Sectional View And Working of Two Stroke Engine

 

Chapter-1

INTRODUCTION

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). This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform simultaneously the intake and exhaust (or scavenging) functions. In this way, two-stroke engines often provide strikingly high specific power, at least in a narrow range of rotational speeds. The functions of some or all of the valves required by a four-stroke engine are usually served in a two-stroke engine by ports that are opened and closed by the motion of the pistons, greatly reducing the number of moving parts. Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications, such as chainsaws, and the concept is also used in diesel compression ignition engines in large and weight insensitive applications, such as ships and locomotives.

Invention of the two-stroke cycle is attributed to Scottish engineer Dugald Clerk, who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day (and Frederick Cock for the piston-controlled inlet port).

Chapter-2

ENGINE AND TRANSMISSION SPECIFICATIONS (SUZUKI MAX 1OO)

Engine
Type	2 Stroke, air cooled
Intake system	Reed valve
Bore	50.0 mm
Stroke	50.0 mm
Piston displacement	98.2 cc
Carburettor	Mikuni VM 18 SS
Lubrication System	Suzuki "CCI"
Compression ratio	6.7:1
Fuel consumption	Around 67 kmpl under standard specified test conditions (at a steady speed of 40 kmph with a payload of 130 Kgs)
Transmission
Clutch	Wet multi-plate type
Transmission	4 speed constant mesh
Gear  shift pattern	All Down, heel-toe shift

 

Chapter-3

 TWO STROKE CYCLE

Fig 3.1

 

A: intake/scavenging

B: Exhaust

C: Compression

D: Expansion(power

Two stroke cycle are very widely employed where small power required for motor cycle ,auto rickshaw ,scooters .This type of engines are compact in size, easy for manufacturing and simple in operations. One drawback is there, Specific Fuel Consumption (S.F.C) is more.(means fuel per Break Horse Power (b.h.p.)per hour is more).

There are no inlet and exhaust valves as in four stroke engine but we have inlet and exhaust ports only, due to which suction and exhaust stroke are
eliminated in two stroke cycle engine. Here the burnt exhaust gases are forced out through the exhaust port by a fresh charge of fuel which enters the cylinder nearly  at the end of working stroke through inlet port. This process is called as “Scavenging”. Details about Scavenging will be covered in another post.

As I told above, it has no valves but consists of the inlet port (IP),exhaust port(EP) and transfer port(TP).The ignition starts due to the spark given by spark plug when the piston be nearing the completion of its compression stroke. As a result, piston is pushed down performing the working stroke and in doing so, the air-fuel mixture already drawn from the inlet port in the previous stroke is compressed to a pressure of about 1.4 kilogram/centimeter square.

When 80% of this stroke is completed the exhaust port is uncovered slightly and some of the charge of burnt  gases escape to the atmosphere. As the exhaust port is uncovered by the further downward movement of the piston, the transfer port, which is slightly lower than exhaust port, is also uncovered and a charge of compressed air-fuel mixture enters the cylinder and further pushes out the burnt gases out of the exhaust port.

To facilitate the deflection of the fresh charge upward and to avoid its escape along with exhaust gases, the top of the piston is made of a particular shape.

From bottom dead centre, when the piston moves up, it first closes the transfer port and then exhaust port. The charge of fuel which previously entered in the cylinder is now compressed. When the piston is nearing the upward movement the inlet port opens and afresh charge of air-fuel mixture from the carburetor enters the crankcase. After the ignition of charge takes place the piston moves down for the power stroke and thus the cycle is repeated.

It can be fairly assumed that the crankcase and the bottom of the piston form a compressor assembly. In which the work used up in compressing the charge is negative. The top of the piston is considered as positive and crankcase is as negative. The difference between the positive and negative loop gives us the
net work available from engine.

3.1 MODE OF OPERATION OF TWO STROKE ENGINE

1^st stroke: The piston is at the bottom of the cylinder. A pipe at the left side is opened and lets the fuel mixture, which is already compressed a bit, flow from the lower to the upper part of the cylinder. The fresh gases expulse now the exhaust through an ejection pipe , which is not closed by the piston at this moment.

