Four Stroke and Two Stroke Engines

Fundamental Principles of the Internal Combustion (I.C.) Engine

• Core Operating Principle: The internal combustion engine operates by exploding or burning a mixture of fuel with the correct amount of air inside an engine cylinder that is closed at one end.
• Energy Conversion Process:     - Heat Release: The explosion of the fuel-air mixture releases heat, which causes the pressure of the burning gases to increase significantly.     - Piston Movement: This increased pressure regulates the piston, forcing it to move downward within the cylinder.     - Transmission of Motion: The linear movement of the piston is transmitted to a crankshaft via a connecting rod.     - Rotary Motion: The crankshaft then rotates the flywheel, converting reciprocating motion into rotary motion.
• Requirements for Continuous Operation:     - Repeated Explosions: To maintain continuous rotation of the crankshaft, the explosion process must be repeated cyclically.     - Gas Exchange: Burnt gases must be expelled from the cylinder (exhaust), and fresh charges of fuel and air must be admitted (intake).     - Piston Reset: The piston must be moved back to its original position to begin the cycle again.

Mechanical Components and Structural Design of I.C. Engines

• Cylinder and Piston Assembly:     - The piston reciprocates within the cylinder and is designed as a tight fit.     - Piston Rings: These are inserted into circumferential grooves on the piston to prevent the leakage of gases from the sides of the piston during operation.     - Cylinder Block: The cylinder is typically bored directly into a cylinder block.     - Gasket: A gasket made of copper sheet or asbestos is inserted between the cylinder and the cylinder head to ensure a seal.     - Combustion Space: This is the area provided at the top of the cylinder head where the actual combustion of the charge takes place.
• Connecting Rod and Crankshaft:     - Connecting Rod: The component that links the piston to the crankshaft.     - Gudgeon Pin (Wrist Pin): A pin used to connect the piston to the small end of the connecting rod.     - Small End: The end of the connecting rod that fits over the gudgeon pin.     - Big End: The end of the connecting rod that fits over the crank pin of the crankshaft.     - Main Bearings: These are fitted in the crankcase to support the rotating crankshaft.
• Ancillary Components:     - Flywheel: Attached to one end of the crankshaft, its purpose is to smoothen the uneven torque produced by the engine stages.     - Oil Sump: Located at the bottom of the engine, it contains the lubricating oil necessary for reducing friction between various engine parts.     - Crankcase: The housing for the crankshaft; in two-stroke engines, this must be gas-tight.

Operational Classification and the Four-Stroke Cycle Engine

• Classification by Cycle Duration:     - Four-Stroke Cycle Engine: The engine cycle is completed in four strokes of the piston, reflecting two full revolutions of the crankshaft.     - Two-Stroke Cycle Engine: The engine cycle is completed in two strokes of the piston, reflecting one full revolution of the crankshaft.
• Four-Stroke Mechanical Control:     - Valves: Four-stroke engines use specific inlet and exhaust valves to control the admission of charges and the expulsion of exhaust gases.     - Camshaft: The opening and closing of these valves are controlled by cams fitted on a camshaft.     - Camshaft Drive: The camshaft is driven by the crankshaft using gears or chains.     - Speed Ratio: The camshaft runs at exactly half the speed of the crankshaft.

Detailed Stages of the Four-Stroke Cycle

• I. Suction (Induction) Stroke:     - The piston moves to draw air or a mixture of air and fuel into the cylinder.     - The inlet valve remains open during this admission, while the exhaust valve remains closed.     - The pressure inside the cylinder drops below atmospheric pressure, creating a vacuum that sucks the charge in.
• II. Compression Stroke:     - The piston moves to compress the charge into a small volume called the clearance volume.     - Both the inlet and exhaust valves are closed.     - Ignition Mechanisms:         - Compression Ignition (CI): If only air is compressed, fuel is injected at the end of the stroke, igniting due to high pressure and temperature.         - Spark Ignition (SI): If a mixture of air and fuel is compressed, it is ignited by a spark plug.
• III. Power (Expansion) Stroke:     - Following ignition, a tremendous amount of heat is generated, creating high pressure that forces the piston downward.     - This is the only stroke that develops power; the connecting rod and crankshaft transmit this power to the transmission system.     - Both valves remain closed during this stroke.
• IV. Exhaust Stroke:     - The piston moves to push the burnt exhaust gases out through the exhaust valve.     - The inlet valve remains closed while the exhaust valve is open.     - This clears the cylinder to receive a fresh charge for the next cycle.
• Stroke Dynamics: Out of the four strokes, only one is a power stroke. The remaining three are "idle strokes," which rely on the momentum supplied by the power stroke to complete their movements.

The Two-Stroke Cycle Engine: Mechanisms and Operation

• Mechanical Differences: Unlike the four-stroke engine, the two-stroke engine has no valves. Instead, gas movement occurs through ports in the cylinder. The crankcase must be air-tight.
• First Stroke (Combined Suction and Compression):     - As the piston moves upward, it covers the exhaust port and the transfer port (which are located nearly opposite each other).     - This traps and compresses the fresh charge in the cylinder.     - Simultaneously, the upward movement uncovers the suction port in the crankcase, drawing a fresh mixture into the crankcase.     - The mixture in the cylinder is ignited just before the piston reaches the end of its upward stroke.
• Second Stroke (Combined Power and Exhaust):     - Rising pressure from burning gases forces the piston downward.     - The downward movement covers the suction port, trapping and compressing the mixture inside the crankcase.     - Further downward movement uncovers the exhaust port (allowing burnt gases to leave) and then the transfer port.     - Compressed fresh mixture from the crankcase then flows through the transfer port into the cylinder.     - Deflection: A specially shaped piston crown deflects the incoming fresh mixture upward to help drive out the remaining exhaust gases.

Key Operational Terminology and Scavenging

• Dead Centers:     - Top Dead Centre (TDC): The position of the piston when it is at the very top of its stroke.     - Bottom Dead Centre (BDC): The position of the piston when it is at the very bottom of its stroke.
• Scavenging:     - Definition: The process of removing burnt or exhaust gases from the engine cylinder.     - Two-Stroke Requirement: Since burnt gases do not fully exit during a normal stroke, two-stroke engines often use a blower or compressor to assist in removing exhaust gases.
• Piston Effectiveness: In a two-stroke engine, both sides of the piston are effective in the cycle (one side for crankcase compression, the other for cylinder compression/power), which is not the case for four-stroke engines.

Comparative Analysis: Four-Stroke vs. Two-Stroke Engines