I. C. Engine Parts

 I. C. ENGINE PARTS

The principal parts and functions of a Four Stroke I.C. Engine are shown in Fig. 1(ii).

1. Engine Cylinder: The heart of the engine is the cylinder in which fuel is burnt and power is developed. The cylinder allows the piston to move to and fro. Combustion of fuel takes place inside the cylinder. The cylinder has to withstand a high pressure (more than 500 NI sq.cm.) and temperature (around 1500°C to 2000°C).

2. Cylinder Head: The cylinder is closed by the cylinder head at one end and the other end is covered by the moving piston. The cylinder head contains inlet and exhaust valves for admitting fresh charge and for exhausting the burnt gases.

In petrol engines, the cylinder head also contains a spark plug for igniting the fuel-air mixture. But in diesel engines, the cylinder head contains nozzle for injecting the fuel into the cylinder.

3. Piston: Piston is sliding within the cylinder. This sliding movement changes the volume of the cylinder and provides the combustion space. The space formed between the cylinder head and top of the piston during the process of combustion is known as Combustion Chamber. Piston transmits the force exerted by the burning of the charge to the connecting rod.

4. Piston Rings: Piston rings are circular rings used to maintain a pressure tight seal between the moving piston and the cylinder wall.

5. Piston Pin (also known as Gudgeon Pin): A Piston Pin (Gudgeon Pin ) connects the piston to the small end of the connecting rod.

6. Connecting Rod: As the name suggests, the connecting rod connects the piston with the crank. It is attached to the piston by piston pin. It converts the up and down motion (reciprocating motion) of the piston to a rotary motion of the crankshaft.,

7. Valves: Valves are needed to let the air and fuel into the cylinder (Intake Valve) and also to let out the burnt or spent gases after they have done their work (Exhaust Valve). Valves are operated by cams, rotated by a camshaft, driven by the crankshaft. [However, two stroke cycle engines have only ports at the cylinder walls and have no valves.]

8. Crank and Crankshaft: Crank is a lever. It is connected to the end of the connecting rod by a pin joint. Its other end is connected to a shaft called Crankshaft. It is the rotating member of the engine. Its function is to convert the reciprocating motion of the piston into a rotary motion with the help of the connecting rod.

9. Crank Case: The main body of the engine which contains the crank and crankshaft is known as Crank Case. It serves as sump for the lubricating oil.

10. Flywheel: It is a heavy wheel, mounted on the crankshaft. Its function is to store the excess energy during power stroke of the engine and help the movement of the piston during the remaining idle strokes, thus maintaining uniform rotation (speed) of the crankshaft.

1. I.C. ENGINE TERMINOLOGY (Fig. 1]

1. Cylinder Bore: The inside diameter of the engine cylinder is known as Cylinder bore.

2. Stroke or Stroke Length: It is the linear distance through which the piston moves inside the cylinder during one stroke. In other words, stroke length is the distance between the extreme upper and lower positions of the piston. Numerically, the stroke length is equal to twice the crank radius.

3. Top Dead Center (TDC) or Inner Dead Center (IDC): Top Dead Center (TDC) in the vertical engine is the extreme position of the piston on the top of the cylinder (head side). At this position, piston motion reverses direction and the cylinder volume is at a minimum.

In the case of horizontal engine, this position is known as Inner Dead Center (IDC). At TDC or IDC, the crank angle is zero.

4. Bottom Dead Center (BDC) or Outer Dead Center (ODC): Bottom Dead Center (BDC) in vertical engine indicates the extreme position at the bottom of the cylinder. At this position, the piston motion reverses direction and the cylinder volume is at a maximum.

In the case of horizontal engine, this position is known Outer Dead Center (ODC). At BDC or ODC, the crank angle is 180^o.

5. Compression Ratio: It is a ratio of the volume when the piston is at bottom dead center to the volume when the piston is at top dead center.

Compression Ratio = Maximum cylinder volume / Minimum cylinder volume

6. Stroke Volume or Swept Volume or Displacement Volume: It is the volume generated by piston movement in one stroke from one dead center to other.

Swept Volume = π/4 D² × L (D = cylinder bore and L = piston stroke length )

7. Clearance Volume: The volume contained in the cylinder above the top of the piston, when the piston is at TDC is called Clearance Volume.

Thus, when the piston is at BDC, total volume = Swept Volume + Clearance Volume.

8. Air-Fuel Ratio: This is expressed as a ratio of the mass of air to the mass of the fuel.

9. Indicated Horse Power (I.H.P.): It is the power produced within the engine cylinder. It is called indicated power as it can be measured with the help of an Indicator. Indicator is an instrument that draws pressure-volume diagram for the engine.

10. Brake Horse Power (B.H.P.): This is the net output of an engine. It is called Brake Power, since it can be measured by absorbing the power with a brake system. (The brake system consists of a brake pulley mounted on the engine shaft.)

11. Friction Horse Power (F.H.P.): It is the difference between I.H.P. and B.H.P. This is the power absorbed by the moving parts of the engine (piston bearings, etc.,).

12. Thermal Efficiency: It is the ratio of work done inside the engine cylinder (I.H.P.) to the fuel energy supplied to the engine.

13. Mechanical Efficiency: Mechanical Efficiency of an I.C. engine is defined as the ratio of power delivered (B.H.P.) to the power provided to the piston (I.H.P.).

Mechanical Efficiency = B.H.P./ I.H.P.

14. Brake Thermal Efficiency: It is defined as the ratio of energy in the B.H.P. to the fuel energy.

Brake Thermal Efficiency.= B.H.P./ Fuel Energy

i.e., Brake Thermal Efficiency= Thermal Efficiency × Mechanical Efficiency.

15. Volumetric Efficiency: The engine output is limited by the maximum amount of air taken during the suction stroke, because only a certain amount of fuel can be burnt effectively with a given quantity of air. Volumetric Efficiency is an indication of the breathing ability of the engine. It is defined as the ratio of the air actually indicated to the swept volume of the engine.

Volumetric Efficiency = Mass of charge actually induced / Mass of charge represented by cylinder volume