Comparison between two and four stroke engine.

Advantages: 

1)  The two-stroke cycle engine gives one working stroke for each revolution of the crankshaft. Hence theoretically the power developed for the same engine speed and cylinder volume is twice that of the four-stroke cycle engine, which gives only one working stroke for every two revolutions of the crankshaft. However, in practice, because of poor scavenging, only 50-60% extra power is developed. 

2) Due to one working stroke for each revolution of the crankshaft, the turning moment on the crankshaft is more uniform. Therefore, a two-stroke engine requires a lighter flywheel. 

3) The two-stroke engine is simpler in construction. The design of its ports is much simpler and their maintenance easier than that of the valve mechanism.  

4) The power required to overcome frictional resistance of the suction and exhaust strokes is saved, resulting in some economy of fuel. 

5) Owing to the absence of the cam, camshaft, rockers, etc. of the valve mechanism, the mechanical efficiency is higher. 

6) The two-stroke engine gives fewer oscillations. 

7) For the same power, a two-stroke engine is more compact and requires less space than a four-stroke cycle engine. This makes it more suitable for use in small machines and motorcycles.  

8) A two-stroke engine is lighter in weight for the same power and speed especially when the crankcase compression is used.  

9) Due to its simpler design, it requires fewer spare parts.  

10) A two-stroke cycle engine can be easily reversed if it is of the valve less type.

Disadvantages: 

1. The scavenging being not very efficient in a two-stroke engine, the dilution of the charges takes place which results in poor thermal efficiency. 

2. The two-stroke spark ignition engines do not have a separate lubrication system and normally, lubricating oil is mixed with the fuel. This is not as effective as the lubrication of a four-stroke engine. Therefore, the parts of the two-stroke engine are subjected to greater wear and tear. 

3. In a spark ignition two-stroke engine, some of the fuel passes directly to the exhaust. Hence, the fuel consumption per horsepower is comparatively higher. 

4. With heavy loads a two-stroke engine gets heated up due to the excessive heat produced. At the same time the running of the engine is not very smooth at light loads. 

5. It consumes more lubricating oil because of the greater amount of heat generated. 

6. Since the ports remain open during the upward stroke, the actual compression starts only after both the inlet and exhaust ports have been closed. Hence, the compression ratio of this engine is lower than that of a four-stroke engine of the same dimensions. As the efficiency of an engine is directly proportional to its compression ratio, the efficiency of a two-stroke cycle engine is lower than that of a four-stroke cycle engine of the same size.

Comparison of 2 and 4 stroke cycles. 

The present state of engine development appears to favor the two-stroke cycle for the large, slow-running, direct coupled engines and the four-stroke cycle engine for medium speed and high speed engines used for generators and indirect propeller drives 

The two-stroke engine gives an engine which: 

(i) Has high power/mass ratio 

(ii) Is easy to reverse 

(iii) Has a reduced number of moving parts and consequently requires less maintenance 

Whereas the 4-stroke engine: 

(i) Good volumetric efficiency 

(ii) Good combustion characteristics 

(iii) Positive exhaust scavenging 

(iv) Less arduous lubrication requirements due to idle strokes which allow spread of lubricating oil through bearings and around the liner. 

     Two Stroke Engine has the advantage of compactness in relation to the power output and requires not to increase brake mean effective pressure or the engine speed to increase rating. High bmep increases the stresses on nearly all the engine components, with a consequently greater rate of cylinder wear, whilst the  alternative of higher speed, valve flutter may become a serious problem. Each out-stroke being a working stroke gives more even turning for the same number 

of cranks, consequently a lighter flywheel may be employed. The operation of reversing the direction of rotation is simplified since there are not so many cylinder head valves to contend with. The followings are also the advantages of using two stroke engines: 

Fewer moving parts and lower maintenance.

Lower specific fuel consumption No gear loss.

Simplicity in construction. 

Longer life time Higher reliability.

Low lube oil consumption 

Better ability to burn low quality fuel oil. 

    Four Stroke Engine has the advantage of good volumetric efficiency, good combustion characteristics and positive exhaust scavenging. The thermal and mechanical efficiencies are slightly better than those of two stroke engine. In this type of engines only half the quantity of the heat generated in the cylinders has to be dealt with a given time, so that efficient lubrication of the piston and cooling of the cylinder is more easily accomplished. The followings are also advantages of  using four stroke engines: 

Lower initial cost for equivalent power 

Ease of installation 

Lower weight per power 

Saving in weight and engine room length 

Increase cargo capacity 

Free choice of propeller speed through gearing 

Suitable for electrical power take off. 

    Practically all large, slow-speed, direct drive marine diesel engines operate on the two-stroke cycle. As its name implies a two-stroke cycle takes place in two consecutive strokes of the engine piston, or one revolution of the crankshaft. Thus each operation in the cycle is repeated during every revolution of the engine. The two strokes of the cycle may be termed: Compression stroke and Power or Expansion Stroke. 

    The majority of medium and high speed diesel engines for main or auxiliary drive operate on the four-stroke cycle, which takes place during four consecutive strokes, or two complete revolutions, of the engine. Exhaust Stroke, and Aspirating or Air Induction stroke.

Difference between Connecting rods of 2 and 4 Engines

a. In service, swing movement of the con-rod sets up an inertia load in a transverse direction and the initial load depends upon mass and velocity.

b. In 2-s engines, the transverse inertia loading is not so much and the round section con-rod of normal length is strong enough to sustain the connecting gas loading in the slow speed engines.

c. In 4-s engines the transverse inertia is of such magnitude that the weight of the con-rod must be considered together with its strength to resist these loads. rectangular and I-section con-rods fulfill this requirement.

d. In 2-s engine, gas load on the piston is unidirectional. In single acting 2-s cycle engine the load from the end thrust on the con-rod is usually purely compressive.

e. In 4-s engines load reversal takes place at the end of the exhaust stroke. The end loading on the con-rod may be push-pull with con-rod being subjected to high compressive load in the push stage of power stroke and low tensile load in the pull stage of exhaust stroke.

In 4-s engines only the big end bearings are split, usually in an oblique direction to:- Reduce the big end with, lessen load on bolts and to increase crankpin diameter. Top end bearing may be a bush type.