2 stroke Engines

1.      Explain how a two stroke engine works?

A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft (an up stroke and a down stroke of the piston, compared to twice that number for a four-stroke engine).

2.      What is happening below and above the piston?

In a 2-stroke gasoline engine the events occurring below the piston are: intake of the mixture into the crankcase and its pre-compression. The events occurring above the piston are: compression of the mixture, combustion, and scavenging of exhaust gases.

3.      Explain what is meant, by scavenging when applied to 2 stroke SI engines?

In two stroke engine, each outward stroke of the piston is a power stroke. To achieve the operating cycle, the fresh charge must be supplied to the engine cylinder at a high- enough pressure to displace the burned gases from the previous cycle. Raising the pressure of the intake mixture is done in a separate pump or blower or compressor. The operation of clearing the cylinder of burned gases and filling it with fresh mixture (or air) – the combined intake and exhaust process – is called Scavenging.

4.      Why must roller and ball race bearings be used on 2 stroke SI engine crankshafts?

The two-stroke engine requires a needle roller bearing because the big end has less lubricating oil due to the structure. The needle rollers held in the retainer are inserted into the big end and run on the outer raceway of the big end and the inner raceway of the crankpin. The roller itself receives high stress and also exerts high  stress on the rolling surfaces.

5.      Why are rollers caged in crankshaft bearings?

Cages support high centrifugal and acceleration forces and are suitable for high speeds. They give bearing arrangements with high run out accuracy.

6.      Why must ‘split’ type bearings be used on one piece crankshafts?

A split (two halves) bearing is used on one piece crankshafts because the bearing must be assembled around the journal with the bearing housing being of two parts also, including a cap that holds the assembly together.

7.      Why do we have pins between the piston ring gaps on a 2 stroke?

We have a gap and pin at the centre because when the piston expands and contracts, the pin is used to align the ring at the centre

8.      How does a reed valve work on a two stroke?

Reed valve acts as a one way valve .It lets the fuel and air mixture into the crank case because of the low atmospheric pressure. As the piston travels down, the air pushes the reed valve to close acting as a one way valve.

9.      How do you check a reed valve? 

Remove the reed cage and examine the valves carefully exposing to the sunlight. If the corners are chipped or cracked, then they need to be replaced.

10.  Explain the term ovality and taper and how are they formed?

Ovality: It is a condition where circular object is deformed to an oval shape. Usually, ovality occurs on the Thrust side of a piston.

Taper: Taper means cylinder will be wide enough to accept both piston and rings. As you push piston upward, decreasing taper will squeeze rings to fit cylinder

11.  How does the piston ring seal in the bore?

Because of the gap in the piston ring, the piston ring expands and contracts during the power stroke and the exhaust stroke. During the expansion it can seal in the bore.

12.  What is meant by the terms groove depth, side clearance and end gap on the pistons rings?

Groove Depth: The distance between the cylinder walls with the piston centred and the back of the groove of the ring is known as Groove Depth.

Side Clearance: Clearance between the ring and the ring groove is known as Side Clearance.

End Gap: When fitting new piston rings or breaking them in within an engine, the end gap is a crucial measurement. In order that a ring may be fitted into the "grooves" of the piston, it is not continuous but is broken at one point on its circumference

13.  Explain the difference between a 2 stroke and a 4 stroke engine?

2 stroke engine

4 stroke engine

In a 2-stroke engine, the last three strokes are combined into one. 1.      Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel.) 2.      Power Stroke. The fuel is ignited and drives the piston down. As the piston goes down it not only turns the crankshaft but also pressurizes the fuel/air mixture in the crankcase which is about to be admitted to the piston for the next cycle. Near the bottom of the stroke, an outlet opens and the exhaust gases are released. Even nearer the bottom of the stroke, an inlet opens and new fuel/air rushes in ready for compression.

In a 4-stroke engine the sequence can be written: 1.      Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel). 2.      Power Stroke. The fuel is ignited (by a spark in a gas engine, by being injected into high temperature air in a diesel.) The energy released drives the piston down. This provides the momentum necessary to keep the crankshaft turning and make the other three strokes happen. 3.      Exhaust Stroke. The piston goes up and pushes the burned gases out the exhaust valve. 4.      Intake Stroke. The piston goes down and draws in new air, or fuel/air mixture, ready for the next compression stroke.

14.  What is meant by each of the following terms:

(a)   Mechanical Efficiency

It is the performance of the machine and it is known as a useful power output divided by the total consumed electrical power.

(b)   Thermal Efficiency

It is reciprocal to the actual heat energy stored within the fuel and the power produced in the engine.

(c)    Volumetric Efficiency

It is the percentage of the quantity of the fuel and air which enters the cylinder during induction to the actual capacity of the cylinder under static conditions.

15.  List 2 methods of lubricating the internal parts of 2 stroke S.I. engines?

2 stroke S.I. engines have a dry slump (except diesel).Henceforth, right amount of oil is needed to be mixed according to the specifications of the manufacturer.

OR

Another method is by premixing gasoline with oil.

16.  What happens if the piston gap is too big?

If the piston gap is too big, it can cause a leak in compression resulting lack of power.

17.  What happens if the piston gap is too small?

If the piston gap is too small, it can cease the piston during expansion and contraction or might result in breakage of the piston rings.

18.  How does a 2 stroke diesel engine work?