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 Compression 1
Gasoline and diesel engines differ not only in terms of quality respectively quantity control. Much more widespread is the knowledge of the different compression of the two engine principles. First of all, the essence of
compression should be clarified a little. Put simply, the compression ratio determines the amount by which the filling is compressed. Compression was once one of the essential features that Nikolaus August Otto originally
added to the luminous gas engine originally from Lenoir, thus putting a temporary end to the unsatisfactory power output of the atmospheric gas engine.
Properly applied, compression is a key not only to more power or torque, but also to better efficiency, of course not necessarily both at the same time. If I mercilessly retrieve the generated increased performance,
consumption will probably rise. However, if I stay within the limits before a compression increase with the demand for performance, I can count on lower consumption. Actually, the designers of internal combustion engines
should use this miracle cure as extensively as possible.
But they don't do it, because there are catches, as with all miracle cures. Let's start with the gasoline engine, which this drug only gets to a certain extent. The electric ignition is actually the boss here. It is intended to
determine when combustion and thus an increase in pressure may take place, namely in such a way that the piston can escape with the increase in pressure. However, if the pressure increase occurs too early, when the
piston is still before top dead center, the situation becomes critical.
This scenario is called 'self-ignition' and one fears it like the devil avoids holy water. Why? Because one provokes damage with it, such as burned through, overheated pistons, bent connecting rods, ruined crankshafts and
their bearings. So you have to keep the compression within limits. First of all, these limits are determined by a certain quality of the fuel, referred to here as the 'octane number'. Roughly speaking, the design of the engine
must be adapted to this quality of fuel.
In the past, the octane number of the fuel to be filled up was simply specified in the operating instructions. You were allowed to use fuel with a higher octane rating, but not with a lower one. The improvement in octane rating
can be beautifully understood by following the compression ratios of petrol engines over the decades. Enormous progress has been made here, especially since the Second World War, in good old Europe even more than
in America, where people still drive around with comparatively lower octane ratings.
In the early days of the petrol engine, for example, there were still compression ratios of 5:1 or 6:1, while today we have already reached 12.5:1 and at the peak at 14:1. Do me a favor and be sure to relate these numbers to
naturally aspirated engines, supercharged ones obey different laws. But we'll get to that. At least it is now clear that for decades the engine builders had to move in unison with the chemists of the fuel manufacturers. It was
also about possible areas of application for the vehicles going into series production, because the octane number for these should actually be guaranteed at the filling stations.
Gradually, the engine builders learned that the octane requirement of an engine could also be improved, so that something had to be done on their part. To pick just one from the abundance of mechanical possibilities, the
famous pinch edges should be mentioned here, which found their way into engine construction in the seventies of the last century. To pick just one from the abundance of mechanical possibilities, the famous pinch edges
should be mentioned here, which found their way into engine construction in the seventies of the last century. Here the engineer Michael May must be mentioned, whom we will meet again in the history of supercharging.
How does this work? Apart from the valves, there are still vacancies on the cylinder head. Now either the valves themselves or (much more often) the vacancies are shaped in such a way that the piston approaches them to
within less than a millimeter. As a result, the air or air-fuel mixture located there is squeezed and accelerated into the middle of the combustion chamber, where it causes turbulence. The in turn propagates through the entire
combustion chamber.
Self-ignition mainly occurs at particularly hot spots in the combustion chamber. The more even the temperature throughout the combustion chamber, the lower the risk of so-called hot spots. And that's what the turbulence
prevents. Of course there are more ways to impede self-ignition, mostly electronic these days. Extensive use is made of the option of retarding the ignition and thus lowering the overall combustion temperature.
As you might have guessed, the latter isn't exactly a performance-enhancement measure, but rather the opposite. So you really shouldn't use fuel with a lower octane rating than the owner's manual says, even if the allows
you to do so. Because everything that has to do with a reduction in performance through a reduction in compression also affects efficiency. It should be exciting to determine the additional consumption of an engine designed
for Super Plus when running on premium petrol.
And what about the diesel engine? Maybe a little easier, but only a little. The diesel knows no knocking limits, on the contrary, it practically always knocks, because self-ignition is its elixir. But if you screw the already
significantly higher compression to even greater heights, you will initially get a bunch of disadvantages. The engine running, which is not always civilized anyway, becomes significantly rougher and the stress on the engine
seems to increase exponentially. Even exhaust gases are not getting any better, e.g. the soot content.
And more performance is not necessarily to be expected. Engines that are still mechanically regulated may be regulated strictly according to rotational speed, and electronic regulation may not notice the increase in
compression. In any case, the diesel engine is better equipped for its higher load, but a bit more sensitive when the limits of this mechanical condition are further exploited. Even its birth, e.g. as a car engine, was not easy
and the significantly more expensive production has been maintained to this day, if not even increased.
In summary, one can say that the level of the compression ratio in petrol engines is characterized by the knock limit and the associated threat of mechanical and thermal destruction, and in diesel engines it has a no less
critical effect on the running mechanics and running satisfaction. Here, the compression ratios in naturally aspirated engines have often been the result of years of experience. Why is that in the past tense? Because you have
to look for naturally aspirated diesel with a magnifying glass.
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