Jet engines have transformed air travel since their widespread adoption more than half a century ago. The power delivered by these revolutionary engines has allowed humans to fly further, faster, and cheaper than ever before. But how do these engines work?
The gas turbine
Turbofan engines power many modern commercial aircraft. These are part of a family of engines called gas turbines, which includes engines for some helicopters, smaller powerplants, and even for some types of tanks.
The name turbine gives some clue as to the way this type of engine functions. Other turbines, such as wind turbines or steam turbines, all rely on something spinning in order to generate power. Gas turbines are no different. While wind spins a wind turbine and steam drives a steam turbine, another type of pressurized gas spins a gas turbine – air.
Gas turbines need to produce this highly pressurized air themselves to ensure a flow of power to the engine. They do this by burning something that is very energy-dense, like jet fuel, kerosene, or natural gas. Burning the fuel expands the air, and it is this rush of hot air that causes the turbine to spin.
Suck, squeeze, bang, blow
The process by which this happens is sometimes bluntly explained by the concept of ‘suck, squeeze, bang, blow.’ Air is sucked into the engine from the front using the big fan visible when looking at a plane straight on.
That air is then compressed in the next stage of the engine – this is the ‘squeeze’ part. A second fan increases the pressure in the air by around eight times, which also significantly increases its temperature.
Fuel is mixed with the air and ignited – bang – producing the power. This hot, high-pressure air rushes past a set of turbine blades, causing them to spin. This turbine is connected by an axel to the compressor and the fan, so as the gases turn the turbine, this causes both the inlet fan and compressor fan to spin too.
Finally, the hot exhaust gases exit the engine via a tapering exhaust nozzle. Just as putting your thumb over the end of a hosepipe (reducing the exit aperture for the water) causes water to squirt out at high speed, this tapered exhaust has the effect of speeding up the exiting gases. The hot air leaving the engine is moving at over 2,100 km/h (1,300 mph), around twice the speed of the cold air entering at the front.
It is this rapidly moving air that pushes the vehicle forward. Military jets (and one very special passenger plane) sometimes use afterburners. This is simply fuel squirted directly into the exhaust jet to create additional thrust. But for most passenger planes, the shove from the moving air is more than enough to provide sufficient forward motion for their wings to generate lift.
Sounds simple enough? Essentially, it is, but the pressures and high temperatures involved make designing jet engines a rather specialist task. In the combustion chamber, where the compressed air is mixed with the fuel, the burning temperatures reach in excess of 900 °C (1,650°F).
This means that engines need to be made from strong yet lightweight, thermally stable and corrosion-resistant components that will not bend, break or become weak under extreme heat and pressure. In the early days of the jet engine, Sir Frank Whittle’s prototypes relied on steel. This was a strong and hard material, but would not cope with the stresses of the modern gas turbine. Steel begins to degrade at around 500 °C (932 °F).
The unsuitability of steel meant engine manufacturers needed to look for another type of material. The Goldilocks metal that manufacturers settled on was nickel with a bit of chromium mixed in. It was light, it was cheap, and it was strong. It resisted corrosion, and would retain its integrity to 85% of its melting point, which is a staggering 1,455 °C (2,651 °f).
These early superalloys allowed jet engines to become cheaper, more efficient and far easier to mass-produce. Descendants of this mix still provide the structure in the hottest part of the gas turbine engine, operating in temperatures of as much as 1,700 °C (3,000 °F), somewhat higher than the melting point of the metal. So how do engine manufacturers ensure the integrity of these parts?
Cool your jets
The first strategy is to apply a ceramic coating, which reduces the penetration of the heat. Secondly, cool air is fed to the surface of the blades, drawn from further up the engine and distributed by tiny holes in the blade’s surface. In an interview with The Engineer, Rolls-Royce chief of materials Neil Glover explained,
“The blades operate in an environment several hundreds of degrees hotter than the melting point of the nickel alloy, but because of the cooling mechanisms, the metal is never above its melting point, even though the environment is.”
Materials technology has gone even further than this, rearranging the metal’s atomic structure to avoid loss of integrity. The tiny crystals that make up metals are engineered to grow all in the same direction, to eliminate the weaknesses usually found at the boundaries of the crystals. This means the blades are effectively like a gemstone, with a single atomic lattice right through their structure.
The nickel alloys have further been refined over the years by creating new mixes and adding new elements. This gives the turbine designer the latitude to create the perfect mix for each engine component.
A balancing act
As engine designs have evolved and improved, turbofan engines have typically gotten larger. This is because a large proportion of the thrust generated is the result of incoming air diverted around the compressor and turbine. The difference in the volume of the air delivered to the turbine versus that which is diverted around it is known as the ‘bypass ratio.’
This ‘bypass thrust’ does not require fuel to be burned directly. As such, engine efficiencies have been improved by increasing the bypass ratio, which means creating a larger diameter engine. But there’s a downside to this too. Making the engine larger means making the fan sections larger too, which makes for a heavier engine. Every extra kilo of weight in the fan section requires 2.25 kg of additional support structure in the engine and wing.
In order to mitigate some of the increased weight of more fuel-efficient engines, manufacturers have started to turn towards composite materials as a replacement for metals. Ceramic matrix composites are as tough as metals but as just a third of the weight of nickel alloys.
The world’s current largest engine, the GE9X for the 777X, uses composite materials in the fan blades and case. It also uses ceramic matrix composites in the turbine and combustor. This large, light, and strong engine promises to be 10% more fuel-efficient than its predecessor, the GE90, and is also the quietest engine ever produced by GE.