Engine Instrumentation — Page 524, Lesson 611

I want to walk you through the torquemeter — the instrument that measures engine power output. We're starting with the hydraulic system, then we'll move to the electronic one. First, the hydraulic torquemeter. The idea is that engine oil is fed to a torquemeter pump, which boosts its pressure — in some cases up to 800 psi. That high-pressure oil is allowed into cylinders, and those cylinders form the bearings within which the helical gear shafts rotate. So the gear shafts sit inside these cylinders, and the oil pressure is what supports them. Here's the clever part. There's a small bleed hole in the wall of each cylinder. If the gear shaft moves into the cylinder under increasing axial load — that is, when the engine is producing more power and pushing harder along the shaft — the shaft covers the bleed hole. That traps the oil, so the pressure inside the cylinder builds up until it's high enough to push the gear shaft back to its original position. It's a self-balancing system. Now the reverse. If the load on the helical gear shaft decreases, the existing oil pressure forces the shaft slightly out of the cylinder. That uncovers the bleed hole, which lets the balancing oil pressure reduce, and so the gear shaft moves back into its correct position within the cylinder. So the shaft is always held in equilibrium — the oil pressure exactly balances the axial force. And here's the payoff: if you measure that oil pressure that's balancing the axial force, you can compare it with reference figures. Those 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. That's the whole purpose of the torquemeter — it turns a mechanical force into a measurable oil pressure that tells you how much power the engine is producing. Now let's look at the electronic system, which is shown in Figure 38.4. This one comprises two concentric shafts — one 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. So one shaft carries the full load path through to the propeller, the other just follows the engine. At the forward end of each shaft there's an exciter wheel — that's a toothed gear. These exciter wheels rotate past an electromagnetic pick-up, and as the teeth pass, they produce an AC voltage. So each shaft generates its own AC signal. Now, the exciter wheels are aligned at assembly — they start off in phase. But as power is increased, the torque shaft twists. That twisting displaces the phase relationship of the voltages produced by the two wheels. The displacement is proportional to the change in power, and that phase displacement is what drives the indicator. So the more the shaft twists, the more the phase shifts, and the more power the gauge shows. This electronic system is simple, and lighter than other systems — lighter than the hydraulic one — and it has proven to be very reliable in service. One more thing about the torque indicator itself. It may indicate negative torque as well as positive torque. Negative torque happens with a windmilling propeller — that's when the propeller is being driven by the airflow rather than driving the aircraft, so the torque reverses. 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 tie it together: the hydraulic system balances axial load with oil pressure and measures that pressure; the electronic system measures the twist of a torque shaft against a reference shaft via phase shift. Both give you the same thing — a measure of engine power output, with limits colour-coded on the gauge and negative torque shown for windmilling conditions.