ENGINE PRINCIPLES

Engine Principles

Chapter 1.

What is the engine?

1. What is the engine?

This book introduces about the automobile engine especially the gasoline engine.

However, it is very complecate to define about the engine, so called, what the engine is. In general concept, the engine is 『the devices driving something by changing the energy in the natural source such as fire, wind or electric material to the mechanical energy continuously』. There are many types of the engine and they are driven in different ways.

Therefore, we can define the gasoline engine, as a kind of combustion device in other word, the 『device changing the heat acquired by combusting the gasoline to the mechanical force for driving the vehicles』.

How is the heat energy changed into the mechanical energy? For example, the bottle or pot. As they are heated, the covers move with a noise.

The heat acquired from the gas or electric energy boils the water so as to make the water vapor pushing up the cover of the bottle or pot.

There is an important thing. The moving force is not come from the heat energy, but the hot air or vapor by the heat works.

That is, the media is need for changing the energy form. This media is the working fluid in the technical terminology. The working fluid for the gasoline engine is the air in-taken with the gasoline into the engine and then combusted and exhausted.

The procedure for transmitting the fuel to the mechanical energy in the vehicle engine will be explained in the following sections.

In this process, the working fluid is the air. If there is no working fluid, the energy transformation shall not be performed.

In contrary, think about the transforming the mechanical energy to the heat energy. In the vehicle, the brake is the represented example. The principle is the friction heat coming from the rubbing or striking the two materials. We can warm our hands by rubbing each other, that is, the moving force(rubbing) can easily transform to the heat energy(warming hands). At this time, there is no working fluid. The force is changed into heat directly.

However, when the heat energy is transformed into the mechanical energy, there must be a working fluid. Being the media for the transforming the energy, there should be a lot of loss of energy, at any case. Therefore, how much energy of the heat can be transformed into the mechanical energy, the efficiency, is an important factor in the engine.

2. Kinds of engine

There are many kinds of the engines.

The engines can be classified by the number of cylinder, the array style of the cylinder or the equipped method at the vehicle, and so on.

As you know, the engine makes the driving force by the reciprocal movement of the piston in the cylinder so that the power is decided by the number of the cylinder.

Therefore, the engine is mainly classified by the number of the cylinder. The commercial vehicles are classified into the 2, 3, 4, 5, 6, 8, and 12 cylinders. More displacement volume has the more cylinders.

According to the array of the cylinder, there are three types including the In-line type with serial arraying of the cylinder, the V type with V-shaped arraying of the cylinder and the opposed type in which the cylinders are arrayed facing each other.

According to the engine installation type, there are two types; the one is the lengthwise type and the breadthwise type. When the engines are arrayed in length direction of the vehicle is called as the lengthwise type, when the engines are arrayed in width direction of the vehicle is called as the breadthwise type.

Lengthwise type       Breadthwise type

For example, the FR type car having the engine at front and driving the rear wheels has the lengthwise type engine. The reason is that the propeller shaftt transmitting the driving force to the rear wheels shall be run underneath the floor. In the case of FF type car, the most small car, having the engine at front and driving the front wheels, the engine is breadthwise type because that the rotation axis of the engine and the driving axis rotating the wheels should be are parallel. However, when the FF type car has the 6-cylinder engine, if the serial engine is installed in width direction, then the width of the car is too wide. Therefore, in that case, the engine rather is the V-type. In this manner, considered with the displacement volume and vehicle type, the engine array type and the installation type are selected for the best combination in design.

3. Engine layout

FF : Front Engine Front Drive

FR : Front Engine Rear Drive

MR : Midship Engine Rear Drive

RR : Rear Engine Rear Drive

It is possible to assume that the engine will be installed at the front of the car. However, the all vehicles do not have their engine at front side. In 1770, the origin of the car, the engine of the steam car of Cugnot was located at the front ends of the body. In 1885, the first car equipped the gasoline engine, the engine of the Daimler’s car is located at the under the seat and the front of the rear wheel axis.

In the history, many researches had been performed to find the best condition where the engine was installed. In 1891, the FR (front engine rear drive) car was in France.  The FR type is that the engine was installed at the front side and the rear wheels were driven. After that, this type would be the set as the standard layout of the engine. Until now, the most large passenger cars and the sports cars accept this type.

The typical characteristics of the FR type car is that the cabin space is located to rear side, the steering is controlling the front wheels and the driving is contributed to the rear wheels so that the operation and weight load are evenly divided to the front side and rear side and the movement performance and passenger convenience are balanced as well as the vibration and noise are less than other types.

