What is the purpose of a glow plug on a diesel engine?
Glow plug is a heating plug used with some diesel engines for easy starting. They are small electric heater plugs that are fitted to the small combustion chambers of in direct-injection engines. They are used to provide heat during cold starting. 

Whats the difference between an two stroke and four stroke petrol engine?

For presentation

Lubrication 

There are two types lubrication system of two-stroke engine.

Mixed lubrication - lubricating oil mixed with fuel; 

Separate lubrication - use of oil pump to pump the lubricating oil into the crankshaft, etc., then the oil burned with air-fuel mixture inside the cylinder. 

Due to this lubrication system carbon build build inside engine and will stick onto piston rings, causing leaks, or may block the exhaust port, causing power loss, and wear the crank and connecting rod bearing. 

In four-stroke engine lubrication system, there are also two types. 

One is splash lubrication - the oil is splashed up from the oil pan or trays in the lower part of the crankcase by moving components. The oil is thrown up as droplets or fine mist to provide lubrication to valve mechanisms, piston pins, cylinder walls, and piston rings. 

The other type is the combination splash and pressure lubricating system, The lubricating oil circulating through the engine to all moving parts requiring lubrication by an oil pump, it takes oil form the oil pan and forces oil through holes drilled in the engine block and crankshaft to lubricate all moving parts, in this system, cylinder walls are lubricated by splashing oil thrown off from the connecting-rod bearings.

Compare the lubricating system of two-stroke and four-stroke engine, we can see the two-stroke lubricating system is less effective than four-stroke engine. Therefore, the life of most of two-stroke engine only about half of four-stroke engine.

Other:
In two-stroke engine the main thermal parts are the cylinder head, cylinder, piston, 
piston rings and other parts. In four-stroke engine the main thermal parts including the cylinder head, cylinder head cover, cylinder, piston, piston rings and valve timing mechanism.Valve timing mechanism including valves, valve guides, valve seats, valve springs, camshaft, cam belt or chain, cam gear, belt or chain tensioner, rocker arm and other component parts. Two-stroke engine has small size, simple structure, light weight, easy manufacture and maintenance compare to four-stroke engine.
In four-stroke engine, it needs four strokes to complete a work cycle. First, intake air-fuel mixture into the cylinder, then the mixture is compressed, after that, ignite the mixture to generate power, last exhaust out from the cylinder. This intake, compression, combustion and exhaust processes are the four movements for four-stroke engine. Two-stroke engine crank 360 ° to complete a work cycle, that is, crankshaft rotate one revolution to complete the cycle, which is air-fuel mixture intake through the transfer port on cylinder wall, and push the burnt mixture out of exhaust port to complete the intake and exhaust process. but four-stroke engine crank 720 ° to complete a work cycle.Because two-stroke engine rotate one revolution to do a cycle, but four-stroke engine needs rotate twice, so the torque generated by two-stroke engine are more evenly than four-stroke engine. While at the same working speed and the same volume of cylinder and combustion chamber, two-stroke engine should outputs twice much power than the  four-stroke engines but there are power loss during intake and exhausts, and the piston working distance is short than four-stroke, so the output of the two stroke engine only about 50% to 70% more than the four-stroke engine.


Reference:
http://zhidao.baidu.com/question/78387415.html
http://www.doc88.com/p-04866931808.html
http://www.google.com/search?hl=zh-CN&source=hp&biw=1364&bih=597&q=engine+lubricating+system&oq=engine+lubri&aq=0&aqi=g2&aql=&gs_sm=e&gs_upl=19820l22511l0l12l11l0l0l0l0l653l2209l2-2.2.1.1
http://www.britannica.com/EBchecked/media/93029/Typical-gasoline-engine-lubrication-system
http://www.dansmc.com/4_stroke_oilpump.htm