Table of Contents
- Introduction
- ICEV powertrain
- Drivetrain
- Driveline
- Powertrain vs. Drivetrain vs. Driveline
- PHEV powertrain
- BEV powertrain
- FCEV powertrain
- Conclusions
Introduction
In the world of automotive engineering, it is common for technical terms to be misunderstood or used one in place of the other, even when they refer to different components or systems. One common confusion happens between the terms Powertrain and Drivetrain. At the first instance, they sound similar, as both are related to the components which are producing and transmitting torque to the wheel. However, while they share some similarities, they refer to distinct systems with different functions.
This article aims to clarify these two concepts, explaining what each term means and what is the relationship between them. By the end, you’ll have a clear understanding of the differences between a Powertrain and a Drivetrain, and why using the correct term is important when discussing automotive systems.
ICEV powertrain
The Powertrain refers to the system of components in a vehicle that work together to generate power and deliver it to the wheels, enabling movement. In an internal combustion engine vehicle (ICEV), the powertrain is responsible for converting fuel energy into mechanical energy and transmitting that energy to the wheels.
The architecture of a powertrain depends on the type of traction system of the vehicle, which can be: front-wheel drive (FWD), rear-wheel drive (RWD), all-wheel drive (AWD) or 4-wheel drive (4WD). An example of powertrain components is presented in the image below for a vehicle powered by an internal combustion engine and with RWD.
The primary components of the powertrain in an internal combustion engine vehicle are as follows:
- internal combustion engine (ICE): the engine is the power source of the vehicle, where the chemical energy in fuel is converted into mechanical energy through combustion. This process occurs within the engine’s cylinders, which use controlled explosions to drive pistons, creating the rotational force (torque) that powers the vehicle.
- transmission: the transmission is responsible for managing and regulating the engine’s output to ensure the vehicle operates efficiently under different driving conditions. By changing gear ratios, the transmission allows the engine to run within its optimal speed range, providing the appropriate amount of torque to the wheels based on the vehicle’s speed and load.
- transfer case (for 4WD and AWD vehicles): the transfer case is found in vehicles designed with both front and rear axles powered by the engine. It distributes power between the front and rear axles, allowing for improved traction and performance, especially in off-road or low-traction conditions.
- propeller shaft: the propeller shaft is a rotating shaft that transmits torque from the transmission to the differential (located closer to the wheels). This component is crucial in vehicles with front-mounted engines and RWD, as it links the engine to the rear axle.
- differential: the differential is a set of gears that divides the torque coming from the engine between the left and right wheels, allowing them to rotate at different speeds when cornering. This is essential for smooth handling and stability, as the outer wheel needs to travel a greater distance than the inner wheel during a turn.
- driveshafts: sometimes called half-shafts in FWD vehicles, transmit the torque from the differential to the wheels. These shafts are responsible for delivering the rotational energy needed to move the vehicle.
Now let’s analyze in detail each component.
Engine
The internal combustion engine (ICE) has been the driving force behind most vehicles for over a century. This engine operates by burning fuel inside cylinders to create controlled explosions, converting chemical energy into mechanical energy that powers the vehicle. While internal combustion engines come in various forms, the two most common types in vehicles are gasoline (petrol) and diesel engines. Though they share the same basic principle, they differ significantly in terms of fuel, efficiency, and power characteristics.
Petrol engines use spark plugs to ignite a mixture of air and gasoline, producing power through high-revving, smooth combustion. These engines are known for their quicker response and higher RPM (revolutions per minute), making them suitable for lighter, performance-oriented vehicles.
Diesel engines rely on compression to ignite the fuel. Diesel fuel is injected into the cylinder, where it ignites due to the high temperature of the compressed air. This results in a slower burn but produces significantly more torque at lower speeds [rpm]. This makes diesel engines more fuel-efficient and better suited for heavy-duty applications such as trucks and buses, where pulling power and load capacity are key.
