ENGINE Chapter 11 Combustion and Combustion Chamber

ENGINE

Chapter 11.

Combustion and Combustion Chamber

1. Combustion Process

To get high output and to enhance the fuel efficiency, it is necessary to combust the mixture of fuel and air perfectly as soon as possible while the combustion process. Therefore, to enhance the engine performance is to learn the relationship with the combustion and to study how to increase the fuel efficiency.

The mixture of fuel and air by the carburetor and injector is, at first, inhaled into the cylinder through the intake valve with the swirling flow. And then it is compressed with the swirl flow by the piston going up from the BDC (botton dead center). At this time, the fog state fuel is converted into a vapor state by the heat from the chamber wall and adiabatic compression, and the strong flow of the mixture. Some components may change to the flammable gas.

When a flame is applied to the gas with high temperature, then a flame kernel is made between the electrodes of the plug. This flame kernel is a combusting gas unit having high temperature made from the reaction the fuel gas and the oxygen in the air.

This combusting gas unit immediately heats the mixture there-around. The more mixture surrounding the kernel reacts with more oxygen by this heat and then converts into larger combusting gas unit. Within a short time, this sequence is widely spread so the whole mixture is converted into the combusting gas. This is the combusting process of the mixture.

As the time for being the spark is only about 2/1000 seconds (2 milli seconds: ms), if the temperature around the flame kernel is low or the kernel is blown out by the swirl of the mixture, then the mixture can not be combusted. This phenomenon is called the misfire.

In the process of the combustion, the boundary between the combusting gas and the combusted gas is called the flame surface. The expansion velocity of the flame surface is the flame velocity. The flame velocity is the same mixing the combustion velocity which is the speed of flame developing with statistic fuel gas, the expansion velocity which is the speed of gas expansion by the combusting heat, and the velocity of the gas flow.

The combustion velocity is changed by the component of the fuel and air-fuel ratio which is the weight ratio between the fuel and air. However, it is very slow, i.e. several cm per second, As adding the gas expansion velocity and flow velocity to the combustion velocity, the flame velocity is about 15∼20m per second, even it can be 30m per second. Therefore, the flow of mixture is very important.

2. Air-fuel ratio and Flame Velocity

To enhance the engine performance, the flame velocity should be fast and the amount heat energy which will be converted into the kinetic energy should be as large as possible.

The flame velocity is decided by the three main elements including the combustion velocity, the gas expansion velocity, and the mixture flow velocity. To combust the mixture fast, these elements should be maintained in the best condition.

Considering the combustion velocity and gas expansion velocity, the flame velocity is decided by the mixture ratio which is the ratio between the fuel and air and the temperature and the pressure of the mixture. The temperature and the pressure are decided by the temperature of chamber and compression ratio. To consider the temperature and pressure is very complicated, so here, we assume these conditions are constant. We focus on the fuel component and the mixing ratio.

The gasoline is a liquid consisting of 4∼12 carbon atom in chain link and various molecules including hydrogen atom. If the component ratio is changed or a material is added to accelerate the combustion, then the combustion velocity and the gas expansion velocity shall be faster.

The mixing ratio is a number representing the ratio of fuel amount and the air amount. It can affect to the combustion velocity. So it can be represented by the three indicating number such as the air-fuel ratio (or A/F ratio), the excess air ratio, and the equivalency ratio.

The air-fuel ratio is the value calculated by which weight of air included into the mixture is divided by the weight of fuel included into the mixture. It is called the AIR/FUEL RATIO, or A/F ratio. When the air and fuel are mixed, the A/F ratio for complete combustion theoretically is called the theoretic A/F ratio. The theoretic A/F ratio of the regular gasoline is about 14.7.

If the actual A/F ratio is less than the theoretic A/F ratio, then the amount of the gasoline is more than the theoretic A/F ratio so it is indicated as 『RICH』, otherwise, as 『LEAN』.

For the mixture is combusted in the best condition and for the flame velocity is fastest, the A/F ratio is little smaller than the theoretic A/F ratio, that is 13.5∼14. This means that when the fuel is little more than air, the combustion is better. The combustion velocity has the maximum value at the A/F ratio of 12∼13, with more gasoline amount.

Therefore, the engine power output will be maximum at the A/F ratio of 12∼13. Otherwise, the output will be reduced. In the aspect of fuel consumption ratio, the consumption ratio will be minimum value about the A/F ratio of 16, that is, little lean state has the best fuel efficiency. After combusted, if any oxygen is not remained, then the gasoline is not completely combusted.