In the layout of the relatively small public car, there had been two big developments for about hundred years. The first one is the Volkswagen in 1936 having the engine at the rear side for driving the rear wheel. After the World War II, this type was leading the worldwide design of the mainstream the passenger car of rear engine and rear drive (RR) type. The second one is the Mini of England in 1959. The Mini accepted the front engine and front drive (FF) type having the breadthwise type engine at front side for driving the front wheels. Nowadays, this type is applied to the small cars as well as the middle sized sedans.

The FF type car has the engine and driving devices at the front side so that it should not applied to the car of which weight is concentrated to the front side. The defect of this type is that it is not easy to steer the car. However, it has the relatively large space of indoor and trunk, and the safety is better than other types. Therefore, it is the best design for the layout of the utility cars.

The midship engine and rear drive (MR) type is focused on the performance rather than the convenience of the passenger so that it is mainly applied to the sports cars. When the main parts of the engine are located at front side than the rear wheel type, it is called the midship type. When the main parts are located at rear side, it is called the rear engine type.

4. Expansive Force and Inertia Force

Almost of the commercial gasoline engines for the vehicles are the Recipro-engines except for the rotary engine of which working principle is different. The word “Recipro” comes from the “Reciprocating”. The reciprocation is the mechanical movement of go and back so that the Recipro-engine is the device transforming the reciprocal movement to the rotational movement using the crank which is the device having the prominence and depression shape.

The cross sectional view of the Recipro-engine shows that there is a piston reciprocal moving in the cylinder at the upper side and there is a crankshaft at the lower side and the piston and the crankshaft are connected by the connecting rod.

The gasoline engine uses the air as the media of working fluid for transforming the heat energy to the mechanical energy. The air is mixed with the mist of gasoline in the cylinder. When the mixture of air and gasoline are compressed using the piston and combusted, then the expansive gas will press the piston. The expansive force pressing the piston shall drive the car.

At this time, except for the expansive force, there is an inertia force which we are considering. This force is somewhat strange because it is not generated intentionally; however, this force is followed in the moving part of the engine naturally. Therefore, it can affect to the performance of engine or is the source of the vibration or noise.

Considering the reciprocal movement of the piston, the piston starts moving from the halt state at the highest position of the stroke, has the maximum speed at the middle of the stroke, After that, the speed is reduced and stops to the lowest position of the stroke, and then goes back to the highest position again. During performing these movements, the inertia forces shall be generated when the speed of movements are changed. For example, from the highest position of the piston to the middle of the stroke, there is an ascending inertia force. After that there is a descending inertia force from the middle of the stroke to the lowest position of the piston.

When this inertia force is resonated with the other inertia forces from the other pistons, there may be vibrations or noises.

5. Reciprocal Engine

The Recipro-Engines are classified into two types, the 2-cycle type and the 4-cycle type. After developed by Daimler of Germany in 1883, the 4-cycle type which had been completed its standard system until 1900 is the mainly used in the most of vehicles.

The principle of the reciprocal engine is that the mixture of the air and the gasoline are injected into the cylinder, the mixture is combusted using the electric spark to explode, the combustion force drives the piston in reciprocal movement, and the reciprocal movement is changed to the rotational movement by the crankshaft.

The operation of the 4-cycle engine is like that. When the piston is at the highest position, the intake valve is opened. Being down the piston, the mixtures of fuel gas is injected into the cylinder and then the intake valve is closed [Intake Stroke]. Next, the piston will go to upward to compress the mixtures of gas [Compression Stroke]. Then, the compressed mixtures of gas will be burnt by an electrical ignition [Combustion Stroke]. The burnt gas having the high pressure and temperature will press the piston downward. At this time, the exhaust valve shall be open to take out the burnt gas [Exhaust Stroke]. These strokes are repeated.

Among these 4 strokes, only at the combustion stroke, the engine makes the power for work. Therefore, there are needed the additional forces, for intake and exhausting the gas at the intake and exhaust stroke, and for compressing the mixture at the compression stroke. To do so, a flywheel is installed to the crankshaft to make these additional forces using the  inertia force to ensure the continuous rotational movement.

The 2-cycle engine comprises of the two strokes. The operation of this engine comprises the four components such as the intake, the compression, the combustion, and the exhaust as the 4-cycle engine.