Torque is the main output in both types of engines, representing the twisting force that causes rotation—essentially, what gets the vehicle moving. Torque is produced when the expanding gases from combustion push the piston downward, turning the crankshaft, which ultimately powers the wheels. In general, diesel engines are known for generating more torque than petrol engines, especially at low speeds, making them ideal for towing and heavy-load tasks.
Transmission
The transmission is a key component in the vehicles powered by ICEs, adapting the speed and torque between the engine and the wheels. Its primary role is to manage and distribute the power generated by the engine, converting it into usable speed and torque for various driving conditions. By adjusting gear ratios, the transmission ensures that the engine operates within its optimal range, regardless of the vehicle’s speed or load.
There are two primary types of transmissions found in modern vehicles: manual and automatic.
In a manual transmission, the driver is responsible for selecting and engaging the gears, using a clutch pedal and gear stick/lever. This type of transmission offers greater control over the vehicle’s power and efficiency, as the driver can manually choose the most appropriate gear for any situation. Manual transmissions are often favored by driving enthusiasts for their engaging nature and precise power management, particularly in performance-oriented vehicles.
Automatic transmissions handle gear shifting automatically, without the need for driver intervention. Using hydraulic pressure and complex electronic controls, the automatic transmission determines when to change gears based on vehicle speed, engine load, and throttle input. This system provides a more convenient and smooth driving experience, particularly in stop-and-go traffic, making it the most popular choice in modern cars.
Regardless of type, the transmission is crucial in converting torque and speed to match the operating load of the vehicle. At lower gears, the transmission increases torque, providing greater pulling power for acceleration or climbing. At higher gears, it reduces torque and increases speed, allowing for efficient highway cruising with lower engine speed. This balance between torque and speed is essential for optimizing both performance and fuel economy.
Transfer case
In 4WD vehicles, the transfer case is used to distribute power to all four wheels, enhancing traction and control on challenging terrains such as mud, snow, and off-road environments. It is a key component in any 4WD or AWD systems, receiving torque from the transmission and distributing the torque between the front and rear axles.
The primary function of the transfer case is to ensure that the engine’s power reaches both axles, allowing all four wheels to engage and work synchronously. This power distribution can either be in a fixed manner—where power is constantly sent to both axles—or in a selectable manner, where the driver can choose between two-wheel drive (2WD) and 4WD modes depending on road conditions. Additionally, many transfer cases offer low-range gearing, which significantly multiplies torque at lower speeds, making it easier to navigate steep inclines, rough trails, or pull heavy loads.
There are different types of transfer cases, including part-time and full-time systems. Part-time 4WD systems allow drivers to manually engage 4WD when needed, offering more flexibility and efficiency during regular driving. Full-time 4WD systems, on the other hand, continuously send power to all four wheels, providing constant traction without the need for driver intervention.
Propeller shaft
The propeller shaft is a component used in vehicles equipped with a longitudinal powertrain layout, especially in rear-wheel drive (RWD), four-wheel drive (4WD), and all-wheel drive (AWD) configurations. Its primary function is to transmit the power and torque, generated by the engine and transmission to the vehicle’s axles.
The front propeller shaft is responsible for transmitting power to the front axle in 4WD or AWD vehicles, especially when the transfer case engages all four wheels for improved traction. It connects the transfer case to the front differential, enabling the front wheels to receive torque.
The rear propeller shaft is used in RWD and 4WD vehicles to transmit power from the transmission or transfer case to the rear axle, driving the rear wheels. In 4WD vehicles, the rear shaft operates alongside the front shaft to provide balanced power to all wheels.
These shafts are designed to handle high rotational speeds and significant torque while maintaining smooth power delivery. They are typically made of strong, lightweight materials such as steel or aluminum to ensure durability and efficiency. Universal joints or constant velocity (CV) joints are often incorporated into the shafts to accommodate the movement and flexing of the vehicle’s suspension and axles.
