What Is A Petrol Engine? Its Application, Parts and How it Works

A petrol engine is a spark-ignited internal combustion engine rather than the compression-ignited systems used in diesel vehicles. In the spark-ignited system, the fuel is injected into the combustion chamber and combined with air. A spark from the spark plug ignites the air-fuel mixture.

In this reading, we’ll explore what a petrol engine is, its diagrams, application, parts, types, and how it works.

Let’s get started!

What Is A Petrol Engine?

A petrol engine is a type of internal combustion engine with spark ignition (spark plug), which uses petrol and similar volatile fuels to run the engine.

The earlier petrol engine was designed to pre-mix fuel and air before compressing, which occurs in the carburetor. However, today’s fuel injection is electronic-controlled.

A two-stroke petrol engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution using petrol/air mixture.

This is in contrast to a four-stroke petrol engine that requires four strokes of the piston to complete a power cycle during two crankshaft revolutions.

The earlier petrol engine was designed to pre-mix fuel and air before compressing, which occurs in the carburetor. However, today’s fuel injection is electronic-controlled.

Well, the process differs in a small engine where the cost or complications of electronics do not suit the engine’s efficiency.

Diagram of a Petrol Engine

Diagram of a Petrol Engine

Applications

Petrol engines are widely used in different fields where it has become inevitable for humans to use them due to their distinctive service.

The applications of petrol engines include automobiles, motorcycles, aircraft, and motorboats. Also, Small engines, such as lawnmowers, chainsaws, and portable engine-generators

Today, four-stroke gasoline engines are generally used in the automobile sector and vehicles. It is used in petrol cars, scooters, small motor-powered boats, motorbikes, small propeller aircraft, water spray systems, etc.

These engines have been highly applicable to the higher value of the low punch end capability of the engines.

The application of a two-stroke or two-cycle petrol engine is an internal combustion engine that is often found in smaller, lower-power engines such as scooters, dirt bikes, jet skis, smaller outboard motors, and lawn equipment such as lawnmowers and chain saws.

Parts of a Petrol Engine

The various parts of a petrol engine include:

1. Cylinder

These engine parts are located in the engine block, also known as the cylinder block. It contains a liner or sleeves around it. This liner wears out when subjected to work and can be easily replaced.

The cylinders have part or space for the piston to move upward and downward, making the combustion take place.

Cylinders are characterized by their bore and stroke. The bore is the inner diameter, and the stroke is the effective length along the piston reciprocate, i.e., the movement of the piston from the TDC to the BDC. They are the uppermost and lowestmost points of the stroke.

2. Piston

The piston is a cylindrical part that moves upward and downward in the cylinder, allowing the complete combustion cycle to take place (intake, compression, combustion, and exhaust) check out how this process works below.

The diameter of the piston is a bit less than the bore of the cylinder to avoid quick wear of the piston surface. There are three rings, known as piston rings, fitted in the circular recesses on the piston surfaces.

These rings are made of aluminum and have direct contact with the cylinder liner, thus preventing piston wear.

The two rings are compression rings; it are chamfered on the outer part, helping to prevent waste gases inside the combustion chamber from entering into the crankcase.

The third ring, known as the oil ring, prevents oil from entering the combustion chamber and ensures proper distribution of oil along the cylinder walls.

3. Crankshaft

These engine parts help to convert the sliding motion from the piston into rotary motion via the connecting rod. It is located below the cylinder block inside a casing called the crankcase. The crankshaft has projections bent and offset from the shaft axis.

In a multi-cylinder engine, each cylinder is provided with its own crankpin, which is used to attach the piston to the connecting rod. A part of the crankshaft named crankpin journal bearing is known as a big end, having a sliding bearing.

Another part of it is called counterbalance weights. It is available to lessen the tensional vibrations that the crankshaft experiences as a result of the reciprocating unbalance of the moving piston during combustion.

The crank balance is either bolted to the crank body or forms an integral part of it.

Crankshafts are either produced in parts or as a single piece. The single-piece design is preferable because it leaves no space for vibration and offers better fiber flow and good stress-bearing capabilities.

4. Connecting Rod

These engine parts are provided to connect the piston to the crankshaft. Just as mentioned earlier, it converts the linear motion of the piston into the rotary motion of the crank.

One of its end parts is attached to the piston through a piston pin, also known as a gudgeon pin, and a wrist pin. Another end is attached to the crankpin journal using bolts to hold down the upper and lower bearing caps, called the big end.

The bearing is in the form of two half-shells placed in the crank journal by the big end connecting rod. Both ends are not rigidly fixed in order to rotate at an angle. Hence, both ends are in continuous motion and under tremendous stress from the pressure of the piston.

5. Cylinder Head

These engine parts serve as a cover for the cylinder block, valve, rocker arms, and ignition element. It is bolted to the cylinder block with the head gasket in between.

The cylinder head is made from cast iron and sometimes aluminum alloy when a lightweight part is needed, as aluminum conducts heat more quickly than cast iron.

In an overhead camshaft engine, the camshaft is placed in the head in the absence of a pushrod arrangement for the valve mechanism.

Some other parts, like the inlet, exhaust ports, and combustion chamber, also have space beneath the cylinder, making it one whole engine component.