3. Ignition Timing

The ignition timing is when the compressed mixture is fired, that is the timing for making a electrical flame at the spark plug. Generally, it can be thought when the mixture is fully compressed and the piston reaches at the TDC (top dead center) is the best timing for the ignition. However, it is too late. The reason is that the combustion velocity of the mixture is changed by the gas flow velocity. As the engine speed is increased, the gas flow will be faster and faster. Therefore, the flame velocity will be faster. So, to ignite when the piston is at the highest point is too late. The best timing is when the piston is almost at the highest point that is, when the area of flame surface is almost half of the combustion chamber.

The ignition timing is represented by the rotation angle of the crankshaft about the TDC of the piston. In terms of the angle, if the ignition timing is set to 40∼30° before the TDC, then the combustion chamber has the maximum pressure at the 15∼20° after the TDC.

If the ignition timing is too early performed, then the combustion is occurred before the piston reaches at the highest point. In this case, the combustion force will press the up rising piston, so the force will be reduced. If the ignition timing is too late, then combustion force will press the downing piston. So the combustion force will not be work effectively.

As the flame velocity is as fast as the engine speed, the ignition timing should be corresponded with the engine speed in order to maximize the pressure of combustion chamber at the TDC of the piston. This operation is to advance the angle of the ignition in considering of the crankshaft rotating angle, so it is called the advance angle.

In the system for performing the advance angle, there are the mechanical type and the electrical type. The mechanical advance angel device is assembled between the distributors applying currents to the spark plug. By detecting the engine speed mechanically, the timing for applying current is controlled according to the engine speed to advance the ignition timing of the spark plug. For example, in the vacuum advance angle device, the advance angle is performed by the operation proportional to the negative pressure of the device connected to the carburetor with pipe using the phenomenon in which the negative pressure in the intake port is increased according to the engine speed.

The electrical advance angle device is that the engine speed and the negative pressure are detected by the sensor, and the best ignition timing is decided by the computer.

4. Swirl Effect

As the flame velocity is fast, more heat energy can be converted into the kinetic energy. Ideally, the mixture should be exploded when the piston just passes the highest point to transmit the expansion force of the combusted gas to the piston most effectively. For full combustion, in terms of crankshaft rotation angle, the time of 40∼60° rotation should be needed. So, the actual situation differs from the ideal situation.

To ensure the fast combusting, the gasoline should be mixed with air well to be enable to perform the chemical react between hydrogen carbon and oxygen.

To do so, the gasoline particle from the injector should be tiny and easy to be vaporized as possible. And the injector orifice should face to the intake valve in order not to adhere the gasoline particles to the intake port wall. For some racing engines, two injectors may be attached at each cylinder.

Additionally, in order to be make the flame velocity be fast, the flow velocity of gas should be faster. When the engine is rotating in slow speed, the flow velocity of the mixture is very important element. When the engine is rotating in high speed, the flow of mixture is high, so the mixing is well and the flame velocity is enough fast. However, when the engine speed starts to be decelerated, the piston downing speed is low, so the mixture flow velocity is lowered and the gasoline fog within the mixture can not be easily vaporized.

Therefore, some researches and developments for direction of intake port, for reducing the size of intake port and for using two intake ports in which one intake port is closed to flow in whirl when the engine works in low speed, to mix the fuel with air enough. The flow of whirl is divided into the swirl of which direction is in horizontal and the tumble of which direction is vertical.

The important thing in the swirl is that the swirl generated at the intake stroke should be remained even should be much stronger in the ignition-combustion stroke.

  

        Swirl               Tumble

To do so, one method is that a little gap called squish area is made between the most far position from the plug and the end portion of the piston crown, to blow the mixture by squish area when the piston is near the highest point.

5. Knocking

Even it is rarely occurred in nowadays, the engine makes a noise when the car is accelerated in high load condition.

This is the typical knocking. This comes from that the combustion is not started from the flame kernel of the spark plug and expansion of the flame surface, but from the early combusting of the mixture in the end zone which will be combusted at last.

As the flame surface is a boundary, inside of the surface is filled with the combusted gas and outside of the surface is filled with un-burn gas. That is the combustion is spreading from the flame surface. Before this flame surface is not reached, the un-burn gas is self combusted by the pressure of the gas expansion. This gas with the high pressure and high temperature knocks the cylinder head and piston, so the engine has harmful damages. The knocking is occurred at once, then the piston and cylinder have the abnormally high temperature, so the sequential knockings can be easily following.

