What is an Engine?
Ever wondered how engines work their magic? Well, an engine is like that ingenious wizard that transforms one kind of energy into the cool stuff that makes machines move – mechanical energy.
So, let’s dive into the world of engines, shall we? First up, we’ve got heat engines. These babies, also known as internal combustion engines, work their mojo by burning fuel right inside the engine cylinder. Now, picture this – on the flip side, there are external combustion engines. These guys, like the classic steam engines, burn their fuel outside the cylinder.
But here’s the fascinating part – when the fuel gets all fired up, it’s like a party for energy. This energy dances over to the steam, which then goes full throttle on a piston inside the cylinder. Speaking of internal combustion engines, they’re like energy treasure chests. They stash away chemical energy and use it to power their incredible moves.
And guess what? The real magic happens when that sizzling heat energy transforms into mechanical energy. How, you ask? Well, it’s a bit like a superhero story – gases expand, pushing against a piston that’s best friends with a crankshaft, and voila! The crankshaft starts spinning, making the world go ’round.
So there you have it, the scoop on engines – turning fuel into fantastic motion. Pretty cool, right?”
Types of Engine
Now we get into serious business. Basically, engines are classified into two types, which are: internal combustion engines and external combustion engines.
#1 External Combustion Engine
An external combustion engine operates uniquely by burning fuel outside the engine cylinder. This setup, seen notably in steam engines, heats water to produce steam that propels a piston, converting heat energy into mechanical motion.
#2 Internal Combustion Engine
The internal combustion engine is a modern engineering marvel, igniting fuel directly within the engine cylinder. This controlled ignition drives pistons, which transmit force to a crankshaft, generating rotational energy that powers vehicles and machinery. The energy from the combustion is then partially converted into work by the engine. Examples of internal combustion engines include two- and four-stroke petrol and diesel engines.
Internal combustion (I.C.) engines come in many types, and different criteria are used to classify them.
#1 Classification by Types of Fuel used
According to the type of fuel used, the engines are classified into three categories
- Petrol engine (or Gasoline engine)
- Diesel engine
- Gas engine
A petrol engine, also called a gasoline engine, is a type of internal combustion engine. It works by igniting a mixture of air and petrol inside cylinders, creating controlled explosions. The correct air-petrol mixture is obtained from the carburettor.
These explosions move pistons, which turn a crankshaft, converting the energy into rotational motion. This engine powers vehicles, machinery, and generators, known for their efficiency and performance.
A diesel engine is a type of internal combustion engine that runs on diesel fuel. Air is sucked into the cylinder during the suction stroke and compressed to high pressure, and the compression ratio is as high as 22:1. Its temperature also rises by about 1,000°F. Fuel is then injected directly into the highly compressed, hot air, causing spontaneous ignition. This controlled explosion drives pistons, converting energy into motion. Diesel engines are renowned for their efficiency and durability, commonly used in vehicles, trucks, and industrial equipment.
The diesel oil is injected by an injector at the end of the compression stroke which catches fire and burns due to the high temperature of the compressed air. No separate ignition system is required. The burnt gases expand pushing the piston down during the power stroke and finally, the gases are pushed out during the exhaust stroke.
The gasifier consists of a compressor with a rotor with a series of blades around its outer edge. As the rotor rotates, the air between the blades is carried around and thrown out by centrifugal force into the burner. Thus air pressure rises in the burner. The fuel is injected into the burner, where it burns and further raises the pressure.
A gas turbine essentially consists of two sections-a gasifier section and a power section. The fuel used in a gas turbine can be gasoline, kerosene, or oil. The gasifier section burns fuel in a burner and delivers the resulting gas to the power section, where it spins the power turbine. The power turbine then turns the vehicle wheels through a series of gears.
#2 Classification by Cycle of Operations
According to the cycle of operations, automobile engines may be of the following three types:
- Otto cycle engine.
- Diesel cycle engine.
- Dual cycle engine.
Otto Cycle or Constant Volume Cycle
The Otto Cycle, named after its developer Nikolaus August Otto, is a theoretical thermodynamic cycle that elucidates the operation of spark-ignition internal combustion engines, commonly found in gasoline/petrol engines. It serves as a foundational framework for analyzing the thermodynamic processes within such engines, shedding light on the efficiency and performance parameters.
The cycle comprises four distinct processes, executed sequentially within the engine cylinder:
- Intake: In this phase, a mixture of air and fuel is drawn into the cylinder as the piston moves downward. The intake valve is open, allowing the mixture to fill the cylinder.
