stefan skrev:
til henrik, tak for dit arrogante og værdiløse svar! prøv at læse mit spørgsmål igen! hvordan du vil flytte nogen form for gas, (varm eller kold) uden at have en trykforskel er mig stadig en gåde? mener du måske at udstøningsgassen bliver ved med at (være lige varm) udvide sig til enden af bagpotten? ps. til saabmanden ingen gentager ingen turbomotor gentager turbomotor har gavn af modtryk i udstødning! som henrik var inde på kan der være tilfælde hvor dette viser sig ikke at holde stik i virkeligheden. men så er det fordi der er noget der ikke er rigtig afstemt et andet sted!
Beklager da meget hvis det blev opfattet som et arrogant svar!
En turbolader er ikke nogen simpel sag at forklare, og af samme grund valgte jeg indledningsvist at diske op med en meget forsimplet model.
Selvf. er der en trykforskel når der er flow. Som jeg skriver så er denne forskel ikke nær nok til at drive turboen. Groft sagt(og stadigt lidt forsimplet) så er det energi og temperaturforskellen henover turboen gange masseflowet gennem den der udgør langt størstedelen af drivkraften. trykforskellen udgør en forsvindende lille del.
Se denne snippede tekst, når det nu absolut SKAL være indviklet (teksten er iøvrigt sakset fra nettet efter ca 10minutters søgen, så mon nu mit svar var så værdiløst alligevel???), så nu kan du jo bare slå dig løs:
In general there are two types of compressors, displacement compressors (roots, screw) and dynamic compressors. Dynamic compressors are also called turbomachines and the centrifugal compressor is of this type.
Displacement compressors work by reducing the volume of the trapped gas and hence increasing its pressure. The gasflow is proportional to the rotational speed of the shaft and the flow is almost independent on the pressure increase.
The centrifugal compressor works by increasing the velocity of the flowing fluid with the impeller, and therefore its kinetic energy. In the following diffusor the velocity is reduced and the kinetic energy is transfered into a static pressure.
When the pressure is increased thermodynamic laws says that the temperature will rise, if no heat is supplied to or rejected from the working fluid (no friction, heat radiation, cooling and so on) the process is called adibatic. In reality this is not the case for any compressor, therefore there’s something called adiabatic efficiency which relate how much the temperature will rise in reality compared to what thermodynamic laws says. If the adiabatic efficiency is 100% it's an adiabatic process.
Compressors are doing a negative work, this means that they require power from an external source. This source can be an exhaust driven turbine or the engine itself, in any case the amount of power required depends on a few things. The power needed is the enthalpy change over the compressor multiplied with the mass flow per second, this can be written as:
P=delta h * m, where delta h is:
delta h = h outlet - h inlet
or if we consider the fluid to be thermally perfect:
delta h = (cT outlet)-(cT inlet)
where c is specific heat and T the absolute temperature
If we consider that the compression is an adiabatic process the outlet temperature would be:
T outlet = T inlet * PR^((g-1)/g)
Where g is gamma, specific heat ratio and PR is the pressure ratio defined below
PR = pressure outlet/pressure inlet
As we now can se the power needed to drive the compressor depends on the adiabatic efficiency, pressure ratio and mass flow.
The centrifugal compressor is one of the compressor types which has a high adiabatic efficiency, the ones in modern turbochargers have up to 80% efficiency while the ones that are engine driven usually have a little lower efficiency (I think this is because they use lower rpms). Other compressors like roots compressors have an extremely low efficiency, which usually is below 50%.
If we use and exhaust driven turbine to power the compressor we will use the heat in the exhausts which had otherwise been wasted. The power delivered by the turbine can be written, similar to the power needed by the compressor:
P = ((cT inlet)-(cT outlet)) * m
Under ideal conditions a turbocharger robs almost no power from the crankshaft, this means that the power output and engine efficiency can be higher with a turbocharger than with an engine driven compressor. However, exhaust flow and temperature is too low to at low speeds for the turbine to produce enough power to create a boost pressure, and if a smaller turbine is used it will cause a restriction at high speeds/loads and therefore a drop in peak power. But if variable nozzle area (usually called VNT or VGT) is used on the turbine we can increase the width of the turbines range so it will work earlier and cause less restriction at high speeds/loads, sadly it can only be used in diesels because of their lower exhaust temperature, at least yet.
Under low engine load the turbocharger will run with a low speed, and under high engine load it will run with a high speed. Since there is no connection with the crankshaft we must wait until we have enough exhaust for the turbine to supply enough power to reach a boost pressure. This will cause what is called “lag”, today we can however eliminate lag by using an anti lag system, ALS, but this is very hard on exhaust valves, exhaust manifold, turbocharger and exhaust system and of course very loud so it’s only used on racing cars. VNT turbines, ball bearing turbos and so on are also ways of reducing the lag.
Jeg har ikke tænkt mig at gå ind i en dybere diskussion omkring dette emne herinde, da det vil være totalt off-topic, og hvis man ikke har lidt indsigt i termodynamik er det iøvrigt omtrent nonsens.
Log ind
Tilmeld
OSS
Søg
