Four-Stroke Diesel Engines
Introduction;- In this post on four-stroke diesel engines, we will discuss about the four stroke diesel cycle, difference between four stroke and two stroke and advantages –disadvantages of Four stroke engines.
A Four-Stroke Diesel Engines works on the four-stroke cycle. The four strokes are as under.
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Suction stroke
28 Deg before the movement of the piston in exhaust stroke, inlet valve starts to open and Intake stroke starts from this moment and Exhaust valve still in open condition. Flow of exhaust gases will create a suction effect and the fresh charges will start to suck in.
The piston continues its upward moment TDC; the fresh charges will enter and exhaust gases leave. Ta about 12 deg after TDC the exhaust valve will start to close. The exhaust valve closes fully at about 22 Deg AFTDC (After Top dead Centre) The time during inlet valve and exhaust valve are open is known as Valve overlap period. In this case valve overlap period will be 50 Deg.
The cool air keeps the engine parts cool.
After the piston passes Top dead centre, fresh charges keep coming in due to the suction effect in natural aspirated engine and with force in supercharged engines.
This stroke is known as induction/ suction stroke.
Compression stroke
The inlet valve starts to close at 35 deg after Bottom dead centre and it is in fully closed position 43 Deg after bottom dead centre. Charge air is at normal pressure and temperature. As the piston moves upward the pressure and temperatures keep rising.
Fuel injection starts at 10 to 20 Deg before TDC. Injection fuel gets ignited due to the ignition temperature.
In this stroke inlet valve closes and the upward movement of the piston compresses the air in the cylinder.
Power stroke
Just before the endcompression stroke, fuel is injected inside the cylinder. This fuel gets ignited due to the heat developed in the compression stroke. Cylinder pressure rises during the spread of combustion and forcing the piston downward in the Power stroke.
Exhaust stroke
- Piston moves upward in this stroke pushing the burnt gases through the open exhaust valve.
At the end exhaust valve closes and before the closing of the exhaust valve inlet valve opens. The four-stroke cycle begins again in repetition.
In four stroke engines camshaft rotates at half the speed of Crankshaft
In four-stroke engines, almost complete expulsion of burnt gases takes place due to the availability of full exhaust stroke.
The complete piston stroke in the induction stroke ensures the full induction of the air/fuel mixture. This is the reason for the production of more power than the two-cycle engine.
Another advantage of the four strokes is the simple and reliable lubrication system making it more popular as unattended manning of engine installation.
Internal combustion engines work on theoretical air standard cycle of duel combustion.
Duel combustion cycle is the mixture of constant pressure and constant volume diesel cycle.
Difference between Four-Stroke Diesel Engines and Two stroke engines
| Sr. No. | Four Stroke Engine | Two Stroke Engine |
| 1 | One cycle completes in two revolution of Crankshaft | One cycle completes in each rotation of crankshaft |
| 2 | There is one power stroke in every four stroke of piston | Power stoke in two stroke of piston |
| 3 | Due the valves and valve operating mechanism engine design is complicated | Design of Two stroke engine is simple due to the non requirement of valves and valve mechanism. |
| 4 | Fuel doesn’t need addition of lubrication. | Addition of lubrication is required in two stroke engines |
| 5 | Piston top is simple and flat | A Bump/ pro-turbulence is the requirement of piston top. |
| 6 | The fuel mixture is limited to the combustion chamber. | Air fuel mixture routes through the combustion chamber. |
| 7 | Four stroke engines are heavy | Two stroke engines are lighter comparatively. |
| 8 | Noise level is low in four stroke engines | Two stroke engines are noisy. |
Four-Stroke Diesel Engines advantages
Listing below the advantages of four strokes Engine
- Torque made by four stroke engines at low RPM is more than two stroke engines.
- Fuel efficiency of four stroke engines is higher than the two stroke engines, since fuel is consumed once in four strokes.
- Four stroke engines produce less pollution.
- Ife of four stroke engines is more than the two stroke engines due to the engine RPM.
- There is no extra requirement of lubricating oil in four stroke engines other than the lubrication of moving parts.
Four-Stroke Diesel Engines disadvantages
- Due to the valves and valves operating mechanism, design of four stroke engines is complicated. It becomes difficult to carry out the troubleshooting due to the more parts.
- Four stroke design engines are less powerful than the two stroke engines, since there is one power stroke i four strokes.
- Maintenance cost is expensive in four stroke engines due to the involvement of more parts.
Conclusion;- I hope the post on Four-Stroke Diesel Engines has been useful to the readers.!
Good Luck!!
