Here is some information taken from my project, which was also my major project at uni. Try not to fall asleep.
Compressor efficiencyThe compressor efficiency of the turbo plays an important part in the turbo selection. It will have a particular combination of boost pressure and airflow at which its efficiency is highest. It is important that the most useful part of the rev range of an engine coincides with the maximum efficiency of the compressor. This is mainly because as the efficiency drops, the heat produced by the turbo goes up. If an engine were fitted with a small turbo where maximum efficiency occurred at one quarter of the rev range, the efficiency at high rpm would be low and the inlet charge temperatures would be extremely hot. Similarly, if maximum efficiency was at maximum revs, then temperatures in the mid-range of the engine could rise to an unacceptable level. Therefore the most useful part of the rev range of the engine should coincide with the compressor’s maximum efficiency.
The compressor wheel itself does not have a large effect on the boost threshold or lag of a turbo engine, due to it being the lightest part of the rotational assembly. Rather, the turbine wheel is the part which is heaviest and causes any lag. This must be sized carefully to avoid excessive back pressure and temperature, or excessive lag and a high boost threshold.
Pressure ratioThe pressure ratio of a turbo engine is defined as the total absolute pressure divided by atmospheric pressure:
Pressure ratio = 14.7 + boost / 14.7
Since atmospheric pressure is 14.7psi, and the desired boost for the project engine is 12psi, the pressure ratio becomes:
Pressure ratio = 14.7 + 12 / 14.7
=
1.82This means that approximately 82% more air will be going through the engine. The pressure ratio is later plotted onto a compressor map to help choose the correct turbo.
Airflow rateThe airflow rate through an engine is also required before compressor selection can take place. The generally used airflow rate is measured in cubic feet per minute (cfm), although some use lb‘s per minute. It is first necessary to calculate the airflow rate through an engine with no boost:
Airflow rate = cid x rpm x 0.5 x Ev / 1728
Where:
-Cid is engine displacement in cubic inches
-Rpm is revs per minute
-0.5 is due to the fact that a four stroke engine only fills its cylinders on half of the revolutions
-Ev is volumetric efficiency
-1728 converts cubic inches into cubic feet
So for the project engine cid = 97, max rpm = 7500, and approximate volumetric efficiency at this rpm = 75%
Airflow rate = 97 x 7500 x 0.5 x 0.75 / 1728
=
158 cfmThis is the basic engine flow rate. The flow rate under boost can be calculated from this by multiplying by the pressure ratio worked out earlier:
Airflow rate = pressure ratio x basic engine flow rate
= 1.82 x 158
=
288 cfm or 23.25 lb’s / minPlotting the values onto a compressor mapWith the airflow rate and pressure ratio known, it is now time to plot the values onto a compressor map(s) until a suitable match is found.
Figure3-10 shows a T3 50 trim compressor map with the project engine’s values plotted. When plotting onto a compressor map, the first value to plot is the airflow rate, which is 288 cfm or 23.25 lb’s per minute. This is plotted by drawing a line across from the pressure ratio of 1.82 and another one up from the air flow of 23.25 (shown by black, straight lines). This intersection shows the compressor efficiency at the engine’s maximum rpm. In this case, the efficiency here is about 66%. It also represents the maximum flow that the compressor can provide at the given pressure ratio for this engine.
The next stage is to check the surge characteristics of the compressor with regard to the project engine. This can be accurately approximated in a simple manner. Assume that the desired pressure ratio is achieved at 50% of the maximum rpm, and plot this new point onto the map. For the project engine 50% of maximum rev’s is about 3750 rpm. Therefore the non-boosted airflow is:
Airflow rate = 97 x 3750 x 0.5 x 0.75 / 1728
=
78.94 cfmThe boosted airflow is therefore:
Airflow rate = 1.82 x 78.94
=
144 cfm or 11.65 lb’s / minAgain, a line is drawn across from the y-axis at 1.82, and another up from the x-axis at 11.65 (see figure3-10).
The next stage is to plot a point at pressure ratio = 1 and airflow = 20% of maximum, or 4.65 lb’s / min in the case of the project engine. These three points can then be joined together (shown by the purple line). If the line lies completely to the right of the surge line, then the compressor should be suitable for the engine with regards to avoiding surge.
Since the purple line lies completely within the efficiency islands of the compressor map, this turbo would seem to be a suitable match for the engine.