 Compression
In reciprocating piston engines, a distinction must be made between individual and total displacement. Both are only the same in the single-cylinder. Incidentally, we are not aware of any engines in which the cylinders
involved have different displacements. Probably also because the crankshaft and mass balance would be unable to handle it well.
The total displacement is therefore calculated as the product of the individual displacement and the number of cylinders. And if you look at the individual displacement, it is part of the cylinder space when the
piston is at TDC. More precisely, it is the space between UT and OT. The direct distance between the two is referred to as the stroke. And although the displacement is a cylindrical volume,
the diameter is referred to here as the bore.
The compression ratio is relatively simple compared to what comes next. While the importance of bore and stroke is slowly dimishing today, that of the compression ratio is still or once again quite high. It's like a
seismograph that tells you a little bit about the design of the engine.
First of all, it tells us that the displacement as the space between TDC and BDC is not the only space in the cylinder. There is always a little space between the TDC and the cylinder head. This space is referred to as VC
(compression space) in the formula above. Basically, the formula is quite easy to understand because it compares the two volumes before and after the compression stroke.
In the past, the compression ratio allowed certain conclusions to be drawn about the expected fuel quality and the quality of the combustion chamber. If, for example, super gasoline was required instead of regular gasoline
and only a relatively low compression ratio of, say, 8.5 to 9.0:1 was specified, this indicated that the combustion chamber was not quite optimal. In the case of diesel engines, the possibilities for drawing conclusions were
more limited.
That changed with the turbocharging. Take, for example, a diesel engine that has a charger despite a compression ratio of 16.5:1. You can actually feel the slightly weaker boost. Advantage: It is not so dependent on the
supercharger and therefore only has to contend with a slightly smaller turbo lag, if any.
Conversely, Mazda currently has a compression ratio of 14:1, which is the same as that of a gasoline engine. This diesel probably has a lot of turbo pressure. Here, you should look at the torque curve in the lower
speed range and provoke a turbo lag during the test drive. However, if it can overcome this hurdle, i.e., if it doesn't always have to be revved up to accelerate spontaneously, you can look forward to an efficient engine.
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