2nd stroke:  After being hurried upward, the piston now covers the pipe on the left side and the ejection pipe. Because there is no way out any more, the upper, fresh gas mixture gets compressed now. At the same time in the part below fresh gas is taken in by the piston driving upward through the open suction pipe. At the upper dead-center, the compressed fuel mixture is ignited by the sparking plug, the piston is pressed downward while he compresses at the same time the fresh gas below. The process begins again as soon as the piston arrives at its lowest point.

Fig 1.1

3.2 REED INLET VALVE

The reed valve is a simple but highly effective form of check valve commonly fitted in the intake tract of the piston-controlled port. They allow asymmetric intake of the fuel charge, improving power and economy, while widening the power band. They are widely used in ATVs and marine outboard engines.

Chapter-4

TRANSMISSION

The transmission systems in vehicles are installed to send signals from one part to another such as from engines to wheels to ensure mobility. In this process, several components that are fixed in the back of the engine in a car play integral part. There is limited variety we find in the transmission devices across the globe. However, some of the essential components of transmission system are modulator, torque converter, planetary gears, governor, computer, seals, output shafts, differential, hydraulic designs, axles and pressure plate.

Fig 4.1  Cut-away look at 4-Speed Overdrive transmission internals

4.1 CONSTANT-MESH GEARBOX

 

Fig 4.1  

            The main feature is the use of the stronger helical of double helical gears which lead to quieter operation. In this design, the main shaft pinions revolves freely on bushes or needle-roller bearings and are all in constant engagement with the corresponding lay shaft wheels.  The gear operation is obtained by locking the respective gear to the main shaft by means of a dog clutch. The layout of the box is shown in the figure.

With this arrangement the quieter-running helical gears can be employed, and during gear changing the noise and wear are reduced by the simultaneous engagement of all the dogs instead of only a pair of gear teeth as on the sliding-mesh gearbox.

With single helical pinions (double helical is economically impractical), the driving loads on the teeth cause an axial thrust which must be resisted by thrust washers, or shoulders, on the main shaft.

4.2. CLUTCH

A clutch is a mechanical device which provides for the transmission of power (and therefore usually motion) from one component (the driving member) to another (the driven member). The opposite component of the clutch is the brake.

Clutches are used whenever the ability to limit the transmission of power or motion needs to be controlled either in amount or over time (e.g. electric screwdrivers limit how much torque is transmitted through use of a clutch; clutches control whether automobiles transmit engine power to the wheels).

4.3 WET TYPE MULTI-PLATE CLUTCH

1.      A wet type multi-plate clutch comprising a first friction engaging element coaxially arranged within a clutch housing, a second friction engaging element alternately disposed with  first friction engaging element, and a piston for applying an axial load to engage  first and second friction engaging elements with each other,  piston being provided in  clutch housing and at a side of a closed end of  clutch housing, wherein a projection having an axially extending pawl portion and axially protruding toward  piston is provided on  clutch housing and a recessed portion is provided in  piston so that a relative rotation between  clutch housing and  piston is prevented by engaging  pawl portion with  recessed portion.

2.      A wet type multi-plate clutch according to claim 1, wherein projection is provided with a distal end capable of engaging with recessed portion.

3.       A wet type multi-plate clutch according to claim 2, wherein   distal end is covered by resin material.

4.      A wet type multi-plate clutch according to claim 1, wherein  projection is integral with the closed end of the clutch housing

5.       A wet type multi-plate clutch comprising a first friction engaging element coaxially arranged within a clutch housing, a second friction engaging element alternately disposed with  first friction engaging element, and a piston for applying an axial load to engage  first and second friction engaging elements with each other,  piston being provided in  clutch housing and at a side of a closed end of  clutch housing, wherein a projection having an axially extending pawl portion and axially protruding toward  piston is provided on the closed end of  clutch housing and a recessed portion is provided in  piston so that relative rotation between  clutch housing and  piston is prevented by engaging  pawl portion with  recessed portion.

