Components of an Engine;
Even though reciprocating internal combustion engines look quite simple, they are highly complex machines. There are hundreds of components that have to perform their functions satisfactorily to produce output power. There are two types of engines, viz., spark ignition (S1) and compression-ignition (CI) engine. Let us now go through the important engine components and the nomenclature associated with an engine.
Terms connected with i.c. engines;
· Bore: The inside diameter of the cylinder is called bore
· Stroke: The linear distance along the cylinder axis between two limiting position s is called stroke.
· Top Dead Center ( T.D.C.) : the top most position of the piston towards cover end side of the cylinder is called T.D.C.
· Bottom dead Center ( B.D.C.) : The lowest position of the piston towards the crank end side of the cylinder is called B.D.C.
· Clearance Volume : The volume contained in the cylinder above the top of the piston , when the piston is at top dead center , is called the clearance volume.
· Swept Volume: The volume swept through by the piston in moving between T.D.C. and B.D.C, is called swept volume or piston displacement.
· Compression Ratio: It is the ratio of Total cylinder volume to clearance volume
Definition of ‘Engine’
An engine is a device, which transforms one form of energy into another form.
Normally, most of the engines convert thermal energy into mechanical work and therefore they are called ‘heat engines’.
The major components of the engine and their functions are briefly described below.
The cylinder block is the main supporting structure for the various components. The cylinder of a multicylinder engine is cast as a single unit, called cylinder block. The cylinder head is mounted on the cylinder block.
The cylinder head and cylinder block are provided with water jackets in the case of water-cooling with cooling fins in the case of air-cooling. Cylinder head gasket is incorporated between the cylinder block and cylinder head. The cylinder head is held tight to the cylinder block by number of bolts or studs. The bottom portion of the cylinder block is called crankcase. A cover called crankcase, which becomes a sump for lubricating oil is fastened to the bottom of the crankcase. The inner surface of the cylinder block, which is machined and finished accurately to cylindrical shape, is called bore or face.
As the name implies it is a cylindrical vessel or space in which the piston makes a reciprocating motion. The varying volume created in the cylinder during the operation of the engine is filled with the working fluid and subjected to different thermodynamic processes. The cylinder is supported in the cylinder block.
It is a cylindrical component fitted into the cylinder forming the moving boundary of the combustion system. It fits perfectly (snugly) into the cylinder providing a gas-tight space with the piston rings and the lubricant. It forms the first link in transmitting the gas forces to the output shaft.
The space enclosed in the upper part of the cylinder, by the cylinder head and the piston top during the combustion process, is called the combustion chamber. The combustion of fuel and the consequent release of thermal energy results in the building up of pressure in this part of the cylinder.
The pipe which connects the intake system to the inlet valve of the engine and through which air or air-fuel mixture is drawn into the cylinder is called the inlet manifold.
It forms the link between the small end of the connecting rod and the piston.
The pipe that connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere is called the exhaust manifold.
Inlet and Exhaust Valves
Valves are commonly mushroom shaped poppet type. They are provided either on the cylinder head or on the side of the cylinder for regulating the charge coming into the cylinder (inlet valve) and for discharging the products of combustion (exhaust valve) from the cylinder.
It interconnects the piston and the crankshaft and transmits the gas forces from the piston to the crankshaft. The two ends of the connecting rod are called as small end and the big end. Small end is connected to the piston by gudgeon pin and the big end is connected to the crankshaft by crankpin.
It converts the reciprocating motion of the piston into useful rotary motion of the output shaft. In the crankshaft of a single cylinder engine there is pair of crank arms and balance weights. The balance weights are provided for static and dynamic balancing of the rotating system. The crankshaft is enclosed in a crankcase.
Piston rings, fitted into the slots around the piston, provide a tight seal between the piston and the cylinder wall thus preventing leakage of combustion gases
The camshaft and its associated parts control the opening and closing of the two valves. The associated parts are push rods, rocker arms, valve springs and tappets. This shaft also provides the drive to the ignition system. The camshaft is driven by the crankshaft through timing gears.
These are made as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open for the necessary duration.
The net torque imparted to the crankshaft during one complete cycle of operation of the engine fluctuates causing a change in the angular velocity of the shaft. In order to achieve a uniform torque an inertia mass in the form of a wheel is attached to the output shaft and this wheel is called the flywheel.
Basic Parts of the Gasoline Engine:
Basic Parts of the Gasoline Engine are listed below;
· Cylinder block
· Piston rings
· Piston pin
· Connecting rod
· Cylinder head
· Intake valve
· Exhaust valve
· Timing gears
· Spark plug
Cylinder Block Basic frame of gasoline engine. Contains the cylinder.
Piston A sliding plug that harnesses the force of the burning gases in the cylinder.
Piston rings seal the compression gases above the piston keep the oil below the piston rings.
Piston Pins Also known as the wrist pin, it connects the piston to the small end of the connecting rod. It transfers the force and allows the rod to swing back and forth.
Connecting Rod Connects the piston and piston pin to the crankshaft.
Crankshaft Along the the piston pin and connecting rod it converts the up and down motion (reciprocating) of the engine to spinning (rotary) motion.
Flywheel Carries the inertia when there is no power stroke.
Cylinder Head Forms the top of the combustion chamber. Contains the valves, the passageways for the fuel mixture to move in and out of the engine.
Intake and Exhaust Valves:
Intake and Exhaust Valves Doorway that lets the gases in and out of the engine.
Camshaft Through the use of an eccentric the cam lobes push the valves open. The valve springs close them.
Timing Gears These gears drive the camshaft from the crankshaft.
Why not diesel engines are not preferred in commercial ?:
1. Diesel engines, because they have much higher compression ratios (20:1 for a typical diesel vs. 8:1 for a typical gasoline engine), tend to be heavier than an equivalent gasoline engine.
2. Diesel engines also tend to be more expensive.
3. Diesel engines, because of the weight and compression ratio, tend to have lower maximum RPM ranges than gasoline engines .This makes diesel engines high torque rather than high horsepower, and that tends to make diesel cars slow in terms of acceleration.
4. Diesel engines must be fuel injected, and in the past fuel injection was expensive and less reliable
5. Diesel engines tend to produce more smoke.
6. Diesel engines are harder to start in cold weather, and if they contain glow plugs, diesel engines can require you to wait before starting the engine so the glow plugs can heat up.
7. Diesel engines are much noisier and tend to vibrate.
8. Diesel fuel is less readily available than gasoline
Advantages diesel engines:
The two things working in favor of diesel engines are better fuel economy and longer engine life. Both of these advantages mean that, over the life of the engine, you will tend to save money with a diesel.
However, you also have to take the initial high cost of the engine into account. You have to own and operate a diesel engine for a fairly long time before the fuel economy overcomes the increased purchase price of the engine.
The equation works great in a big diesel tractor-trailer rig that is running 400 miles every day, but it is not nearly so beneficial in a passenger car.