The overall efficiency of a turbocharger has an important effect on the performance of a turbocharged engine. When the overall efficiency is high, less turbine power is required to drive the compressor in order to deliver a desired boost pressure to the engine intake manifold. A relatively larger turbine casting A/R (see our Bulletin No. 5 for a description of A/R) can be used, and this results in a lowered average pressure in the exhaust manifold. Since the pistons act against this lowered pressure on their upstroke when pushing remaining exhaust gas out of the engine cylinders, the parasitic loss of the engine is minimized and the engine fuel consumption is improved. Therefore, it is desirable to strive for the highest possible turbocharger overall efficiency when matching a turbocharger to an engine. The overall efficiency of a turbocharger is obtained by multiplying the compressor efficiency, mechanical efficiency, and turbine efficiency together to produce a single value of efficiency.
The compressor efficiency can be found from the compressor's performance map. These maps are produced by turbocharger manufacturers by running the turbocharger on a performance test stand and measuring the performance over the complete air flow and speed range. Several of these maps are presented in our Bulletin No. 4 on matching a turbocharger to an engine. The Graph Number 1 in Bulletin No. 4 us reproduced below and shows an engine air requirement superimposed on a compressor map, where the compressor efficiency remains over 75% over the complete engine operating range and peaks at 80% in the center of the range. This graph shows an ideally matched engine air requirement with a turbocharger compressor flow range. The maximum compressor efficiency would be taken from the compressor map when calculating a turbocharger overall efficiency.
Next, the mechanical efficiency of the turbocharger can be determined by analysis of the bearing system.
The friction loss in the floating sleeve bearings used in most commercial turbochargers can be calculated using the following formula:
HP = .1363(NS⁄ 10000)2 L (1⁄1000C1 ⁄ D13 + 1000C2 ⁄ D23)
Where:
N = turbocharger speed -RPM
L = bearing length – in.
C1 = I.D. Clearance – in.
C2 = O.D clearance – in.
D1 = inside diameter – in.
D2 = outside diameter – in.
The power loss should be calculated for both minimum and maximum bearing clearances. The minimum bearing clearances produce the higher power loss.
The power loss of the stationary thrust bearings used in commercial turbochargers can be found using the following formula:
HPT = .1363 (N⁄10000) 2 R24 – R14 ⁄1000C
Where:
N = turbocharger speed – RPM
R1 = inside radius of thrust surface – in.
R2 = outside radius of thrust surface -in.
C = oil film thickness – in.
The loss of the loaded side of the thrust bearing and the loss of the unloaded side must both be calculated and added together to obtain the total HP loss (see our Bulletin No. 1).
The mechanical efficiency of a typical floating sleeve bearing system can be calculated using the above formula and should be of the order of 96%.
The full compliment ball bearings with ceramic balls used in Comp Turbo turbochargers have minimal friction loss since there is no oil film shear in the bearings. The rolling friction loss is very low and the mechanical efficiency of Comp Turbo ball bearing turbochargers is of the order of 99%.
The efficiency of the small gas turbines used in turbochargers has been very difficult to quantify. Some testing has been done by turbocharger manufacturers using a small high-speed dynamometer to measure the HP output of the turbine and determine the efficiency on steady hot gas flow. The values thus obtained are applicable to undivided exhaust manifold systems, however, most turbocharged engines used divided exhaust manifolds to take advantage of their superior performance. In the case of a divided manifold system, the turbine must operate on pulsating exhaust gas flow and, as a result, the turbine efficiency is a variable with time and changes as the magnitude of the exhaust pulses change. A detailed discussion of turbine efficiency is complicated and will be the subject of a subsequent technical bulletin.
In summary, if an average turbine efficiency of 75% is assumed, a Comp Turbo turbocharger can have an overall efficiency of .80 x .99 x .75, which equals 59.4%. A value of turbocharger overall efficiency of 60% has been the objective of turbocharger designers and developers since their inception.