INTERNAL COMBUSTION ENGINE

INTRODUCTION

First of all the question arises that what is an engine? An engine is a machine that converts heat energy into mechanical energy. Now the engines can be classified as Internal combustion engine and external combustion engine. In this blog we will focus on internal combustion engines only.

Automotive engines are internal combustion engine (I.C engine)because the fuel that runs them is burned internally or inside the engine. I.C engine are further classified as reciprocating and rotary engine. Now a days reciprocating engines are mainly used in automotive engines. Reciprocating means moving up and down, or back and forth. These engines have pistons that move up and down, or reciprocate, in cylinders. These are also known as piston engines.

Rotary engines have rotors that spin, or rotate. Rotary engines are not used in automobiles widely. 

The basic difference between reciprocating engine and rotary engine is that in rotary engines crankshaft remains stationary, while cylinders, crankcase and other units rotate in this operation.

TYPES OF RECIPROCATING ENGINES

There are two types of reciprocating engines
1. spark-ignition
2. compression-ignition 

They differ in 3 ways
a. The type of fuel used.
b. The way the fuel gets into the cylinders.
c. The way the fuel is ignited.

SPARK-IGNITION ENGINE

1. It usually runs on gasoline or an alcohol blend which are liquid fuels.
2. The fuel must be highly volatile so that it vaporizes quickly.
3. Fuel vapor mixes with air before entering the engine cylinders.
4. Combustion occurs due to spark produced in the cylinder.

COMPRESSION-IGNITION ENGINE

1. Mostly diesel is used in this kind of engines.
2. Fuel vapor is directly sprayed into the cylinder, not mixes with air before entering into the cylinder.
3. Combustion occurs due to pressure in the cylinder.

ENGINE CONSTRUCTION

Both types of piston engines are similar in construction. They have following main parts:-

1. Combustion chamber
2. Cylinder 
3. Cylinder Head
4. Camshaft
5. Valves
6. Crankshaft
7. Connecting rod

The figure shows a three-cylinder spark-ignition engine. The cylinder head covers the top of the cylinder. The bottom of the cylinder is open. As the engine runs, the piston slides up and down in the cylinder. The piston at the lowest position is said to be at bottom dead center(BDC), while at the top most point is said to be at top dead center(TDC) as shown in the diagram below.

WORKING OF S.I ENGINE

Engine could be two stroke or four stroke engine. Let us see four stroke engine first

The movement of piston can be divided into four parts or four strokes 

1. Intake stroke 
2. Compression stroke
3. Power stroke 
4. Exhaust stroke

These strokes are movement of piston from TDC to BDC. The crankshaft makes two complete revolutions to complete the four strokes. This makes the engine a four stroke engine.

INTAKE STROKE 

During the intake stroke of a spark ignition engine, the piston is moving down. The intake valve is open. Air-fuel mixture flows through the intake port and into the cylinder. The fuel system supplies the mixture.

As the piston moves down, air fuel mixture enters the cylinder. This is because the piston creates a partial vacuum above it. Due to pressure difference(outside is atmospheric pressure) the air gets in the cylinder. So intake stroke is the movement of the piton from top dead center to bottom dead center. (It should be noted that it is only ideal not in practical.)

COMPRESSION STROKE 

As the piston moves from BDC to TDC the air fuel mixture is compressed. The space which is there after compression is combustion chamber. In this stroke both the valves are kept closed so that proper pressure can be developed. The amount the mixture is compressed is the compression ratio. It is the ratio between the original volume and the compressed volume in the combustion chamber. If the mixture i compressed to 1/8 of its original volume than the compression ratio is 8:1

POWER STROKE 

As the piston is approaching at TDC at the end of compression stroke, an electric spark is produced by the spark plug. Due to spark the air fuel mixture gets burnt. It produces high temperature upto 6000°F. Due to very high temperature it cause very high pres sure which pushes the piston down. The connecting rod carries the motion to the crankshaft, which turns the wheels.

