Overview of Two-Stroke Engines PDF

Overview of Two-Stroke Engines Power Output: Produces power with every revolution of A two-stroke engine is a type of internal combustion the crankshaft, leading to higher power-to-weight ratios. engine that completes a power cycle in just two strokes of...

Overview of Two-Stroke Engines Power Output: Produces power with every revolution of A two-stroke engine is a type of internal combustion the crankshaft, leading to higher power-to-weight ratios. engine that completes a power cycle in just two strokes of Lightweight: Generally lighter and more compact, making the piston, as opposed to the four strokes required by a them suitable for portable applications. four-stroke engine. This design allows for a more compact Disadvantages of Two-Stroke Engines and lightweight engine, often used in applications like Efficiency: Less fuel-efficient due to potential fuel loss motorcycles, chainsaws, and lawn equipment. during the exhaust phase. Key Components Emissions: Typically produce more emissions than four- Ports: stroke engines due to incomplete combustion. Inlet Port: Allows air and fuel mixture to enter the engine. Summary Exhaust Port: Releases burnt gases from the combustion Two-stroke engines are efficient for specific applications process. due to their lightweight and high power output. Transfer Port: Connects the lower chamber (below the Understanding the operation cycle—comprising the piston) to the upper chamber (above the piston). upward and downward strokes, along with the role of the Working Cycle ports—is crucial for grasping how these engines function The operation of a two-stroke engine can be broken down effectively. into two main strokes: DIFFERENCE OF 2 AND 4-STROKE ENGINE 1. First Stroke A two-stroke engine differs from a four-stroke engine The piston moves upwards in the cylinder. primarily in its power cycle completion and design. A two- All ports are initially covered, causing the pressure in the stroke engine completes a power cycle in just one lower chamber to drop below atmospheric pressure, which crankshaft revolution and two strokes of the piston, allows the inlet port to uncover. whereas a four-stroke engine requires four strokes to Air and fuel are sucked into the lower chamber through the complete the cycle. This design allows two-stroke engines inlet port as the piston approaches the top of the cylinder. to produce power with every revolution, making them 2. Second Stroke lighter and more compact, which is ideal for applications Just before the piston reaches the top, the air-fuel mixture like motorcycles and chainsaws. In contrast, four-stroke in the upper chamber is ignited. engines are generally more fuel-efficient and produce The combustion creates high pressure, forcing the piston fewer emissions due to their more complex operation downwards. involving intake, compression, power, and exhaust strokes. As the piston descends, it uncovers the exhaust port, The simplicity and higher power-to-weight ratio of two- allowing burnt gases to escape while simultaneously stroke engines often make them preferable for portable compressing the new air-fuel mixture in the lower machinery, while four-stroke engines are favored in chamber. vehicles requiring better fuel efficiency and lower 3. Bottom Dead Center (BDC) emissions At BDC, the exhaust port is fully uncovered, releasing burnt gases from the upper chamber. WHY DO VEHICLES CHOOSE ONE ENGINE OVER The transfer port allows the fresh air-fuel mixture from the ANOTHER? lower chamber to enter the upper chamber, preparing for Vehicles choose between two-stroke and four-stroke the next cycle. engines based on their specific performance needs and Advantages of Two-Stroke Engines design considerations. A two-stroke engine completes a Simplicity: Fewer moving parts compared to four-stroke power cycle in just one crankshaft revolution and two engines. strokes of the piston, allowing for a more compact and lightweight design. This makes them ideal for applications to complete a power cycle, yielding one power stroke every requiring high power-to-weight ratios, such as motorcycles two revolutions. This design allows for better fuel and portable tools. They produce power with every efficiency and lower emissions, making four-stroke revolution, which can be advantageous in situations where engines preferable for vehicles that prioritize these aspects, quick acceleration is needed. such as