Engines {Week 4}

Describe the terms Conduction, Convection, and Radiation, and where would you find them on an car engine?

Conduction: The transfer of heat through a metal. This can be found in the combustion chamber, heat produced by the combustion is transferred to the cylinder walls which are made of metal.

Convection: The transfer of heat through water. E.g. when the heat from the cylinder walls is transferred to the coolant circulating around the engine through the water jackets.

Radiation: The transfer of heat through air. E.g. when the vehicle is being driven, air passes through the radiator which takes the heat away.

What is the purpose of a thermostat?

The purpose of the thermostat is to hold the coolant from flowing around when the engine is cold. When the engine has reached operating temperature, the coolant heats up, and when the coolant reaches the specified temperature required for the thermostat to open. When the thermostat opens, it lets the coolant flow throughout the system.

Why do engines have coolant in them?

Coolant in the engine raises the boiling point of water to prevent it from boiling in normal engine operating temperature.

Coolant is used to prevent the corrosion of valuable internal engine parts. (Precious aluminium head) Water alone in the cooling system would have attacked and corroded the metal. Coolant also acts as a lubricant for the water pump.

When should an oil filter be replaced (miles)?

Around every 6000km – 10000km, different vehicles have different specs.


What projects the oil pressure and where is it situated?
The oil pump projects the pressure. The oil pump is mounted on the engine block behind the crank pulley.

ENGINES {Week 3}

What is the purpose of a glow plug on a diesel engine?
The purpose of the glow plug on a diesel engine is to heat up the combustion chamber before the pistons start to move, this is because in a diesel engine, the air and fuel mixture ignites in high compression and heat. The glow plug is there to assist in starting up the cycles and later on turns off when the combustion chamber is hot enough.

What is the difference between a two stroke and four stroke petrol engine?


2 Stroke Engine: In a two stroke engine, while the piston travels down on the first stroke, the piston head unblocks the exhaust port. The inlet port is blocked and the transfer port is opened which lets the air, fuel and oil mixture from the bottom of the cylinder travel upwards and come out through the transfer port and into the combustion chamber.

With the centrifugal force, the piston travels upwards, this is the second stroke. In this stroke, while the piston is travelling upwards, it now opens the inlet port, further on it blocks the exhaust and transfer ports. The fuel mixture gets compressed and is ignited by the sparkplug at TDC. This process is repeated over and over again.

 In a two stroke engine, when the inlet port opens, air with fuel and oil is directed to the big end of the con rod and crankshaft which is lubricated by the mixture.

4 stroke engine:  In a four stroke engine, there are four cycles.

Stroke 1: When the piston is travelling down to BDC, the inlet valve is open to let in the air and fuel mixture. This is the inlet stroke.

Stroke 2: The piston is travelling upwards to TDC. In this stroke, both the inlet and the exhaust valves are closed, this stroke is called the compression stroke. The mixture of fuel and air is getting compressed and the area is getting much smaller.

Stroke 3: Once the piston is fully up to TDC, the mixture is ignited with a sparkplug at the top of the cylinder head. This ignition causes and explosion, the force drives the piston down to BDC. Both valves remain closed. This is the combustion stroke.

Stroke 4: The piston is being driven upward again by centrifugal force. In this stroke, only the exhaust valve opens and while the piston is travelling upwards to TDC, the burnt gases are driven out of the combustion chamber.

ENGINES {Week 2}

1. What is the firing order of a four stroke engine?

1 , 3 , 4 , 2.

2. What is the difference between a S.I and C.I engine?

S.I stands for spark ignition, this means that the fuel and air mixture in the combustion chamber is ignited by a spark plug. This ignition type is on gasoline engines.

C.I stands for compression ignition, this means that the air is introduced in the chamber before fuel, the air is compressed on compression stroke, the compression is very high, as soon as the piston is right to the top, diesel fuel is injected into the chamber, due to the very high compression rate and temperature, the fuel ignites immediately.

3. Is there any difference between a S.I and C.I piston if so what?

C.I pistons a heavily built, these pistons are designed to be able to withstand high compression and heat. Diesel engines will be used for heavy work so the pistons are much more stronger the gasoline engine pistons.

4. What year was the diesel engine invented and by whom?

Diesel engine was invented in 1887 by Rudolf Diesel.

5. What is the purpose of the oil jet on the connectng rod, and where does it spray?

The oil jet sprays oil on the cylinder walls to keep it lubricated while the piston is moving.

6. On the connecting rod where is the big end and little end?

The big end is where it will connect with the crankshaft and the little end is where it will connect with the piston with a gudgeon pin.

7. What is the purpose of the thrush washers on the crankshaft?

Thrust Washers stop the play of the connecting rods on the crank shaft.

http://wiki.answers.com/Q/What_does_a_thrust_washer_do#ixzz1JIiIDpXa

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.