Let's know about one of the most successful engines - the four Stroke Engine. Get to know of its working and the processes comprised in this type of Engine.
About four stroke internal combustion engine:
A very common type(or variation) of Internal Combustion Engine is the 4-Stroke Engine. Most modern Vehicles on Road are powered by a Stroke Engine whether by Gasoline(or Petrol) or Diesel. These Engines are more powerful, efficient and reliable.
The 4 denotes the number of events that happen in one combustion cycle in every cylinder. The events overall comprise the upward and downward motion of Piston. This reduces the amount of Carbon Monoxide(CO) significantly by not mixing oil with fuel. Further due to better combustion, the amount of unburnt Carbon being released in the environment reduces. These amazing features present in this Engine justifies its popularity amongst manufactures.
Strokes of a 4-Cycle Engine (Working):
The working of this engine requires high precision to set up correct synchronisation of Camshaft and the timing belt(or chain) to achieve the best results. These days camshaft’s rotation is aided by computer for best results and maximum power output. Also in most of the modern cars, there are either 3 or 4 valves(2 intake 1 exhaust or 2 each). The camshaft is designed in such a manner that when the car is at low speeds it opens just one valve to prevent high Torque while at high speeds both of them work together to achieve high power. Timing belt(or chain) is required in an overhead camshaft set up which is the most common orientation, even though direct roation of camshafts are also used but generally for higher power commercial vehicles. The 4 events in the four stroke internal combustion engine are:
Intake Stroke
Compression Stroke
Power Stroke
Exhaust Stroke
Intake stroke
The piston moves down the cylinder bore from the top dead centre (TDC) to the bottom dead centre (BDC). This creates the pressure difference above piston in the cylinder.
The intake valve(s) is/are open with camshafts rotation and the exhaust valve(s) is/are closed.
Downward piston motion creates a vacuum (negative air pressure) that draws that air/fuel mixture into the engine via the open intake valve.
Compression stroke
The piston moves up the cylinder bore from the bottom dead centre to the top dead centre.
Both the intake and exhaust valves are closed at this point.
Upward piston motion compresses air/fuel mixture in the combustion chamber and increases the temperature tremendously.
Power stroke
At the end of the compression (previous) stroke, the high rise in temperature ignites the air/fuel mixture(in diesel engines) or a spark plug ignites the mixture(in petrol or gasoline engines). This leads to combustion of fuel and high amounts of energy is released.
This forces the piston down the cylinder bore from top dead centre to bottom dead centre causing rotation of the Crankshaft.
Both the intake and exhaust valve are closed at this point also.
Exhaust stroke
The piston moves up the cylinder bore from the bottom dead centre to the top dead centre. The momentum caused by the power stroke is what continues the crankshaft movement and the other 3 strokes consecutively. This is aided by weights attached to Crankshaft at each piston to store its inertia. Also a flywheel attached to Crankshaft’s End, which prevents sudden surge or depression in power by maintaining Moment of Inertia. The revolution of Engine is counted by the rotations of crankshaft in a minute.
The intake valve(s) is/are closed, the exhaust valve(s) are open.
This final stroke forces the spent gasses/exhaust out of the cylinder. The cycle is now complete and the piston is ready to begin the intake stroke.
Air is compressible. When the air and fuel is compressed before ignition, the combustion efficiencies are improved significantly. This is the reason for higher efficiency of a diesel engine. Compression ratio is the relationship of total cylinder volume to the compressed volume. For example, a compression ratio of 10:1, would indicate that the chamber squeezes 10 parts of air and fuel volume into 1 part of the mixture's volume at the end of the compression stroke.
A higher compression ratio can offer more power in some applications. The downsides to a higher compression ratio can be decreased durability and the need to run higher-octane (expensive) gasoline. The engines also have anti freeze to cool the Engine. The tubes supplying this fluid extracts the heat from the chambers and takes it to the Radiator which has many fins or cool the fluid and make it ready for use again. The inbuilt fans further aids this process. If this fluid becomes too hot then fractions of the amount is replaced by the storage tank to distribute heat evenly. This is controlled by electronics and a thermostat and incase it gets too hot it warns the driver for a rest.
Engine Oils are used to extract heat and lubricate the cylinder to prevent wear and tear and also cool the cylinder chamber. This is stored in an oil tank and is pumped through small tubes.
The fuel tank is located at the rear end of the car and is supplied via pressure tubes to the front where it is filtered and then mixed with fuel.
Also the Engine is tuned in such a manner that all the pistons don’t undergo same stoke at a time. In a 4 inline cylinder the order goes as 4 2 3 1 for the power stroke and hence providing the car with continuous energy.
2-Stroke vs 4-Stroke:
“The smaller, lighter and more powerful, 2-Stroke, is less preferred”
The differences in the 2 and the 4-stroke Engines are listed below:
No Valves present in 2 Stroke:
The 2-stroke engine uses ports instead of Valves for flowing air, fuel, oil and releasing exhaust.
Each piston pressurises 2 chambers- The airspace in the Piston and the main cylinder simultaneously.
The absence of valves in the 2-Stroke Engines eliminates the Camshaft, therefore, reducing the size and weight.
Mixing of Oil and Gas takes place in the former:
This process has its own benefits and drawbacks.
Benefits:
There is no need for a separate oil lubrication reservoir.
The oil keeps on lubricating the chamber in each cycle hence reducing the wear and tear.
2. Drawbacks:
The mixing of fuel and oil lowers the oil's viscosity to lubricate better.
This mixture leads to inefficient combustion of fuel leading to smoke, hence causing pollution.
Delivers more power than the latter:
The 2-Stroke Engine provides a power pulse twice as often as the 4-Stroke. This could result in the generation of twice the power of a 4-Stroke Engine of the same displacement.
This also increases the Torque produced and hence producing better acceleration and more engine revolutions.
Disadvantages of a 2-Stroke Engine:
Less fuel Efficiency is achieved through this type of Engine.
This Engine is noisier because of higher revolutions and fewer chances of Damping.
The Engine also vibrates more compared to a 4-Stroke But that doesn't make much difference.
The 2-Stroke Engines are much more polluting because of unburnt Carbon and burning of Oil
Get to know more about the structure and working of the internal combustion 4-stroke Engines:
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