Four Stroke Cycle Petrol Engine

How does the petrol engine of your vehicle work? It has become the most promising question to you. Let us take a look at the operation and engine cycle of a petrol engine. Theoretically, a petrol engine cycle is also known as the Otto cycle. Your petrol powered engine requires four strokes of the engine piston to complete a cycle.  These four strokes are discussed below:

a. Suction Stroke: At the very beginning of the suction stroke, the inlet valve of your engine opens. As the piston moves downward from TDC, it aids to suck the air fuel mixture into the cylinder. The valve remains open until the piston reaches the BDC.

b. Compression Stroke: This is one of the most important strokes of an engine operation. In this stroke, both valves (Inlet and Outlet) of your engine remain closed. Here, the piston moves from BDC to TDC, and the charge gets compressed in the cylinder. As you know, temperature of any gas mixture increases with the increased pressure. To complete this stroke your engine crankshaft gives one revolution of 180o.

c. Expansion Stroke: You can also say it the working stroke of an engine. In this stroke the combustion of air-fuel mixture takes place. Shortly before the piston reaches the TDC during the compression stroke, the air fuel mixture gets ignited by means of a spark plug. Then, the pressure and temperature of the cylinder increases significantly. But, the volume the volume remains constant. This increased pressure creates a thrust to the piston that moves from TDC to BDC, and the crankshaft gets a revolution of 180o. As the piston of your engine moves at high speed, the flue gas expands. During the expansion of gases, the heat energy gets converted into mechanical energy. All valves remain closed in this stroke.

d. Exhaust Stroke: This is the final stroke of your four stroke cycle petrol engine. In this stroke, the outlet valve opens and the inlet valve remains closed. From the timing diagram you can see that the piston moves from BDC to TDC in this stroke, and the burnt gases escapes from the engine cylinder due to the push of the piston. With the completion of this stroke, the engine completes a successful operation cycle. Then, your engine gets ready to such fresh charge again. This is how your petrol engine works by completing the engine operation cycle.

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What is Thrust Bearing and Why it is Important for Your Engine?

 

   

The thrust bearing is one of the most important bearings of your engine, which limits the endplay of your crankshaft. You will see flanges on both sides of this bearing, and the crankshaft flanges fit very close to thrust bearing flanges. This special structure of thrust bearing aids to resist forward and rearward movement of your engine’s crankshaft. Thus, it aids to resist vibration. 

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Main Bearings and Bushing

You may find bearings and bushing in places where your engine contains rotary parts. You will find sleeve bearings, which fit like sleeves around rotating shafts of your engine. Here, you will find most bearings are journal, and a few ball bearings. Due to durable service and low wear and tear, journal bearings are used in automotives. The shaft part of your engine that rotates in the bearing is known as the journal. The following is a list of main bearings and bushing of your OHV (Over Head Valve) engine.

a. Crankshaft Thrust Bearing
b. Connecting-Rod Bearing
c. Intermediate Main Bearing
d. Alternator Main Bearing
e. Front Main Bearing
f. Rocker-Arm Bushing
g. Valve Guide Bushing
h. Upper Distributor Bushing
i. Lower Distributor Bushing
j. Camshaft Bushing
k. Clutch Pilot Bushing
l. Water Pump Bushings
m. Oil Pump Bushing
n. Piston-Pin bushing and so on

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Why Your Engine Requires a Flywheel?

- Firstly, a flywheel is advantageous to provide continuous energy. If your engine’s energy becomes discontinuous, then the flywheel will aid the engine to make it continuous by supplying the stored energy.

- Basically, the flywheel is capable of supplying energy at rates that exceed the abilities of your engine’s energy source.

- It also aids to control the orientation of your engine’s mechanical system.

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What is a Flywheel and How Does It Work?

Flywheel

 

You will find the manual transmission flywheel at the end of the crankshaft. Basically, a flywheel is a rotating mechanical device, which aids to store rotational energy. Most interesting matter is that it poses significant moment of inertia, and it also aids to resist changes of your engine’s rotational speed. In most cases, you will find flywheels made of steel. Now a day, automakers are bringing carbon fiber flywheels to retain high strength and durability.

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Theoretical Timing Diagram of a Four Stroke Cycle Petrol Engine:

Theoretical Valve Timing Diagram:

The theoretical timing diagram of a four stroke cycle engine is shown in the figure. Here, you can see that at the initial stage of the suction stroke, the valve opens and takes place between A to B. To complete this, the crankshaft revolves 180^o and the piston moves from TDC to BDC. At BDC the inlet valve closes and the compression takes place between B to C. The crankshaft revolves 180^o, and the piston of your engine moves from BDC to TDC.

At the C position, the fuel of your engine gets fired, and the expansion takes place in 180^o from TDC to BDC. Then, again the crankshaft revolves 180^o from BDC to TDC at C to D allowing the expansion of flue gases. Finally, from D to E the crankshaft revolves another 180^o, and moves from BDC to TDC to complete the exhaust stroke. This is how the engine valve timing of your engine is maintained, and the crankshaft revolves 360^o to complete the cycle. In a complete cycle, the crankshaft gets two rotations of 360^o. 

