Two-Stroke Engine Operation Flashcards
When does scavenging occur in the two-stroke engine?
When exhaust and transfer ports are open at the same time.
List the five events of two-stroke engine operation (remember, same as 4 stroke except one extra).
A. Intake B. Combustion C. Compression D. Exhaust E. Transfer
What is the purpose of loop scavenging?
Minimize loss of the intake charge mixture out of the exhaust port.
what are the three major moving parts of a two-stroke engine?
A. Piston
B. Connecting Rod (one piece)
C. Crankshaft (multi-piece)
Name two purposes of the expansion chamber.
- Aids in scavenging
2. Shapes the engine powerband
When does piston port design intake open?
- Opens at BTDC
2. Closes at ATDC
When does the piston port design exhaust open and close?
- Opens at BBDC
2. Closes at ABDC
When does the piston port design transfer open and close?
Transfer opens at BBDC
Transfer closes at ABDC
The five events occur in how many strokes of the piston?
Two
The five events occur in how many revolutions of the crankshaft?
One
Where in the engine does primary compression take place?
In the primary area.
Where in the engine does secondary compression take place?
In the secondary area.
Name four variations from the basic piston-port intake design.
- Cylinder Reed Valve
- Crank case Reed Valve
- Rotary Valve
- Piston Port Crankcase reed valve
Which is the most common piston-port intake design?
The Cylinder Reed Valve.
What is the essential purpose of these variations from the basic piston-port intake design?
To make intake time asymmetrical.
Which induction system uses a a disc to control the intake?
The Rotary Valve.
What does an “exhaust power valve” do?
Alters the exhaust ports type.
What are the three main ports in the two-stroke cylinder, and what are their purposes?
A. Exhaust; routes combustion
B. Intake; routes mixture into the primary
C. Transfer; routes mixture from primary to secondary
Port
- Part of the cylinder
2. Controls the airflow into and out of the engine.
Primary
All the area under the piston crown (primary compression occurs here)
Secondary
All the area above the piston crown (secondary compression occurs here)
Major areas
Primary and Secondary
Intake port
- Lowest port in the cylinder wall
2. Controlled by Skirt
Exhaust port
- Controlled by the piston Crown
2. Routes spent gases out of the cylinder
Transfer port
- Controlled by the piston Crown
2. Transfer mixture from Primary to Secondary
Upstroke
- Intake
- Compression
- Combustion
Downstroke
Exhaust and transfer occur
Exhaust
Longest duration
Transfer
Shortest duration
Symmetrical
A. Open and close equal distance from dead center
B. Narrow tuning
Scavenge
- Evacuation of spent gases exhaust gases by force of incoming mixture
- Minimize charge loss
Loop scavenge
Transfer ports angled to minimize loss of charge out the exhaust port
Cross scavenge
Piston crown designed to minimize loss of charge out the exhaust port
Expansion Chamber
Shapes engine powerband (sonic waves move up and down the pipe)
Piston port design variations
Make intake port timing asymmetrical
Cylinder Reed Valve
A. Most common
B. Widest possible power-band
Rotary Valve
A. Rotary disc valve on crankshaft end, driven by crankshaft
B. Most tunable
Piston port/crankcase reed (combo)
- Good overall power
2. Parallel passages
Power Valves
A power valve alters the exhaust port’s height, width, or length.
Cylinder head
A. Centrally located spark plug and large amount of squish aids combustion.
B. No Valves
Types of Cylinder head
- Cast-iron
- Aluminum w/ cast-iron sleeve
- Plated Aluminum.
Cylinder, Servicing
- Must be chamfered after boring the cylinder.
- Very wide ports are bridged to prevent rings catching on port edges.
- Measured 6 x’s X and Y axis, Top, Center, and bottom of cylinder.
Crosshatch
- Seats piston rings
2. Controls cylinder lubrication
Piston
- Cast (common) and Forged (high performance)
- Transfers power to connecting rod
- Tapered - Crown smaller than skirt
- Elliptical - Allows for expansion
Piston Ring purpose and Ring types:
- To seal primary area from secondary area.
A. Standard: Rectangular cross section.
B. Keystone: Wedge cross section. Cleans groove.
C. Dykes: Prevents ring flutter
Wrist pin
Lubricated by slot or hole in connecting rod
Connecting Rod
- Needle bearings on small end
2. Ball Bearing on Large end
Crankshaft
Ball main bearings
Multi-cylinder crankshafts
Labyrinth seal used between cylinders to keep positive and negative pressures from each other
Crankcase
Wet side seal (clutch): if it leaks engine smokes excessively
Dry Side seal (ignition): if it leaks, engine runs lean and may seize
Crankcase
Leakdown test
How much oil to a gallon?
128 / ratio x number of gallons = oz oil.
More oil
32:1
More Fuel
50:1
What are the eight causes of two-stroke engine seizure?
A. Lean B. Tight
C. Lubrication D. Timing (not adjustable)
E. Loading F. Torque
G. E. Temperature
An oil injection lubrication system is regulated by which two things?
Throttle opening and engine RPM
What are two methods of lubricating a two-stroke engine?
Pre-mix and oil injection
What test is made if you suspect that a two stroke primary is no longer air tight; and what areas of the engine must be sealed before beginning test?
Leak down test and the intake and exhaust must be sealed before starting tests.
What type of crankshaft is used in a two-stroke engine?
Multi-piece
What type of case is found in a single-cylinder two-stroke engine?
Vertical split
