This sometimes plays a very important role in the most various places in automobile technology. If, e.g., in your own car, through frequent sharp braking when driving downhill, the brake fluid starts to boil, the next time
you put your foot on the brakes, it may be that nothing happens. Perhaps the cavitation of the Diesel fuel, whose (equal size) mass is made available to certain injection pumps and converts it (without significant
temperature increase) from a liquid- to a gaseous state.
By this, one can appreciate that solid and liquid substances have fixed volume limits, which are lifted when in the gaseous state. By the way, it is possible for every substance, to take on one of the three
physical states, solid, liquid or gaseous. Iron only becomes gaseous, when it is heated to 2750°C. Helium only becomes solid when cooled down to -272°C, the basic principle however, is the same.
As a rule, a substance expands more and more when heated, once the limit of the physical state has been reached, it moves on, with it's entire volume, to the next state with an increased distance between the
molecules. Take water as an example, at 100°C, one would need more than five times the heat necessary to warm it from 0- 100°C, before it finally turns into steam.
Indeed, water, at 4°C is the great exception among all substances, because with further cooling, it expands in size. Nature probably needs this property in water, e.g., to erode mountains or for other changes in the
landscape. After all, expanding ice has an explosive effect. Indeed, motorists find this unpleasant, it means that, at least in winter, one has to drive with frost-protection, which not only costs money, it also reduces the
cooling efficiency. However, before the entire engine block is pressed apart or at least the frost-plugs are forced out …
Why do the molecules, when heated, need more space? Because their oscillations increase. The thermal energy applied to them, appears as kinetic energy, which of course, is never lost. Should however, the
temperature be lowered again, there must be a point where there are no oscillations at all. Indeed, there is this point, absolute zero at -273°C. This is where the much more logical Kelvin-scale begins.
One can imagine the oscillating molecules as small balls, but this is in fact, only a model. This means, that with this notion, one may be able to explain something, but in reality, it only concerns the processes to be
explained. Indeed, if one can determine, at an exactly defined point, the position of a certain electron, with only a certain amount of probability (Heisenberg), see what we mean, that's how complicated the reality is, as
far as it has even been discovered.
Indeed, since our concern is still the combustion engine, the physics of the gases is particularly interesting to us. As we've already seen in the cavitation of Diesel fuel, one can influence the temperature of a
substance not only through the application of heat. It also rises, when the pressure is increased, which can easily be noticed when pumping (by hand) air into a bicycle tyre. If the pressure is reduced to below the
prevailing level, the temperature accordingly drops.
The pressure (p) multiplied by the volume (V) is constant. Thereby, the limits of the state of substances can be shifted. Thus, e.g., in power stations, water in it's liquid state having a temperature of 300°C is not
unusual. However should such a high pressure water pipe burst, it could mean mortal danger. If there are also different temperatures, then pressure (p) multiplied by volume (V), divided by temperature (T) is even
constant. Indeed, the latter must then be converted into Kelvin.
There are at least two conditions for these formulae. This can only work properly with ideal gases and the gas amount must, e.g., be in the combustion chamber of a combustion engine, and completely
encapsulated. Thus, we find ourselves in the middle of the nicest engine calculations. Would you like an example? Have a look at the page about the turbo-charger, then you'll know, that in the meantime, even for displacement there are substitutes, but there is no substute for pressure.
Should one wish to learn something about the skills of the engineers who construct these engines, one must put the displacement and the performance into relation. We find everything summarised in one value, the
Mean Effective Pressure (BMEP), theoretically derived from the average pressure determined by all the cylinder strokes, the Indexed Mean Effective Pressure (IMEP). Through consideration of the engine efficiency, e.g.,
the friction losses in the engine, the effective pressure is determined.
|Effective mean pressure in modern, naturally aspirated engines: distinctly above 10 bar|
So, we have found a pretty good parameter for indicating the quality of an engine, even if it can only be calculated. We'll stay with air, which we will henceforth, treat as the ideal gas. Now in fact, the calculations really
start. Even before the engine goes into construction Using the gas equations, one can calculate the necessary air-mass and the displacement volume for one particular performance aspect of a naturally aspirated
engine and - in the meantime particularly important - get an idea of it's thermal efficiency.
The whole thing is based on a model by Mayer from the beginnings of thermodynamics in the 19th century. It was discovered that heat represents a form of energy, exactly like, e.g., movement (1st main clause),
where one can be converted into the other, it is never lost, but not added, e.g., to keep a perpetual motion machine moving forever. Heat is thus, the kinetic energy of the molecules, which with increasing temperature
oscillate more and more.
Now one can also understand, what happens when a substance crosses over from one physical state to the others. At some point, e.g., with solid substances, the oscillations become so great, that the molecules
give up their solid form and arrange themselves in a new, looser formation but retaining the same volume (liquid). Should we then, using water as an example, heat it further, under normal pressure at 100°C another
break from the system limits would take place. After all, steam takes up enormously more space than does water.
Molecular changes in ferrous materials, are really exiting, particularly when they are cooling down. Iron belongs to the materials, which when crossing from the solid- to the liquid state, can form very compact and long-
lasting molecular structures, which are called 'crystals'. Now, this crystallisation, when cooling down, must begin somewhere. Of course, not only in one, but in several places at the same time. Indeed, because that
does not occur uniformly in all temperature ranges, one can, by timing the cooling down process, extract enormous material properties from the ferrous material. 08/11
|Read more about the structure of ferrous materials here. |