Air intake and exhaust system

The intake and exhaust system means that the engine needs the assistance of the intake system and the exhaust system when working, and these two systems constitute the intake and exhaust system.

Composition of air intake system The air intake system consists of air filter, air flowmeter, intake pressure sensor, throttle body, additional air valve, idle speed control valve, resonant cavity, power cavity, intake manifold, etc.

Variable valve distribution technology of variable valve distribution intake system can be divided into two categories: variable valve timing and variable valve stroke.

What is variable valve timing?

First of all, let's talk about the valve train of an ordinary engine. Everyone knows that the valve is driven by the crankshaft of the engine through the camshaft, and the valve timing depends on the rotation angle of the camshaft. When the engine is running, we need to let more fresh air into the combustion chamber and exhaust the exhaust gas as much as possible. The best solution is to let the intake valve open ahead of time and the exhaust valve close later. In this way, between the intake stroke and the exhaust stroke, the intake valve and the exhaust valve will open at the same time, and this overlap of the intake valve and the exhaust valve is called the valve overlap angle.

On an ordinary engine, the opening and closing time of the intake valve and the exhaust valve are fixed, and the valve overlap angle is also fixed, which is the best valve timing obtained according to experiments and cannot be changed during engine operation. However, the engine speed has an influence on the intake and exhaust flow in the cylinder and the combustion process. When the rotating speed is high, the intake flow rate is high and the inertia energy is large, so it is hoped that the intake valve will open earlier and close later, so that fresh gas can be smoothly filled into the cylinder and as much mixed gas or air as possible. On the contrary, when the engine speed is low, the intake air flow rate is low and the flow inertia energy is also small. If the intake valve is opened too early, because the piston is exhausting upwards at this time, it is easy to squeeze fresh air out of the cylinder, which makes the intake air less and the engine unstable. Therefore, there is no fixed valve overlap angle setting that can make the engine output perfect at high and low speeds. If there is no variable valve timing technology, the engine can only choose the best fixed valve overlap angle according to the needs of its matching model.

For example, the engine of a racing car generally adopts a small valve overlap angle to facilitate the power output at high speed. However, ordinary civilian vehicles adopt a moderate valve superposition angle, taking into account both high-speed and low-speed power output, but they will lose a lot of power at low and high speeds. Variable valve timing technology solves this contradiction by realizing variable valve overlap angle through technical means.

What is variable valve stroke?

That is to say, on the basis of variable valve timing, a function that can lift the valve is added. The advantage of this is that it can control the opening and closing time and lift of the valve at the same time, and Toyota VTEC is the first company to adopt this technology. For the general engine, the valve group of each cylinder is driven by only one group of cams, while the engine of VTEC system has two groups of different valve driving cams, which can be changed automatically through the automatic control of the electronic control system. VTEC system ensures different requirements for valve timing and air intake at low speed and high speed, and makes the engine achieve a unified and excellent state of power, economy and low emission at any speed. It should be noted that the engine adopts variable valve timing technology, which has not any negative effects while obtaining the above benefits, in other words, it has not put forward higher requirements for the working intensity of the engine.

Classification of intake system The intake system of modern automobile is mainly variable intake system. Variable intake systems are mainly divided into four types: VVT (variable valve timing), CVVT (continuously variable valve timing), VVT-i (electronically variable timing) and i-VTEC (electronically variable valve lift).

1)VVT (variable valve timing)

The crankshaft drives the camshaft to rotate through the toothed transmission device, so that the valve will form a certain corresponding relationship with the rotation angle of the crankshaft when it is opened and closed. The gas flow will change with the engine speed. How to make the cylinder get good intake efficiency at different speeds? Therefore, it is necessary to change the opening and closing time of the valve. Through the oil pressure device installed at the front end of the camshaft, the camshaft can rotate at a small angle, so that when the speed increases, the intake valve can be opened in advance.

Variable valve timing mechanism can improve the performance of the engine. Different engine speeds require different valve timings. This is because when the engine speed changes, the intake flow and exhaust flow will also change during forced exhaust, so the effect of increasing intake and promoting exhaust by using airflow inertia in the later stage of valve closing will be different.

