How Is Your Engine’s Performance?
When asked by my company to submit an article, I thought it might be interesting to take a look at engine testing. Many people are experienced in the operation, maintenance, repair, etc., of the PT6A engine, but engine testing seems to remain somewhat of a mystery! Let’s take a look at it!
Maybe it’s the confusion of mathematical calculations, or the use of terminology and units of measure that are unfamiliar to everyday life. Unfortunately, some aspects of engine testing are sort of complicated, such as “correction” of performance data. However, it is not really necessary to get caught up in all that, because computers do the calculations very reliably. It may be better for us to look at what’s done during engine testing in more common terms; that is, to consider the “big picture” of what can be accomplished with a test cell.
Even though we all know what a PT6A is, we need to properly define what this thing consists of. The PT6A is a lightweight free turbine engine. The engine utilizes two independent turbine sections: one driving the compressor in the gas generator section and the other driving the propeller shaft through the reduction gearbox. The engine’s oil system, while being driven by the gas generator, provides the necessary lubrication for all areas of the engine. It also supplies the torquemeter system which gives an indication of engine power output, and the primary governor which changes oil pressure to the propeller in order to maintain control of rpm, reverse pitch, and feathering.
Testing consists of running the engine to prescribed levels of power output while various data are monitored and recorded. An apparatus is required to absorb engine power and maintain rpm while running, and in our case a dynomometer (dyno) is used. The dyno is a housing containing a rotor and stator, and is fitted with inlet and outlet valves and a load cell. The prop shaft of the engine is connected so that the dyno’s rotor is driven by the engine. As water passes into the dyno, it provides resistance between the rotor and stator. The water outlet valve is controlled automatically to absorb engine power while maintaining rpm, and the load cell indicates power output by measuring the “twisting” force being imparted on the dyno by the engine.
When an engine is stabilized at a prescribed power output, various data is recorded. The data is then compared to the acceptance limits set forth by the manufacturer. The gas generator speed must be within limits, as well as Tt5 (ITT) temperature and fuel flow, to name a few. What this really tells us is how well the engine is doing its job of producing power. If the engine is in good working order, it will achieve its prescribed power level and be within acceptance limits on all of its data (Ng, ITT, etc). If, however, the acceptance data is not within limits, the engine must be repaired. Knowing what to do to repair the engine depends on which data is unacceptable. In general, if the gas generator speed is high and the ITT temperature is high, a cold section problem (bleed valve, compressor) is indicated. If the gas generator speed is low and the ITT is high, a hot section problem (tip clearance, vane damage) is indicated, or vane classing may be the issue. It may be as simple as that. But it can also be much more complex. There are many things that can affect the performance of turbine engines: high air temperature, air inlet restriction, accessory malfunction, dirt/erosion/F.O.D., fuel flow restriction, parts not correct or incorrectly installed, etc. The real beauty of a test cell is the ability to know with certainty what an engine’s performance actually is.
The test cell’s instrumentation is checked and certified regularly, and includes independent systems for measuring power output. Also, our test cell is correlated to the manufacturer, which means it has been proven against another “known good” cell. So, if proper performance is what you’re after for your turbine engine, a certified test cell may
prove to be instrumental in achieving that goal.