What does it mean to use engine braking on long downhill slopes?
3 Answers
Using engine braking on long downhill slopes means utilizing the engine's power to prevent the vehicle from speeding excessively. Here are the relevant explanations: 1. Principle: When the vehicle is moving, the engine pulls the vehicle forward, but when going down a steep slope, due to inertia, the vehicle will automatically accelerate if not controlled by the engine. Therefore, using a lower gear allows the engine's work to counteract the inertia and maintain a certain speed for the vehicle. 2. Advantages: The vehicle speed is always limited within a certain range, which is beneficial for timely deceleration or stopping, ensuring driving safety. It effectively reduces the frequency of using foot brakes, avoiding the rise in temperature of the brake friction pads due to prolonged use, which could lead to a decrease in braking force.
I used to do this often when driving on mountain roads, which is to intentionally shift into a lower gear instead of pressing the accelerator or putting it in neutral when going downhill. The principle is to use the cylinder compression resistance from the piston movement inside the engine to slow down, effectively converting the vehicle's downward kinetic energy into internal engine friction to dissipate it. For example, shifting into 2nd gear downhill restricts the car's speed due to the engine RPM, making it much safer than just relying on the brakes. This is especially crucial for fully loaded trucks descending long slopes like those on the Yunnan-Guizhou Plateau, where brake fade can be deadly. However, note that manual transmissions can directly shift into a lower gear, while automatic transmissions need to be switched to manual mode or L gear; otherwise, the transmission will automatically upshift.
Drivers of manual transmission cars should understand best that this operation essentially involves downshifting to a lower gear and maintaining high RPMs for deceleration. When descending a slope in 1st or 2nd gear, the engine RPM is forcibly dragged up to 4,000-5,000 RPM by the wheels, and the operation of the crankshaft and connecting rod mechanism generates reverse traction. I've tested it myself: descending a 5-kilometer slope from an altitude of 2,000 meters in 3rd gear without touching the brake pedal, the speed was steadily maintained at around 30 km/h, and the brake pedal wasn't used at all. Compared to continuous friction from braking, which can generate temperatures of 500-600 degrees Celsius, using engine RPM control causes zero wear on the braking system and saves brake pads. However, when downshifting on steep slopes, the gear shift must be decisive—shifting into neutral first and then trying to engage a lower gear might cause gear grinding.
