Wednesday, July 6, 2011

2 Stroke Engines


2 Stroke Engines

Explain the difference between a 2 stroke and a 4 stroke engine?
The difference between them is the amount of strokes needed to complete the four stages of an engine cycle. Intake, Compression, Ignition/Power, Exhaust. A 2 stroke does this in only two strokes or revolutions of the engine. A four stroke does this in four revolutions.

How does a 2 stroke diesel engine work?
A diesel 2 stroke has a wet sump and is more like a four stroke in respect to the fact that everything happens above the piston. All diesel 2 stroke engines are artificially aspirated using either a turbo or supercharger. As the piston moves down the bore the exhaust and intake valves are exposed. As the exhaust evacuates the combustion chamber pressure changes allow the inlet reed valve to open and the turbo forces fresh air into the chamber. As the piston starts to rise the inlet and exhaust ports close and pressure begins to build. As the air is compressed its temperature rises.Just before T.D.C the diesel fuel is injected and ignites forcing the piston down. This cycle is repeated.

Explain how a two stroke engine works?
This engine is designed to complete the four stages of the engine cycle in two strokes. It works by drawing an Fuel/Air/Oil mixture into the crankcase(Pre-combustion chamber) as the piston moves up the cylinder, at the same time the Fuel/Air/Oil mixture above the piston is being compressed. As the piston reaches T.D.C the mixture above the piston is ignited, forcing the piston downward exposing the exhaust and transfer port. As the piston moves down the cylinder pressure pushes the reed valve on the inlet port shut and starts to build pressure in the pre-combustion chamber. As the tranfer port opens the Fuel/Air/Oil mixture flows from the pre-combustion chamber to the combustion chamber. The shape of the top of the piston causes the mixture to swirl as it enters, this helps expell exhaust fumes and reduces fresh mixture leaving the chamber before combustion.

What is happening below and above the piston?
Above the piston we have Compression, Ignition and Transfer/Exhaust of the fuel.
Below the piston we have Intake, Pre-compression and Tranfer.
Note - As the Fuel/Air/Oil mixture is compressed it lubricates the system.

Explain what is meant, by scavenging when applied to 2 stroke SI engines?
This is the proccess of pushing the exhaust gases out and drawing the new mixture in. The scavenging effect can be helped by making sure the flow of gases through the system is uninterupted.

Why must roller and ball race bearings be used on 2 stroke SI engine crankshafts?
2 Stroke engines are lubricated using oil in the fuel. Because of this we use open bearings such as roller/ball race bearings. As the mixture is compressed the oil within the fuel mixture works its way through the bearings lubricating them. If a sealed type bearing was used then the oil would not enter the bearing when compressed.

Why are rollers caged in crankshaft bearings?
Caged roller bearings are used because the cage holds each roller in place stopping them from bunching up and unevenly distributing the load.

Why must ‘split’ type bearings be used on one piece crankshafts?
If split type bearings were not used on a one peice crankcshaft then it would be impossible to service/remove the bearing should it wear out. Split type bearings can be fitted around the crankshaft.

Why do we have pins between the piston ring gaps on a 2 stroke?
We have pins between the ring gaps to locate the piston rings. As the rings pass over the valves friction can cause them to move, these pins keep the rings from moveing.
How does a reed valve work on a two stroke?
This is basicly a one way valve. It opens as pressure increases on one side and closes as pressure builds on the opposite side. This is used on the intake side of a 2 stroke. As the piston move up the cylinder the reed valve opens allowing mixture to enter. As the piston move down and compresses the mixture it forces the valve shut.

How do you check a reed valve?
A Reed valve can be checked by visual inspection, the reed valve must completely close. You can also blow againsted the valve from each direction to make sure the valve acts as it should.


Explain the term ovality and taper and how are they formed?
Ovality - This refers to the deformation of a circular object to an oval. Wear on the thrust sides of a piston can cause it to become slightly oval.
Taper - For example wear on the cylinder wall can cause a taper or slope of the wall. This wear occurs on the thrust sides of the piston. A tapered bearing has a cone shape.

How does the piston rings seal in the bore?
As the piston rises pressure above the piston increases. This pressure pushes down past the piston, as it reaches the piston ring grove the pressure acts on the ring pushing it outward againsted the cylinder wall. Sealing the bore.

Measuring Side clearance

Explain these terms:
Groove depth - This is the depth of the groove the piston ring sits in. this is measured with a Feeler gauge.
Side clearance - This is the clearance between the piston ring and the side of the ring groove. This is measured using a Feeler gauge.
End gap - Place the ring in the bore. As it rests level in the bore there is a small gap, this is the end gap. We measure this with a Feeler gauge.


B - Measuring end gap


What do the following terms mean:
Mechanical Efficiency - This is a measure of effectiveness of a machine. A low friction machine would have a higher mechanical efficiency that a machine with high friction or resistance.
Thermal Efficiency - This refers to a dimensionless measure of performance in a device that uses thermal energy
Volumetric Efficiency - This refers to the engines ability to move a fuel/air/oil charge into and out of the cylinder. Artificialy aspirated or forced induction engines would operated above 100% efficiency.

List 2 methods of lubricating the internal parts of 2 stroke S.I. engines?
Lubrication of 2 stroke internal parts is done by introducing oil into the fuel.
This can be done two ways;
The oil can be added directly to the mixture in the fuel tank.
The oil is injected into the intake manifold and mixes during intake. When not using pre-mixed fuel there is a small pump that runs off the crankshaft. This increases oil delivery as the engine speed increases.

What happens if the piston ring gap is too big?
If the piston ring gap is too big then the piston ring will not seal when it expands. This will drop compression causing power loss. This could also cause blow back through the carburettor.
What happens if the piston ring gap is too small?
If a piston ring gap is too small it means that the piston ring cannot retract and may seize or cause excessive wear.
References
http://www.wikipedia.com/
http://www.google.com/
Ed May Vol 1
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