric but the power comes from a generator attached to an internal combustion engine. Some, called “plug-in hybrids,” can also charge from an outlet. 


Hybrids win out on range. Usually IC vehicles beat out electric, but that’s not necessarily the case. 

Hybrid and IC are evenly matched for fuelling speed, both needing only a few minutes at most. Electric on the other hand, needs several hours, meaning it’s range is really per day rather than between recharges. 

IC leaves both hybrid and electric in the dust in terms of service life. Hybrid and electric vehicles typically become uneconomical to maintain within five to ten years, when the battery needs to be replaced. IC on the other hand, can be economically maintained indefinitely, assuming the owner is able to do most of the work himself. 

IC also has that powerful roar and manual and e-shift transmissions. All in all, hybrids are okay, IC is great, and electric is absolute garbage. 


The drivetrain is the subsystem that converts the power into movement. Electric and hybrid vehicles use electric motors to turn the drive wheels, but IC drivetrains are more complicated. 


The transmission, or more accurately transaxle in front wheel drive vehicles, is the first and most complex component in the drivetrain. It houses a complex series of gears that reduce that reduce the rotational speed from the high speed of the crankshaft to the approximate speed of the wheels. The gears can be rearranged on the fly by the driver or onboard computer into several predetermined arrangements to make the most efficient use of power and reverse direction. On front wheel drive vehicles, the transaxle is a part of the drive axle assembly ;and all others feed into the driveshaft that connects to the differential. 


The differential is the complicated part of the drive axle assembly. Its purpose is to split power between the drive wheels on both sides, and possibly additional drive axles. The cool thing is that it actually sends more power to the side that’s spinning more freely so you can turn more easily and without dragging either wheel. In front wheel drive vehicles, the differential is incorporated into the transaxle.

Drive Axle & Wheels

The drive axle assembly ends with the wheels, which I assume you understand. The wheels are mounted to the frame with a system of springs and shock absorbers, collectively called suspension. One or two u-joints, which can bend in any direction, are included in each drive axle to allow the wheels to move vertically while the rest of the drivetrain stays in place. 

Drive Wheels

The drive wheels are the wheels that actually propel the vehicle, rather than just hold it off the ground. The only strict rule is that they must be symmetrical around the centreline, but there a four common arrangement on two axle vehicles; front wheel drive, rear wheel, drive, four wheel drive, and all wheel drive. Front wheel drive and rear wheel drive have only two wheels. Four wheel drive and all wheel drive use all four, but not in the same way. 

Four wheel drive vehicles operate as rear wheel drive most of the time but have a second drive axle assembly the driver can engage at will. All wheel drive vehicles redistribute power between all four wheels in real-time to maintain traction, without excessively sacrificing fuel economy. 

Front wheel drive is typically used on cars and vans because they’re fuel efficient and get good traction under most conditions. Rear wheel drive is reserved for trucks because they only get good traction with a load or under near perfect conditions. Four wheel drive gets traction somewhere between what front and rear wheel drive can when on the road but can maintain traction off road, so it’s used on trucks and off road SUVs. All wheel drive is used on everything but trucks because it gets good traction on virtually all road surfaces but still get decent fuel economy. 

Traction Control Systems

Some cars now have automated traction control systems. They maintain maximum traction by overriding the differential and throttle and braking inputs. They can suck power in good conditions but they also make ice like dry pavement. 

Now that you understand your powertrain, watch out for more articles like this. 


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