Differential
The differential is responsible for allowing the wheels to rotate at different speeds while cornering. This function is crucial because, during a turn, the outside wheels travel a greater distance than the inside wheels, requiring them to rotate faster. Without a differential, the wheels would be forced to rotate at the same speed, leading to poor handling, increased tire wear, and potential drivetrain damage.
There are two common types of differentials found in vehicles: open differentials and limited-slip differentials (LSD). Both serve the same basic function of distributing torque between the wheels, but they handle traction and power distribution differently.
Open differentials are the most common type and are designed to split power evenly between the wheels on an axle. However, in low-traction situations (like snow, mud, or gravel), an open differential will send the majority of power to the wheel with the least resistance, often resulting in one wheel spinning while the other remains stationary. This can lead to traction problems, particularly on slippery surfaces.
Limited-slip differentials (LSD), on the other hand, are designed to overcome the limitations of an open differential. An LSD can detect when one wheel is slipping and will transfer more power to the wheel with better traction. This improves handling and performance, especially in low-traction conditions or during high-performance driving. Limited-slip differentials come in various forms, including clutch-based, gear-based, and electronically controlled systems, each with varying levels of power distribution and responsiveness.
Driveshafts
The driveshaft is responsible for transmitting torque from the differential to the wheels, enabling the vehicle to move.
For FWD vehicles, driveshafts connect the front differential (often integrated with the transmission) to the front wheels, while in RWD and 4WD systems, driveshafts also connect the rear differential to the rear wheels.
Drivetrain
While the powertrain includes all components that generate and transmit power from the engine to the wheels (i.e., the engine, transmission, driveshafts, and differentials), the drivetrain refers specifically to the components that transfer power from the engine to the wheels, excluding the engine.
The components of the drivetrain include:
- manual/automatic transmission
- transfer case (only for 4WD or AWD vehicles)
- propeller shaft (for RWD, 4WD or AWD vehicles)
- differential
- drive shafts
The drivetrain components work together to ensure that the power generated by the engine (part of the powertrain) is delivered to the wheels, allowing the vehicle to move. The powertrain, on the other hand, also includes the engine, which are responsible for generating the power that is sent through the drivetrain.
In summary:
- the powertrain includes everything from the engine to the wheels.
- the drivetrain includes the components that transfer power from the engine to the wheels, but excludes the engine itself.
Driveline
The driveline is a subset of the drivetrain. It specifically refers to the components that transfer rotational power from the transmission to the wheels. The driveline excludes the transmission itself but includes:
- transfer case (only for 4WD/AWD vehicles)
- propeller shafts (front and rear)
- differentials
- driveshafts
In simpler terms, the driveline is everything involved after the transmission that delivers power to the wheels. The driveline’s main function is to transfer torque from the transmission components to the wheels while accounting for the dynamic motion and angles of the suspension and wheels.
Powertrain vs. Drivetrain vs. Driveline
The image below provides a clear breakdown of the relationship between the powertrain, drivetrain, and driveline, showing how each system is nested within the other and which mechanical components belong to each group.
The powertrain includes everything that generates and transmits power to the wheels, including the engine, transmission, and all downstream components in the drivetrain and driveline.
The drivetrain includes everything from the transmission to the wheels but excludes the engine itself. It transmits power that is already generated by the engine to the rest of the vehicle.
The driveline specifically refers to the parts that deliver rotational power from the transmission to the wheels, ensuring they receive the necessary torque to rotate.
PHEV powertrain
The image below shows a Plug-in Hybrid Electric Vehicle (PHEV) powertrain, which integrates both an internal combustion engine (ICE) and an electric motor to drive the vehicle. PHEVs offer the benefits of both gasoline and electric power, providing flexibility in fuel use and increased efficiency.
