When driving down a steep slope, shifting to a low gear is a proper way to fully utilize the engine's braking, which can reduce the pressure on the braking system. But some people have doubts that although the pressure on the brakes has decreased, the engine speed has increased, reaching two to three thousand or three to four thousand revolutions. So, would the engine speed be so high that it would consume a lot of fuel? Going downhill could have saved some fuel, but it's not cost-effective to waste fuel in gear instead?

    First of all, let's talk about the answer. This will not consume fuel, and even save fuel in specific road conditions and gears. This is the same principle as sliding with gears. In low gear up and down steep slopes, although the engine speed is very high, it is not gasoline that provides power to the engine. Its high speed is not driven by the combustion of gasoline, but by the gravity of the car. On a slope, the gravity of a car has a downward force component. If the brake is not applied, the car will continue to move downwards under this force, and the wheels will inevitably rotate when the car moves downwards. At this time, in low gear, the wheels are connected to the engine through the transmission shaft and gearbox. When accelerating normally, the engine drives the wheels to rotate, and at this time, the power transmission is reversed, and the wheels drive the engine to rotate. The faster the wheels turn and the lower the gear, the faster the engine speed will be. Even if the engine is turned off at this moment and there is no more oil spraying, the engine speed will still be so fast. Therefore, a fast engine speed does not necessarily consume fuel, it depends on what power source is driving it to speed up.

      From the perspective of the engine itself, the engine computer controls the amount of fuel injected, and the computer determines the amount of fuel injected based on the amount of intake air. What the driver can control while driving is the intake volume. Stepping on the accelerator is actually controlling the intake volume. Stepping on the accelerator deeply increases the intake volume, and the fuel injection volume will increase accordingly. When going downhill, we definitely didn't step on the accelerator, we must have stepped on the brake, and the engine was in idle mode. Without stepping on the accelerator, the intake volume will not change, and the fuel injection volume will definitely correspond to the intake volume. No matter how fast the engine speed is, the fuel injection volume will not change too much. There is no possibility of the intake volume remaining unchanged and the fuel injection volume increasing exponentially. Since you haven't stepped on the accelerator pedal, the computer knows you're not accelerating, so there's no need to spray more fuel. Therefore, from the perspective of the engine's control over fuel injection, even if the engine revs high, it will not consume fuel when going downhill.

    Why do we sometimes save fuel? Because the engine has the fuel cut-off function, which is exactly the same as the principle of fuel cut-off when coasting in gear. If the vehicle speed is high and the engine speed is high, the fuel cut-off condition under the current working condition of the vehicle is met, the computer will not inject fuel directly. The engine speed is very high, and spraying a little fuel is not very useful. Instead, it wastes fuel and pollutes the environment, so it is not injected directly. Of course, the fuel cut-off conditions of the engine are quite harsh. To meet many conditions, it is not certain that fuel cut-off will occur at a certain speed. Our goal is not to save fuel, but to drive more safely and without additional fuel costs, which is enough.

From: WONDEE Autoparts

2024-1-29