For example, when the automobile engine is running at low speed, the airflow inertia is small. If the valve timing is unchanged at this time, part of the intake air will be pushed out of the cylinder by the piston, which will reduce the intake air volume and increase the residual exhaust gas in the cylinder. When the engine is running at high speed, the airflow inertia is large. If the intake lag angle and valve overlap angle are increased at this time, the intake volume will increase and the residual exhaust volume will decrease, thus making the engine ventilation process more perfect.

In a word, the valve timing of four-stroke engine should be that the intake lag angle and valve overlap angle increase with the increase of engine speed. If the valve lift can also be improved with the increase of engine speed, it will be more conducive to obtaining good engine high-speed performance.

Technical analysis of VVT 2)CVVT (continuously variable valve timing)

CVVT is the abbreviation of English Continue Variable Valve Timing, which is translated into Chinese as continuously variable valve timing mechanism. It is one of many variable valve timing technologies that have been gradually applied to modern automobiles in recent years. For example, BMW is called Vanos, Toyota is called VVTI, and Honda is called VTEC, but whatever they are called, their purpose is to match the best valve overlap angle (valve timing) for different engine working conditions, but the methods are different.

3)VVT-i (electronic variable timing)

Its working principle is that VVT-I system consists of sensor, ECU, camshaft hydraulic control valve and controller. The ECU stores the optimal valve timing parameter values, and feedback information such as crankshaft position sensor, intake manifold pressure sensor, throttle position sensor, water temperature sensor and camshaft position sensor is collected in the ECU and compared with the predetermined parameter values to calculate the correction parameters and send instructions to the hydraulic control valve to control the camshaft timing. The control valve controls the position of the oil pan valve according to the instruction of ECU, that is, changes the hydraulic flow, selects and sends signal instructions such as advance, delay and invariability to different oil circuits of VVT-I controller.

Working principle of VVTI technology

3)i-VTEC (Electronic Variable Valve Lift)

I-VTEC system is the abbreviation of Honda intelligent variable valve timing system, and I-VTEC system has been widely installed in the engine of Honda's latest car. Honda's i-VTEC

The system can continuously adjust valve timing and valve lift.

Its working principle is that when the engine changes from low speed to high speed, the electronic computer automatically presses the engine oil to the small turbine in the driving gear of the intake camshaft, so that the small turbine rotates at a certain angle relative to the gear box under the pressure, and the camshaft rotates forward or backward within 60 degrees, thus changing the opening time of the intake valve and achieving the purpose of continuously adjusting the valve timing.

Honda i-VTEC technology explains the composition of exhaust system in detail. Automobile exhaust system refers to the system for collecting and discharging exhaust gas, which generally consists of exhaust manifold, exhaust pipe, catalytic converter, exhaust temperature sensor, car muffler and exhaust tail pipe.

Working principle of exhaust system After fresh air and gasoline are mixed and burned in the engine, high-temperature and high-pressure gas is generated to push the piston. When the gas energy is released, it is no longer valuable to the engine, and these gases are discharged from the engine as exhaust gas. The exhaust gas enters the exhaust manifold immediately after it is discharged from the cylinder. 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 a pulse way in the exhaust manifold, so the length and curvature of the exhaust manifold of each cylinder should be designed to be as same as possible, so that the exhaust of each cylinder can be as smooth as possible. After the waste exhaust manifold is connected with a catalytic converter, the incompletely burned pollutants are converted into harmless substances to protect the environment. When it comes out of the catalytic converter, it is connected to the muffler. Mufflers are circular or oval objects in cross section, mostly welded by thin steel plates, and installed in the middle or rear position of the exhaust system. There are a series of baffles, chambers, perforated pipes and pipes inside. Using the mutual interference and cancellation of acoustic reflection, the acoustic energy is gradually weakened to isolate and attenuate the pulsating pressure generated every time the exhaust valve is opened.

Working principle of exhaust system

Classification of variable air distribution of intake system Composition of intake system Composition of exhaust system Working principle of exhaust system @20 19