Engine Instrumentation — Page 530, Lesson 618

I want to walk you through the temperature sensing side of engine instrumentation, because this is where a lot of the "how do we actually know what's happening inside the engine" questions get answered. First, the big picture. Piston engines and gas turbine aero engines are both heat engines. That's the defining characteristic. The power they produce is directly proportional to the heat released during combustion of fuel. So more heat, more power. But here's the catch — every engine component and system is designed to withstand a certain temperature. If you exceed those limits, the component may fail. So to allow safe operation, we have to monitor engine temperatures continuously. And it's not just the combustion heat we care about. We also have to consider ambient temperatures — the outside air temperature — because that affects the whole picture too. Now, which temperatures do we actually monitor? On both piston and gas turbine engines, we watch: air inlet temperature, piston cylinder head temperature, piston exhaust gas temperature, gas turbine compressor outlet temperature, turbine temperature, oil and fuel system temperatures, and the internal air system temperature. Each of those is a separate sensing job. The temperatures we're dealing with can range from -56°C up to +1200°C. That's a huge span. And because of that span, no single sensor works for everything. Different sensors are used depending on the temperature range to be monitored. They fall broadly into two categories: high temperatures and low temperatures. Now, there are four major types of measuring devices. Let me walk you through each one, because each works on a completely different physical principle. First, the Expansion Type. This relies on the principle that most solids, liquids, and gases expand and contract with temperature changes. The classic examples are the mercury thermometer and the bimetallic strip. The bimetallic strip is two different metals bonded together — when they heat up, they expand at different rates, so the strip bends, and that bending is what we measure. Second, the Vapour Pressure Type. Here's the idea: liquids, when subjected to a rise in temperature, change their state from liquid to vapour. So by measuring the pressure of that vapour, we can gain an indication of temperature. The vapour pressure tells us how hot things are. Third, the Electrical Type. This one is important. A change in temperature of an electrical conductor can cause a change in resistance of that conductor. So by measuring the change in resistance, we can indicate the temperature of the conductor. This sensor is called the Resistance Type — sometimes called a temperature bulb. That's the resistance-based approach. And fourth — and this is where the excerpt cuts off, but I want to set it up properly — there's the thermocouple principle. Two dissimilar metals, when joined together at their ends — that junction — can produce an electrical potential called a thermo EMF. That's the thermoelectric effect, and it's the basis for high-temperature measurement in gas turbines. So to tie it together: we have four families of sensors — expansion, vapour pressure, electrical resistance, and the thermocouple junction. Each one is suited to a different temperature range within that -56°C to +1200°C envelope. The expansion and vapour pressure types handle the lower end, and the electrical and thermocouple types handle the higher ranges where you're measuring turbine and exhaust gas temperatures. That's the foundation. When you're ready, we can move into how each of these is actually installed and connected in the engine — the probes and their electrical connections.