"Insights Of Exploring Technology"

How does a car engine inhale and exhaust air?

By, CoolMotor

• 2023-12-27
• 29 View

Intake and exhaust systems are often considered linked systems. On the one hand, the gas passes through the entire system in the form of fresh air and exhaust gas; on the other hand, some engine systems are intrinsically linked (e.g. turbochargers). The intake system is responsible for providing fresh air to the engine (picture), and the exhaust system is responsible for "carrying away" the combustion exhaust gases.

Engine air intake

The intake valves we see are larger than the exhaust valves. This is because generally the intake air is sucked in by vacuum, and the exhaust gas is pushed out through squeezing, so exhaust is relatively easier than intake. In order to obtain more fresh air for combustion, the intake valve needs to be larger to obtain more intake air.

The working principle of a four-stroke engine is as follows.

①Intake stroke: During the intake process, the intake valve opens and the exhaust valve closes. The piston moves from the top dead center of the cylinder to the bottom dead center, the volume of the cylinder above the piston increases, and the pressure in the cylinder decreases. When the pressure drops below atmospheric pressure, a vacuum is formed in the cylinder, and the combustible mixture is sucked into the cylinder through the intake valve.

Engine intake stroke

②Compression stroke: During the compression process, both the intake and exhaust valves are closed, and the piston moves from bottom dead center to top dead center to compress the combustible mixture, causing its volume to decrease, density to increase, and temperature to rise. When the piston reaches top dead center, After compression is completed, the mixture is compressed into the combustion chamber.

Engine compression stroke

③ Power stroke: During the power process, the intake and exhaust valves are still closed. When the piston approaches the compression top dead center, the spark plug installed on the cylinder block emits an electric spark to ignite the compressed combustible mixture. After the combustible mixture is burned, the high-temperature and high-pressure gas pushes the piston to move rapidly from the top dead center to the bottom dead center, causing the crankshaft to rotate through the connecting rod and output mechanical energy.

Engine power stroke

④Exhaust stroke: During the exhaust process, the exhaust valve opens and the intake valve closes. The piston moves from bottom dead center to top dead center, forcing the exhaust gas out of the cylinder for the next working cycle.

Engine exhaust stroke

The intake system consists of an air filter, air flow meter, intake pressure sensor, throttle body, additional air valve, idle control valve, resonant chamber, power chamber, intake manifold, etc.

The main function of the air intake system is to deliver clean, dry, sufficient and stable air to the engine to meet the needs of the engine, and to prevent impurities and large particles of dust in the air from entering the engine combustion chamber and causing abnormal engine wear. Another important function of the air intake system is to reduce noise. The air intake noise not only affects the passing noise of the entire vehicle, but also affects the noise inside the vehicle, which has a great impact on ride comfort. The design of the air intake system directly affects the power and noise quality of the engine, and is related to the ride comfort of the entire vehicle.

Engine air intake route

When the engine is running, the driver controls the opening of the throttle valve through the accelerator pedal to change the amount of air intake and control the operation of the engine. After the air entering the engine filters out dust and other impurities through the air filter, it flows through the air flow meter, enters the power chamber along the throttle channel, and is then distributed to each cylinder through the intake manifold; when the engine is cold and idling, some The air bypasses the throttle valve and enters the cylinder through the additional air valve or idle speed control valve.

The exhaust system refers to the system that collects and discharges exhaust gases, including exhaust manifolds, exhaust pipes, muffler pipes, and tail pipes.

After fresh air and gasoline are mixed into the engine for combustion, high-temperature and high-pressure gas is produced to push the piston. When the gas energy is released, it is no longer valuable to the engine, and these gases become exhaust gas and are discharged out of the engine. After the exhaust gas is discharged from the cylinder, it immediately enters the exhaust manifold. After the exhaust manifolds of each cylinder are collected, the exhaust gas is discharged through the exhaust pipe. Just like the intake manifold, the gas leaves the engine in pulses in the exhaust manifold, so the length and curvature of the exhaust manifold for each cylinder should be designed to be as consistent as possible so that the exhaust of each cylinder is equally smooth.

Engine exhaust route

What is an engine self-test?

Today's engines are controlled by electronic devices and are not purely mechanical. There are various electronic components and electronic sensors on the engine. The so-called engine self-test means that every time the engine is powered on, the engine central controller, similar to a computer CPU, will perform a logical test on these electronic components. If a fault is found, it will Prompt the car owner through the indicator light. If there is no fault, you can start driving normally.

What are the advantages and disadvantages of front-mounted, mid-mounted, and rear-mounted engines?

