Internal combustion engines are the type of engines commonly used in automobiles. Basic Engine Operation by burning fuel within the engine itself, resulting in the direct production of mechanical energy. In contrast, external combustion engines, such as steam engines, burn fuel outside of the engine and use the generated steam to produce power. In automotive engines, fuel is burned internally to generate the power necessary to propel the vehicle.
This assignment will involve an examination of the fundamental components of a gasoline engine and their respective roles in the functioning of the engine as a whole. After gaining familiarity with the engine’s basic structure, the discussion will progress to an exploration of the engine’s operational mechanisms, providing a concise overview of the topic.
Basic Engine Construction
The diagram depicted below illustrates a basic representation of a section of an engine. It features a circular cylinder housing a piston inside. The cylinder, a hollow metal tube, is intricately drilled into the engine block. The piston, a cylindrical metal component, has the ability to vertically move within the cylinder and is a crucial moving part of the engine. In a functioning engine, a metal cover known as the cylinder head seals the top of the cylinder and is securely fastened in place with bolts.
Presented here is a simplified diagram depicting a portion of an engine, highlighting the specific placements of the cylinder, piston, connecting rod, crankshaft, combustion chamber, cylinder head, and spark plug.
It should be noted that when the piston reaches the highest point within the cylinder, there is a remaining gap between the piston and the cylinder head known as the combustion chamber. Within this space, a combination of air and gasoline is ignited to generate power. This combustion results in a controlled explosion, which creates enough force to move the piston downwards and convert its reciprocating motion into rotary motion of the crankshaft, thus giving rise to the term reciprocating engine.
The diagram also illustrates the presence of the spark plug situated above the combustion chamber. The spark plug is securely fastened into a threaded opening in the cylinder head, with its tip extending into the combustion chamber. Its primary function is to generate sparks that ignite the mixture of air and fuel within the cylinder, initiating the combustion process. The ignition of the spark plug is regulated by the engine’s ignition system, which will be further elaborated on in the following discussion.
The lower part of the piston is linked to a rod and crankshaft system. As the piston moves downward within the cylinder, this movement is transmitted to the rod and crankshaft. Subsequently, the rod and crankshaft transform the reciprocating motion of the piston into rotational motion.
The process of converting vertical motion to rotational motion can be likened to the mechanism found in a typical bicycle. Just as pedaling converts the up-and-down movement of your feet into circular motion in the rear wheel, an engine operates on a similar principle. The vertical motion of the piston is transformed into rotational motion in the crankshaft, which can then be utilized to drive machinery.
This diagram illustrates the basic components of a cylinder and piston system. In the process of engine operation, the piston undergoes a continuous cycle of upward and downward movement within the cylinder. It is important to observe the specific positions of the cylinder at both bottom dead center (BDC) and top dead center (TDC) during this process.
In the operation of an engine, the piston undergoes a reciprocating motion within the cylinder, transitioning between top dead center (TDC) and bottom dead center (BDC) to denote its highest and lowest positions, respectively.
In Part A, a piston ring is depicted, while Part B displays the ring grooves on a piston. The rings are designed to extend over the exterior of the piston and securely fit into the grooves.
It is important to observe that the exterior surface of a piston is etched with multiple horizontal grooves, each of which accommodates a metal ring known as a piston ring. These piston rings, which are split at one juncture, are constructed to be flexible in order to expand over the piston’s surface and retract into the grooves securely.
After being installed, the piston rings protrude like ridges on the surface of the piston. When the piston is positioned within the cylinder, the piston rings exert outward pressure against the cylinder walls. This process is essential for creating a secure seal between the piston and the cylinder, crucial for optimal engine performance.
Bore and Stroke
The bore refers to the diameter of the cylinder, while the stroke is the distance traveled by the piston from the top to the bottom of the cylinder. At top dead center (TDC), the piston is at its highest point and the crankshaft is closest to the cylinder. At bottom dead center (BDC), the piston is at its lowest point and the crankshaft is farthest from the cylinder. The space between the piston and the cylinder head at TDC is known as the clearance space, also referred to as the compression or combustion space. The volume of this space is called the clearance volume. An engine is considered oversquared when the bore is larger than the stroke, leading to power being primarily dependent on rpm and higher rpm values. Conversely, an engine is undersquared when the stroke is larger than the bore, with power primarily reliant on torque.
