Engines week 2

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

The firing order is the sequence in which the cylinders ignite the air/fuel mixture and use the resulting energy to turn the crank. In a four stroke engine the crankshaft is shared by the pistons and each piston will need two revolutions of the crank shaft to complete its cycle and the firing order will need to be in sequence that will put even load on the crank.

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

Compression Ignition engine 4 stroke cycle

A spark ignition engine uses a spark from a spark plug to ignite the air/fuel mixture when the cylinder is top dead center on the compression stroke.

Compression ignition engine uses compression it ignite the air/fuel mixture. on the intake stroke air is introduced into the cylinder then at the top of the compression stroke fuel is introduced into the hot compressed air which evaporates and expands and then ignites. Compression ignition engines use a less volatile fuel and have more compression than SI engines







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

Diesel piston

A compression ignition engine piston have a deep groove in the top so that when the air is compressed the fuel has somewhere to be injected to. The pistons are usually larger and stronger to be able to handle more pressure

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

 Rudolf Diesel. The first successful compression ignition engine was in the year1897

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

The oil jet sprays oil onto the cylinder wall to provide lubrication between the piston and the bore. Without this the piston could seize or damage the bore. Also provides cooling to the engine.

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

The big end is connected to the crankshaft and the little end is connected to the piston


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

Thrust washers are in between the connecting rod big end and the crankshaft with a film of oil it helps stop the unwanted movement. When the hole in the thrust washers lines up with the hole in the connecting rod oil travels through and sprays the bore and piston

8.On a diesel engine whats the difference between direct injection and a indirect injection type?

 In a direct injection the fuel is injected directly into the combustion chamber. An indirect injection the fuel is injected into a prechamber which is off the combustion chamber. The combustion starts in the prechamber and then spreads into the combustion chamber

9.Explain the term valve timing?

 Valve timing is the precise timing of  the opening and closing of the valves controlled by the camshaft. The valves need to be able to open and close at the right time for the fuel/air mixture to enter the combustion chamber and for the exhaust to exit

10.Why do we have valve clearance?

 Correct valve clearance ensures that the valves open and close properly. if the clearance was too much then the valves wouldnt open to their full capacity. If the gap was too small then the valve wouldnt fully close

Theory questions

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

Taper and ovality is the wear of the cylinder bore caused by the side to side movement of the piston. If the cylinder bore is tapered then you measure the diameter of the top and bottom and the middle of the cylinder bore

To check the ovality of the cylinder bore you measure the circumference of the cylinder bore, instead of the bore being round it will be oval shaped

What is side clearance on a piston and how is it checked?
The distance between the piston and the cylinder wall. Piston rings close this gap under pressure to create compression in the cylinder. to check the side clearance on a piston insert the ring into the piston and use a feeler gauge in between. 

How can you tell the difference between a inlet and exhaust vavle and why?

The inlet valve is larger than the exhaust valve to allow more air/fuel mixture into the engine. The exhaust valve only needs to get rid of the burn remains

What sort of tempatures do the inlet and exhaust valves get upto?
inlet valves get up to around 250 degrees Celsius
Exhaust valves get up to around 750 degrees Celsius

What temperatures and speeds must the piston be able to cope with?
pistons must be able to handle temperatures up to 300 degrees celsius and handle rotations up to 7500rpm

Why do we have piston ring end gap clearance ?

Pistons rings ensure there is a good combustion seal in the cylinder and help with heat transfer from the piston to the cylinder wall. The end gap clearance allows for expansion of the ring under heat and compression

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

When the engine gets up to operating temperature and the piston rings expand from heat it can cause overlapping or bending. can also cause closure of the end gap, damage to the bore, the ring breaking and even the pison siezing up.

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

Over time the valve spring lose tension. Adding a steel shim would bring the spring tension back up to spec

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

S.I is spark ignition. It uses a spark plug to ignite the air/fuel mixture at the top of the compression stroke like in petrol engines

 
C.I is Compression ignition. It uses highly compressed hot air without spark to ignite the fuel like in diesel engines. On the compression stroke only air is introduced to the combustion chamber and at the very top of the stroke the fuel is injected into the hot compressed air.

What is the purpose of the margin on the valve?

Helps dissipate heat from the valves. The exhaust valves have a thicker margin than the intake valves because the exhaust valves encounter more heat. if the valve margin is too thin then the valve will become too hot when exposed to combustion and it will burn.

 What is the purpose of a core plug?
Core plugs are used to plug the core holes that are left by the casting of the block. Sand is used to create the paths for the water cooling and leaves holes on the outside of the block which are plugged by core plugs. If the engine doesn't have antifreeze in it and the water in the engine freezes, core plugs are supposed to pop out as the freezing water expands. without the core plugs the engine block could crack.

What is end float and run out on a crankshaft, hows is measured? 
 End float is the slight movement you crankshaft is allowed to move in the main bearings while the engine is cold. Using a DTI gauge at the end of the crankshaft, lever the crankshaft side to side to get the measurements of the crankshaft play. Alternatively you can push the crankshaft to one side and use feeler gauge inbetween the crankshaft and a thrust bearing.

4 stroke engines

4 stroke engine – induction compression combustion exhaust

induction – intake valve open atmospheric pressure draws in air and fuel mixture when piston is traveling to BDC. Exhaust valve is closed.

Compression – both valves are closed piston is traveling to TDC and compresses the air and fuel mixture

Combustion – Both valves are closed. Air fuel mixture is ignited by spark plugs when piston is near TDC piston then heads to BDC from explosion

Exhaust – Exhaust valve is open and piston travels to TDC in take valve is closed

The four stroke engine was invented by Nicolaus Otto in the year 1976. The four stroke engine was theorised in 1961 by Alphonse Beau de Rochas but Nicolaus Otto was the first to make it practical


Crankshaft - Converts the up and down movement for the pistons to a rotational movement of the crankshaft.

Camshaft - Connected directly to the crankshaft by timing belt. Controls the inlet and oulet valves and the air/fuel mixture

Valve collets - Work in pairs to hold the inlet and exhaust valves in the engine 

Valve stem seals - Control the amount of oil to the valves and to the combustion chamber for lubrication. sit on top of the valve stem

Head gasket - Sits inbetween the engine block and the cylinder head to create maximum compression and prevent leaks from coolant and oil into the cylinders

Pistons - Attached to the crankshaft by con rods. creates compression of the air fuel mixture which ignites and the force created turns the crankshaft

Piston rings - Open ended rings around the piston to seal the combustion chamber, transfer heat from the piston to the cylinder wall, and control oil consumption


Cambelt - The cambelt is connected to the crankshaft and the camshaft and it controls the timing of the engines valves

Flywheel - Connected to the crankshaft and provides the momentum to keep the crankshaft turning from the energy created by the combustion stroke

Bore - The diameter of the combustion chamber
 


Stroke - The movement of the piston from Top Dead Center to Bottom Dead center