What does it mean to use engine braking on long downhill slopes?

[DeEvelyn]

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.

[VonAiden]

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.

[OFinn]

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.

[TravisLee]

Simply put, it's about using the engine as a decelerator. Automatic transmission cars have torque converters, so the effect might not be obvious, but it's particularly effective for manual transmissions. Once, I was driving an old Jetta on a winding mountain road, using third gear for engine braking on a downhill slope. The RPM shot straight into the red zone, and the exhaust roared loudly, but the speed remained stable. The key point is not having to worry about brake fade leading to brake failure. Players who modify brake discs should take note: the original calipers will experience heat fade under continuous braking, whereas low-gear engine braking doesn't generate heat, making it safer when combined with intermittent braking.

[LunaLee]

What I learned from driving a training car: coasting in neutral and using the foot brake is the worst thing to do on a long downhill. If the engine stalls, the vacuum booster fails, and the brake pedal becomes as hard as a rock after two presses. Keeping the engine running in a low gear not only maintains operation but also provides continuous deceleration. My neighbor, who drives a heavy truck in Yunnan, once saw a car with smoking brakes plunge off a cliff. Now, when driving my SUV down to the underground garage, I use the M mode to lock in 1st gear, coasting down slowly without pressing the brake—quiet and easy on the brake pads.

[Gordon]

Novice drivers should pay attention to this key operation point: reduce speed before shifting to a lower gear. For example, shifting directly to 2nd gear at 60km/h can cause the engine to over-rev and get damaged. It's better to slow down to 30km/h before reaching the top of the slope and then shift to a lower gear. Once, I saw a young guy driving a modified car downhill, suddenly shifting to 2nd gear at 80km/h, which caused the transmission to grind gears and get completely wrecked. When switching an automatic transmission to manual mode, it's advisable to practice and get familiar with the speed limits of each gear on flat roads first, to master the proper control.