cars and larger motorcycles 6. Why does the exhaust stroke happen before the intake In contrast, four-stroke engines are generally more fuel- stroke? efficient and produce fewer emissions due to their more The exhaust stroke needs to occur before the next intake complex operation involving intake, compression, power, stroke to ensure that the burned gases are effectively and exhaust strokes. This makes them preferable for expelled from the cylinder, allowing for a fresh charge of vehicles that prioritize fuel economy and environmental air and fuel to enter without contamination. During the considerations, such as cars and larger motorcycles. The exhaust stroke, the exhaust valve opens, and the piston design of four-stroke engines allows for better combustion moves from bottom dead center (BDC) to top dead center control, resulting in cleaner emissions and improved (TDC), pushing the burned gases out of the cylinder 8. efficiency. If the exhaust stroke does not happen properly, residual INTAKE VALVE MALFUNCTIONS exhaust gases can remain in the cylinder, which can dilute If the intake valve of an engine malfunctions and fails to the incoming air-fuel mixture and reduce engine efficiency. open, the engine cycle is significantly disrupted. During the To optimize this process, the exhaust valve is designed to suction or intake stroke, the piston moves downwards from open before the piston reaches BDC, allowing the pressure the top dead center (TDC) to the bottom dead center (BDC), from the burnt gases to assist in expelling them 9. This creating a vacuum that draws in the fuel-air mixture timing is crucial as it helps maintain the engine's through the open intake valve 4. If the intake valve does volumetric efficiency and ensures that the fresh charge is not open, this suction effect cannot occur, preventing the drawn in effectively during the intake stroke. fresh charge from entering the cylinder. FOUR-STROKE ENGINE As a result, the engine will not receive the necessary fuel- A four-stroke engine is a type of internal combustion air mixture for combustion, leading to incomplete or failed engine that completes a power cycle in four distinct strokes ignition during the subsequent compression and power of the piston. Understanding the basic components and strokes. This can cause a variety of issues, such as processes is crucial for grasping how these engines misfiring, reduced power output, or even engine stalling. function. Additionally, the exhaust valve may still open, which could Key Components lead to the expulsion of unburnt fuel and air, further Piston: Moves up and down within the cylinder. exacerbating the inefficiency of the engine cycle. Connecting Rod: Connects the piston to the crankshaft. A two-stroke engine differs from a four-stroke engine Crankshaft: Converts the linear motion of the piston into primarily in its operational cycle and design. In a two- rotational motion. stroke engine, the power cycle is completed in just one The Four Strokes of the Cycle crankshaft revolution and two strokes of the piston, Intake Stroke: resulting in one power stroke per revolution. This makes The piston moves downward, opening the intake valve. two-stroke engines lighter and more compact, which is A mixture of air and fuel enters the cylinder. advantageous for applications like motorcycles and This stroke sets up the next phase of the cycle. chainsaws 1. In contrast, a four-stroke engine requires two Compression Stroke: revolutions of the crankshaft and four strokes of the piston The piston moves upward, compressing the air-fuel Removal of Burned Gases: mixture. During the exhaust stroke, the piston moves upward, and This creates a highly combustible environment for ignition. the exhaust valve opens to release the burned air-fuel Combustion Stroke: mixture from the cylinder. This step is essential to clear out The spark plug ignites the compressed mixture, causing an the combustion byproducts, ensuring that the cylinder is explosion. free of any residual gases before the next intake stroke This explosion forces the piston downward, generating begins. If this step is skipped, the remaining gases can power. dilute the fresh air-fuel mixture, leading to inefficient Exhaust Stroke: combustion. The piston moves upward again, opening the exhaust valve. Preparation for Fresh Intake: The burned gases are expelled from the cylinder, After the exhaust stroke, the cylinder needs to be filled