The PHEV powertrain in the image combines a petrol/gasoline engine with an electric motor to deliver power to the wheels via an automatic transmission and a propeller shaft connected to the rear differential. The vehicle can operate on either electric power or gasoline, or a combination of both, optimizing for efficiency, performance, and range. The high-voltage battery provides the necessary electric energy for short trips or low-emission driving, and the vehicle can be charged by plugging it into an external power source.
The image below shows an 8-speed automatic transmission with an integrated electric motor, commonly used in PHEVs. This transmission is designed to combine the benefits of traditional internal combustion engine power with electric motor assistance, providing smooth gear changes and optimized fuel efficiency. In this system, the electric motor is housed within the transmission, enabling hybrid functionality without requiring a separate electric drive system. The dual-mass flywheel helps reduce vibrations and improves driving comfort, while the planetary gearbox ensures smooth and efficient power delivery through multiple gear ratios.
This transmission works by integrating the electric motor with the traditional automatic transmission’s gear set. During all-electric driving mode, the electric motor drives the vehicle independently, bypassing the engine. When additional power is required or the battery is low, the internal combustion engine starts, and the transmission shifts between the gears to maintain optimal performance. The planetary gearbox allows for smooth gear changes, while the electrohydraulic control module manages the coordination of gear shifts and the interaction between the electric motor and the engine. This setup allows for smooth transitions between electric, hybrid, and engine-only modes, enhancing fuel efficiency and performance in various driving conditions.
BEV powertrain
The powertrain of a battery electric vehicle (BEV) is much simpler, compared to an ICE vehicle. The main difference is that in a BEV, the multi-gear transmission usually is missing since the electric motor has better traction characteristics and does not need a transmission attached to it.
A BEV powertrain is powered by electricity stored in batteries, whereas an ICE vehicle powertrain relies on the combustion of fuel to generate power. In an ICE vehicle, fuel is burned inside the engine to create a controlled explosion that moves pistons, which in turn rotate the crankshaft and ultimately drive the wheels. This process involves multiple moving parts including cylinders, pistons, crankshafts, and a complex transmission system to manage gear changes.
In contrast, a BEV powertrain works by using electrical energy stored in its high voltage battery pack to power one or more electric motors that drive the wheels. When acceleration is required, electricity from the battery is delivered to the electric motor(s), which converts it into mechanical energy to turn the wheels. The simplicity of this system allows for fewer moving parts compared to an ICE vehicle’s drivetrain. Additionally, BEVs can regenerate energy through regenerative braking; when braking or decelerating, instead of solely using traditional brakes to slow down, kinetic energy from motion can be converted back into electrical energy and stored in the battery for future use.
The powertrain of an BEV is the Electric Drive Unit (EDU). Usually, the EDU contains the:
- electric machine (which can work as a motor or generator)
- power electronics module (which controls the torque of the electric machine)
- single-speed or multi-speed (2 gears) transmission
- differential
If the BEV had AWD capability, each axle has an independent EDU.
In conclusion, the EDU is the powertrain of the electric vehicles, and is responsible for converting electrical energy from the battery into mechanical energy to drive the wheels.
FCEV powertrain
The powertrain of a fuel cell electric vehicle (FCEV) is exactly the same as in a BEV. The only difference is that in an FCEV the elecal energy is mainly supplied by the fuel cell, which used hydrogen stored in a tank and oxygen from air to generate electricity.
The powertrain of an FCEV includes several key components: the hydrogen fuel tank, which stores the hydrogen gas; the fuel cell stack, where the hydrogen reacts with oxygen to produce electricity; the electric motor, which uses this electricity to drive the vehicle; and a battery pack, which stores energy recovered from regenerative braking and provides additional power when needed.
This combination allows FCEVs to offer the extended range and quick refueling times of conventional vehicles, while maintaining the environmental benefits of electric propulsion.
Conclusions
In a nutshell:
Powertrain = Engine + Drivetrain
Drivetrain = Transmission + Driveline
Driveline = Transfer case (if AWD or 4WD vehicle) + Propeller shaft + Differentials + Driveshafts