Gas Turbines - Introduction — Page 206, Lesson 269

Let’s start with the big idea, because everything else hangs off it. The gas turbine engine and the piston engine/propeller combination are doing the same fundamental job: they both propel a mass of air backwards. That’s the whole point. And the reason that propels the aircraft forward comes straight from Newton’s Third Law: for every force acting on a body, there is an equal and opposite reaction. So when the engine shoves air backwards, the air shoves the engine—and the aircraft—forwards. Now, why does shoving air backwards create thrust? Because of the equation Force = Mass × Acceleration. In a gas turbine engine, the Mass is the air delivered by the compressor. The Acceleration is the difference between the outlet velocity of the air, which we write as Vo, and its inlet velocity, which we write as Vi. That velocity change happens because of the addition of heat energy—the fuel burning inside the engine speeds the air up. So the thrust force is therefore m × (Vo − Vi), plus something called Pressure Thrust. Written scientifically, it’s Thrust = W(Vo − Vi) + Pressure Thrust. Let me unpack that. The m is mass, and W is weight—in this context they’re being used interchangeably for the air flow. The (Vo − Vi) is the velocity change, outlet minus inlet. And the Pressure Thrust is an extra term that comes from pressure differences at the engine’s exhaust—we’ll deal with that properly in Chapter 20, but for now just know it’s part of the full thrust equation. Here’s the key contrast between the two engines. The propeller drives a relatively large mass of air backwards fairly slowly. The gas turbine throws a small mass of air backwards relatively quickly. Both generate thrust, but they do it with different combinations of mass and velocity. Now, one very important warning that I want you to remember, because it’s a classic misconception: the jet reaction does not result from the pressure of the jet on the atmosphere. In all instances, the resultant reaction—or thrust—exerted on the engine is proportional to the mass, or weight, of air expelled by the engine, and the velocity change imparted to it. So it’s the momentum change of the air that matters, not the jet pushing against the outside air. Let’s move to the working cycle. The gas turbine runs on what’s called the Brayton cycle, and the four-stroke piston engine runs on the Otto cycle. They’re very similar. The Otto cycle has induction, compression, combustion, power, and exhaust. The Brayton cycle matches that with induction, compression, combustion, and exhaust. Notice the difference: the Otto cycle has that separate power stroke, but in the gas turbine, power is developed in the turbine of the engine. There are two big differences between the cycles. First, in the gas turbine engine, combustion theoretically occurs at a constant pressure, whereas in the piston engine it occurs at a constant volume. That’s a fundamental thermodynamic difference. Second, the gas turbine processes occur continuously, whereas the piston engine processes are intermittent—they happen in discrete strokes. Here’s the efficiency payoff. In the piston engine, only one of the four strokes actually produces power; the other three effectively absorb power. In the gas turbine engine, those three ‘idle’ strokes have been eliminated, which allows more time for the burning of fuel. And that is one of the reasons why the gas turbine engine has a greater power/weight ratio than the piston engine. More time burning fuel, no wasted strokes, means more power for the weight. Let me show you the comparison visually. shows the Athodyd and the Whittle engine—the principles of the gas turbine engine. And compares the working cycles of the piston engine and the gas turbine engine side by side, so you can see how the Otto cycle’s induction, compression, combustion, power, and exhaust line up against the Brayton cycle’s induction, compression, combustion, and exhaust. One more thing worth noting from the history: the idea of using a jet reaction engine for aircraft is not new. Back in 1913, there was already a design for an aerodynamic jet engine. shows that early design. So the concept predates the practical engine by decades. So to sum up where we are: thrust comes from accelerating a mass of air backwards, governed by Newton’s Third Law and the force equation. The gas turbine does it with a small mass at high velocity, continuously, at constant pressure, and that continuous operation with no idle strokes is why it beats the piston engine on power/weight ratio. That’s the foundation. Next we’ll build the actual components onto this.