Because that the knocking is generated at the end zone of the combusting chamber, the bore will be enlarged by the SHORT STROKE and it is easily generated in the engine having longer flame spread distance. Therefore, the modern engine is equipped with the spark plug, especially the center plug, at the center of the chamber or with a squish area enhancing the mixture flow by making the end zone be narrower.

Nowadays most car doesn’t make any knocking during driving. The engine is developed to prevent from knocking.

On the other hand, there is a research for enhancing the engine performance using the knocking. The knocking, as the firstly concerned, is occurred at the low engine speed in which the combustion of mixture is lag behind of the abnormal combustion. Generally, it is occurred at the ignition timing is advanced when the compression ratio is increase or the flame velocity is high. Therefore, by detecting the knocking, if the engine is run with maximum advance of ignition timing, the best combusting condition can be made.

6. Abnormal Combustion

The all combustion contrary with the normal combustion in which the combustion starts from the spark plug and spread over all chamber are called abnormal combustions. The knocking is the representative example. There are also other types of abnormal combustions.

⑴ PRE-IGNITION & POST-IGNITION

As the PRE is “before” and the POST is “After”, these ignition means that the mixture can be combusted by the other flame before or after the normal ignition is occurred. The PRE-IGNITION is occurred at the compression stroke by any reason sush as remaining at the carbon slug attached at the plug, chamber wall, piston or valves. The POST-IGNITION is that the mixtures not combusted at the normal flame period by misfire, un-burned gas is combusted at the combustion stroke. Both of them are very similar with the knocking, so they can make a great affects at the parts around the chamber.

⑵ RUN ON

As being also called as dieseling, this is the phenomenon that the engine is still working even the ignition switch is off. Very similar with the PRE-IGNITION, the carbon slug works as a flame seed. This is generally occurred when the key is off with the overheated carburetor engine. This is named from that the diesel engine combusts without ignition.

⑶ AFTER FIRE

This is also called as the AFTER BURN. This is that the incompletely combusted gas is exploded at the exhaust system with a big combustion sound. When the accelerator is turn to open or close abruptly, the exceeded gasoline is exhausted into the chamber and then the incompletely combusted mixture is exploded at the catalyst converter or at the muffler. This can make damage to the exhaust system.

⑷ BACK FIRE

In the state that the almost of the combusted gas is taken out at the exhaust stroke, there are some amount of remained gas. The remained gas with high temperature make a ignition the air/fuel mixture at the beginning of intake stroke. In some cases, the fire can reach back to the air cleaner. This is mainly occurred at the carburetor system.

These abnormal combustion is not often occurred in normal driving situations, however, be careful to maintain the engine.

7. Shape of Combustion Chamber

According to the combustion method, the engine performance shall differ. Then, which shape of the combustion chamber is the best for engine performance.

It may be true that the faster flame velocity is the better in order to increase the engine output. With the same  gasoline and A/F ratio, we can consider the following five items for the engine power.

⑴ The amount of the inhaled mixture shall be plentiful (More fuel, more heat)

⑵ The flow just before the ignition shall be proper.

  (The faster is the better, however, too fast makes a misfire)

⑶ The ignition plug should be installed at the center of the combustion chamber (to ensure fast combustion of mixture)

⑷ The compression ratio should be as high as possible (With high compression, heat efficiency is good)

⑸ Combustion chamber should be compact size to prevent heat from losing.

  (to ensure the heat energy converted into kinematic energy)

First of all, concerning the inhalation amount of mixture in ⑴, this is decided by the attaching angle, number, size, lift and shape of the intake valve. It is explained in the intake-exhaust valve section in detail.

In the mixture flow in ⑵, here, how the mixture is taken into the cylinder is the most important point. Even the mixture flow is well, if the shape of valve inside and piston crown are complicated, then the gas will not be expanded smoothly, so it should have the simple shape as possible.

The plug position in ⑶ is decided by the number and position of the intake-exhaust valves. In the 4-valve engine most used nowadays, the plug shall be installed at the center of the combustion chamber, ideally.

As the compression ratio mentioned in ⑷ is higher, the combustion will be faster because the temperature and pressure of the combustion chamber just before ignition is high. However, if the combustion is too fast, then the combustion is performed abnormally. So the chamber can be damaged by this abnormal combustion such as knocking.

