Engine Instrumentation — Page 524, Lesson 611

Right, let's pick this up with the torquemeter system. We've already covered how the engine produces torque and why we need to measure it, so now we're looking at the two ways we actually do that measurement. First, let's look at the hydraulic system. The principle here is that we use oil pressure to balance the axial force created by the helical gear shafts. Now, these helical gears are the ones that transmit the torque, and as the torque increases, they try to push themselves along their axis of rotation. That's the axial force we're talking about. So here's how it works. The oil from the engine's lubrication system goes to a torquemeter pump. This pump boosts the oil pressure, in some cases up to as much as 800 psi. That's a very high pressure. This high-pressure oil is then allowed into cylinders, and these cylinders form the bearings within which the helical gear shafts rotate. Now, here's the clever part. Each cylinder has a small bleed hole in its wall. If the gear shaft moves into the cylinder because the axial load is increasing, it covers that bleed hole. With the hole covered, the oil can't escape, so the pressure inside the cylinder builds up. This increased pressure pushes the gear shaft back to its original position. It's a balancing act. Conversely, if the load on the helical gear shaft decreases, the existing oil pressure forces the shaft slightly out of the cylinder. This uncovers the bleed hole, which allows the balancing oil pressure to reduce. And so the gear shaft moves back into its correct position within the cylinder. So the system is constantly self-correcting to keep the shaft in equilibrium. Now, the key point is this: if we measure that oil pressure that's balancing the axial force, we can compare it with reference figures. These reference figures take into account the ambient pressure and temperature, and the performance of the engine. From that comparison, the engine's power output can be judged. So the oil pressure becomes a direct indication of torque. Now, let's move on to the electronic system, which is a completely different approach. This system comprises two concentric shafts. That means one shaft is inside the other. The first is the Torque Shaft, and it's connected to both the engine and the propeller's reduction gear box. The second is the Reference Shaft, and it's connected only to the engine. At the forward end of each shaft, there's an exciter wheel, which is a toothed gear. These exciter wheels rotate past an electromagnetic pick-up, and as the teeth pass by, they produce an AC voltage. So we get an AC voltage from each shaft. Now, the exciter wheels are aligned at assembly. But here's the key: as power is increased, the torque shaft twists. This twisting displaces the phase relationship of the voltages produced. In other words, the two AC voltages are no longer in sync with each other. The amount of displacement is proportional to the change in power, and that displacement is used to drive an indicator. So the phase difference between the two voltages tells us the torque. This electronic system is simple, and it's lighter than other systems. And it has proven to be very reliable in service. Now, one important point about the torque indicator itself. It may indicate negative torque as well as positive torque. Negative torque happens when you have a windmilling propeller — that's when the airflow is driving the propeller, and the propeller is driving the engine, rather than the other way around. The torque limits are colour coded and shown on the gauge. A red coloured band or marker indicates the maximum limits. And on a FADEC system — that's Full Authority Digital Engine Control — these limits may be adjusted and set by the crew, and the indication can be presented in a digital read-out. So to summarise: we have two ways to measure torque. The hydraulic system uses oil pressure to balance the axial force on helical gears, and we measure that balancing pressure. The electronic system uses two concentric shafts and measures the phase displacement between their AC voltages. Both give us the engine's power output.