Compression and Expansion

Exhaust and Intake

However, before and After the piston is located at the highest position, the compression and combustion operations are performed, and before and After the piston is located at the lowest position, the exhaust and intake operation at the same time. So, through the two strokes, the one cycle of engine operation is completed. The 4-cycle engine performs the combustion stroke at one time per two cycles of the crankshaft, but, the 2-cycle engine performs the combustion stroke at every cycle of crankshaft. Therefore, the 2-cycle has higher efficiency.

Scavenging

Additionally, it has no intake and exhaust valves so that it has simple structure and low cost. However, this merit can be a defect.

The exhaust gas is exhausted by the entering new mixtures of gas when the piston is located at the lowest position. Therefore, some new mixtures will be mixed the combusted gas and unburned gases will be exhausted. It makes the air pollutions and the large fuel consumptions.

6. Exhaust & Intake Stroke

This content will explain about the engine on focusing the 4-cycle gasoline engine, the most used type.

To understand the 4 strokes, the intake, the compression, the combustion, and the exhaust, it is helpful to refer the figures, the indicator diagram (P-V diagram), showing the process of the engine operations.

It looks somewhat complicated, but it is easy to understand just following the figures.

The indicator diagram is the graph consisting of the horizontal axis representing the pressure of the chamber and the vertical axis representing the volume of the chamber. At the left ends of the graph, C and F, the piston is located at the highest position of the cylinder, and at the right ends of the graph, G and H, the piston is located at the lowest position of the cylinder.

By comparing the lines of the graph to the 4 strokes, the line of A-B is the intake stroke, the line of B-C is the compression stroke, the line of C-D is the combustion stroke and the line of D-E is the exhaust stroke.

In act, the cycle of the strokes is starting from the intake stroke. To understand the engine operation, it is easy to start from the exhaust stroke. To inhale more air as possible, the engine uses also the force from the exhausted gas outgoing to the outside of the engine through the exhaust port.

The exhaust stroke is for pushing the combusted gas to outside of the chamber by moving the piston from the lowest position to the highest position with being opened exhaust valve. In principle, it is think that the exhaust valve will be opened when the piston is reaching at the lowest position.

Exhaust stroke

However, actually, the exhaust valve prefers to be opened before the piston reaches to the lowest position, that is, at the position of D in the drawing. Remaining the pressing force of the burent gas, the exhausting the used gas is more effective by opening the exhaust valve in advance. After that, the piston will push out the remained used gas thoroughly to finish the exhaust stroke.

At the intake stroke, the intake valve is opened, and the piston goes down from the highest position to the lowest position so that the mixtures of fuel gas and the air are inhaled into the cylinder from the intake port. At that time, the intake valve shall be opened just before the piston reaches the highest position, that is, the E in the drawing. Doing so, the intake operation is enhanced somewhat because of the exhausting force of the used gas will pull the intake gases.

Intake stroke

With the same manner, the intake valve shall be closed when the piston is at the B. Doing so, more mixtures of gas shall be inhaled into the cylinder by the inertia force of the intake gas. To inhale air into the engine is performed by the difference of air pressure. When the piston goes down, the air pressure in the cylinder is lower than that of the outside of the cylinder, so that the air around the intake valve shall be inhaled to the cylinder. The opening timing of the valve is different from the stroke position is to use this force to inhale more air as possible

7. Compression and Combustion Stroke

At the compression stroke, the mixtures of gas are compressed by the piston, so that the pressure is increased and the temperature is high by adiabatic compression. Therefore, the gasoline is vaporized by the compressive heat of the air, ready to be combusted. The injected gasoline in the cylinder like the misty with the air is vaporized to be gas state by the heat from the adiabatic compression. Then it is ready to be combusted easily. This space for combustion is called the combustion chamber.

The reason of that it is hard to start the engine in winter is that it is hard for the gasoline to be vaporized. To solve this problem, one method is to mix more gasoline with the air. Another problem is that; when the gasoline is vaporized, the ambient heat is used to the vaporization, so the temperature of the chamber is lowered somewhat. It may decrease the fuel efficiency of the engine. To prevent from lowering the fuel efficiency, the amount of the gasoline is reduced. However, to do so, the temperature of the chamber is so high that the combustion may be performed prior to the ignition, so called the abnormal combustion.

The most important fact from the intake stroke to the compression stroke is the flow of the mixtures of the gasoline and the air. It is not proper that the flow is too strong to be ignited. The little particles of gasoline should be mixed with air to be the mixtures. Therefore, many manufacturers research and develop the shape of the intake port and flow pattern of the mixtures so that the strength and the pattern of the mixture flow are maintained until the combustion stroke for the best efficiency of the engine.