6.      A wet type multi-plate clutch according to claim 5, wherein the projection is annular and there are a plurality of pawl portions spaced circumferentially along the annular projection and there are a plurality of the recessed portions engaged by respective pawl portions.

7.      A wet type multi-plate clutch according to claim 5, wherein  projection is integral with the clutch housing.

Chapter-5

ADVANTAGES OF 2 STROKE ENGINE

1.      Two stroke cycle engines is more compact, requires less space and lighter in weight than four stroke cycle engine for same output or power. Power developed per kilogram of engine weight is higher in two stroke cycle engine. That's why it is more suitable for motor bike and scooters.

2.      The two stroke cycle engine gives one working stroke for each revolution of crankshaft. Hence theoretically the power developed for the same engine
speed and cylinder volume is twice is that of four stroke cycle engine which gives one working stroke for every two revolution of the crankshaft.

3.      We can use lighter flywheel in two stroke cycle engine as their is one working stroke for each revolution of crankshaft.

4.       The design and construction is much simpler than four stroke engine that's why it is easy to manufacture and cost effective.

5.      As there is no suction and exhaust valve used in two stroke cycle engine hence no need of cams, camshaft and rockers etc. of the valve gear .Resulting in higher mechanical efficiency.

6.      Due two suction and exhaust ports only, there is work saved avoiding friction.

7.      Two stroke cycle engine gives less torsional oscillations.

8.       This type of engine requires less spare parts.

9.      A two stroke cycle engine can be easily reversed if it is of valve less type.

10.  Due to simplicity in construction and fewer parts, two stroke cycle engine is easy for maintenance. Hence reduce the initial and maintenance cost also.

Chapter-6
DISADVANTAGES OF 2 STROKE ENGINE

1.      The fuel consumption is higher in case of two stroke engine working on Otto cycle .This happens due to some of the fuel or unburnt gas likely to be wasted by escaping through the exhaust port.

2.      Since the ports remains open during up word stroke, the actual compression starts only after both inlet and exhaust ports have been closed. So actual compression ratio and the thermal efficiency of the two stroke cycle engine is less than that of four stroke engine of the same dimension.

3.      The dilution of charges takes place due to incomplete scavenging.

4.      Two stroke engines consumes more lubricating oil.

5.      There is greater wear and tear of moving parts hence it gives loud noise.

Chapter-7

APPLICATION OF 2 STROKE ENGINE

The two-stroke engine was very popular throughout the 20th century in motorcycles and small-engined devices, such as chainsaws and outboard motors and was also used in some cars, a few tractors and many ships. Part of their appeal was their simple design (and resulting low cost) and often high power to weight ratio. Many designs use total-loss lubrication, with the oil being burned in the combustion chamber, causing "blue smoke" and other types of exhaust pollution. This is a major reason for two-stroke engines being replaced by four-stroke engines in many applications.

Two-stroke engines continue to be commonly used in high-power, handheld applications such as string trimmers and chainsaws. The light overall weight, and light-weight spinning parts give important operational and even safety advantages. For example, only a two-stroke engine that uses a gasoline-oil mixture can power a chainsaw operating in any position.

These engines are still used for small, portable, or specialized machine applications such as outboard motors, high-performance, small-capacity motorcycles, mopeds, under bones, scooters, tuk-tuks, snowmobiles, karts, ultra lights , model airplanes (and other model vehicles) and lawnmowers. The two-stroke cycle is used in many diesel engines, most notably large industrial and marine engines, as well as some trucks and heavy machinery.

Chapter-8

CONCLUSION

From the analysis of above study we here by conclude that two-stroke engine is less used now a days because of the unavoidable disadvantage. Even though two stroke engine produce higher power and lighter in weight as compared to four stroke engine the disadvantages are more noticed.

             From the sectional view of two stoke engine we can develop a clear idea of piston movement, positioning of clutch, fly wheel, gearbox, gear shaft, dynamo, spark plug, intake and exhaust port.