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Valve Timing System

The timing system of valve is really an interesting matter on an automotive engine operation. You may wonder to think about the valve timing and all responsible elements to control it. Firstly, you have to know what the valve timing is. Generally, valve timing refers to cylinder valve opening and closing times in relation to the position of your engine piston. Remember that all specifications of timing system are calculated in degrees, and these degrees are calculated in between TDC (Top Dead Center) and BDC (Bottom Dead Center). Now, you have to understand first what are TDC and BDC. TDC is the farthest distance of the piston away from the crankshaft, and BDC is the closest distance of the piston towards the crankshaft. Things could be easier to you from the following timing diagram of your four stroke cycle petrol engine.

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Rocker Arm

You can say it a pivoted lever, and it aids to transfer pushrod or cam motion to engine valves. Basically, these are made of aluminum alloys. Often stamped steel is used to make these arms. You may find one of the following rocker arm adjustments on your engine: - Stud Mounted Rocker Arms - Rocker Arms with Adjustable Screws If you look closely, then you will find that rocker arm pivot points are offset from the center. This arrangement aids to gain the rocker arm ratio of 1:5:1. You may also find smaller cam lobe due to this rocker arm ratio. It places high loads on pushrod, cam lobe, and valve lifter.

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Automotive Valve Assembly

In a basic engine construction, there exist at least two valves, an inlet valve and an exhaust valve. The inlet valve of your engine opens just before the suction stroke, and the exhaust valve opens just before the exhaust stroke. The inlet valve allows your engine to suck fresh air-fuel mixture for combustion, and the exhaust valve releases burnt gases after combustion. You may find any one of the following valve trains on your engine-

 a. Overhead-Valve Engines: If your engine is an overhead-valve engine, then you will find valves in the cylinder head of your engine, and the cam shaft in the cylinder block. In case of an inline engine, you will find valves in a single row, and in V-type engine valves may be in double rows in each bank, or in single row in each bank. If your vehicle engine is of V type, then you must remember that one row contains intake valves and the other row contains exhaust valves.

Automotive Valve Assembly

b. L-head Engines: In case of an L-head engine, you will find the camshaft and valves in the cylinder block of your engine. In early days, this type of arrangement was highly popular for automotive engines. Now a day, this type of valve arrangement is used in small engines like lawn mowers. L-Head engines have some drawbacks. This type of engines can to accomplish high compression ratio. Another mentionable drawback is that it cannot achieve complete combustion of air-fuel mixture, and the flue gas contains too much partially burned and unburned gases.

c. Overhead-Camshaft Engines: This type of engines has become practical in newer engine designs. If you have such an engine, then you will find camshaft on the cylinder head of your engine. This type of engines is highly beneficial as they resist changes in direction and speed. If you closely look to the engine action, then you will find that push rod and rocker arm bend slightly before opening the valve. In case of DOHC (Double Overhead-Camshaft) engines, you have to remember that one camshaft operates the suction valve, and the other one operates the exhaust valve. In case of a V-type engine, you may find one or two camshaft in the cylinder head.

 d. Multivalve Engines: What is a multivalve engine and how it is beneficial? Basically, multivalve engines have 2-6 valves per cylinder. Two and four valves are most common in the recent time. In modern engine designs, you will find four valves per cylinder. This increased number of valves boosts power output and improves engine breaching. This arrangement allows more charge to enter into the cylinder, and aids to remove the exhaust gas easily than two or three valve engines.

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Push Rods

In an Over-Head-Valve engine, you will find push rods connected between rocker arm and cam, which actuate engine inlet, and exhaust valves to aid with proper air-fuel and exhaust flows. If your vehicle’s engine is an old one, and an over head engine, then you can think about these push rods. In modern automotive designs, automakers are avoiding push rods, and they are placing cam shaft in the cylinder head to aid the valve timing. 

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Cam Shaft

What is an Engine Cam Shaft and How Does it Work?

Probably, you are thinking that what is a cam shaft and why it is for. The shaft of your engine to which cam is connected as an integral part is known as the cam shaft. Basically, cam shaft is made of cast iron. Chilled iron casting process is used industrially for high volume production.

As a matter of fact, the `cam shaft aids to control engine valves for proper charge and exhaust flowing. Practically, the connection between cam shaft and crank shaft varies from engine to engine. The cam shaft is connected either directly with the crank shaft or via gear arrangement. You could also find your engine cam shaft connected with the crank shaft via belt or chain mechanism.

Practically, direct connection and chain-sprocket arrangements are more efficient than gear or belt mechanisms. Due to high wear and tear, gear arrangement has become unusual in auto industries. You may find some vehicles using cam shaft to drive the power steering pump.

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Crank Case

The case that houses your engine cylinder, crank, crankshaft, and cam is called the crank case. Basically, engine crank case is made of cast iron. You can also call it the sump of lubricating oil. This case stores lubricating oil to give all inner moving parts like cam shaft, connecting rod, and crankshaft and so on, a smooth movement and rotation with reduced frictional losses. You will find the lower end of your engine crank case fixed with bolts, which is known as bed plate. 

