Two-Stroke Engine Operation

Question 0

When does scavenging occur in the two-stroke engine?

Answer 0

When exhaust and transfer ports are open at the same time.

Question 1

List the five events of two-stroke engine operation (remember, same as 4 stroke except one extra).

Answer 1

A. Intake 
B. Combustion
C. Compression
D. Exhaust
E. Transfer

Question 2

What is the purpose of loop scavenging?

Answer 2

Minimize loss of the intake charge mixture out of the exhaust port.

Question 3

what are the three major moving parts of a two-stroke engine?

Answer 3

A. Piston
B. Connecting Rod (one piece)
C. Crankshaft (multi-piece)

Question 4

Name two purposes of the expansion chamber.

Answer 4

1. Aids in scavenging

2. Shapes the engine powerband

Question 5

When does piston port design intake open?

Answer 5

1. Opens at BTDC

2. Closes at ATDC

Question 6

When does the piston port design exhaust open and close?

Answer 6

1. Opens at BBDC

2. Closes at ABDC

Question 7

When does the piston port design transfer open and close?

Answer 7

Transfer opens at BBDC

Transfer closes at ABDC

Question 8

The five events occur in how many strokes of the piston?

Answer 8

Two

Question 9

The five events occur in how many revolutions of the crankshaft?

Answer 9

One

Question 10

Where in the engine does primary compression take place?

Answer 10

In the primary area.

Question 11

Where in the engine does secondary compression take place?

Answer 11

In the secondary area.

Question 12

Name four variations from the basic piston-port intake design.

Answer 12

1. Cylinder Reed Valve
2. Crank case Reed Valve
3. Rotary Valve
4. Piston Port Crankcase reed valve

Question 13

Which is the most common piston-port intake design?

Answer 13

The Cylinder Reed Valve.

Question 14

What is the essential purpose of these variations from the basic piston-port intake design?

Answer 14

To make intake time asymmetrical.

Question 15

Which induction system uses a a disc to control the intake?

Answer 15

The Rotary Valve.

Question 16

What does an “exhaust power valve” do?

Answer 16

Alters the exhaust ports type.

Question 17

What are the three main ports in the two-stroke cylinder, and what are their purposes?

Answer 17

A. Exhaust; routes combustion
B. Intake; routes mixture into the primary
C. Transfer; routes mixture from primary to secondary

Question 18

Port

Answer 18

1. Part of the cylinder

2. Controls the airflow into and out of the engine.

Question 19

Primary

Answer 19

All the area under the piston crown (primary compression occurs here)

Question 20

Secondary

Answer 20

All the area above the piston crown (secondary compression occurs here)

Question 21

Major areas

Answer 21

Primary and Secondary

Question 22

Intake port

Answer 22

1. Lowest port in the cylinder wall

2. Controlled by Skirt

Question 23

Exhaust port

Answer 23

1. Controlled by the piston Crown

2. Routes spent gases out of the cylinder

Question 24

Transfer port

Answer 24

1. Controlled by the piston Crown | 2. Transfer mixture from Primary to Secondary

Question 25

Upstroke

Answer 25

1. Intake 2. Compression 3. Combustion

Question 26

Downstroke

Answer 26

Exhaust and transfer occur

Question 27

Exhaust

Answer 27

Longest duration

Question 28

Transfer

Answer 28

Shortest duration

Question 29

Symmetrical

Answer 29

A. Open and close equal distance from dead center | B. Narrow tuning

Question 30

Scavenge

Answer 30

1. Evacuation of spent gases exhaust gases by force of incoming mixture 2. Minimize charge loss

Question 31

Loop scavenge

Answer 31

Transfer ports angled to minimize loss of charge out the exhaust port

Question 32

Cross scavenge

Answer 32

Piston crown designed to minimize loss of charge out the exhaust port

Question 33

Expansion Chamber

Answer 33

Shapes engine powerband (sonic waves move up and down the pipe)

Question 34

Piston port design variations

Answer 34

Make intake port timing asymmetrical

Question 35

Cylinder Reed Valve

Answer 35

A. Most common | B. Widest possible power-band

Question 36

Rotary Valve

Answer 36

A. Rotary disc valve on crankshaft end, driven by crankshaft | B. Most tunable

Question 37

Piston port/crankcase reed (combo)

Answer 37

1. Good overall power | 2. Parallel passages

Question 38

Power Valves

Answer 38

A power valve alters the exhaust port's height, width, or length.

Question 39

Cylinder head

Answer 39

A. Centrally located spark plug and large amount of squish aids combustion. B. No Valves

Question 40

Types of Cylinder head

Answer 40

1. Cast-iron 2. Aluminum w/ cast-iron sleeve 3. Plated Aluminum.

Question 41

Cylinder, Servicing

Answer 41

1. Must be chamfered after boring the cylinder. 2. Very wide ports are bridged to prevent rings catching on port edges. 3. Measured 6 x's X and Y axis, Top, Center, and bottom of cylinder.

Question 42

Crosshatch

Answer 42

1. Seats piston rings | 2. Controls cylinder lubrication

Question 43

Piston

Answer 43

1. Cast (common) and Forged (high performance) 2. Transfers power to connecting rod 3. Tapered - Crown smaller than skirt 4. Elliptical - Allows for expansion

Question 44

Piston Ring purpose and Ring types:

Answer 44

1. To seal primary area from secondary area. A. Standard: Rectangular cross section. B. Keystone: Wedge cross section. Cleans groove. C. Dykes: Prevents ring flutter

Question 45

Wrist pin

Answer 45

Lubricated by slot or hole in connecting rod

Question 46

Connecting Rod

Answer 46

1. Needle bearings on small end | 2. Ball Bearing on Large end

Question 47

Crankshaft

Answer 47

Ball main bearings

Question 48

Multi-cylinder crankshafts

Answer 48

Labyrinth seal used between cylinders to keep positive and negative pressures from each other

Question 49

Crankcase

Answer 49

Wet side seal (clutch): if it leaks engine smokes excessively Dry Side seal (ignition): if it leaks, engine runs lean and may seize

Question 50

Crankcase

Answer 50

Leakdown test

Question 51

How much oil to a gallon?

Answer 51

128 / ratio x number of gallons = oz oil.

Question 52

More oil

Answer 52

32:1

Question 53

More Fuel

Answer 53

50:1

Question 54

What are the eight causes of two-stroke engine seizure?

Answer 54

A. Lean B. Tight C. Lubrication D. Timing (not adjustable) E. Loading F. Torque G. E. Temperature

Question 55

An oil injection lubrication system is regulated by which two things?

Answer 55

Throttle opening and engine RPM

Question 56

What are two methods of lubricating a two-stroke engine?

Answer 56

Pre-mix and oil injection

Question 57

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?

Answer 57

Leak down test and the intake and exhaust must be sealed before starting tests.

Question 58

What type of crankshaft is used in a two-stroke engine?

Answer 58

Multi-piece

Question 59

What type of case is found in a single-cylinder two-stroke engine?

Answer 59

Vertical split