What is the working principle of engine braking?
3 Answers
Simply put, it means the engine stops injecting fuel and sparking, but the pistons inside the engine continue to move, constantly compressing air without ignition. At this point, the energy for compressing the air comes from the vehicle's own kinetic energy, gradually reducing the vehicle's speed, effectively turning the engine into an air compressor. The main principles are as follows: 1. Engine Braking: Engine braking refers to the process where, after releasing the throttle, the exhaust valve opens instantaneously near the top dead center during the compression stroke, releasing the high-pressure gas; 2. Cylinder State: After closing the exhaust valve, there is very little gas left in the cylinder. During the expansion (power) stroke, when the piston moves from the top dead center downward, the cylinder is in a negative pressure state; 3. Effect Generation: The gas resists the downward movement of the piston (the piston acts like a vacuum pump), creating a torque in the opposite direction to the crankshaft, thereby generating a braking effect.
I've always been fascinated by engine technology, and the engine brake is particularly useful in diesel vehicles. When you're driving a heavy truck downhill, traditional brakes can easily overheat and fail, that's when it comes into play. How it works, simply put, is a clever conversion inside the cylinder: during normal engine operation, the piston pushes up to compress air for ignition, but the engine brake system opens the exhaust valve early, releasing this high-pressure air. The energy that was supposed to push the piston to do work and accelerate is now wasted, turning into resistance to help you slow down. This is more fuel-efficient than stepping on the brakes and also protects the brake pads. Don't underestimate it; on long-haul freight or mountainous routes, this thing can be a lifesaver! Remember to check the valve components regularly, don't slack off. I've tried it myself, with good maintenance, it can last for years.
I've been driving trucks for over a decade, and engine braking has been my trusted companion on steep descents. When activated, the engine automatically switches modes: it compresses air after the intake stroke but doesn't ignite fuel for combustion. Instead, it releases the compressed air when the exhaust valves open. This released energy creates a counterforce, effectively acting like an internal drag on the vehicle, allowing for smooth speed reduction. This technique prevents frequent use of the foot brake, avoiding overheating and potential failure of the braking system. Simply put, it utilizes the engine's own motion to waste power for deceleration. It's incredibly useful on long hauls—both convenient and safe. I've also noticed that proper maintenance can enhance overall fuel efficiency and reduce wear. However, avoid overusing it in urban areas as it can be quite noisy.
