 Exhaust gas 1
Once again in plain language: Given infinite time, a diesel engine actually needs exactly the same lambda as a gasoline engine, perhaps slightly adjusted by its slightly higher carbon content. But the gasoline engine
always has more time to form the mixture, even with direct injection. Furthermore, electric ignition is more reliable than autoignition in a diesel engine.
With its always short time for mixture formation, the diesel runs as constantly as possible with at least lambda equal to 1.4. Swirl systems can help, but they don't change the fundamental difference. A swirl system could,
for example, be a 45° rotation of the cylinder head for each cylinder or a spiral intake port extending to the intake valves.
Now, of course, one could ask the reverse question: why is lambda 1 so often emphasized in gasoline engines? Of course, ignition is also successful with a lambda of 1.1, which means a 10 percent leaner mixture. Or a
correspondingly richer one with a lambda of 0.9. This burns so well that power increases, while fuel consumption is optimal with the 10 percent leaner mixture.
No, both ranges, and certainly not the ones behind them, are possible for the gasoline engine, although it would probably still run at a 20 percent deviation until at some point the spark plug would get wet due to too much
fuel or the mixture would be too lean to ignite. No, all ranges beyond 0.05 percent deviation are eliminated from our next considerations.
And this has been an iron law since the advent of exhaust gas detoxification through three-way catalytic converters. We'll see some relatively complex exceptions to this rule later, but for now, let's stick with it. The reason
this catalytic converter is called 'three-way' is because it corrects the imbalance between CO, CH, and NOx.
The three (in reality, NOx stands for more) are signs of a failed chemical reaction. As described above, not every C and H atom has found its O atom(s). And if you don't find what you're looking for, you resort to a
compromise. In addition, oxygen doesn't like being alone and will grab onto another oxygen atom.
So something went wrong with the combustion. Either there wasn't enough time for mixture formation, or a leaner mixture formed in one part of the combustion chamber and a richer mixture in the other. Regardless, the
three-way catalytic converter corrects this, much to the benefit of the environment. That's why it's sometimes referred to as a 'twin-cylinder' engine.
What does that mean again? If you take a closer look at the three pollutants, it should become clear that only CO must convert to CO2, CH also to CO2, and H2O and NOX
to nitrogen. Hopefully, it's also clear now that the first two cases belong to the category of 'oxidation', and the last case to 'reduction'. 'Two-bed catalyst' means exactly that: both chemical processes take place
simultaneously.
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