completing the cycle. with a new mixture of air and fuel during the intake stroke. Important Concept If the exhaust stroke does not occur first, the intake valve RPM (Revolutions Per Minute): Measures how many times may not be able to open properly, or the incoming mixture the crankshaft rotates in one minute. At wide open throttle, may not be effective due to the presence of exhaust gases. it can reach up to 5,000 RPMs. This can hinder the engine's performance and power output. Displacement: The volume of gas and air displaced by the Maintaining Engine Cycle Efficiency: piston during its stroke. Longer strokes result in larger The four-stroke cycle relies on a specific sequence of displacements. operations. Each stroke has its designated role, and Impact of Intake Valve Malfunction skipping the exhaust stroke would disrupt this sequence, Intake Stroke Disruption: leading to potential engine misfires or stalling. The cycle During the intake stroke, the piston moves downward, and consists of the intake, compression, combustion, and the intake valve should open to allow a mixture of air and exhaust strokes, and each must happen in order for the fuel to enter the cylinder. If the intake valve does not open, engine to operate smoothly and efficiently. this essential mixture cannot enter, leading to a lack of fuel The Otto Cycle for combustion. As a result, the engine will not be able to The Otto Cycle is an ideal thermodynamic cycle that generate the necessary power. powers gasoline-fueled vehicles. It consists of four distinct Subsequent Strokes Affected: strokes performed by the engine's piston, which convert The failure to draw in the air-fuel mixture means that heat energy from combustion into mechanical energy to during the compression stroke, there is insufficient material drive the vehicle. to compress. This can lead to a situation where the piston Key Terms moves upward without any significant resistance, which Cylinder Displacement: The volume of the cylinder that the can cause misfires or incomplete combustion during the piston travels through during its strokes. combustion stroke. Top Dead Center (TDC): The highest point of the piston’s Overall Engine Performance: travel. The engine cycle relies on the proper functioning of all Bottom Dead Center (BDC): The lowest point of the valves. If the intake valve is malfunctioning, the engine piston’s travel. may experience rough idling, reduced power output, or The Four Strokes of the Otto Cycle even stalling, as it cannot complete the cycle efficiently. Intake Stroke Additionally, the crankshaft's RPMs may drop significantly Description: The piston moves downward from TDC to due to the lack of power generation. BDC. Importance of the Exhaust Stroke Function: Draws in the air and gasoline mixture into the mixture would not ignite properly, leading to incomplete cylinder. combustion and reduced power output Volume Change: Volume increases as the piston moves down. Key Differences Between Petrol and Diesel Engines Compression Stroke Engine Operation Description: The piston moves upward from BDC to TDC. Both petrol and diesel engines operate on the four-stroke Function: Compresses the air and gasoline mixture to make cycle: it combustible. Intake Stroke: Piston descends, drawing air into the Volume Change: Volume decreases, and pressure increases. cylinder through open intake valves. Power Stroke Compression Stroke: Valves close, and the piston ascends Description: The spark plug ignites the compressed to compress the air-fuel mixture. mixture, causing combustion. Power Stroke: The mixture ignites, forcing the piston back Function: The heat from combustion increases pressure, down. pushing the piston down. Exhaust Stroke: The ignited mixture is expelled through Volume Change: Volume increases as pressure decreases exhaust valves. after combustion. Ignition Method Exhaust Stroke Petrol Engines: Use a spark plug to ignite the air-fuel Description: The piston moves upward from BDC to TDC mixture. again. Diesel Engines: Rely on self-ignition due to high Function: Evacuates waste gases from the cylinder through compression, which raises the temperature enough to ignite the exhaust valve. the mixture without a spark. Volume Change: Pressure decreases as waste gases are Compression Ratio expelled. Definition: The ratio of the maximum to minimum volume The spark plug in the cylinder. The spark plug plays a crucial role in the Otto cycle, Example: