Why is it difficult to turn the steering wheel after turning off the engine?

A few days ago, I took a friend’s car out to play. I put the car into gear manually and enjoyed the joy of "shifting to second gear, 7300" along the way, and soon reached the destination. Unfortunately, the parking space is relatively narrow, and it took a long time for the engine to be turned off before it came to a smooth stop. After getting off the car, my friend said: "When the engine was turned off just now, the steering wheel suddenly became heavier. I don't know why."

In fact, it is not just my friend, I believe many people will also have this question: Why Is it difficult to turn the steering wheel after turning off the engine? You will find that driving is not difficult. Normally, the steering wheel is easy to turn, but the moment the engine is turned off, it immediately becomes extremely heavy and almost impossible to turn. why is that? In fact, the answer is not difficult, but only if you keep looking down.

How does the car steering system work?

Before solving this problem, we first have a brief understanding of the car's steering system. The steering system generally consists of a steering wheel, steering column, steering gear, steering rod and other parts. The driver rotates the steering column by turning the steering wheel, causing the steering gear to drive the steering rod. The steering rod is generally connected to the steering knuckle of the wheel, thereby indirectly steering the vehicle. As for the details, such as rack and pinion type, circular ball type, worm type, etc. , once Baidu searches a lot, I won’t copy and paste. Everyone just needs to know how the steering system works.

Because it is unreliable to turn a car weighing one or two tons by hand, we added a power-assist mechanism to the steering system to help us turn the wheels. With the help of "foreign aid" you can easily turn the steering wheel even if you are a cute little girl's paper. But having said so much, it seems that the questions raised before have not been answered yet! Don't worry, you still need to know about power steering.

Currently common models are mainly equipped with three power steering systems, namely mechanical hydraulic power steering (HPS), electro-hydraulic power steering (EHPS) and electronic power steering (EPS).

Mechanical power steering mainly uses a hydraulic pump to provide oil pressure to push the piston, thereby generating auxiliary thrust to push the steering gear to pull the steering tie rod, thereby steering.

Take the most common constant flow as an example. Generally, during normal driving, the steering control valve does not work and the oil pressure is equal everywhere. When the steering wheel rotates, it will cause a relative angle difference between the valve core and the valve body of the steering control valve. At this time, the gap in the oil return pipe will decrease and the pressure will increase, so that the pressure will be unequal, resulting in a pressure difference that drives the piston of the power cylinder and generates power to turn the steering wheel. Due to the complex structure, it can be simply understood that when the steering wheel is turned, the hydraulic pressure on one side of the piston decreases, causing the piston to move to one side and generate power.

The principle of electro-hydraulic power steering is similar to that of mechanical hydraulic power steering, except that an electronic pump is used instead of a hydraulic pump. Since the switch of the electronic pump can be adjusted through a computer program, compared with the mechanical hydraulic system, the electronic hydraulic system can accurately control the opening of the valve through processing and calculation of the electronic program based on the relevant sensors of speed and steering angle, thereby adjusting the steering power. More diverse, creating different feel under different working conditions. For example, when power is reduced at high speed, the steering feel will be more stable, which is what we call "sinking", and the driving feel will be enhanced. It will be "light" at low speeds, making it easier to turn at a large angle or make a U-turn.

Finally there is electronic power steering. Compared with the first two, electronic power steering does not have a complex oil circuit system, and there is no pump body to provide oil pressure. It uses the rotation of the motor to assist steering. Typically, this auxiliary motor is mounted on the steering column or steering gear. When the steering wheel turns, the motor generates torque to "help" us turn the steering wheel, thus achieving a power-assist effect.

Because the torque of the motor can be adjusted at will through voltage and other methods, electric power steering often makes the steering feel more delicate. It is no longer limited to the stage of "sinking at high speed and light at low speed". Taking advantage of the "initiative" of the motor and the more powerful ECU electronic program, everything from automatic parking to lane departure assistance, ACC adaptive cruise and even automatic driving are inseparable from electric power steering. Because of this, coupled with its simple structure and huge potential, electronic power steering is increasingly used in various vehicles.

Why is it difficult to turn the steering wheel after turning off the engine?

Having said that, let’s get down to business and talk about why it is difficult to turn the steering wheel after turning off the engine. In fact, after knowing the above knowledge, it seems that this problem has become very simple now. Since the car's power is mainly provided by the engine running to drive the generator, turning off the engine means that the car is cut off. Don't expect the battery to provide this part of the power, after all, it also shoulders the important task of powering the starter. The hydraulic pump in mechanical hydraulic power steering is driven by the engine's belt. When the engine stops working, the hydraulic pump will stop working. Without the oil pressure difference, there will naturally be no extra thrust to help you turn. At this point you are dealing with a car weighing one or two tons, so it makes sense that the steering wheel would be difficult to turn. Likewise, electro-hydraulic and electronic assist were initially powered by electricity. There's no electricity, so don't expect anyone to help you, of course. The above is the reason why the steering wheel is difficult to turn after turning off the engine. Hope it helps. We will also cover the steering system in more detail in the future.

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