Four Stages (Strokes) of Engine Operation
Having acquired knowledge about fundamental elements of a standard engine, it is now essential to analyze the synergistic functioning of these components in enabling the operation of an engine. In order to function, all gasoline engines must adhere to the four essential processes known as the four-stroke or Otto Cycle.
Here are the four things an engine must do:
- Take in air and fuel (Intake)
- Compress (squeeze) the air and fuel (Compression)
- Ignite and burn the air-and-fuel mixture (Power) Get rid of the burned fuel gases (Exhaust)
The operation of an engine is comprised of four essential stages known as intake, compression, power, and exhaust. These stages are consistently repeated as the engine functions.
Stage 1: During the intake stage, air that has been mixed with fuel is drawn into the cylinder.
Stage 2: During the compression stage, the piston rises and compresses the air-and-fuel mixture that’s trapped in the combustion chamber.
Stage 3: During the power stage, the air-and-fuel mixture is ignited by a spark, and the contained explosion of the fuel presses the piston back down in the cylinder. !e downward motion of the piston is transferred to the rod and crankshaft.
Stage 4: During the exhaust stage, the burned exhaust gases are released from the cylinder. !e four stages then begin all over again.
An engine cycle encompasses a full progression through the intake, compression, power, and exhaust phases of operation. It is important to recognize that these stages occur rapidly and continuously during engine operation. These fundamental stages are universal to all automotive engines and are essential for the proper functioning of the engine.
Once you grasp these four stages, all other aspects of engine functioning will easily align.
Two-Stroke and Four-Stroke Engines
There exist two fundamental categories of gasoline engines: the two-stroke engine and the four-stroke engine. The classification of each engine type is based on the number of strokes required for its piston to complete a full engine cycle. It is important to note that a piston stroke refers to the entire distance traveled by the piston from the top to the bottom of the cylinder, and an engine cycle encompasses all four stages of operation.
In order for an engine to function properly, it must go through all four stages or strokes of operation. However, the methods by which two-stroke engines and four-stroke engines achieve this differ. Two-stroke engines complete one full engine cycle of intake, compression, power, and exhaust in just two strokes of the piston, while four-stroke engines require four strokes to complete the same cycle.
The vast majority of modern vehicles are equipped with four-stroke cycle engines. These engines operate through a series of four piston strokes, each necessary to complete the four stages of operation.
- Piston stroke 1: Intake stage
- Piston stroke 2: Compression stage
- Piston stroke 3: Power stage
- Piston stroke 4: Exhaust stage
Following the conclusion of the fourth piston stroke, the engine has successfully traversed all four stages of operation, marking the commencement of a new cycle.
On the other hand, a two-stroke engine requires only two piston strokes to carry out all four stages of operation.
- Piston stroke 1: Intake/compression stage
- Piston stroke 2: Power/exhaust
The intake and compression stages in a two-stroke engine are consolidated into a single piston stroke, while the power and exhaust stages are consolidated into a second piston stroke. It is important to recognize that despite this consolidation, the two-stroke engine still carries out all four stages of operation, albeit in only two piston strokes instead of four.
Although two-stroke engines are not utilized in modern automobiles, they are frequently found in motorcycles and small motors due to their inefficiency in fuel consumption and higher levels of pollution compared to four-stroke engines. It is important to acknowledge the existence of two-stroke engines, but this course will focus solely on the study of four-stroke engines.
Four-Stroke Engine Operation
In the initial phase of the engine cycle, the downward movement of the piston results in the aspiration of the air-fuel blend into the cylinder. The intake valve remains open while the exhaust valve remains shut during this process.
The components of the four-stroke cycle are clearly identified in the following four images. Analyze the diagram, taking note of the piston, cylinder, and crankshaft positions. The crankshaft is linked to the piston through the connecting rod, and the spark plug is situated at the top of the engine above the combustion chamber.
Additionally, it is important to observe the presence of valves located on the cylinder head. In a four-stroke engine, there will be a minimum of two mechanical valves, specifically an intake valve and an exhaust valve. These valves are responsible for regulating the flow of air and fuel into the cylinder and allowing exhaust gases to exit during engine operation. The intake valve opens to permit the entry of the air-and-fuel mixture, while the exhaust valve opens to release the exhaust gases generated by the combustion process.
