Two Stroke Engine Operation

Automotive Technology 1

Mr. Wasacz

Objectives

The student will be able to explain the operation of a 2 stroke engine

The student will be able to identify the differences between a 2 and 4 stroke engine.

The students will be able to compare the lubrication systems of a 2 and 4 stroke engine

Quick Reminder

Stroke is the movement of the piston from one end of the cylinder to the other

TDC to BDC BDC to TDC

Two Stroke Engine

Performs the same tasks as a 4 strokeHowever, intake, compression, power, and exhaust take place simultaneously

The crankshaft only rotates ONCE, so the cycle is complete after only 2 strokes

The design of two strokes is much more simple!

Variations in Design

Cross Scavenged Has contoured piston head to deflect air upward to prevent intake from flowing out through exhaust

Uses a reed valve or rotary valve to hold the incoming charge in the crankcase

The downward movement of the piston creates pressure in the crankcase

The Piston acts as a valve for the cylinder

Cross Scavenged Diagram

Variations in Design

Loop- Scavenged Does not deflect incoming gases Flat or Domed piston Ports are positioned and shaped so that incoming gases swirl in the cylinder

This swirl keeps the intake gases in, and forces the exhaust gases out

May have more than one transfer port to create this swirling effect

Loop-Scavenged Diagram

Intake into the crankcase

As the piston moves upwards in the cylinder, low pressure in the crankcase is created

This pressure allows the higher outside atmospheric to rush into the crankcase through the carburetor

When the air moves through the carburetor, it takes a charge of gasoline and oil with it.

This newly in took charge lubricates the bearings and moving parts in the crankcase

Fuel Transfer

As the piston moves downwards, it compresses the air fuel and oil in the crankcase

When it travels far enough downward, it exposes the transfer port into the cylinder

The newly compressed air, fuel, and oil then rush upwards towards the cylinder

This new charge of air, fuel, and oil also cool the hot combustion area, and push the previous exhaust gasses outward

Ignition-Power

The piston travels upwards, compressing the charge to approx 1/10th its original volume

Spark ignites charge when piston reaches TDC

The ignited gasses force the piston downward

Exhaust

As the piston moves downwards, the exhaust port is exposed

This allows for the removal of gasesThe remainder of gases are forced out when the transfer ports are exposed

This completes one cycle of operation

What it looks like all together

Scavenging

When 2 strokes are properly designed, all exhaust gasses will be scavenged

This allows for a new charge to enter more rapidly for cleaner combustion

The charge needs to be held within the combustion chamber while the exhaust port is open

Scavenging Con’t

Well engineered exhaust systems use the energy of sound waves to control this

As the exhaust is moving out, a megaphone shaped pipe allows the sound waves to be reflected back towards the cylinder

These reflected sound waves create back pressure for the exhaust system, and keeps the incoming charge held within the cylinder

Most 2 stroke engines will not use a straight exhaust pipe for this reason

Rotary Disc Valves

The Rotary disc controls the flow of a new charge into the crankcase

Intake port is located directly in the crankcase (this allows for additional transfer ports in the engine block)

The crankshaft holds the rotary valve, and provides the rotation for it to open and close

The valve only has one cutout, allowing to to spin into position to allow for the transfer of a new charge into the cylinder

Reed Valves

This also permits the intake of a charge directly into the crankcase

Reed is made of a thin flexible spring steel which is connected at one end

The Reed stop is made for thick inflexible steel. This prevents the reed from opening too far and becoming permanently bent

Reed Valves Con’t

The reed is controlled by the outside atmospheric pressure

It closes due to the springiness of the steel, and the built pressure within the crankcase

The reed valve comes in multiple designs

Advantages of 2 Strokes

Have less moving parts (less parts to fail)

Simpler DesignOperates at extreme anglesLighter constructionMore power strokes per rotation

Disadvantages of 2 Strokes

Eratic OperationSmellsBurns oilMust have premixed oilVery Hot running

Comparison of Lubrication systemsTwo stroke engines are lubricated by the charge Since the charge is what lubricates, it can operate at extreme angles

Four strokes use a pressure system or an oil slinger If the slinger is not touching the oil, lubrication will fail. This is why it does not operate at extreme angles

Comparison to Four Strokes

Look in Book!