Main Engine Parts and Functions
1. CYLINDER Block: (Refer Fig 1)
The cylinder block is the main support for the other basic engine parts. The block is usually a one piece casting made from cast iron. Blocks are cast with and walls and centre webs to support the crankshaft and camshaft and have enlargement in their walls to make room for oil and coolant passages. The cylinders are often cast into the block and may be bored into the block or machined to receive replaceable liners.
2. Camshaft: (Refer Fig 2 & Fig 3)
Camshaft for small and medium size engines are generally made of a one piece casting or forging. One intake and one exhaust cam provided for, each cylinder, along with bearing journals. The journal diameters are large to permit removal of the shaft from the bore. The crankshaft is normally driven by gearing from crankshaft.
How the cams are arranged on the shaft determines the firing order of the engine. The contour of each cam decides the time and rate of opening of each valve. Camshafts are made of a lowcarbon alloy steel with the cam and journal surfaces carburised before finish grinding.
Some high speed engines use alloy cast-iron camshafts with hardened cams and journals. Overhead camshafts for car engines are supported on pedestals mounted on the cylinder head with removal caps or else in a tunnel formed in the valve housing.
Camshafts may also drive oil pumps, fuel pumps and distributors using extra lobes for a gear on the shaft.
Camshaft Timing: (Refer Fig 4)
On 4 cycle engines, the camshaft turns at one half, the speed of the crankshaft, so that each valve is opened and closed once during two revolutions of the crankshaft.
The exhaust valve should open before the end of the power stroke and close after the completion of the exhaust stroke.It should open before the end of the power stroke because due to the angularity of the connecting rod and crankshaft,the pressure in the cylinder near the end of power stroke has very little effect in turning the crankshaft. The valve should close after the end of the exhaust stroke to permit better scavenging due to the enertia of the outflowing gases in the exhaust manifold.
The intake valve should open before the end of the exhaust stroke and close after the end of the intake stroke. The intake valve opens before the end of the exhaust stroke to take advantage of the inertia of the outflowing gases in the exhaust manifold. The valve should close after end of the intake stroke to take advantage of the ramming effect, due to enertia, of the incoming air or air-fuel mixture, therby more completely filling the cylinder. To Summarise:
1. Exhaust valve opens before end of power stroke, closes after end of exhaust stroke.
2. Intake valve opens before end of exhaust stroke, closes after end of intake stroke.
The intake valve opens at 10 degrees before top dead centre (TDC). It stays open until 50 degrees after BDC. During this time the engine has taken in its full charge of air-fuel and the piston has started its compression stroke. The fuel is then compressed and ignited, and the power stroke begins. When the piston reaches a point 50 degrees before bottom dead centre (BDC), the exhaust valve opens and stays open until the piston has traveled to 10 degrees beyond TDC, when it closes.
3. Main Bearing: (Refer Fig 5) All the major wear and load points in an engine use bushings or bearings to reduce friction. Let’s define the two words:
1. Bushing – small full round sleeve, pressed in, for lighter loads or slower speed.
2. Bearing – full round or halves, for heavier loads and higher speeds. Bushing are used at the piston end of the connecting rod, rocker a oil pump, etc. Bearings are used at crankshafts, main journals, connecting rod journals, camshafts, etc.
4. Flywheel: (Refer Fig 5) The flywheel does 3 things. for the engine:
1. Store energy for momentum between power strokes.
2, Smooths out speed of crankshaft.
3. Transmit power to the machine. Mounted on the rear of the crankshaft, the heavy flywheel is a stabiliser for the whole engine.
In a 4 cycle engine, the flywheel must be heavy enough to turn the engine during the exhaust, intake and compression strokes. At the same time it must transmit power to the driven machine.
The more cylinders in the engine, the less needed for a flywheel. This is because the power impulses are closer together and the engine has more momentum of its own.
Flywheel may also have two other jobs:
1. Provide a drive from the starting motor via the ring gear.
2. Serve as a facing for the engine clutch.
Next article we will discuss further on the topic Main Engine Parts and Functions.