Gas Turbines - Introduction — Page 206, Lesson 271

Let's start with the pressure-volume diagram, because that's the heart of this whole section. I want you to picture a graph with pressure on one axis and volume on the other. That's what a pressure-volume diagram is — it shows how pressure and volume change together through a cycle. The diagram we're looking at is called the Brayton Cycle, and it represents the working cycle of the gas turbine engine in its simplest form. So when you hear "Brayton Cycle," that's the name for the idealised pressure-volume relationship of a gas turbine. Now let's trace the cycle. Air at atmospheric pressure enters the engine at point A. So point A is the starting condition — the air coming in is at the same pressure as the surrounding atmosphere. Then the air is compressed along the line A-B. That's the compression stage, where pressure rises and volume shrinks. At point B, fuel is added in the combustion chambers. So point B is where combustion begins. The fuel is burnt — and here's the key theoretical assumption — in theory, at a constant pressure. That's a big deal, and we'll come back to it. Now, in actual fact, there are pressure losses in the combustion chamber. Why? Because you have to produce swirl and turbulence to mix the fuel and air properly. That turbulence causes a pressure drop throughout the length of the chamber of between 3 and 6 percent. So even though we say "constant pressure," the reality is a small pressure loss of 3 to 6 percent. Nevertheless, despite that loss, a considerable increase in the volume of the air is generated within the combustion chamber. That's the whole point of burning fuel — you add heat, and the gas expands dramatically in volume. Then, between points C and D, the gas generated through combustion expands in the turbine and the jet pipe. So C to D is the expansion stage. Theoretically, the gas attains a value equal to atmospheric pressure before being ejected. So the cycle ends where it began — back at atmospheric pressure, ready to start again. Now let's talk about constant pressure combustion, because that's the defining feature that separates a gas turbine from a piston engine. As I said, theoretically combustion occurs at a constant pressure. This is achieved partly through the continuous process of the Brayton cycle — it's a continuous flow, not a batch process — and partly because the combustion chamber is not an enclosed space. The chamber is open at the back, so pressure can't build up like it would in a sealed cylinder. These circumstances ensure that there are no fluctuations of pressure in the engine, as there are in the piston engine. In a piston engine, peak pressures greater than 1000 pounds per square inch have to be accommodated. That's over a thousand psi — a huge number. Those pressures necessitate extremely strong and heavy construction in the piston engine. And if detonation is to be avoided — detonation being the uncontrolled, explosive burning of fuel — you need high octane fuels. Now here's the contrast, and it's a beautiful one. In the gas turbine engine, the use of low octane fuels and relatively light construction methods are the rule rather than the exception. So the gas turbine can burn cheap, low octane fuel, and it doesn't need the heavy, strong construction that a piston engine demands. That's a direct consequence of the constant pressure combustion — no pressure spikes, no need for massive strength. So the whole story here is: the Brayton cycle gives you continuous, constant-pressure combustion, which means low pressure fluctuations, which means light construction and low octane fuel. The piston engine, by contrast, has to survive over 1000 psi peaks, so it's heavy and needs high octane fuel. That's the fundamental trade-off that makes the gas turbine so attractive for aircraft. Take a look at Figure 13.4 to see the Brayton cycle drawn out — you'll see the A-B compression line, the combustion at B, and the C-D expansion.