What is friction loss? How engine and drivetrain power is lost and the mechanism of frictional resistance

Even if an engine produces 100 units of power, not all 100 units are converted into force that propels the tires forward.

Inside the engine, pistons, bearings, and other components move, and the transmission, differential, and driveshafts also rotate.

Friction and resistance inevitably occur there.

Thus, one type of energy lost during the transmission of power in a car is "friction loss."

Friction loss is an important factor when considering acceleration feel, fuel efficiency, and the smoothness of engine rotation.

This article will clearly explain what friction loss is, where it occurs in the engine and drivetrain, and how friction and lubrication are related.

What is friction loss?

Friction means "摩擦" (friction) and loss means "損失" (loss).

Thus, friction loss refers to the energy lost due to friction when parts move against each other.

There are numerous rotating and sliding parts in a car.

To move them, force is required to overcome frictional resistance.

Therefore, a portion of the power generated by the engine from fuel energy is used not to propel the car forward, but to overcome internal friction.

And much of that energy is ultimately dissipated as heat.

Research by SAE International also indicates that friction within the powertrain is a form of energy loss, and that the energy consumed by friction is converted into heat.

Where is there friction inside the engine?

There are many friction points inside the engine.

Typical examples include:

• Piston and cylinder

• Piston rings

• Crankshaft

• Connecting rod

• Camshaft

• Valvetrain

• Bearings in various parts

• Oil pump

• Water pump

and so on.

The piston, in particular, repeatedly moves up and down at high speed inside the engine.

During this movement, friction constantly occurs between the piston rings and the cylinder wall.

Past SAE research has also shown that the piston-ring system, valvetrain, bearings, and auxiliary components contribute to mechanical losses inside the engine.

In other words, not all of the power generated by combustion becomes crankshaft output.

Friction between piston and cylinder

One of the areas where significant friction occurs inside the engine is around the piston.

The piston moves up and down inside the cylinder at high speed.

The piston rings, meanwhile, seal combustion gases and properly manage oil.

Therefore, a certain amount of contact pressure is necessary.

However, as contact pressure increases, friction also tends to increase.

Conversely, if contact is too weak in an attempt to merely reduce friction, it can lead to other problems such as compression leakage or oil consumption.

This means that engine design requires balancing:

"ensuring a tight seal"

and

"minimizing friction as much as possible."

This is where the importance of lubrication technology and surface treatment technology lies.

Why is engine oil necessary?

Engine oil is essential when considering friction.

If metals rub directly against each other, friction becomes extremely high, leading to wear and seizure.

Engine oil creates an oil film on the surface of parts, reducing direct metal-to-metal contact.

This is lubrication.

Proper lubrication can be expected to:

• Reduce friction

• Suppress wear

• Dissipate heat

• Carry away contaminants

• Protect metal surfaces

On the other hand, oil has viscosity.

While higher viscosity helps maintain an oil film, it can increase the resistance involved in moving the oil itself.

Lower viscosity tends to reduce resistance, but it becomes crucial to ensure adequate oil film under certain operating conditions.

Therefore, it is fundamental to adhere to the viscosity and specifications designated by the manufacturer.

SAE research has also shown that friction varies depending on multiple conditions such as oil viscosity, temperature, surface roughness, and rotational speed.

Friction tends to be greater when the engine is cold

The state of the engine is not the same immediately after starting it first thing in the morning and after it has warmed up sufficiently.

At low temperatures, oil viscosity tends to be higher, increasing the resistance to moving parts.

Therefore, the engine may feel a bit heavier when cold.

Conversely, when the engine warms up to its proper temperature, the oil approaches its intended viscosity characteristics, and all parts tend to move more smoothly.

This is one reason why the engine feels "lighter to rev" when it warms up.

However, this does not mean that raising the oil temperature excessively is a good thing.

It is important for oil to function within the appropriate temperature range.

Power is also lost in the drivetrain

The power generated by the engine is not directly transmitted from the crankshaft to the wheels.

In a typical car, it passes through many components, such as:

Engine → Transmission → Differential → Driveshaft → Hub → Tire

Each component, including gears, bearings, and seals, generates friction when rotating.

Therefore, there is a difference between the engine output and the power actually delivered to the tires.

This is commonly referred to as "driveline loss" or "drivetrain loss."

Energy is also required to rotate the transmission, driveshaft, differential, half shafts, hubs, and tires.

Why horsepower doesn't directly reach the tires

The engine output listed in catalogs is usually a value measured at the engine itself.

However, what actually accelerates the car is the force ultimately transmitted to the tires.

Along the way, losses occur due to factors such as:

• Gear meshing

• Bearings

• Seals

• Viscosity of oil or fluid

• Inertia of rotating parts

• Friction in various parts

In other words,

it's not just "how much horsepower the engine produces,"

but also "how efficiently that power is transmitted to the tires" that relates to actual driving performance.

This point is deeply connected to a previous article, "Reasons why cars with the same 100 horsepower have different speeds."

Even with the same 100 horsepower, the driving feel changes depending on not only vehicle weight, gear ratio, and torque characteristics, but also the overall efficiency of the drivetrain.