- Compression: The intake valve closes, and the piston starts moving upward, compressing the air-fuel mixture within the cylinder. This compression significantly raises the pressure and temperature of the mixture.
- Power (Combustion): At the peak of compression, a spark plug ignites the compressed mixture. The rapid combustion of the air-fuel mixture generates a high-pressure explosion. The resulting increase in pressure forces the piston to move downward, converting the energy of combustion into mechanical work. This is the stage where the engine generates power.
- Exhaust: After the power stroke, the exhaust valve opens, and the piston moves upward again. This action expels the burned gases from the cylinder to make room for the fresh intake of air and fuel in the next cycle.
The Otto Cycle’s thermodynamic analysis involves considering the First Law of Thermodynamics (conservation of energy) and the Second Law of Thermodynamics (entropy increase), providing insights into the engine’s efficiency, work output, and heat exchange processes.
While the Otto Cycle serves as a simplified model, real-world engines exhibit deviations due to factors like heat loss, incomplete combustion, and valve timing. Nevertheless, understanding the Otto Cycle provides a valuable foundation for engineers and researchers seeking to optimize internal combustion engines’ performance, efficiency, and environmental impact.
The engines operating on this cycle are known as Otto-cycle engines.
An I.C. engine does not undergo a cyclic change, but it is assumed here that the working medium is pure air which does not undergo any chemical change. The air is heated and cooled to undergo a cycle. It is also assumed that the ideal indicator diagram is strictly followed.
|The ideal Otto cycle consists of the following operations:
|1-2 Adiabatic compression.
2-3 Heat addition at constant volume.
3-4 Adiabatic expansion.
4 1 Heat rejection at constant volume.
Diesel Cycle or Constant Pressure Cycle
Diesel cycle was introduced by Dr. Rudolph Diesel in 1897. The engines operating on this cycle are known as Diesel engines. The figure shows the p-v diagram for a Diesel cycle.
|It consists of the following operations:
|1-2 Adiabatic compression.
2-3 Heat addition at constant pressure
3-4 Adiabatic expansion.
4-1 Heat rejection at constant volume
The Diesel cycle differs from the Otto cycle in one respect. In the Diesel cycle, the heat is added at constant pressure instead of a constant volume.
The air is compressed in the cylinder during the compression stroke from point 1 to 2. Now the heat is added at constant pressure from point 2 to 3, and then the air is expanded adiabatically from point 3 to 4. Finally, the heat is rejected at constant volume from point 4 to 1. The air returns to its original condition, and the cycle is complete.
Dual Cycle (or Dual Combustion Cycle)
In these types of engines, more time is allowed for the combustion of fuel in the Diesel engine without adversely affecting the efficiency.
The fuel is injected in the cylinder before the end of the compression stroke so that combustion proceeds partly at constant volume and partly at constant pressure. Such a cycle is known as Dual cycle. In fact, all Diesel engines actually operate on this cycle. The figure shows the dual cycle on the p-v diagram.
|It consists of the following operations.
|1-2. Adiabatic compression
2-3. Heat addition at constant volume
3-4. Heat addition at constant pressure
4-5. Adiabatic expansion
5-1. Heat rejection at constant volume.
Because the fuel is injected into the cylinder before the end of the compression stroke in the dual cycle, it takes care of the ignition lag characteristic of the fuel.
#3 Classification by No. of Strokes per Cycle
According to the number of strokes per cycle, automobile engines are classified as
- Four-stroke engine.
- Two-stroke engine.
The four-stroke completes a cycle of operations during the four-piston strokes, namely suction, compression, power, and exhaust. These four strokes require two revolutions of the crankshaft. Thus, during every two crankshaft revolutions, there is only one power stroke of the piston.
The two-stroke engine completes a cycle of operations during the two-piston strokes. These two strokes require one revolution of the crankshaft. Thus, during every revolution of the crankshaft, there is one power stroke of the piston. Therefore, a two-stroke engine produces twice as much horsepower as a four-stroke engine of the same size, running at the same speed.
In the two-stroke engine, the intake and compression strokes, and the power and exhaust strokes are, in a sense, combined. Two-stroke engines are used in motorcycles, Scooters. Four-stroke engines are used in cars, trucks, and buses.
#4 Classification by Type of Ignition
According to the type of ignition used, modern automobile engines are classified mainly into two groups:
- Spark ignition engines.
- Compression ignition engines.
Spark Ignition Engine
In a spark-ignition engine, a spark plug is fitted at the cylinder head, which gives an electric spark at the end of the compression stroke to ignite the fuel. Petrol engines are spark-ignition engines.
