 Petrol - Diesel
| Left diesel, right petrol, each top in summer, bottom in winter |
One thing must be clear: diesel fuel has a density of approximately 0.83 g/cm³, which is about 10 percent higher than gasoline at approximately 0.76 g/cm³. This means that you definitely get more energy from
a liter of diesel than from a liter of gasoline. But you may have always suspected that. It's actually more than 10 percent—due to a different composition with a higher carbon content, it's about 13 percent.
So what's the actual difference between a diesel engine and a gasoline engine? For this comparison, we'll use two direct-injection engines. You probably already know that since the mass introduction of
gasoline direct injection, there's only one clearly noticeable difference, even from the outside: The gasoline engine requires an electric ignition system, while the diesel engine doesn't.
A
nd this difference says everything about the requirements for the firing process. The diesel engine must have enough internal heat to ignite the fuel spontaneously upon injection. If it doesn't obtain this heat
through its geometric compression ratio, then perhaps through supercharging, usually an exhaust turbocharger. If that's still not enough, e.g., during a cold start, the combustion chamber usually has to be
preheated electrically.
If you have the appropriate video, take a look at how such a finely atomized injection spray initially ignites at the edges during injection. Usually, before it hits anything, it almost burns through, all a result of
this incredibly high pressure and the minimal amounts with an injection duration of less than 1 millisecond. A gasoline engine doesn't need all of that, even with direct injection.
If you have the appropriate video technology, take a look at how such a super finely atomized injection jet initially ignites at the edges during injection. Usually, before it hits anything, it almost burns through,
all a result of this incredibly high pressure and the minimal amounts with an injection duration of less than 1 millisecond. A gasoline engine doesn't need all of that, even with direct injection.
Did you know that the first engines of their kind began injecting while the intake valve was still open? The only thing that mattered was that none of the precious fuel escaped unburned into the exhaust ports
during any overlap. In principle, injection can occur throughout the entire period from the end of the overlap to the start of ignition.
A complete difference from diesel, despite the nominally identical process. It's simply that a gasoline-air mixture has to wait until an ignition spark occurs, and which it does with a well-functioning engine. And
then there are the differences in pressure. If you inject during intake, you obviously need considerably less than at peak compression.
In principle, the pressures of 3 to 4 bar of a conventional intake manifold injector would be sufficient. However, even in the past, injection was carried out at higher pressures, such as 18 bar. Today, injection
occurs during intake and/or compression, but still earlier than in a diesel engine. This creates the famous difference from a maximum of 200 to more than 2,000 bar, which, apart from the timing control, makes
for a significantly more complex and therefore more expensive system.
Apparently, going straight to the fuel tank is always more expensive than waiting. And now we come to the crux of the matter: the time required for mixture formation. This is very limited in diesel engines. What
can be done during this time?
| C8H16 + 12 O2 = 8 CO2 + 8 H
2O |
If you look at the above formula again, you might realize that it's not that easy for the 8 C and 16 H atoms to get to the 24 O atoms. Especially since they're hidden under almost four times as many nitrogen
atoms in the air. And when time is of the essence, complete combustion is only possible if statistically 1.4 times the amount of air is available.
| 14.8 kg air to 1 kg fuel -> Lambda equals 1 |
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