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 Petrol - Diesel 2
Both are refueled using similar nozzles, differing only in diameter. And although both burn in almost the same way in a completely enclosed space, thus generating pressure, this by no means
means that the composition is the same and that combustion occurs the same in both cases.
Despite direct injection in gasoline engines, one must assume a mixing period, albeit a short one. First, the fuel comes into contact with air with maximum turbulence, and only then does it
reach the ignition point. There is a certain time delay between the two events.
Diesel fuel, on the other hand, forms the basis for ignition through its injection into the higher-compression combustion chamber. In some cases, it doesn't even reach its target, such as a wall,
where it can burn in layers. Especially at the edges of the injection spray, tiny droplets ignite beforehand and evaporate then.
Whereas with a gasoline engine, the flame front can be assumed to spread in a more punctiform manner due to the central triggering of combustion, in a diesel engine, this spread is more
indefinable along an injection jet, with varying degrees of access to its center. The oxygen essential for combustion is simply lacking there.
Before one can perhaps synthetically produce fuels on a small scale, thus giving them a more clearly defined composition, one must assume that fuel derived directly from petroleum contains an enormous
variety of basic substances, all of which contain OH groups, some of which are even combined with oxygen.
In the image above, you can see the two groups into which fuels for internal combustion engines are divided: on the left, the alkanes or aliphatics, and on the right, the aromatics or alicyclic hydrocarbons. This
simply shows the basic arrangement: on the left, in chains and on the right, in (benzene) rings. There are countless variations for both, except that carbon always forms the supporting framework, so to speak.
This can also be reinforced by double and triple bonds.
Generally speaking, a chain-like structure generally indicates greater ignitability and thus a higher cetane number, while the ring-like structure not only appears more stable, but also demonstrates this with its
higher octane number. This is determined by comparing it with a mixture of isooctane with an octane number of 100 and n-heptane, the latter of which, with an octane number of zero, belongs to the alkanes.
However, one can only say that gasoline contains slightly more alkanes and diesel more aromatics, despite a number of other additives. The cetane number is determined using a similar process, although it
behaves in a relatively reversed manner. Here, we dare to repeat a previously frequently used theorem: that cetane equals 100 minus octane.
Direct-injection diesel engines, due to their short residence time until auto-ignition, have even higher requirements for a cetane number of at least 50. One of the reasons for this chapter is to promote a
fundamental rethink. A light fuel, which vaporizes or even evaporates much faster, is less likely to ignite than a heavy fuel, whose vaporization curve is approximately 150°C higher.
Yet, a prerequisite for combustion is that the fuel becomes gaseous. One should not confuse the tendency to (self-)ignite with flammability. Try holding a match (preferably outdoors) to a very small amount of
gasoline and, alternatively, to diesel. Gasoline even forms gases that, heavier than air, can evaporate in all directions. This is why its danger is so difficult to predict.
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