A 2015 VW Golf TDI has a compression ratio of particularly during the power stroke. Its primary function is 16.2:1, while a VW Golf TSI has 9.6:1. Diesel engines to ignite the air and gasoline mixture that has been have higher compression ratios to ensure ignition. compressed in the cylinder. This ignition is essential for Fuel Characteristics producing the combustion that generates the power needed Energy Content: Diesel contains about 15% more energy to drive the vehicle. by volume than petrol: Ignition of the Mixture: During the power stroke, the spark Diesel: 36.9 MJ/L plug ignites the compressed air-fuel mixture, which leads Petrol: 33.7 MJ/L. to combustion. This process releases heat energy, causing a Efficiency rapid increase in pressure within the cylinder. The pressure Diesel engines are generally more fuel-efficient due to: generated from this combustion is what ultimately drives Higher compression ratios. the piston down, converting thermal energy into Thermal efficiency from igniting air with high compression. mechanical energy that powers the car's wheels 23. Ability to produce more torque at lower RPMs. Fuel Efficiency and Engine Performance: The spark plug Summary ensures that the combustion process occurs at the right Petrol engines: Lower compression, spark ignition, less moment, optimizing fuel efficiency and engine fuel-efficient. performance. Without a functioning spark plug, the air-fuel Diesel engines: Higher compression, self-ignition, more fuel-efficient.  Thermal Efficiency: Diesel engines benefit from higher PETROL VS DIESEL ENGINES thermal efficiency due to their design and operation. Petrol and diesel engines operate on the same four-stroke Overview cycle but differ significantly in their ignition methods, The Diesel Cycle is an ideal thermodynamic cycle that efficiency, and design features. describes how diesel engines operate. It consists of four main strokes that work together to convert fuel into 1. Ignition Method mechanical energy. Petrol Engines: Use a spark plug to ignite the air-fuel The Four Strokes of the Diesel Cycle mixture. Intake Stroke Diesel Engines: Rely on compression to ignite the air-fuel The piston moves down, creating a partial vacuum. mixture without a spark plug. This is due to the higher The intake valve opens, drawing air into the cylinder until compression ratios in diesel engines, which increase the the piston reaches the bottom dead center (BDC), which is temperature enough for ignition. the lowest point of the stroke. 2. Compression Ratio Compression Stroke Definition: The compression ratio is the ratio of the The intake valve closes, and the piston moves up, maximum to minimum volume in the cylinder. compressing the air. Example: A 2015 VW Golf TSI has a compression ratio of This compression raises the pressure and temperature of 9.6:1, while a 2015 VW Golf TDI has a compression ratio the air to levels sufficient for combustion. of 16.2:1. Diesel engines have higher compression ratios to Power Stroke ensure ignition. Fuel injectors introduce atomized diesel fuel into the high- 3. Throttle Mechanism pressure, high-temperature air. Diesel Engines: Lack a throttle body; pressing the The fuel combusts, forcing the piston down and rotating accelerator increases fuel supply directly. the crankshaft, which ultimately powers the vehicle's Petrol Engines: Use a throttle body; pressing the wheels. accelerator opens the throttle to allow more air into the Exhaust Stroke engine. The piston moves up again, expelling burnt gases from the 4. Fuel Efficiency cylinder through the exhaust valve. Diesel Engines: More fuel-efficient, producing This occurs due to the pressure difference between the approximately 33.7 mega joules per liter. This is due to cylinder and the atmosphere. higher thermal efficiency from compression ignition. Petrol Engines: Generally less efficient than diesel engines due to lower compression ratios. 5. Torque and RPM Diesel Engines: Produce more torque at lower RPMs due to longer strokes and higher turbo boost pressure. FACTORS PETROL DIESEL Petrol Engines: Typically require higher RPMs to produce COMPRESSION LOW HIGH similar torque levels. LEVEL FUEL CONSUMPTION HIGHER LOW CONSUMPTION Important Concepts to Remember WEIGHT LIGHT HEAVY NOISE LOW (QUIETER) LOUDER Self-Ignition Temperature: The temperature at which an FUEL COST HIGH COST LOW COST air-fuel mixture ignites without a spark. RELATIVE TO FUEL CONSUMPTION