Engine {Week 1}

How does a four stroke engine work?

In Automobile engines, there is a series of events which take place, ‘the four-stroke cycle’. A stroke is one complete down or one complete up movement of the piston. There are two downstrokes and two upstrokes for the internal combustion engine. A cycle is a round of events, which occurs in a certain fixed order. There are four events in the automobile-engine cycle. These four events correspond to the four strokes.

On the first downstroke, the first event or operation is the drawing in of air and
fuel. This is called the Intake Stroke.

The second operation is compressing the air and fuel. The compression occurs on the upstroke
of the piston. The mixture is fired at the end of this stroke.

The piston is forced downward on the second downstroke. On the second upstroke, it drives
the burned gases before it, and they pass out of the cylinder. The order, then,
of the four strokes of the cycle is this: First downstroke, intake; first
upstroke, compression; second downstroke, power ; second upstroke, exhaust. The
cycles occur as follows: Intake, compression, power, exhaust, over and over
again..

The camshaft carries two cams for each cylinder. It turns once for each four
strokes of the piston. Each cam has a valve lifter resting on it. Each cam lifts
the lifter resting on it, once for each full revolution of the camshaft. Each
valve is thus lifted once for each four strokes of the piston or two
revolutions of the crankshaft. The time at which each valve is lifted is called
"engine or valve time."

The intake valve must start to open as intake (first down-stroke) starts. It must close
when compression (first upstroke) starts. Both valves must remain closed during
compression and power (first upstroke and second downstroke). The exhaust valve
must open at the beginning of the exhaust stroke and remain open to the finish
of this, the second upstroke. The intake valve opens on the first stroke to let
the fuel charge in. The exhaust valve opens on the fourth stroke to let the
burned gases out.

http://www.carbasics-1950.com/four-stroke-engine.htm

What is taper and ovality in the bore, how is it caused how do you check it?

A Ball gauge is used to check
for taper and ovality

Taper is the shape of the cylinder bore which starts of wide at the bottom and narrows on the way up; Ovality is the shape of the circumference of the bore.

Taper is caused by the piston head scraping the cylinder walls on the way up. The position of maximum cylinder wear is the point of maximum piston velocity on the thrust side of the
cylinder. This over time causes ovality.

Ovality is checked by using an internal micro meter as shown in the picture. Taper is checked at different points inside the cylinder bore.

What is side clearance on a piston and how is it checked?

                 

Side clearance of the piston is the gap between the cylinder bore and the piston head; this is checked by inserting a feeler gauge in between the gap.

How can you tell the difference between an inlet and exhaust valve and why?

The inlet valve has a larger head when compared to the exhaust valve. Inlet air and fuel mixture has a greater pressure then the exhaust gas wastes flowing out of the chamber; therefore the inlet valve needs to be bigger to let the mixture flow into the burning chamber as quickly as possible under higher pressure.

Why do we have piston ring end gap clearance?
 
This is to allow the piston to expand and take the shape of the ring.

What could the result be if the piston ring end gap is too small?


It could cause the rings to scratch the cylinder bore walls, this will later on cause taper in the cylinder bore.

Why do aluminium cylinder heads usually have a steel shim (washer) between the valve spring and the cylinder head surface?

This is to help the valve spring retain its specified tension.

What is meant by the terms S.I and C.I in a four stroke engine?

S.I stands for Sparkplug ignition for petrol engines; C.I Stands for Compression Ignition for diesel engines.

What is the purpose of a core plug?

When the water in the cooling system freezes, it expands and takes up more space, this builds up too much pressure, therefore, the core plugs pop out to prevent the head from cracking.