Front-engine, as the name suggests, means that the engine is installed above the front axle of the car. The front-engine layout is also the most common layout. Currently, 99% of family cars on the market adopt the front-engine layout. The driving modes are divided into three types: front-wheel drive, front-wheel drive and front-wheel drive.

Advantages: Highly practical, most convenient to operate, and able to maximize the space in the car.

Disadvantages: A lot of power is wasted, and braking performance is lost.

Representative models: Ford Mustang, Honda Civic, BMW, Audi.

front engine vehicles

① Front-wheel drive is the method used by most vehicles currently, and it is also the most practical. The engine is mounted transversely, and the engine and wheels run in the same direction, so there is no drive shaft. The advantage is that the engine efficiency is relatively high and the space utilization rate is also high.

However, its handling is poor, so many high-end car companies only apply this layout to low-end models, or simply abandon it. Many car fans also reject this form of driving, after all, the control feeling is too weak.

As for the reason, it is not difficult to understand. Because in terms of performance, rear-wheel drive vehicles have the best performance. If a front-mounted front-wheel drive engine is used, the engine power must be output and transmitted to the entire vehicle in order to drive the rear wheels. This wastes a lot of power, so there will inevitably be a certain loss of power performance.

In the actual maneuvering process, since the center of gravity of the entire vehicle is pressed on the front axle, the cornering limit and cornering flexibility will be compromised, and the "pushing head" phenomenon will also be serious.

② Front-mounted rear drive, the engine is installed longitudinally, the crankshaft is perpendicular to the front axle of the car, and power is mainly output to the rear wheels through the transmission shaft. The advantage is that the controllability is okay, and the control fun is greater than that of a front-wheel drive vehicle. Therefore, most car enthusiasts who like to play with cars prefer the front-wheel drive model.

In actual control, the cornering limit of front-wheel drive and rear-wheel drive is relatively large, but the requirements for driving skills are relatively high. If the driver's skills are in place, he can also complete difficult actions such as "drifting", but if It is very dangerous when encountering slippery roads in rain or snow.

As for the disadvantages, they are also related to the drive shaft. Due to the extra drive shaft, some power will be lost during the output process. There is another disadvantage, which is the transmission problem, which will also affect the space inside the car.

③ Compared with front-wheel drive and front-rear wheel drive, the front-wheel drive mode is much better, and the controllability is also better. Furthermore, since it is a four-wheel drive, the technical requirements for the driver are relatively low. For example, the Audi quattro four-wheel drive model uses this type of drive. In addition, many off-road vehicles are mostly equipped with front-mounted four-wheel drive. There is no doubt about its ability to get out of trouble.

The engine is mid-mounted, which means the engine is installed in the middle position between the front axle and the rear axle of the car (picture). There are two main drive forms: mid-mounted four-wheel drive and mid-mounted rear drive.

Mid-engine vehicles

Advantages: The front and rear weight ratio is more balanced and the control is better. This is the layout of most supercars.

Disadvantages: takes up a lot of space.

Representative models: Bugatti, Koenigsegg.

The mid-engine layout is the least common of the three layouts, but in fact, its engine is in the best position in the entire vehicle. The reason is that its position is close to the outside of the rear wheel, which can achieve the optimal balance of weight distribution, center of gravity and pulling force of the vehicle.

In addition, since the center of gravity of the vehicle is in the middle, the wheels will not be unbalanced due to extra weight, which is also very helpful in effectively improving the accuracy of the vehicle's steering, especially when making sharp turns on the track. Better control and higher limits have far exceeded practicality.

The engine is rear-mounted, which means that the engine is installed on the rear axle of the car (picture). This kind of layout is relatively rare. Apart from daily buses and buses, the one that everyone is more familiar with is the Porsche 911. This layout mainly has two forms: rear-mounted rear drive and rear-mounted four-wheel drive.

Rear engine vehicles

Advantages: The front of the car is more flexible and the control experience is better.

Disadvantages: High technical requirements for drivers.

Representative model: Porsche 911.

The rear-engine layout is more popular in sports cars, because placing the power source above the rear axle can improve acceleration and braking performance, and also provide a more flexible control experience.

In addition, vehicles equipped with rear engines are smaller than ordinary passenger cars, so the size advantage, that is, the compact layout, also significantly improves driving control. The specific performance is that part of the structure is omitted, and the body is lighter, which is more conducive to breakthroughs in acceleration performance of 100 kilometers per hour.

However, this type of sports car is also more difficult to control due to the increased pressure on the rear axle. For example, rear-wheel drive is prone to sideslip; and when the rear wheels of the vehicle slide more than the front wheels, a tail-wagging effect will occur, so the driving skills of the driver are relatively high. But this is also the reason why most racing drivers like it, after all, its driving effect is really "too cool".