On the day of my visit to Honeywell’s engine flight-test center at Phoenix Sky Harbor International Airport, the 757 carried an HTF7000-series engine undergoing testing in preparation for certification flight trials aboard the Embraer Legacy 450 and 500 business jets. The plan was to run the engine at various altitudes and airspeeds and under loads atypical of normal corporate jet operations. In other words, torture testing.
“We try to sniff out anything unusual in the engine so that the customer isn’t the first one to discover it,” said Ron Rich, vice president of propulsion systems for Honeywell.
That involves slamming the power full forward and then immediately pulling it all the way back (all controlled by computers) as well as doing fun things like inducing compressor stalls — yes, on purpose — at altitude. Modern turbofan engines are carefully designed to avoid full compressor stalls — also known as surging — when operating in their normal range. Surging was a common problem on early jet engines, but the phenomenon has been virtually eliminated by better design and the use of electronic control systems such as full authority digital engine controls (fadec).
Still, with the press of a few buttons, a flight-test engineer sitting at a workstation in the cabin can cause a test engine to surge.
For our trials we headed south from Phoenix and spent a few hours following a racetrack pattern high above the Arizona desert near the border with Mexico. Because of the scarcity of air traffic and perpetually good weather, this sliver of airspace is perfect for these types of evaluations. Our trip aloft followed a predetermined test regimen involving step climbs to 15,000 feet, 25,000 feet and, finally, 31,000 feet. At each new altitude, the engineers commanded the HTF7000 engine to transition instantaneously from max power to flight idle and back to max power — something you’d probably never do during a regular flight, but because you can, it has to be tested.
Next, the test engineers seated at various stations in the 757’s cabin prepared for the intentional surge. The sequence involved closing and locking two of the engines’ three bleed air valves and then slowly reducing N1 power. Soon, the engine let out a one-second belch. To me it sounded like the cannon burst of an AC-130’s side-facing 20 mm guns — not as loud, perhaps, but enough to get your attention.
All of the testing and minute attention to detail appear to be paying off for Honeywell. The HTF7000 engine — selected to power the Legacy 450 and 500, Bombardier Challenger 300 and Gulfstream G250 — boasts the highest dispatch reliability (99.96 percent) of any turbofan the company has built.
How does a turbine engine work?
The easiest way to understand what makes a turbine engine work is to compare what’s happening with another type of internal combustion engine: the piston engine in your airplane.
The four basic steps of any internal combustion engine are:
In a piston engine, the intake, compression, combustion and exhaust steps occur in the cylinder head many times per minute as the piston goes up and down. In a turbine engine, these same four steps occur simultaneously in a continuous flow in different places.
As a result of this fundamental difference, the turbine engine has sections called:
Air enters the inlet section at the front of the engine and passes more or less straight through, front to back. Along the way, a compressor squeezes the air in the compressor section to about one-10th or one-15th of its original volume. Fuel is added and burned in the combustion section, and finally the air is ejected through the exit nozzle to produce thrust. Just after the combustor but before the exhaust nozzle is a turbine (connected by a shaft to the compressor) that harnesses some of the energy in the discharging air to keep the compressor spinning.
To achieve the 10:1 to 15:1 compression needed to develop adequate power, modern turbine engines are built with many stages of compressors stacked in a line and usually separated into segments. Most turbine engines built today are dual-rotor designs, meaning there are two distinct sets of these rotating components. The rear compressor, or high-pressure compressor, is connected by a shaft to a high-pressure turbine. This is the high rotor (the guts of the engine), often referred to as N2. The front compressor, or low-pressure compressor, sits in front of the high-pressure compressor and is connected to another rotor, called N1.
Turboprops are turbine engines that drive a propeller, and turbo-shafts are turbine engines that drive a shaft connecting to a helicopter’s rotor blades.
A turbofan engine is a type of turbine engine in which the N1-stage compressor rotor is much larger in diameter than the rest of the engine, and is called the fan. The air that passes through the fan near its inner diameter reaches the remaining compressor stages in the core of the engine and is further compressed. The air that passes through the outer diameter of the fan rotor does not pass through the core of the engine, but instead passes along the outside of the engine. This air is called bypass air, and the ratio of bypass air to core air is called the bypass ratio. High-bypass-ratio turbofan engines used in most business jets are quieter and more fuel-efficient than turbojet engines.