6. Camshaft

This internal combustion petrol engine component is a shaft containing a cam fitted on it. Its function is to control the valves directly by sitting over them or through the rocker arm and pushrod mechanism.

The valve timing is determined by the size of the camshaft. That is, the opening and closing of valves are governed by the camshaft, which is mounted on the crankshaft either directly through a reduction gear or indirectly through a pulley and a timing belt.

A pushrod and tappet mechanism, along with the rocker arms, were necessary for the camshaft’s gear-driven coupling to the crank. The camshaft is commonly made of chilled iron castings and billet steel used in making high-quality ones.

The purpose of the chilled iron is to offer greater wear resistance and surface hardness.

7. Valves

Valves, also known as poppet valves on IC engines,. It is made of a long, thin circular rod called a valve stem and a flat circular disk called a valve head, which is tapered along the thin rod.

The function of the valve is to enable a fresh intake of fuel and air and the release of waste gases (exhaust.) The sliding motion of the camshaft and its associated linkages is what causes the valve to open and close.

Engine valves are made from steel alloys filled with sodium to increase their heat transfer capacity. Finally, the valves are of two sections: the intake/inlet valve, which allows fresh charge to enter the chamber when open, and the exhaust/outlet valve, which allows the escape of the exhaust gases.

8. Rocker Arm

This internal combustion engine part plays an important role as it transmits the rotary motion of the cam or crankshaft through a tappet/latch and converts it into a linear motion of the valve stem, helping to depressing the valve head

The rocker head is made from steel stampings for light and medium-duty engines, whereas the heavy-duty diesel engine rocker head is made of cast iron and forged carbon steel as it offers greater strength and stiffness.

The rocker’s arms oscillate about a fixed pivot rod in the cylinder head.

9. Crankcase

These internal combustion engine components are located below the cylinder block containing the bearings that rotate the crank. This main bearing is a sliding bearing with an adequate oil supply in it.

Four-cylinder inline petrol engines contain three bearings in the crankcase, one at each end and one in the middle, while diesel engines have five main bearings, one at each end and one between each cylinder.

The crankcase is made from cast iron and aluminum, the same materials used in making the cylinder block. A crankcase serves many purposes for the engine, as it helps to protect its inner mechanism from dust, dirt, and some other materials.

It also serves as housing that encloses the crankshaft and the connecting rod, keeping the oil and air.

10. Oil Pump and Sump

The function of the oil pump is to pump the oil to various parts of the engine for proper lubrication, cleaning, and cooling. The crankshaft gear in the engine is what drives the oil pump.

The oil is pressurized to various parts of the engine components, which helps in lubricating and cooling the system.

The oil sump serves as storage, containing a chamber that keeps the oil. In order to prevent dirt and debris from entering the engine, the oil pump lifts the oil from the sump through a wire mesh strainer.

The oil filter and oil cooler allow the oil to pass through before distributing it to the engine parts. The oil returns to the oil sump after doing its job. Learn more on the components of internal combustion engine here!

Types of Petrol Engine

Petrol engines are classified by configuration, cycle, and ignition method. Here are the main types of petrol engines based on the stroke cycle:

Two-Stroke Petrol Engine

In two-stroke types of petrol engine, the compression and power stroke of the four-stroke cycle are carried out within the inlet and exhaust strokes. This makes it require just one revolution of the crankshaft to complete the cycle.

The reduced number of parts of a 2-stroke petrol engine, the simple design and the lack of oil sump mean these types of petrol engines are more reliable in cold temperatures. This feature makes them also suited for use in machines like snowmobiles and snowblowers.

Four Stroke Petrol Engine

The four-stroke petrol engine that requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. They are used in petrol cars, scooters, small motor-powered boats, motorbikes, small propeller aircraft, water spray systems, etc. 

How Petrol Engine Works

Due to lower compression ratios and the fact that gasoline burns more quickly than diesel, gasoline engines rotate at higher rotation speeds than diesel engines.

Because pistons in gasoline engines tend to have much shorter strokes than pistons in diesel engines, it typically takes less time for a piston in a petrol engine to complete its stroke than a piston in a diesel engine.

However, the lower compression ratios of gasoline engines give them lower efficiency than diesel engines. Typically, most gasoline engines have approximately 20% (avg.) thermal efficiency, which is nearly half of the diesel engines.

However, some newer engines are reported to be much more efficient (thermal efficiency up to 38%) than previous spark-ignition engines. The process differs from a diesel engine in the method of mixing the fuel and air and in using spark plugs to initiate the combustion process.

Petrol Engine Video

Advantages

The following are the advantages of petrol engines:

  • Petrol engines are generally more powerful, by way of horsepower, than diesel.
  • Petrol engines naturally benefit from being easier to tune.
  • It has a better ratio of price to power and burns fuel more eco-friendly than diesel does.
  • Petrol is usually cheaper than diesel fuel.

Disadvantages

The following are the drawbacks of a petrol engine:

  • Generally, the petrol engine’s lifespan is almost always shorter than that of the diesel.
  • Diesel engines usually have a higher residual (resale) value.
  • Petrol engines get fewer miles per gallon (sometimes significantly) than their diesel counterparts.
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