Two-Stroke Engine Principles Flashcards

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)?

Answer: Intake (into crankcase) and compression

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.

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?

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

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.

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

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

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).

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

Answer: Scavenging

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.

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?

Answer: Loop scavenging

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.

In a typical two-stroke engine's port timing sequence, which port is the first to open as the piston moves downward on the power stroke?

Answer: The exhaust port

The exhaust port must open first on the downstroke to release the high-pressure exhaust gases. This process, called 'blowdown', lowers the cylinder pressure enough so that when the transfer ports open shortly after, the fresh charge can effectively enter the cylinder and scavenge the remaining exhaust without being blown back into the crankcase.