ENGINE BRAKING | How Semi Trucks Slow Down Without Brakes | SCIENCE GARAGE

TLDR;

This video explains the mechanics of engine braking in manual transmission vehicles, contrasting it with diesel engine braking methods like the Jake brake.

• Engine braking occurs when you release the gas pedal without disengaging the clutch, utilizing the engine's vacuum to slow the vehicle.
• Diesel engines require a supplemental braking system, known as Jake brakes, to effectively slow down due to their design differences.

Understanding Engine Braking [0:00]

Engine braking happens when driving a manual transmission. By lifting your foot off the gas pedal without engaging the clutch, the car slows down due to the vacuum created within the engine. This method is favored by manual transmission drivers because it allows for more control over the vehicle's speed without damaging the engine or transmission. Understanding the function of the engine as an air pump helps explain the fundamentals of engine braking.

Process in Gasoline Engines [1:06]

The video outlines the four-stroke cycle in gasoline engines, detailing the intake, compression, power, and exhaust strokes. When engine braking, pushing the throttle closes the intake valves, resulting in a limited airflow that creates a vacuum, thus slowing down the vehicle. If the clutch is engaged during this process, the engine's revolutions per minute (RPM) drop significantly, reducing deceleration. Additionally, downshifting increases the number of vacuums generated, enhancing the engine braking effect.

Is Engine Braking Harmful? [4:06]

Using engine braking is generally safe as it merely involves creating air pressure to slow the wheels. However, excessive revving can lead to engine damage. The key is to use engine braking judiciously without pushing the engine into harmful RPM ranges.

Jake Brakes and Diesel Engines [4:30]

Unlike gasoline engines, diesel engines do not have throttle valves, which limits their ability to engine brake naturally. Therefore, a supplemental Jake brake system is necessary, especially in heavy vehicles, to assist in deceleration. This section introduces the history of the Jake brake, created by Cummins, and its function in allowing compressed air to be expelled rapidly, thus slowing down the vehicle by reducing energy from engine movement.

How the Jake Brake Works [9:00]

The video describes the operation of the Jake brake. In contrast to the engine braking in gasoline vehicles, the Jake brake controls exhaust valves to release compressed air during the compression stroke. This mechanic reduces the engine's overall energy output, effectively slowing the vehicle without affecting the brakes significantly.

Summary of Differences [10:00]

In conclusion, the video contrasts engine braking in manual gasoline vehicles—using vacuums to slow down—with Jake braking in diesel engines, which uses released air pressure to achieve deceleration. Understanding these functions helps clarify why different braking methods are used for various engine types.