Does friction loss vary with drive type?

Cars come in various drive types, such as FWD, RWD, and 4WD.

As the number of power-transmitting components increases, so does the tendency for the number of rotating parts, gears, and bearings to increase.

For example, a 4WD vehicle may have additional components to distribute power to both front and rear wheels compared to a 2WD vehicle.

Therefore, a simple comparison may show an increase in drivetrain losses.

However, this does not mean that "4WD is inferior in performance."

4WD offers significant advantages such as traction performance and stability.

The important thing is that the "way power is transmitted" and the "necessary mechanisms" differ depending on the drive type.

Tires also have "rolling resistance"

When considering friction loss, the presence of tires cannot be ignored.

Tires rotate while in contact with the road surface.

However, they do not roll as perfect circles.

At the contact patch, the tire deforms and then recovers, a cycle that repeats continuously.

During this process, some of the energy is converted into heat.

This is one cause of rolling resistance.

If tire pressure is too low, tire deformation increases, and rolling resistance tends to rise.

As a result, you might feel that:

"the car feels heavy"

"it doesn't roll when you take your foot off the accelerator"

"fuel efficiency has worsened"

Checking tire pressure is a very basic maintenance task when considering vehicle resistance.

Brake drag also creates resistance

Brake drag can also contribute to a car feeling heavy.

This occurs when the brake pads do not properly disengage from the rotor even after releasing the brake pedal, maintaining friction with the rotor.

In this state, unnecessary resistance is generated even while driving.

Symptoms may include:

• Poor car coasting

• Worsened fuel economy

• One side becoming abnormally hot

• A burnt smell

• The car drifting to one side

This is not just a "feeling" issue; it may indicate a need for maintenance.

If you notice any abnormalities, it is important to have your car inspected by a dealer or service shop.

What changes can be expected if friction loss is reduced?

If the loss due to friction is reduced, then energy can be utilized more effectively.

Theoretically, this leads to:

• Smoother engine rotation

• Lighter movement of the drivetrain

• Potential improvement in fuel efficiency

• Easier maintenance of power delivered to the tires even with the same output

• More natural response to accelerator input

In fact, reducing friction loss to improve engine mechanical efficiency has long been studied as a means to improve fuel economy and reduce CO2 emissions.

However, it is not as simple as thinking that "less friction is always better."

Not all friction is bad

When we think of friction, we tend to assume it's something that should be eliminated.

However, friction is essential for a car.

For example,

Without friction between the tires and the road, you couldn't accelerate or turn.

Brakes also use friction to slow the car down.

Clutches also transmit power using friction.

So, the important thing is to:

"ensure necessary friction"

while

"minimize unnecessary friction loss as much as possible."

This is the fundamental principle of friction control in cars and machinery.

Does reducing friction always increase horsepower?

This is another common misconception.

Reducing friction loss does not necessarily mean that the power generated by combustion itself will increase.

More precisely, the idea is to

"reduce the power consumed internally and increase the effective power that can be extracted externally."

For example, even if an engine produces a certain amount of power through combustion, if much of it is used for internal friction, less power will be available externally.

When internal losses are reduced, that energy can be utilized more effectively.

Therefore, drivers may feel that "the engine revs more easily" or "responds more naturally to the accelerator."

Why do manufacturers research friction reduction?

Automobile and engine manufacturers have been researching friction reduction technologies for many years.

The reason is that the accumulation of small losses affects the overall efficiency of the vehicle.

For example,

• Reduced tension in piston rings

• Surface treatments

• Low-friction coatings

• Optimization of bearings

• Increased efficiency of oil pumps

• Low-viscosity engine oils

• Lightweight rotating components

and various other methods are used.

SAE research also states that reducing mechanical losses and friction is related to improving powertrain efficiency and fuel consumption.

Although each improvement may seem small, accumulating them enhances the overall efficiency of the car.

Next Science's perspective on "friction and driving quality"

The sensations experienced when driving a car, such as

"it rolls lightly,"

"the engine revs smoothly,"

"it moves forward naturally when the accelerator is pressed,"

and "the machine feels like it's moving lighter than before,"

cannot be explained by peak horsepower alone.

There are many frictional elements throughout the car, including within the engine, drivetrain, bearings, and tires.

At Next Science, we consider "friction and lubrication" to be one of the important research areas when thinking about vehicle and machine performance.

It's not just about adding something to increase output.

How is the energy generated originally lost along the way, and how is it transmitted to the drive?

Focusing on this "energy flow" is also crucial for considering driving quality.

Summary: Friction loss is "invisible resistance"

Friction loss is the energy lost due to friction when parts move.

In a car,

inside the engine,

transmission,

differential,

driveshaft,

bearings,

tires,

resistance occurs in various places.

And some of that is lost as heat.

That's why, when considering a car's driving performance,

it's not just "how much power the engine produces,"

but also "how efficiently that produced power can be delivered to the wheels" that is important.

Horsepower,

torque,

throttle response,

and friction loss.

By considering these together, the "driving performance of a car" that isn't apparent from catalog figures alone becomes clearer.