DIESEL ENGINES

Diesel Engines
Diesel (also know as Gasoleo by the Spanish and Mazout by the French) has an oily texture and a pungent smell. Diesel does not evaporate very quickly and is more difficult to ignite when compared to gasoline. It is also incredibly difficult to remove from clothing so be careful when filling vehicles or bleeding fuel lines.
The main differences from the gasoline engine are :-
The spark plug replaced by an injector nozzle. Which is used for delivering a fine mist of diesel into the cylinder.
Because of the use of an injector, there is no need for a carburettor to supply the fuel in the right form.
Because the fuel self ignites due to the heat caused by the higher compression there is no need foranair THE INDUCTION STROKE (first cycle)
y of the spark generating and distributing components used in the gasoline engine.
THE In this cycle the crankshaft rotates thus drawing the piston down the cylinder bore. This action draws pure air alone into the cylinder via the opened inlet valve. The exhaust valve is closed at this time.
THE COMPRESSION STROKE (second cycle)
Both the exhaust valve and inlet valve are now closed.
The crankshaft continues to rotate and this action forces the piston back up the cylinder bore. As a result of both valves being closed the pure air has nowhere to go, so compression of the air now takes place.
The ratio of compression is typically in the range of between (16 to 1) to (22 to 1) depending on the engines designed performance. This is much higher than that found in the gasoline engine and as a result the air temperature rises to well above 500 degrees Centigrade.This has now completed one revolution of the crankshaft.
THE POWER STROKE (third cycle)
Both the exhaust valve and the inlet valve remain closed.
The fuel is now sprayed into the heated air just before the piston reaches the top of its stroke and as a result it ignites and burns with the oxygen in the air.
This results in a rapid burning (expansion) of the gasses which forces the piston back down the cylinder bore, and in the process turns the crankshaft.
This is Power.....
THE EXHAUST STROKE (forth cycle)
The inlet valve remains closed but the exhaust valve is now opened.
The crankshaft continues to turn, forcing the piston back up the cylinder bore.
Because the exhaust valve is now open the movement of the piston forces the burnt gasses out of the cylinder, through the silencer and out into the atmosphere.
This has now completed the second revolution of the crankshaft. The whole process is now ready to begin again at the induction stroke.
Re-Cap
The petrol "spark ignition" fuel engine, is more wasteful of fuel than the diesel engine. Fuel has a certain heat value, and heat is work. The petrol engine delivers only 22 -- 25 % of the theoretical work value of its fuel, while the diesel engine delivers 30 -- 36 %. (First advantage). Thus the diesel engine is said to have the higher thermal efficiency, which in terms of ordinary usage means that there will be a more economical consumption figure for a given load. The lower temperature, at which petrol ignites compared to diesel, limits the compression ratio within the petrol engine to a lower figure. It is known that the higher the compression the more efficient an engine becomes, so this gives the diesel engine its second advantage, because only pure air is compressed so that there is nothing to "pre-ignite". (You can hear pre-ignition in a petrol engine that has too far advanced timing of the spark. This manifests itself as a "rattle" when you are driving with wide throttle openings and occurs particularly at lower revs. This can also lead to the engine carrying on firing, and running, when the ignition key is turned off)
Just before the piston reaches the end of the compression stroke the diesel is injected into the combustion chamber through an injector nozzle mounted in the cylinder head (in much the same position as the spark plug in a petrol engine). During injection the fuel is split up into finely divided particles, and the mixture of these with the air forms an explosive charge that is ignited by the heat of the compression. Injection, of the fuel, is continued for a short period, during which the piston passes its highest position and begins to descend on the power stroke. The expansion of combustion only begins to have effect when the piston has passed the top of its stroke. When the fuel is cut off expansion of the gases still continues. The overall effect on the piston is a more sustained pressure than that associated with the petrol engine. (In other words the burning and expansion lasts much longer than in a gasoline engine).
To inject the fuel a special type of pump driven by the engine is employed and this is the distinguishing feature of the diesel engine. This pump also has to supply the fuel at just the right instant "injection timing" for the engine to work efficiently. A very important detail is that injection must cease cleanly and abruptly at the end of the delivery period without any trace of "after-dribble" of the fuel from the injector otherwise carbon deposits quickly form on the nozzle tip and excessive smoke will appear in the exhaust.
So, already you should see that the diesel engine has no high voltage electrics to go wrong and be affected by moisture as in the petrol engine (wet high voltage leads in the petrol engine will stop it from running as the high voltage will find an easier path to follow along the wet leads). Also, the diesel's higher thermal efficiency means lower running temperatures so there are less overheating problems in arduous conditions. And because of the higher compression ratios encountered, diesel engines are built far more strongly than the petrol engine thus helping its durability and reliability in the field.
The maximum speed of the diesel engine is also kept to lower revolutions per minute (RPM) than that found in the petrol engine. This is accomplished by the use of a governor. This means that speed regulation (as in power generators) is reliably controlled so that when there is no load on the engine the fuel supply is cut back. But once load is applied then the fuel supply is increased to maintain the set RPM.