Compression 3
What is the relationship between compression and rotational speed? A mixed one. If this is high, that would in principle be an advantage because a smaller proportion of the filling at the piston rings can
escape into the oil sump due to a lack of time. But simultaneously , time is becoming increasingly short to fill the cylinder chamber with fresh gas. And less filling before compression results in less pressure
after it.
That is why the compression and the valve timing have been moving closer together for some time. These can break the pattern determined by the rotational speed. This is not so much about their full
variability, but rather about shifting the opening, mainly of the intake valve.
What can that do? Quite a lot, if you ask me. What comes next is a further development of Miller's principle based on Atkinson. Here it is important to divide the compression, on the one hand into a purely
geometric one as the ratio of the space above the piston in BDC to that of the piston in TDC position.
However, you can also take the increase in pressure in the cylinder as the result of compression, and this is of course also dependent on the pressure of any supercharging present. In the diesel engine, the
geometric compression had to react to the increasing boost pressure. It shrank from just under 24:1 to now even less than 16:1.
The engines have become increasingly more powerful. If you compare two diesel engines and one has less geometric compression, you can almost bet that its boost pressure is higher, which is the opposite
of how it used to be, where the higher compression also meant greater performance strength.
And what about a petrol engine? It's definitely getting more complicated. And that brings us back to the timing. As already mentioned, variable compression would be good for many situations. For example,
after a cold start. The engine will emit roughly more pollutants until it has warmed up sufficiently than it will later after 100 km of gentle motorway driving.
Heating up as quickly as possible would also be beneficial to reduce the greater wear in this condition. As one example of many, greater compression would be possible here, because knocking is associated
with higher temperatures, but these are not present here.
Accordingly, there was and is a wealth of devices for changing the geometric compression during operation of a reciprocating engine, some of which can still be implemented to some extent on existing engines, while
others require a completely new design. We will not discuss any of them here.
Why not? Because there are lso better options for the few non-charged engines. Above all, we will probably no longer see any fundamental changes to the reciprocating piston engine due to electromobility. If anything else
comes, it will be an increase in geometric compression coupled with even more variable valve timing.
So it's not about how to achieve high compression, that's easy to achieve in terms of geometry. Rather, there needs to be a method of avoiding too high compression during operation. And that's easily possible by shifting
the timing, particularly on the intake valve. For example, you only need to leave it open a little too long, and the fresh gas will flow back again.
The method also has the advantage that lambda is equal to 1 and thus the entire exhaust gas aftertreatment is retained. In petrol engines outside the area of sports engines, there is a phenomenon where the
compression specified by the factory increases, far beyond the 11:1 maximum once thought to be possible.
Toyota introduced this forcefully with the Prius, a car that, although it did no downsizing with a 1.8 liter displacement, but consumed little fuel when driven moderately on the highway despite the hybrid drive. The is important
in a car like this without a turbocharger because the engine would feel rather sluggish when driving if the electric motor were not there to provide a few Newton meters.
Finally, there is one more advantage of variable compression, although we will leave the influence of ignition out of this chapter. Previously, when an engine got older, precautions had to be taken to ensure that the
compression remained constant and effective and therefore remained somewhat away from the possible compression. A variable control system can of course do this better.
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