EXHAUST STROKE 

As the piston approaches BDC on the power stroke, the exhaust valve opens. After passing through BDC, the piston moves up again. The burned gases escape through the open exhaust port. As the piston nears TDC, the intake valve opens. The sequence is repeated till the engine is operating  

PISTON AND PISTON RINGS 

Pistons are made of Aluminum alloy. The size of the piston is slightly smaller than the cylinder, which allows the piston to slide up and. This is sliding fit. But there is a problem if we keep some gap between piston and cylinder. Can u think???

Well the problem is that if we keep some gap than there will be pressure loss, leakage of coolant and lubricating oil also. So we should make piston of the same size of a cylinder(in diameter). if we do so the piston will wear very fast and replacement will be needed again and again. This is another problem. So we find an another solution, this solution is piston rings.

Due to pressure difference there is a leakage of compressed gas this leakage is called lowby. Excessive blowby reduces engine power, wastes fuel, and pollutes the air.

To prevent excessive blowby, piston rings are installed on the pistons. The rings are split at one point. This allows them to be expanded slightly, slipped over the head of the piston, and into ring grooves cut in the piston.

There are two types of piston rings

Compression rings These form a sliding seal between the piston and the cylinder wall.

Oil-control rings These scrape off most of the lubricating oil splashed on the cylinder wall, and return the oil to the crankcase.

CLASSIFICATION OF ENGINE

Engine can be classified on the basis of 

1. Number of cylinder. 
2. Arrangement of cylinder. 
3. Arrangement of valves and valve trains.
4. Type of cooling. 
5. Number of strokes per cycle.
6. Types of fuel burned. 
7. Method of ignition.
8. Firing of order. 
9. Reciprocating or rotary.

Engine can has many number of cylinder like three four six or eight cylinder. Now the question is of how they are arranged. Arrangement could in many ways. two major ways are inline and V shape. Other ways are in two rows or banks opposing each other, like spoke on a wheel(radial type).

We won't go deep into these types.

ARRANGEMENT OF VALVES AND VALVE TRAIN

if we notice an engine it is closed from all the sides then how air enters into the cylinder. a point to think. we can't just keep the cylinder open because a pressure should be developed inside the cylinder. so it is supposed to closed. So we should have mechanism which let air in and let air outs so that fresh air is there for combustion and pressure should also be build. Valve are used to do this.

VALVES AND VALVE TRAIN

A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. In an engine a valve is used to control the inlet and outlet of air and fuel. The valve train consists of the valves and a mechanism that opens and closes them. The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down, as shown in.
The red and blue thing are lobes while the shaft on which lobes are there is called camshaft. The whole thing is known as valve train(assembly of valve, lobes camshaft).

 this is a diagram of camshaft.





There are several different arrangements of valve and valve train. Classification is based on the following basis:-

1. location of camshaft.
2. how the camshaft is driven.
3. type of valve train.
4. number of valve per cylinder.

On the basis of type of cooling engine is classified as air cooled or liquid cooled. Most of the engines are liquid cooled. Some of the engine are air cooled like Volkswagen Beetle engine.
Classification of engine is based on the number of strokes. we have already learnt that.
Engine are classified on the basis of fuel also. If engine is using gasoline or diesel or LPG.
Method of ignition- spark-ignition or compression ignition engine.

FIRING ORDER

engines can also be classified on the basis of firing order. Firing order is the sequence in which the cylinders deliver their power strokes. It is decided by crankpin and crankshaft. For a four stroke engine the complete firing order represents two complete revolution of the crankshaft, that is 720 degrees of crankshaft rotation. Most of the engines are even firing(think of the reason). For example in an inline six cylinder engine firing impulse occurs every 120 degrees of crankshaft rotation. Firing order differs for different engines. Several firing orders are 1-3-4-2, 1-5-3-6-2-4, 1-2-3-4-5-6 and many others. Firing order may differs for a V shape or inline whether they have same number of cylinders or not.

RECIPROCATING AND ROTARY(FIND IT YOURSELF)


This was the basic outlet of the reciprocating engine. Read other articles to know more about engines and other parts of a car.