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.