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Engine Crankshaft

What is an Engine Crankshaft?

 

You will find the crankshaft at the bottom of connecting rod. In an engine design, crankshaft is the most impart part that converts the reciprocating motion of the piston into rotary motion. In that portion, connecting rod aids the crankshaft to take the force from piston. This shaft contains cranks, and you will get one or more cranks on your vehicle’s crankshaft according to the engine capacity. Bigger end of the piston rod is directly connected to the crank through crank pin. It is also made of very special alloy steels or aluminum as like other parts of your engine.

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Connecting Rod

What is Connecting Rod and How Does it Work?

 

If you go in depth with your vehicle’s engine, you will find a metallic linkage between piston and the crankshaft. You may wonder to think about such an arrangement with the piston. In a word, it is just a force transmitting element. Its main function is to transmit the force from the piston to the crankshaft, and this force is generated due to the combustion of air-fuel mixture. Most interesting matter is that it also aids to convert the reciprocating motion of piston into rotary motion in the combustion stroke.

According to the basic engine design, the upper end of the connecting rod is connected to the piston, and the lower end of the connecting rod is fitted to the crankshaft. Practically, you will find the bottom end bigger than the top end. According to manufacturer details, very special aluminum or steel alloys are used to manufacture the connecting rod. You need to take special care while designing connecting rod as it is directly subjected to bending, tensile, and compression stresses.

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Piston Rings

Engine Piston Rings

 

You will find piston rings at the top end of the circumferential grooves of the piston’s outer periphery. Basically, these are circular rings and made of very special alloy steels. Though, these rings are made of steel, they can retain elastic property at high temperature and pressure. If you give a close look to it, you will find two rings mounted for the piston of your vehicle’s engine. Basically, the upper ring is used to make air tight seal to prevent leakage of the flue gases (burnt gases) into the lower portion. However, the lower ring prevents oil leakage into the cylinder.

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Engine Piston

What is an Engine Piston?

 

   










Another fundamental element of your IC engine is the piston, which acts as a force transmitting element. It aids to transmit the force exerted by the combustion of air-fuel mixture into the crankshaft. You may think that why this part is for and how it works. The matter is so simple. It gains the reciprocating motion in each stage of combustion, which gets transmitted into the crankshaft via connecting rod. This part is also called the heart of an IC engine. Practically, pistons of your engine are made of aluminum alloys, which give it a stronger yet light weight construction. Aluminum alloys are highly beneficial for such construction as they pose high heat conducting property.

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Engine Cylinder Head

What is an Engine Cylinder Head?

You will find the cylinder head of your engine at the top end of the engine cylinder, which aids to cover the cylinder bore. Basically, it includes inlet and exhaust valves to add new charge (air-fuel mixture) and to exhaust burnt flue gases. In case of a diesel engine the cylinder head of an engine contains nozzle to spray the fuel into a highly atomized form. But, if your vehicle contains a petrol engine, then you will find that the cylinder head includes a spark plug as an additional part of the engine, which aids to ignite the charge.

Engine Cylinder Head

Steel or aluminum alloys are used to cast the cylinder head, and it is casted as a single piece for a cylinder. Asbestos gasket is used in between the bolted joint of cylinder head and cylinder to make cylinder head air tight in its place.

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Engine Cylinder Block

What is an Engine Cylinder Block?

Does your vehicle’s engine contain more than one cylinder? In case of a multi cylinder engine, you will find that all cylinders are cast in a block, which is known as the cylinder block of that engine. The main purpose of this type of block is to support all cylinders in a single block. You may find any one of the following types of cylinder block in your engine.

- Wet liner block
- Dry liner block

In case of a dry liner block, you will find a discrete liner inserted into the engine block that forms the supporting part of the cylinder wall. You will also find additional sleeves, which do not come in contact with the engine coolant. However, if your engine’s cylinder block is in the wet liner category, then you will find cylinder walls, which can be removed entirely. These cylinder walls fit into the engine by means of very special gaskets that can easily withstand high temperature.




You should remember that, in this type of block, liners come in direct contact with the engine coolant. In both wet and dry liner cylinder blocks, you can totally replace or remove liners, while overhauling or rebuilding without replacing the engine block. Practically, it is so tough to remove or install these liners. So, you are advised to never undergo such a procedure within the lifespan of that engine.

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Engine Cylinder Explained

What is an Engine Cylinder?

Engine cylinder is the most important part of your IC engine, where combustion of air-fuel mixture takes place. As the combustion of an engine takes place in the cylinder portion, it is often called as the combustion chamber of an engine. You could hardly believe that this part of your engine has to withstand high temperature and pressure. Practically, the pressure and temperature estimates more than 50 bar and 2000o C accordingly after combustion. So, the material selection has become a prior concern to design and manufacture the engine cylinder. In case a heavy duty engine, the cylinder is made of alloy steel or aluminum alloys.

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IC Engine Construction and Basic Components

Youmay wonder to know that your vehicle’s IC engine involves the most complex mechanical arrangement. It includes more than one hundred important metallic and non-metallic components for its proper functioning. To make things easier to you, most common and important components of an IC engine are discussed below.

 

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