ENGINE CHAPTER 4 Intake System

Chapter 4.

Intake System

1. Enhance the Volume Efficiency

The pressure of the fuel mixture at the end of the intake stroke is lower than the ambient pressure because of the resistance against the flow of the air cleaner and duct.

And the temperature of the fuel mixture is high because that it contacts with the hot valve and cylinder wall when it is inhaled. As the density of the air becomes low when the pressure becomes lower or the temperature becomes higher.

The volumetric efficiency is used for indicating the intake ability of the mixture. As indicating the amount of the inhaled air in rate about the engine displacement, the volumetric efficiency is calculated from dividing the weight of the inhaled air by the weight of the air amount with the displacement at the same temperature and the same pressure. In another method for indicating the efficiency of the intake stroke, the charging efficiency can be used also. This is the volume efficiency when the temperature and the pressure is in the standard condition (25℃, 99kPa).

In order to enlarge the maximum output of the engine, this volume efficiency should be as high as possible. The method for enhancing the volume efficiency is like the followings.

⑴ The ambient air shall be inhaled into the manifold with not high temperature as possible. For the engine having the turbocharger, it should be equipped with the intercooler to prevent from increasing the temperature of the inhaled air.

⑵ Reduce the flow resistance of the inhaled gas by increasing the number of valves, and enlarging the size and the bending radius of the duct and manifold.

⑶ Enlarge the diameter of the valve and the height of the life, and balance the valve timing properly.

⑷ Choose a long intake manifold at the lower speed , and choose a short one at the higher speed to use the inertia and pulsation effect of the air.

⑸ Enlarge the intake gas pressure by installing the turbocharger.

Until now, we mentioned about the enhancing method for volume efficiency related to the intake stroke. It is important to exhaust perfectly the burnt gas at the exhaust stroke as possible in order to enhance the volume efficiency. It is possible to apply the (2) ~ (4) of above mentioned methods applied to the intake system except (1) and (5) to the exhaust system. For example, the “flow resistance of the inhaled gas” of (2) is took place with the “flow resistance of the exhausted gas”, and the “inertia energy of air” of (3) is took place with the “exhausting inertia”. Furthermore, the exhaust interference shall be minimized as possible. Additionally, a turbocharger shall increase the intake resistance.

By tuning up the engine, it can be effective for increasing output even it is very difficult to tune up the engine.

2. Intake Inertia Effect and Pulsation Effect

In order to inhaling air having high density to the engine, to use the inertia of the air flow is called the inertia effect, and to use the characteristics of the longitudinal wave such as sound wave according to the density of the air is called the pulsation effect. In the inertia effect, as the high density air is inhaled into the engine using a inertia energy of air. It is called as the inertia supercharging.

The air into the engine has the flow inertia as a gas and is a media for transmitting the pressure wave. The air flow in the intake manifold is periodically intercepted by the valve, so the manifold pressure has variations from the difference of pressures between high density portion and low density portion.

Valve open (Inhaled air)

Valve close (High density air at valve)

Valve open (Inhaled high density air)

Therefore, the intake inertia effect and the pulsation effect could be made. When this pressure variation affects to the intake stroke at the cycle generating the wave directly, it is called as the inertia effect. When the pressure variation is not reduced and then affects to the next cycle, it is called as the pulsation effect. However, it is not distinguished between them exactly. We, here, will call as the inertia effect when the air flow inertia is mainly governed and as the pulsation effect when the pressure wave is mainly governed.

As the first example, assume that the intake valve is closed during the fuel mixture is inhaled into the cylinder. As the fuel mixture has the flow inertia, the mixture flow in the intake manifold can not stop instantly just at the closing the valve, but pretends to flow continuously. Therefore, the air just before the valve will be pressed by inertia energy of intake air. Consequently, the air density at the port portion will be increased. At that time, if the valve is opened in time, then the air of high density can be inhaled into the cylinder. This is the inertia effect.

Valve close (High density air at valve)

Density of following air is low & pressure wave are reflected by surge tank

High density air by reflection of pressure wave is inhaled

When the air density near the port is increased, the density of the following air is low respectively. So, the boundary portion makes the pressure variations, that is, noise. This air density variation passes through the manifold with the speed of sound. It reflects to the end of the manifold, and then it returns to the port. When the high density air is back to the port, if the port is opened, then the high density air can be injected into the cylinder. This is the pulsation effect.

As these effects are combined, it is hard to separate from each other. However, in order to maximize the effect, it is prefer to make the pressure wave in the manifold to make high air density near the port when the valve is opened. To do so, the diameter and length the intake manifold and the shape of the intake port shall be controlled.

3. Variable Intake System

At low speed

At high speed

The air flow in the intake manifold is not uniformed but variable according to the engine speed. When the high density air flow reaches at the port, if, ideally, the intake speed is maximum just before the closing the valve, then the intake inertia effect will be maximized.

The air pulsation frequency is decided by the diameter and the length of the manifold. When the diameter is same, the frequency of the long length manifold is small. It is the same that the sound has the lower frequency when the distance between the hole and the mouth piece of the recorder is larger.

