TWO STROKE ENGINES

Explain how a two stroke engine works?
A two stroke works in the sense that it is a two stroke cycle.
The piston is an intake valve exhaust valve and a compressor it performs much duty's on it's own making it is lighter than 4stroke engines therefore making it more ideal for outboards, chainsaws, Radio control hobby's.
 Air, fuel and oil enters the crankshaft housing through a reed valve for every rotation of the crankshaft, this allows for fuel to go on the other side of the piston for lubrication as oil is mixed with fuel. lets start with the piston down the reed valve is down with only a bit of fuel around the crank shaft allowing fuel to travel up to the combustion chamber as the piston travels up closing the fuel flow while this happens the reed valve opens. it pushes up and compresses the fuel and air to be ignited at the top. while this is happening the reed valve has allowed more fuel in filling up the area around the crank shaft, then the piston moves down opening the exhaust outlet. just below there the piston will allow that last cycle of fuel from the crank area to come up through the fuel intake in the boar.


What is happening below and above the piston?
 Above: upwards pressure oil fuel/air intake, ignition, exhaust out. Below: air/fuel and cycled around through reed valve, lubrication, downwards pressure.


Explain what is meant, by scavenging when applied to 2 stroke SI engines?
The process of removing exhaust also it tries to bring some back in to burn wasted fuel.


Why must roller and ball race bearings be used on 2 stroke SI engine crankshafts?
These bearings roll over each other and are used in two strokes because only takes on minimum lubrication so the bearings are more effective.


Why are rollers caged in crankshaft bearings?
To help the the conrods move freely around the crankshaft and because four strokes use oil the case inside the conrod is fixed needing oil. So in a two stroke they use rollers to move around the crank shaft because oil doesn't get in there, but these rollers have grease which has a longer life.


Why must ‘split’ type bearings be used on one piece crankshafts?
because the crank shaft is made as one piece the bearings need to go inside the conrod so you can put the conrod back back on the crank shaft also they need to be taken out to be check for wear.


Why do we have pins between the piston ring gaps on a 2 stroke?
To stop the rings from spinning around


How does a reed valve work on a two stroke?
The pressure from the up force when the scavenging process happens opens the reed valves allowing fuel to enter then the piston forces the the reed shut on the way down due to pressure. The reed valve can have two metal thin plates or a circular disk shape with petal like plates going around it, 


How do you check a reed valve? 
Always check these. Look that it clicks open and closes properly then hold it up to the light to make sure you can not see any light coming through when it is closed. Make sure there is no wear or pitting.


Explain the term ovality and taper and how are they formed?
Ovality is when the shaft warps into uneven widths so when you measure with a gauge you spin the shaft on a v stands and look at the gauge for fluctuations if one side is okay the you turn it then it is 2mm out that could well be ovality. Taper is when the haft becomes worn down on one end and starts to slant downwards or the end is already tapered and needs to be checked that the angle still remains the same.


How does the piston rings seal in the bore?
The piston rings have a are wider that the piston and the rings also have a bit of springs so when the piston is set in the boar the rings natrualy want to push out this forms the seal.


What is meant by the terms groove depth, side clearance and end gap on the pistons rings?
The piston ring sits in a groove around the piston this can change when carbon builds up behind the ring.
side clearance is the distance on either side of the rings sitting in the piston. The end gap is when the ring is in the boar making sure the gap at either end is correct using a feeler gague.


Explain the difference between a 2 stroke and a 4 stroke engine?(2 stroke dry sump ,port, piston ring, cycle ,fuel, bearings)
The stroke cycles are different. for a two stroke one cycle in each direction, it has a compression stroke followed by explosion of compressed fuel on return stroke fuel air oil is taken back in. It has a dry sump meaning the oil is mixed with the fuel and the cycle lubricates and cools with oil so there is no direct oil being put through the crank shaft. The port is dictated by the piston pressure. A two stroke only has two rings.
Two strokes need bearings in the conrod and on the crank shaft because they do not cycle oil on it's own these bearings male up for less lubrication also inside the conrod they are cased so you can open them and check for wear.


4 stroke wet sump, valves, piston, cycle, fuel, bearing, camshaft.
four stoke cycle works intake, compression, combustion and exhaust. Fuel/air enters on intake on the down stroke. Up stroke the intake valve has closes and fuel/air undergoes compression. Ignition occurs called a power stroke, the exhaust valve is opened the piston moves up for the second time expelling exhaust.
four strokes have an oil gallery and require oil to be circulated through the engine to lubricate.


What is meant by each of the following terms:

(a) Mechanical Efficiency: Actual work out put of engine to ideal work out put.
(b) Thermal Efficiency: The measurement of efficiency and completeness of the combustion of fuel, if all the potential heat in fuel could be transferred as work the thermal efficiency would be 100%
(c) Volumetric Efficiency: The efficiency which the engine can move the charge in and out of pistons/cylinders, the ratio of fuel/air that actually enters during induction to the actual capacity of cylinders under static conditions. Greater induction manifold pressure means above ambient will have efficiencies above 100%