In these types of engines, the fuel is ignited by the heat of compressed air inside the cylinder. There is no spark plug in it to give a spark. The air is compressed in the cylinder during the compression stroke relatively at higher pressure.
The compression ratio is also higher than that in the spark-ignition engine. Fuel is injected at the end of a compression stroke, which burns due to the heat of the compressed air. Diesel engines are compression ignition engines. Hot spot ignition engines are not practically used.
#5 Classification by Number and Arrangement of Cylinders
The automobile engines may have one, two, three, four, six, eight, twelve, and sixteen cylinders. One-cylinder engine is used in scooters and motorcycles. The two-cylinder engine is used in tractors. Four and six-cylinder engines are used in cars, jeeps, buses, and trucks.
Comet trucks and buses have six-cylinder engines. American passenger cars have eight-cylinder engines. Twelve and sixteen-cylinder engines are also used in some passenger cars, buses, trucks, and industrial installations. The three-cylinder engine is also used in a foreign front-drive automobiles.
The cylinders can be arranged in several ways-vertical, horizontal, in a row (inline), in two rows or banks set at an angle (V-type), in two rows opposing each other (flat, or pancake) or like spokes on a wheel (radial).
Single Cylinder Engine
These types of engines are generally used for light vehicles like scooters and motorcycles. The maximum size of the single-cylinder engine is restricted to about 250-300 c.c. Higher engine size will require heavy engines due to the higher unbalance forces in a single-cylinder engine.
In one cylinder, there is one power impulse in two revolutions of the crankshaft. Thus, out of the four strokes of the pistons, the power is delivered in one stroke, and in the remaining strokes of the pistons, the power is consumed in overcoming the friction resistance of the moving parts. The torque distribution during a cycle is uneven, resulting in rough working and vibrations.
Since there are only one piston and one connecting rod, which reciprocates with no working parts to counterbalance their weight, the one-cylinder engine does not have mechanical balance. However, the engine is balanced to some extent by using the counterweight attached to the crankshaft, and also by using a flywheel so heavy that its momentum produces a comparatively steady movement.
The fluctuations in the engine speed cause vibration, even in the best designs of one-cylinder engines. Hence cylinder engines are undesirable for use in motor vehicles.
This types of engines are used mostly in tractors. They are also used in a small German automobile and DAF of Holland car. The arrangement of the cylinders in the two-cylinder engines may be of three types
- In-line vertical type
- Opposed type
The three-cylinder engine is used on a front-drive car, where the differential is located between the engine and the transmission. The three cylinders are placed in line. This is a two-stroke cycle engine. The crankcase in this engine serves as an intake and precompression chamber.
Each cylinder has its own sealed section of the crankcase. Thus, the main bearings that support the crankshaft are of the sealed type, so the crankcase is divided into three separate compartments, one for each cylinder.
Four Cylinder Engine
Four-cylinder engines are mostly used for ordinary cars. The torque obtained is much more uniform than the two-cylinder engine because two working strokes per revolution are obtained.
The cylinders of a four-cylinder engine are arranged in the following type:
- In-line Vertical Type
- Opposed type
Six and Eight Cylinder Engine
Six and eight-cylinder engines give much smoother torque and higher horsepower. The cylinders of these engines are also arranged in three ways in line, V and opposed type, in the same way as in the four-cylinder engines. In line 6, cylinder engines and V-8 engines are almost universally in use. The angle between the cylinder rows in V-8 engines is usually kept at 90°.
V-8 engines with smaller V-angles have also been made, but the valve operating mechanism is complicated in them. V-6 engines have two three-cylinder rows that are set at an angle to each other. The crankshaft has only three cranks, with connecting rods from opposing cylinders in the two rows being attached to the same crankpin. Each crankpin has two connecting rods attached to it.
The V-8 engine has two four-cylinder rows that are set at an angle to each other. The crankshafts have four cranks with connecting rods from opposing cylinders in the two rows attached to a single crank pin. Thus, two rods are attached to each crankpin, and two pistons work to each crankpin. The crankshaft is usually supported on five bearings.
Twelve-and Sixteen-cylinder engines.
The arrangement of cylinders in twelve and sixteen-cylinder engines may be of the following types.
- V-type or pancake-type has two rows of cylinders.
- W-type has three rows of cylinders.
- X-type has four rows of cylinders.
The twelve-and sixteen cylinder engines have been used in cars, buses, trucks, and industrial installations. The only passenger car now being made with a twelve-cylinder engine is the Ferrari.
