The power output from the engine does not directly drive car wheel but passes through a series of power transmission mechanisms first. So how is the power transmitted to the wheels?
- How is power transmitted?
The power output of the engine must pass through a series of power transmission devices before reaching the drive wheels. The power transmission mechanism between the engine and the drive wheels is called the car's transmission system, which is mainly composed of the clutch, transmission, drive shaft, main reducer, differential and half shafts.
The power output by the engine first passes through the clutch, and after the transmission changes torque and speed, it is transmitted to the main reducer through the drive shaft, and finally transmitted to the drive wheels through the differential and half shafts.
The layout of the automobile transmission system is related to the position of the engine and the drive form. It can generally be divided into four types: front front drive, front rear drive, rear rear drive, and mid rear drive.
- What is front-engine front-wheel drive?
Front-engine, front-wheel drive (FF) means that the engine is placed at the front of the car, and the front wheels are used as driving wheels. Most cars now adopt this layout. Since the engine is placed at the front of the car, the center of gravity of the whole car is concentrated in the front part of the body, which will be a bit "front-heavy". However, since the body will be pulled by the front wheels, the straight-line driving stability of FF cars is very good.
In addition, since the engine power passes through the differential and is directly driven by the half shaft to drive the front wheels, it does not need to pass through the drive shaft, so the power loss is small, which is suitable for small cars. However, since the front wheels are responsible for driving and steering at the same time, the turning radius is relatively large, which is prone to understeering.
- What is front-engine rear-wheel drive?
Front-engine rear-wheel drive (FR) means that the engine is placed in the front of the car and the rear wheels are used as driving wheels. The front and rear weight of the FR vehicle is relatively balanced, and it has better handling performance and driving stability. However, there are many transmission parts and the transmission system is heavy, and the transmission shaft that runs through the passenger compartment occupies the floor space in the cabin.
FR cars have better handling, stability, and braking performance. Today's high-performance cars still like to use this layout.
- What is rear-engine, rear-wheel drive?
Rear-engined, rear-wheel-drive (RR) means that the engine is placed behind the rear axle and the rear wheels are used as drive wheels. Since most of the weight of the vehicle is concentrated at the rear and it is rear-wheel drive, the starting and acceleration performance are very good. For example, the Porsche 911 we are familiar with adopts this layout.
RR cars have sharper cornering performance than FF and FR, but when the rear wheel's grip reaches its limit, there will be skidding and tail drifting, which is not easy to control.
- What is mid-mounted rear-wheel drive?
Mid-mounted rear-wheel drive (MR) means that the engine is placed between the driver's cabin and the rear axle, and the rear wheels are used as drive wheels. This MR design has become the mainstream drive mode for high-end sports cars. Since the engine with the largest inertia in the car is placed in the center of the car body, the weight distribution of the whole vehicle is close to the ideal balance, which ensures that MR cars have the best sports performance.
MR cars have a narrow cabin due to the mid-mounted engine, usually with only two seats, and the engine is close to the driver, and the noise is relatively loud. Of course, people who pursue car driving performance will not care about these.
- The role of the clutch
The clutch is located in the flywheel housing between the engine and the transmission, fixed on the rear plane of the flywheel, and the other end is connected to the input shaft of the transmission. The clutch is equivalent to a power switch that can transmit or cut off the power input from the engine to the transmission. The main purpose is to make the car start smoothly, interrupt the power to the transmission system in time to cooperate with gear shifting, and prevent the transmission system from overloading.
The clutch is mainly composed of four parts: the active part (flywheel, clutch cover, etc.), the driven part (friction plate), the clamping mechanism (diaphragm spring) and the operating mechanism. Automobile clutches include friction clutches, hydraulic couplings, electromagnetic clutches, etc. At present, most of the clutches that match the manual transmission are dry friction clutches. The working principle of the friction clutch is explained below.
The clutch cover is fixed to the rear end face of the flywheel by screws. The friction plate in the clutch is pressed against the flywheel surface by the pressure plate under the action of the spring, and the friction plate is connected to the input shaft of the gearbox. The torque generated by the engine is transmitted to the gearbox through the friction between the flywheel and the contact surface of the pressure plate and the driven plate.
Before the clutch pedal is pressed, the friction plate is pressed against the end face of the flywheel, and the power of the engine can be transmitted to the gearbox. When the clutch pedal is pressed, the force is transmitted to the release fork and release bearing through the operating mechanism. The release bearing moves forward to press the diaphragm spring to the flywheel end. The diaphragm spring moves in the opposite direction with the support ring as the fulcrum, and the pressure plate leaves the friction plate. At this time, the engine power transmission is interrupted; when the clutch pedal is released, the diaphragm spring returns to its position, the clutch re-engages, and the engine power continues to be transmitted.
- Function of universal joint
Universal joint refers to the use of spherical devices to achieve shaft power output in different directions. It is located at the end of the drive shaft and serves to connect the drive shaft and drive axle, half shaft and other parts. The structure and function of the universal joint are a bit like the joints on the human limbs. It allows the angle between the connected parts to change within a certain range.
For example, in a front-engine rear-drive car, the power of the transmission must be connected to the drive axle through the drive shaft. Why use a universal joint? It is mainly to meet the power transmission, adapt to the angle changes caused by the up and down jumps generated when the car is running.
According to whether the universal joint has obvious elasticity in the torsion direction, it can be divided into rigid universal joints and flexible universal joints. Rigid universal joints can be divided into three types: non-constant velocity universal joints (commonly used is the cross-axis type), quasi-constant velocity universal joints (such as double-jointed universal joints) and constant velocity universal joints (such as ball cage universal joints). The constant velocity universal joint commonly used in cars is the ball cage universal joint.
