EETC - Equipment & Engine Training Council Two-Stroke Engine Principles Questions and Answers

Question 1

In a piston-ported two-stroke engine, what two events occur simultaneously as the piston moves from Bottom Dead Center (BDC) to Top Dead Center (TDC)?

Accepted Answer

Intake (into crankcase) and compression

Answer

As the piston travels upward from BDC to TDC, it creates a low-pressure area in the sealed crankcase, drawing the fresh air-fuel mixture in through the intake port. At the same time, the piston is compressing the air-fuel mixture that was previously transferred into the cylinder.

Question 2

A customer complains their chainsaw, a two-stroke engine, is producing excessive blue smoke and has a fouled spark plug. Which of the following is the most likely cause?

Accepted Answer

An incorrect fuel-to-oil ratio (too much oil)

Answer

In a two-stroke engine, lubrication is provided by oil mixed with the gasoline. An incorrect, overly rich oil mixture will not burn completely, leading to excessive blue smoke and oily carbon deposits on the spark plug.

Question 3

Which of the following describes the primary function of reed valves in a two-stroke engine?

Accepted Answer

To act as a one-way check valve for the intake tract, preventing blowback.

Answer

Reed valves are positioned in the intake tract and open due to the vacuum created in the crankcase as the piston rises. When the piston descends, the pressure in the crankcase forces the flexible reeds to close, preventing the air-fuel mixture from being pushed back out through the carburetor (a phenomenon known as blowback).

Question 4

The process of using the incoming fresh air-fuel charge to push the burnt exhaust gases out of the cylinder is known as:

Accepted Answer

Scavenging

Answer

Scavenging is the critical process in a two-stroke engine where the fresh mixture entering the cylinder through the transfer ports displaces the exhaust gases from the previous power stroke, pushing them out through the exhaust port.

Question 5

Which type of scavenging directs the incoming charge upward in a looping path to force exhaust gases out, with both transfer and exhaust ports located on the same side of the cylinder?

Accepted Answer

Loop scavenging

Answer

In loop scavenging, the transfer ports are shaped to direct the fresh charge up one side of the cylinder, across the combustion chamber, and down the other side, effectively 'looping' to push out the exhaust gases.

EETC - Equipment & Engine Training Council Practice Test

EETC - Equipment & Engine Training Council Practice Test

EETC - Equipment & Engine Training Council Two-Stroke Engine Principles Questions and Answers