pv library
Why is the combustion of a diesel and petrol engine diffirent?
http://en.wikipedia.org/wiki/Diesel_cycle
The combustion stage of a diesel is constant pressure (2-3)
http://www.qrg.northwestern.edu/thermo/design-library/otto/Otto-Pv-diagram.gif
while the combustion stage of a petrol is constant volume. (2-3)
Why is this?
It all down to liquid fuel not burning. It’s the vapours that burn
petrol is volatile, and evaporated in to a “gas” before the combustion stroke (outside the engine in a carb, or injected during intake/compression stokes) . This means when combustion happens the mixture doesn’t need take time to evaporate and so burns REALLY fast (like an explosion) before the piston has moved much:- hence constant volume
Diesels inject droplets of heavier fuels during the (start of the) power stoke. This combustion process is a lot like what can happens with splatters of cooking oil when frying bacon. If the droplets pass through flame the ouside of the droplet will heat up and begin to vaporise. If the temperature of the vapours rise further they will ignite, but the “core” of the droplet” will still be liquid, and take some time to burn off. This causes a time lag between the start of fuel injection, start of ignition, end of injection, and end of combustion. This takes place over a few degrees of crankshaft rotation, meaning the cylinder volume changes a fair bit. It’s one of the main reason why diesel engines don’t run as fast as petrol Mechanically diesels could be constructed to cope with RPMs just as fast a petrol, but there’s no point becasuse the fuel simply wouldn’t have enough time to vaporise/burn.
Constant pressure, because ideally fuel is only injected when the pressure is to raised. In practise, the pressure difference between the fuel line, and inside the cylinder dictates how good the fuel atomisation/spray is. If atomisation isn’t not good enough for the engine’s speed/pressure ,the pressure doesn’t rise as much, but stays up for longer. However if it stays up too long, and combustion is still happening when the exhaust valves open (max power/worse fuel efficiency), then black smoke is produced in the exhaust.
Compare:-
http://en.wikipedia.org/wiki/Octane_rating
http://en.wikipedia.org/wiki/Cetane_number
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