To make that the heat can not be lost easily as mentioned in ⑸, consider that; as the inside area of the combustion chamber is bigger, the heat loss when the exploded gas presses the piston will be higher, that is, the heat energy which will be converted into the force energy will be lost. With the same volume of the combustion chamber, as the inside surface area of the chamber is smaller; the heat converting ratio will be higher.

Therefore, when the ratio between the SURFACE and VOLUME of the combustion chamber is the S/V ratio, this ratio represents the combustion efficiency. The smaller S/V ratio is better for the combustion efficiency.

8. Intake-Exhaust Valve & Combustion Chamber

To get better volume efficiency, more amount of intake air is needed, and the flow of intake-exhaust should be made smoother. The size and shape of the intake port is important as well as the attaching angle, diameter and number of the valves should be appropriate to enhance the volume efficiency.

The larger diameter of valve is the better. If the valve is too large, it is heavy so it has large inertia force when it is open and close. Therefore, it will hinder the engine from rotating with high speed. The size of valve should be optimized. The 4-valve engine having two set of intake-exhaust valve is more applied recently than the 2-valve engine having one set of intake-exhaust valve.

The three-valve engine having two intake valves and one exhaust valve was noticed. However, the plug was not installed at the center of chamber, and the exhaust valve was too large so the two intake valve system is worse than 4 valve system.

The chamber types of 4-valve engine are the PENT ROOF type having the roof shaped cylinder head and the Poly-spherical type having the some overlapped spheres. In the both types, a pair of intake-exhaust valve is facing with each other, and the spark plug is located at the center. It satisfies the requirement condition for the excellent volume efficiency.

Big valve angle     Small valve angle

The valve inclined angle is the angle of intake-exhaust valve about the center line of the cylinder. The valve angle is the angle between the center lines of each valve. These angles make an important affect to the chamber shape, the S/V ratio, the compression ratio, and the shape of intake-exhaust ports. If the valve angle is to be larger, then the valve diameter can be made widely, and the intake-exhaust gas will be flown more smoothly. However, the chamber is to be larger also, so it has demerits such that the compression ratio will be reduced, and the S/V ratio is to be large. New type engine has the compact combustion chamber of which valve angle is smaller than ever.

The five-valve engine having the three intake valves and two exhaust valves is for high performance by enlarging the cross area of the valve and lightening the valve weight. However, it has more complicated chamber so the S/V ratio will be larger as well as the mechanism around the valve will be more complicated.

9. Piston & Combustion Chamber

The piston head forms the combustion chamber by facing the intake-exhaust valves portion of cylinder head. To combust the mixture fast, the inside surface of the chamber should have fewer extruded or recessed portions to flow the mixture smoothly, and the S/V ratio should be small as possible. Therefore the piston head should be flattened.

In actual, considering other elements such as the valve angle, the cylinder head shall have the recessed shape. Therefore, to increase the compression ratio, the piston head should be extruded, highly. Furthermore, if the engine has high compression ratio, then the gap between the cylinder head and piston head should be narrow so it need to make the valve recess be larger to prevent the valve from abnormally operating. With these limitations in the mechanism, there are many researches for better combustion.

The piston has an important role to transmit the combustion force to the connecting rod effectively, so the other portions except the piston head should be precisely designed.

The combusted gas is sealed with the piston ring. To ensure the sealing, the gap between the piston and cylinder (piston clearance) should be small as possible. The piston will be cooled by the lubricant oil and the heat will be radiated through the piston ring. The thermal expansion coefficient of the aluminum, the main material of the piston, is 23 relatively higher than steel of which thermal expansion coefficient is 12~15, which is the main material of the cylinder. Therefore it is hard to match the piston size to the cylinder size. For example, as the back side of the piston head is reinforced, it is made little smaller than the skirt part and the piston diameter along to the inserting axis of piston pin is little smaller than the perpendicular axis.

As the connecting rod rotates the crankshaft, the piston will press the connecting rod with inclined direction. Therefore, the piston may be trembling along the lateral direction so the skirt will strike the cylinder wall. This is called the piston slap or the sides knock. This is the cause of the noise or power loss by friction.

To minimize this slap, the center of the piston pin is offset about 1∼2.5mm along to the movement direction of the connecting rod. Doing so, the force pressing the piston to the lateral direction will be reduced. It is called the offset piston.