What is the working principle of engine braking?

[MirandaFitz]

Engine braking utilizes the compression resistance, internal friction, and intake/exhaust resistance generated during the engine's compression stroke to create a braking effect on the drive wheels. The specific principles of engine braking are as follows: 1. Engine braking refers to when the throttle is released: during the compression stroke when the piston is near the top dead center position, the exhaust valve momentarily opens to release the high-pressure gas. 2. Closing the exhaust valve: there is very little gas in the cylinder, and when the piston moves from the top dead center downward during the explosion (expansion) stage, the cylinder is in a negative pressure state. 3. The gas resists the downward movement of the piston: the piston essentially creates a vacuum, generating torque in the opposite direction of the crankshaft, thereby producing a braking effect.

[JustinAnn]

Engine braking sounds quite mysterious, but it's essentially the engine acting as a brake itself. I love using this feature during long-distance hauling, especially on downhill sections. Simply put, when you release the accelerator, the engine suddenly opens the exhaust valve during the piston's upward compression of air, releasing the high-pressure gas. Normally, the compressed air would rebound, pushing the piston downward to generate power, but now all that energy escapes through the exhaust pipe. As a result, the engine not only stops delivering power but also becomes an energy-absorbing device, allowing the vehicle to slow down naturally. Every time I turn it on, it feels like applying an extra 30% of braking force, and the key benefit is that it doesn’t wear out the brake pads. For heavily loaded trucks, even continuous downhill stretches of dozens of kilometers can be tackled without touching the brakes, making it much safer.

[McGrace]

I've been driving heavy trucks for over 20 years, and engine braking is literally a lifesaver. Its core principle is valve ambush: when the piston compresses to the top and is about to exert force, the hydraulically controlled exhaust valve suddenly opens, releasing the compressed high-pressure air. This process is like pushing against a door with all your might only to have someone suddenly remove the door panel. The engine instantly becomes an energy-consuming device, converting kinetic energy into heat that dissipates through the exhaust pipe. In actual driving, it's extremely convenient—just flip the dedicated switch on the dashboard, making it particularly suitable for controlling speed on continuous downhill sections. The best part is that this braking method is completely silent, unlike exhaust braking which produces loud banging noises. Of course, its effectiveness decreases when temperatures are too low, so that's something to keep in mind.

[DiAdrian]

The key to engine braking lies in disrupting the normal engine cycle. When you activate this function, the exhaust valve suddenly opens at the top of the compression stroke, releasing the energy that should have been stored. As a result, there's insufficient suction when the piston moves downward, interrupting the entire four-stroke cycle and generating reverse traction. I often use it on mountain roads, and the feeling is like the car being pulled back to automatically decelerate when you release the accelerator. This is safer than using the brakes, as there's no risk of overheating or failure, and you don't have to worry about the cost of replacing worn brake pads.

[DiLondon]

A vehicle with engine braking feels particularly at ease during long descents. In simple terms, its principle is a clever trick: releasing exhaust gases early at the end of the compression stroke. Normally, compressed gases would rebound and push the piston, but now they're all released, rendering the compression effort wasted. When I drive an automatic transmission car, I activate the dedicated switch, and the car immediately exhibits a noticeable deceleration, much smoother than applying the brakes. The advantage lies in protecting the traditional braking system, avoiding overheating and failure due to frequent braking. Especially in rainy or snowy conditions, using this instead of intermittent braking significantly improves vehicle stability, preventing tire slippage.

[Shelley]

Using engine braking in heavy trucks truly showcases its practicality. Essentially, it disrupts power at the very moment the engine should perform: when the piston compresses air upward, the exhaust valve suddenly opens to release the high-pressure energy. It feels like deflating a tire—the downward-pushing energy vanishes. In practice, you'll hear a brief hiss from the exhaust, signaling the system at work. I prefer engaging it in low gear before descending hills, ensuring smooth speed reduction without surging. The key advantage? It spares the brake system—even with a ten-ton load downhill, there's no worry about overheated brake pads, dramatically boosting driving safety. Note, though, that effectiveness is slightly reduced when the engine is cold after startup.