two stroke operation

7/29/2019 Two Stroke Operation

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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 thedifferences 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 stroke

However, 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 moresimple!



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 incominggases swirl in the cylinder

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

May have more than one transfer port to createthis 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 outsideatmospheric to rush into the crankcasethrough the carburetor

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

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



Fuel Transfer

As the piston moves downwards, itcompresses the air fuel and oil in the

crankcaseWhen it travels far enough downward, it

exposes the transfer port into the cylinder

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

This new charge of air, fuel, and oil also coolthe 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 volumeSpark ignites charge when piston

reaches TDC

The ignited gasses force the pistondownward



Exhaust

As the piston moves downwards, the

exhaust port is exposed

This allows for the removal of gases

The remainder of gases are forced out

when the transfer ports are exposedThis 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 entermore rapidly for cleaner combustion

The charge needs to be held within the

combustion chamber while the exhaustport is open



Scavenging Con’t

Well engineered exhaust systems use theenergy of sound waves to control this

As the exhaust is moving out, a megaphoneshaped pipe allows the sound waves to bereflected back towards the cylinder

These reflected sound waves create backpressure for the exhaust system, and keepsthe 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 newcharge into the crankcase

Intake port is located directly in the crankcase(this allows for additional transfer ports in theengine 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 tospin into position to allow for the transfer of a

new charge into the cylinder



Reed Valves

This also permits the intake of a chargedirectly into the crankcase

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

The Reed stop is made for thick inflexiblesteel. This prevents the reed from opening too far and

becoming permanently bent



Reed Valves Con’t

The reed is controlled by the outsideatmospheric pressure

It closes due to the springiness of thesteel, and the built pressure within thecrankcase

The reed valve comes in multipledesigns



Advantages of 2 Strokes

Have less moving parts (less parts to

fail)

Simpler Design

Operates at extreme angles

Lighter construction

More power strokes per rotation



Disadvantages of 2 Strokes

Eratic Operation

Smells

Burns oil

Must have premixed oil

Very Hot running



Comparison of Lubrication

systems Two stroke engines are lubricated by the

charge

Since the charge is what lubricates, it can operateat extreme angles

Four strokes use a pressure system or an oilslinger

If the slinger is not touching the oil, lubrication willfail. This is why it does not operate at extremeangles



Comparison to Four Strokes

Look in Book!

two stroke operation

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