Compression stroke

As processing the compression stroke, the piston reaches the highest position. When the piston is at the C of the drawing, the ignition will be performed by the electric spark generated from the spark plug. The timing to make the spark is very important. The mixture is not fully combusted at the ignition time, but the combustion is started from the ignition. It is needed some time interval between the time of the ignition and the time to be maximum pressure of the chamber.

Combustion stroke

The ignition timing is determined by considering that the combustion shall be completed between the highest position and the almost half position of the combustion chamber. Additionally, this combustion speed is proportion to the rotational speed of the engine so that the ignition timing should be adjusted with the engine speed.

Beginning the combustion, the vaporized mixtures are combusted in a short time so that the pressure and the temperature are increased. At this time the expanded gas by the combustion shall press the piston. This pressing force should be stronger as possible. The time period of combustion is preferable to be short to strength the force. If the time period for combustion is longer, then the combustion force is not leading the pressing the piston but following the piston. Therefore, the engine efficient is worst. The combustion time period is affected by flow of the mixtures defined by the size and shape of the combustion chamber and the component of the mixture and so on.

8. Engine structure

The gasoline engine is a complicated machine having comprising of parts. Look into how the engine is structured.

The engine is similar with the 3 floors building. The first floor is the crank case including the crankshaft transforming the reciprocal movement to the rotational movement. The second floor is the cylinder block including the cylinder of which a piston is moving with reciprocal movement. The third layer is the cylinder head.

In this structure, the moving parts of the first layer and the second layer are called as the main moving part. It includes the piston, the crankshaft and the connecting rod.

In the third layer, there are the valves controlling the intake and exhaust of the mixtures gas and the used gas and the camshaft operating the valves. These are called the cylinder head system.

On the cylinder head, there are intake manifold sending the gasoline and the air to the cylinder and the exhaust manifold taking out the burnt gas. These are called the intake-exhaust system. The manifold consists the “many” and the “fold”, that is, many materials are combined. In actual, they are many branched pipes distributing the air and gasoline to each cylinder or merging the exhausted gases in one place.

There is the fuel system including the fuel pump taking the gasoline from the fuel tank and the carburetor or fuel injector to make a air fuel mixture.

There is the lubrication system including the oil pump supplying the oil for reducing the frictions and the oil filter for filtering the oil.

Also there is the cooling system including the radiator and water pump for maintaining the temperature of the engine properly.

To drive the engine, the electric power is needed. There are electric devices including igniting spark plug, the alternator generating the electric power and the start motor supplying the initial movement to the engine.

Additionally, there are the auxiliaries such as the oil pump for the power steering the air-con compressor and so on.

9. Diesel Engine

The diesel engine has similar shape and structure with those of the gasoline engine. The different point is the ignition method. The gasoline engine ignites the mixture of fuel with the electric spark. Contrary, in diesel engine, the fuel is injected into the compressed air having the high temperature. When the air is compressed, the temperature of compressed air is increased. The gasoline engine compresses the mixture of fuel gas up to 1/10 of the initial volume.

The diesel engine compresses the air about 1/20 of the initial volume to increase the temperature of the air over 600℃, and injects the fuel compressed with over 100 atm at the injection pump during 1/1000∼2/1000 seconds. The output shall be controlled by the amount of injected mixtures of fuel and air for the gasoline engine. On the other hand, the output of the diesel engine can be controlled by the amount of the injected fuel without controlling the air (the fixed amount of the air).

To combust the fuel perfectly by increasing the temperature of the air, the compression ratio shall be increased. However, doing so, the expansion power will be increased also. Therefore, the engine should be stronger to endure the increased force. Additionally, the high quality of fuel injecting pump may be needed. Then the engine is heavier and the cost is expensive. So diesel engine is not proper to apply to the passenger’s car.

In the diesel engine, because almost constant amount of the air about the volume of the cylinder is inhaled, the load applying to the engine is relatively light. When the fuel amount is small at the low speed, the fuel shall be almost perfectly combusted. However, with the full load, the diesel engine needs more amount of fuel so that the air amount is respectively small. Therefore, it may exhaust a lot of black smoke.

In the gasoline engine, the ignition is performed by the electric spark to the mixture so the combustion time period is very short. However, in the diesel engine, the fuel diesel is sprayed into the compressed air, so it needs some time period to be vaporized. Therefore, the maximum speed of engine is limited relatively lower and the output shall be lower than the gasoline engine.