Generally, the length of the manifold is already decided, so if the engine is running with certain velocity, the intake inertia is effective. However, if it is rotating with variable speed, then the lower density air can reach at the port when the valve is opened so, the charging of air may be worst.

Consequently, the method for varying the length of manifold is developed according to the rpm of the engine. When the engine has high rpm in which the valve is frequently opened and closed within the same time interval, the short length manifold is chosen to make the cycle be short. Contrarily, when the rpm is low, the long manifold is chosen to make the cycle be long. So, it is possible to get the intake inertia effect in wide range of rpm. As being the variable intake system, it is called as the variable inertia charging system or the variable intake control system.

There are many types to control the length of the intake manifold. Mainly, the two types are used. The one type is that the separated two manifold groups are connected together. When the engine is high speed, the path is divided each other, and when the engine is low speed, the two manifold groups are linked each other to elongate the length of the total manifold.

The other is to attach a bypass at the manifold system in which the air pass through the bypass when the engine is low speed, and the path to the bypass is closed to reduce the length of manifold at the high speed.

In case of linking some manifolds, the resonances may be occurred between the manifolds. This comes from the pressure vibration having the same frequency in the separated manifold. In this case, the inertia charging effect can not be expected even at the high speed. This phenomenon can be inhibited by enlarging the volume of the intake collector connecting to the manifold. When the resonance is occurred, at the middle and low speed, the inertia supercharging effect becomes high so the charging efficiency will be increased. This is called as the resonance supercharging effect.

4. Intake System

The intake system takes the air to mix with the gasoline and inhales the mixtures into the cylinder. Generally, the intake system comprises the air cleaner filtering the dust in the inhaled air, the carburetor mixing the air and the gasoline, and the intake manifold (or inlet manifold) inhaling the mixture into the cylinder, at the head portion of the cylinder. Nowadays, the electrical controlled unit for fuel injecting to the intake manifold directly is widely used, so the design of the intake system is changed very much.

Carburetor intake system

MPI intake system

At first, the air inlet port which was near the cylinder head is moved to the front grill to intake the ambient air having lower temperature than the air around the engine room.

As being low temperature air, the air has high density, therefore, the much amount of oxygen will be contained into the inlet air. With the same volume of the inlet air, the lower temperature of the air is the more prefer. For example, in summer of 30℃ temperature, when the air conditioner is working at the low speed such as at the rush hour in the city, the temperature of the engine room could be over 80℃. In this case, by calculation, the amount of the oxygen in the air around engine room is 15% less than that of the ambient air.

The inlet air to the front grill is inhaled into the intake manifold through the long duct via the air cleaner, resonance chamber and throttle body.

In the carburetor system, the dish type air cleaner was look on the carburetor, but nowadays, the box type air cleaner is installed at one corner of engine room. The air cleaner not only cleans the air going into the cylinder but also reduces the noise from the intake operating. The air cleaner element should be maintained periodically.

The resonance chamber is a small box branched from the duct as the device for reducing the intake noise, and it is called as the regenerator chamber or the side branch. According to the opening and closing the intake valve, the vibration of the air inside the air cleaner box or the duct can make a large intake noise or hinder the intake operation. Using the resonance effect by installing a resonance device, this vibration will be terminated.

5. Throttle Valve and Manifold

   

Butterfly type, slide type throttle valve

To rise up the engine rpm, we press the accelerator pedal, and to lower down the rpm, we release the accelerator pedal. As the accelerator pedal is linked to the throttle valve by the wire and linkage, when the pedal is pressed the throttle valve will be opened to intake the air into the cylinder. That is, the carburetor or the electronic control system for fuel injection automatically checks the amount of the air to supply the amount of gasoline proper to the driving situation.

In the carburetor system, the throttle valve is equipped with the carburetor. In the electronic control system, it is installed at the middle of the throttle body (throttle chamber) separately installed in the intake system and being with the air flow sensor detecting the air flow amount and the throttle position sensor checking the status of the throttle valve opening.

In the throttle valve type, there are a butterfly valve in which disk plate having the shape of the butterfly wing is attached at the axis inside the pipe to control the air amount by rotating the axis, and a slide type in which an aluminum plate controls the amount the air without any hindrance at opening the throttle valve, especially for racing engine.

The air passing through the throttle body and the mixture mixed with gasoline at the carburetor are distributed into the cylinder by the intake manifold. The fuel injection is performed before distributing the air at the manifold, or at each cylinder as the mixture. The important thing is that the intake manifold should inhale the mixture into the cylinder as smoothly as possible, so that the manifold should have less bent portions and smooth inside faces.

The mixed gasoline in the carburetor is inhaled into the cylinder as the foggy state in the air. When the temperature is low such as just before starting the engine, this foggy particle of the fuel can attach to the manifold wall during being inhaled. Therefore, the mixture is leaned so the combustion is not enough. To solve this problem, using the heat from the exhaust manifold or from the cooling water for the engine, the intake manifold should be heated up.

The method for heating the intake system using the exhaust temperature is only used for the counter flow type engine in which both the intake and the exhaust manifolds are installed at the same side of the engine. The method for heating the intake system using the cooling water is accepted in the cross flow type engine in which the manifolds are installed at the opposite side of engine each other.