In order for optimal combustion to occur within an engine, fuel must be effectively combined with air. This process takes place within the intake runner of the engine, where the fuel is either indirectly or directly injected, depending on the specific engine design. The piston draws air into the system through the throttle plate, while the electric in-tank fuel pump supplies fuel to the fuel injector. The fuel is then vaporized and mixed with the incoming air before being delivered to the cylinder through the intake valve.
The intake stroke commences with the piston at top dead center (TDC), where a camshaft lobe initiates the opening of the intake valve through a series of components such as followers, pushrods, and rocker arms. This action allows outside air to be drawn into the cylinder as the piston descends in response to the crankshaft’s rotation, creating a low-pressure environment above it. The intake process mirrors the concept of using a straw to draw liquid from a glass, with the air-fuel mixture entering the combustion chamber at a precise ratio of 14.7 parts air to 1 part fuel, known as the stoichiometric ratio. The throttle regulates the amount of air entering the cylinder, with the necessary energy for the downward piston movement derived from the flywheel or power strokes in an engine with multiple cylinders. As the piston approaches bottom dead center (BDC), its speed decreases significantly.
Upon reaching Bottom Dead Center (BDC), the intake valve is shut, effectively sealing the cylinder and initiating the compression stroke. The crankshaft then drives the piston upwards, with both valves closed, creating a sealed environment within the cylinder where air can only escape through the rings.
In the compression stroke, the intake and exhaust valves are shut, allowing the rising piston to compress the air-fuel mixture within the sealed combustion chamber.
The decrease in volume within the cylinder as the piston rises results in the compression of the air-fuel gas mixture. Boyle’s law illustrates that the pressure of a gas is directly related to the volume of its container. During the compression process, the volume decreases, leading to a rise in pressure and temperature as external work is applied to the gas. The compression ratio, defined as the ratio of the volume at Bottom Dead Center (BDC) to the volume at Top Dead Center (TDC) (clearance volume), plays a crucial role in determining the engine’s thermal efficiency. A higher compression ratio translates to improved thermal efficiency, indicating that a larger proportion of the heat supplied to the engine is converted into work. Additionally, an increase in the compression ratio results in a corresponding increase in the expansion ratio, further enhancing thermal efficiency.
The internal energy of the gas increases with the addition of heat. As the compression stroke nears its end, a spark plug will ignite the mixture, and the compression stage will persist as the piston ascends to the cylinder’s apex.
In the power stroke, as the piston reaches its highest point, the spark plug initiates combustion of the air and fuel mixture. The resultant force generated by the combustion propels the piston downward within the cylinder.
The power stroke initiates shortly after the spark plug ignites the air-fuel mixture in the combustion chamber, causing gasoline to burn with the support of oxygen. The burning mixture expands, propelling the piston downward during the power stroke. The pressure generated by the combustion forces the piston down within the cylinder. This downward motion of the piston, connected to the crankshaft through the connecting rod, results in the rotation of the crankshaft, similar to the action of pushing down on the pedals of a bicycle. As the volume increases, the pressure decreases, and the gas cools as it performs work. The gases in the cylinder continue to expand and cool as they release their energy while the piston moves downward. The power stroke is the phase in which energy is extracted from the fuel, with the highest cylinder pressure occurring during this stroke. The combustion of the air-fuel mixture within the cylinder is referred to as the combustion process.
In the exhaust stroke, the upward movement of the piston causes the exhaust valve to open, allowing the expulsion of the remaining combusted gases from the cylinder.
The power stroke in an engine continues until the piston reaches bottom dead center (BDC). As the piston approaches BDC, the exhaust valve, controlled by a camshaft lobe, starts to open, initiating the exhaust stroke. The rising piston pushes the spent gases out of the cylinder through the exhaust valve. Near the top of its movement, the intake valve opens again to begin a new cycle. The exhaust valve closes shortly after the piston starts moving downward, a stroke that expels exhaust gases without producing useful work.
The exhaust stroke persists until the piston reaches top dead center (TDC), marking the completion of the four stages of engine operation. Subsequently, the cycle restarts with the opening of the intake valve and the downward movement of the piston to initiate a new intake stage.
The four-stage cycle of operation in an engine persists during its operation, with these cycles occurring rapidly at a high rate of speed. A typical automobile crankshaft can complete hundreds to thousands of revolutions per minute, showcasing the impressive speed at which an engine’s components move while functioning.
