Ch. 5 Two Cycle and Four Cycle Engines Feb 20 7:43 AM 1
Stroke of the piston is its movement in the cylinder from one end of its travel to the other Feb 20 7:44 AM 2
Four stroke cycle engine 4 strokes 1.Intake 2.Compression 3.Power 4.Exhaust Feb 20 7:46 AM 3
Intake Stroke Piston traveling downward Volume of space in cylinder increases Creates a vacuum, draws in air/fuel mixture Intake valve open, atmospheric pressure forces air through the carburetor Feb 20 7:47 AM 4
Intake Valve Functions 1. Must open at correct instant to permit intake of air fuel mixture 2. Must close at correct time and seal during compression 3. Its shape must be streamlined, so the flow of gases into combustion chamber will not be obstructed Feb 20 7:49 AM 5
Compression Stroke Created by piston moving upward in the cylinder Squeezing action while both valves are closed Valves sealed, piston rings prevent leakage past the piston Feb 20 7:50 AM 6
As the piston moves upward, the air fuel mixture is compressed into a smaller space. This increases the force of combustion for two reasons: 1. When the tiny molecules of air and fuel are squeezed together, heat energy is creaed. The air/fuel mixture is heated close to its flashpoint (will ignite spontaneously) When it ignites, it is instaneous and complete for all of air/fuel mixture 2. The force of combustion is increased because tightly packed molecules(air/fuel mixture) are highly activated and are striving to move apart. This energy, combined with the expanding energy of combustion, provides tremendous force against the piston Feb 20 7:51 AM 7
Power Stroke both valves closed As the piston compresses the charge and reaches the top of the cylinder, an electrical spark jumps the gap and ignites the air fuel mixture Force of explosion forces the piston downward Feb 20 7:56 AM 8
Power Stroke Fuel charge must ignite and expand in short time Amount of power produced by power stroke depends on the volume of air/ fuel mixture in the cylinder and the compression ratio of the engine Feb 20 7:57 AM 9
Compression Ratio proportionate difference in volume of cylinder and combustion chamber at bottom dead center (BDC) and at top dead center (TDC) Feb 20 8:08 AM 10
After the piston has completed the power stroke, the burned gases must be removed from cylinder before introducing a fresh air/fuel mixture exhaust valve opens and rising piston pushes gases from cylinder Feb 20 8:10 AM 11
Exhaust Valve When closed, it must seal When open, it must allow streamlined flow of exhaust gases through the port Scavenging removal of gases from cylinder Feb 20 8:33 AM 12
Heat Transfer Heat must be controlled or the valve will deteriorate rapidly The hottest part of the valve, the valve head, transfers heat through the valve seat to the cylinder block Feb 20 8:38 AM 13
Valve Timing Top Dead Center (TDC) Bottom Dead Center(TDC) If timing marks are aligned on camshaft and crankshaft, the valve timing will take care of itself Feb 20 8:39 AM 14
Valve Overlap occurs when both valves are open at the same time Feb 20 8:41 AM 15
Lubrication Place correct quantity and grade of engine oil in the crankcase Splash and pump are two most common methods Must be drained and and replaced periodically Must be operated in upright position to lubricate properly Feb 20 8:51 AM 16
Two Stroke Cycle Intake, compression, power and exhaust strokes take place during only two strokes of the piston Only takes one revolution of the shaft to complete one two stroke cycle Feb 20 8:53 AM 17
Two stroke Smaller, lighter Will get adequate lubrication at extreme angles because it recieves lubrication as fuel is mixed with oil and is passed through the engine Must install correct mixture of fuel and oil Feb 20 8:56 AM 18
Variations in Design of Two Cycle Cross scavenged Loop scavenged Feb 20 8:57 AM 19
Cross Scavenged special contour on piston head, which acts as a baffle to deflect air fuel charge upward in cylinder Usually have reed valves hold incoming charge in crankcase so it can be compressed Piston acts as a valve in opening and closing intake Feb 20 8:58 AM 20
Loop Scavenged Flat or domed piston. The fuel transfer ports in loop scavenged engines are shaped so that the incoming air fuel mixture swirls Forces gases out and permits new air in Feb 20 9:00 AM 21
Principles of Operation Location of ports is essential to correct timing of intake, transfer, and exhaust functions Feb 20 9:03 AM 22
Intake into Crankcase As piston moves upward into the cylinder, crankcase pressure drops and intake port is exposed Atmospheric pressure is greater than crankcase pressure, air rushes through the carburetor and into the crankcase While passing through the carburetor, the intake air pulls a charge of fuel and oil along Feb 20 9:04 AM 23
Fuel Transfer Piston moves downward Transfer port opens, new charge cools combustion area and pushes out exhaust gases Feb 20 9:07 AM 24
Ignition Power Piston travels upward, compresses airfuel charge in cylinder to about 1/10 of volume Spark ignites at TDC Feb 20 9:08 AM 25
Exhaust Piston exposes exhaust port, most of burned gases are expelled Other gases escape when new air fuel mixture rushes in COMPLETES ONE CYCLE Feb 20 9:09 AM 26
SCAVENGING and TUNING Best efficiency fuel charge must be held in the cylinder momentarily while the exhaust port is open Well engineered exhaust systems use the energy of sound waves from the exhaust gases for proper tuning Feb 20 9:13 AM 27
Rotary Disc valve Engine Intake port located directly in the crankcase, allowing room for additional transfer ports that promote better fuel transfer and scavenging Feb 20 9:15 AM 28
Reed Valve Engine permits fuel intake directly into the crankcase Reed is made of thin, flexible spring steel Reed stop stops reed valve from opening too far, thick and inflexible Feb 20 9:17 AM 29