The Determination of Turbocharger Turbine Efficiency
Our Technical Bulletin No 7 described how the overall efficiency of the turbocharger affects the performance of a turbocharged engine. The overall efficiency is obtained by multiplying the compressor efficiency, mechanical efficiency, and turbine efficiency together, resulting in a single value of overall efficiency.
The Compressor efficiency can be accurately determined by operating the turbocharger on a test stand and measuring its performance over the entire operating field of the compressor. Mechanical efficiency can be found easily by calculation using empirical equations for the power losses in the turbocharger bearing system. (see our Bulletin No. 7) This leaves the turbine efficiency, which has been very difficult to quantify, and several methods have been employed to obtain accurate values for use in calculating turbocharger overall efficiency.
In the early stages of design of small turbochargers, attempts were made to determine turbine efficiencies by using temperature, pressure, and mass flow values taken when the turbocharged engine was run in the laboratory coupled to a dynamometer. Date taken by this method consistently produced turbine efficiencies of over 100% and these attempts were subsequently abandoned.
The following attempts to accurately find turbine efficiencies involved connecting the turbine component to a high-speed dynamometer, where the power output could be measured along with accurate values of mass flow, exhaust temperature and pressure under conditions of steady exhaust gas flow.
Graph No. 1 shows a turbine dynamometer performance test of a contoured turbine wheel that has an exit diameter smaller than the O.D. of the wheel. Most commercial turbochargers use this type of wheel. The Graph vividly shows that the wheel has a choking effect on its mass flow capability and a drop off in turbine efficiency occurs as the mass flow falls off each of the expansion ratios tested.
For comparison purposes, Graph No. 2 shows a turbine dynamometer performance test of a similar sized wheel which has its exits diameter the same as the wheel O.D., knows as a full-bladed wheel. The improvement in mass flow (lbs,/min) is dramatic in that the full-bladed wheel does not choke over its entire performance range. In addition, the turbine efficiency does not fall off as the wheel is operated at high speed and high expansion ratios. Comparing efficiencies at 54,000 RPM and 2.4 expansion ratio, the full-bladed wheel reaches 68%, whereas the contoured wheel only makes 60%. The performance characteristic shown on Graph No. 2 is ideal for racing applications where the turbocharger is operated at very high speed and pressure ratio. Full-bladed turbine wheels are used in many Comp Turbo turbochargers.
As stated previously, the turbine efficiencies obtained from the turbine dynamometer tests correspond to steady exhaust gas flow conditions. These values are useful only when an undivided exhaust manifold is used on a turbocharged engine. In this case, the exhaust gas flow to the turbine wheel approaches steady flow. Most turbocharged engines employ a divided manifold and a divided turbine casting on the turbochargers. There are significant engine performance advantages resulting from the use of divided manifolds, which are described in detail in our Bulletin No. 2.
By dividing the exhaust manifold and turbine casing, the exhaust pulses from the engine cylinders are separated and present the turbine wheel with a highly pulsating flow. (see diagram in Bulletin No.2) Thus, the turbine efficiencies obtained from dynamometer tests are not applicable to turbines operating on pulsating flow.
In an attempt to quantify turbine efficiencies on pulsating flow, a university in Europe devised a test apparatus that could measure the turbine efficiency under simulated pulsating flow. This test apparatus is shown on Graph No. 3. The turbine inlet gas flow is divided to be similar to the gas flow in a divided manifold and the pressure could be controlled in each inlet pipe to mimic pulsating gas flow. This apparatus had the ability to run simulated pulses under steady flow conditions, and thereby obtain accurate turbine efficiencies under simulated gas flow.
The results of this pioneering effort were unique and surprising. Graph No. 4 is a typical example of results obtained from running a turbocharger turbine on the unequal admission test apparatus. Equal gas flow in the separated pipes is represented by the1.0 pressure ration. Pressure ratios less than 1.0 indicate unequal flow in the two gas inlet pipes. It was very interesting to discover that turbine efficiencies are higher when the simulated exhaust gas pulse from the engine cylinders is high, and the turbine efficiencies are higher than with equal flow in the two gas inlet pipes. The pulse turbine efficiencies are continually changing and get higher as the simulated pulse gets higher. When the exhaust pulse is highest during cylinder blow down, the instantaneous turbine efficiency will be maximized. This discovery contributes to explaining why the divided manifolds produce better engine performance than undivided manifolds.
The conclusion that can be reached form consideration of the information and data given in this bulletin is primarily that a turbocharger with a divided turbine casing mounted on a divided manifold and that employs a full-bladed turbine wheel will produce superior engine performance in racing and other commercial applications.