#6 Classification by Valve Arrangements
Automobile engines are classified into four categories according to the arrangement of the inlet and exhaust valve in various positions in the cylinder head of the block. These arrangements are termed as ‘L’ T, F, and T. It is easy to remember the word ‘LIFT to recall the four-valve arrangements. The I-head design is most commonly used in automobile engines.
In I-head or overhead valve engine, the valves are located in the cylinder head. In-line engines usually have the valves in a single row. V-8 engines may have the valves in a single row or in a double row in each bank. Regardless of the arrangement, a single camshaft actuates all the valves.
In an L-head arrangement, the inlet and exhaust valves are located side by side and operated by a single camshaft. The combustion chamber and cylinder from an inverted L. All the valves for an engine are arranged in one line, except for V-8 L-head engines, which are in two lines.
In L-head engines, the valve mechanisms are in the block, and hence the cylinder head may be easily removed when required for overhauling the engine. Although the L-head engine is rugged and dependable, they are not particularly adapted to higher compression.
The I-head valve engine is more adaptable to a high compression ratio. In the I-head valve engine, the clearance volume can be reduced to a greater amount than in the L-head engine. In some I-head engines, there are pockets in the piston heads into which the valve can move when they are open with the piston at T.D.C.
This engine combines L-head and I-head engines, in which one valve, usually the inlet valve, is in the head, and the exhaust valve is in the cylinder block. Both sets are driven from the same camshaft.
The t-head engine has the inlet valves on one side and the exhaust valves on the other side of the cylinder. Thus, two camshafts are required to operate them.
#7 Classification by Type of Cooling
According to the type of cooling method, the automobile engines are classified mainly into two categories:
- Air-cooled engines.
- Water-cooled engines.
Air-cooled engines are used in motorcycles and scooters. In air-cooled engines, the cylinder barrels are usually separate and are equipped with metal fins which give a large radiating surface to increase the rate of cooling.
Many air-cooled engines have metal shrouds that direct the airflow around the cylinders for improved cooling. Since these engines do not use water, the problem of cold weather maintenance is eliminated.
These types of engines are used in buses, trucks, cars, and other four-wheeled, heavy-duty motor vehicles. These engines use water, with an antifreeze compound added to serve as the cooling medium.
The water is calculated through water jackets around each of the combustion chambers, cylinders, valve seats, and valve stems. After passing through the engine jackets in the cylinder block and cylinder head, the water is passed through the radiator, where it is cooled by air drawn through the radiator.
Evaporative cooling engines are not practically used.
#8 Classification by Arrangement of Cylinders
Simply put, an inline engine has all of its cylinders in a straight line. It is a reciprocating engine that consists of banks of cylinders, with each bank having any number of cylinders, although more than six are rare.
In this engine, the crankshaft and cylinders are arranged in a straight line. An inline engine is less expensive in comparison. Due to their small size, these engines are light.
V-type engines have their cylinders arranged in two equal rows, or, to put it another way, in a V-like shape. This type is frequently used because it takes up a smaller space and can fit in most cars.
In this engine, the crankshaft and cylinders are positioned at an angle. V-engines have more parts than inline engines, which can make them more expensive. In addition, they are taller than a flat engine.
It is a type of reciprocating internal combustion engine in which the cylinders stick out from a central crankcase like spokes on a wheel. It is referred to as a “star” engine because it resembles a stylized star when viewed from the front.
In general, radial engines are more reliable. This is because it has a shorter crankshaft, a more straightforward construction, and produces less vibration. Before the gas turbine engine becomes the dominant option, it is often used for aircraft engines.
Opposed Piston Engine
An opposed-piston engine is a piston engine without a cylinder head that has pistons at both ends of each cylinder. In this engine, cylinders are angled 180°, identical to a V-type but with a 180° angle.
Large-scale applications like ships, military tanks, and factories have traditionally used petrol and diesel opposed-piston engines.
Instead of traveling in a V, like a V-6 or V-8 engine, the cylinders in a horizontal engine move horizontally with the ground. In this engine, the cylinders are located on either side of a central crankshaft. These engines are also known as flat engines. This is different from opposed-piston engines, in which two pistons share a central combustion chamber in each cylinder.
In W-type engines, the cylinders are arranged in three rows so that the arrangement is W-shaped. A W engine has three or four groups of cylinders connected to one or two crankshafts, unlike a V engine.
Due to their smaller size and increased power, W engines are used in heavy-duty vehicles, luxurious and exotic cars.
Reverse Cylinder Engine
The cylinders of a reverse-cylinder engine are positioned opposite one another. Connecting rods and pistons operate at comparable speeds. It functions more reliably and evenly, and because of the way it’s set up, the anti-cylinder engine’s size grows.