Compared with the gasoline engine, the expansion force and the inertia force of the moving part are bigger so it makes bigger noise and more vibration. It has merits of easy maintenance resulted from not having the delicate parts such as the ignition system and of good fuel efficiency so that it is used for commercial or business purpose rather than passenger’s car.

10. Internal Combustion & Motor

The force of the 4-cycle gasoline engine, as one of the internal combustion, is changed according to the RPM (Revolutions Per Minute) of the engine compared by the electric motor or the steam engine. So, it is impossible to drive with the lower revolutions than certain RPM value. Therefore, the clutch and the transmission should be equipped when the gasoline engine is used for vehicles.

For the 4-cycle engine, using the four strokes, it makes moving force by combusting the mixture of fuel and air in the cylinder. It is very different with the electric motor used in electric vehicles which can start just by applying the electric power. If the mixtures are not supplied into the cylinder at idle condition, the engine can not continue to run. In order that the engine should be operating continuously when the vehicle is stopped, the device for connecting or disconnecting the moving force of the engine to the wheels such as clutch should be equipped.

In general, the vehicles needs larger power when it is started or accelerated, but when it is driven in constant speed, it does not need larger power. For the motor, it outputs larger force when it rotates with lower RPM, and when the RPM is increased, the output will be lower. Therefore, the electric motor can be applied to the engine of the vehicles without any transmitting device.

However, for the gasoline engine, the power is determined according to the RPM of the engine. The range of the RPM is limited within certain ranges. For example, the RPM of the gasoline engine is about 700∼7000 revolutions per minute, and the RPM for getting the maximum power (torque) is about 4000 revolutions. Therefore, when the vehicles are running with various speeds, it is necessary to control the speed and power of the vehicle by inserting a transmission between the engine and the wheels.

At simple sight, the motor may be the best engine for the vehicles. The important thing is the fuel, the source of power. The gasoline is easy to store during operating the engine, but it is hard for the motor to store the electric power effectively.

To develop the electric vehicles, it is essential to develop the batteries having the high efficiency for charging and maintaining the rechargeable batteries in equivalent state. Many companies are trying developing the method for maintaining the rechargeable batteries. Even though the basic performance has been developed in field test, the manufacturing cost is very high. However there are being suggested some patents and technologies for utilizing.

11. Lean Burn Engine

The exhaust purification system using the 3way catalysts has the characteristics of maintaining the actual air-fuel ratio to the ideal valve to perform the oxidation and reduction of the harmful compounds simultaneously. To do so, the purification of the exhausted gas will be limited, and the fuel amount used for the engine is decided by the engine driving status. Therefore, the engine shall not be developed no more to get higher driving force with less amount of the fuel.

The lean burn system is developed for enhancing the fuel efficiency with the good purification of exhaust gas. To enhance the fuel efficiency is most important point for future. The lean burn engine is one of the most attracted public attention technologies.

With the high A/F ratio by reducing the gasoline in the mixture, how are the three major harmful materials, carbon monoxide, hydrogen carbon, and nitrogen oxide. The oxygen is more plentiful than fuel, so the amount of carbon monoxide will be less or the most carbon monoxide will be changed into carbon dioxide, harmless gas. The hydro carbon will also be completely combusted and changed into carbon dioxide and water. Now we will concern only the last one, nitrogen oxide.

If the A/F ratio is higher, then the temperature will be increase by the plentiful of the oxygen and the amount of the nitrogen oxide will be increased. At about 16 of A/F ratio, the nitrogen oxide will be maximized. If the A/F ratio is higher than 16, then the combustion temperature will be down so the torque is also down. If the A/F ratio is higher and higher, the combustion is not stable, and torque is very unstable, finally combustion will not be performed.

A manufacturer focused on the torque variations according to the lean burn. By adapting the combustion pressure sensor detecting the combustion pressure in the cylinder, the engine is operated with the A/F ratio just before the torque variation is occurred. Therefore, they can make next generation lean burn engine having the low fuel consumption and the less amount of nitrogen oxide. In that system, the lean burn is performed at the condition in which the driving is not hindered by the low torque at low load. When the vehicle in accelerating or high load, the combustion is performed with the theoretical A/F ratio and the exhaust gas is purified by 3way catalyst. Many manufacturers continue to research for enhancing the fuel consumption by focusing on the intake system and combustion chamber with 16∼20 of A/F ratio. Many new engines satisfying this combustion requirement and having less exhaust gas problem are shown more and more.