Making Today’s BizAv Engines More Efficient

One of the most significant developments in Business and General Aviation engine design in recent times, amounting to little less than an operational revolution, is the advent of computerized engine control and sensor-driven digital monitoring of engine performance and health.

From a pilot’s viewpoint, the availability of Full Authority Digital Engine Control (FADEC) improves engine efficiency and engine performance alike. FADEC also helps reduce pilot workload in terms of the frequency of control adjustments pilots have to make to keep the engine(s) operating optimally throughout every phase of flight. FADEC makes the adjustments automatically.

“When we look at the PT6 E-Series, as an example,” says Edward Hoskin, Vice President, Engineering for Pratt & Whitney Canada. (Note: The PT6 E-Series engine powers the Pilatus PC-12 NGX, and also the new Daher TBM 960 single engine turboprop aircraft). “The electronic engine control (EEC) provides full digital envelope protection, and is able to make the necessary adjustments to optimize and deliver the correct engine power throughout the flight.”

Specifically, he explains, the EEC monitors more than 100 parameters continuously, and key engine and aircraft data are used to optimize the engine’s operation and deliver the power needed throughout the flight. 

“In fact, the EEC acts as a sort of cruise control, leveling out the engine’s demand on power, and thus on fuel. And, since the engine is electronically [rather than mechanically] controlled, there are fewer parts to maintain,” Hoskins adds.

Increased Fuel Efficiency

The sophistication of the latest generation of computerized FADEC, the third generation, has allowed manufacturers to introduce two new control technologies into their business and commercial turbofan engines that increase fuel efficiency yet more and reduce emissions still further.

These technologies are: active turbine blade-tip clearance management and modulated blade cooling. Both use cooling air pathways, but they do so in different ways and the pathways are in different parts of the engine.

Active blade-tip clearance: works by feeding cooling air through pathways in the turbine casing which contains the turbine blade stages, in order to actively slightly expand or shrink the interior diameter of the casing. The expansion or shrinkage (controlled automatically) ensures that the tips of the turbine blades are as near to touching the interior of the casing as they can be without actually doing so, every moment of the flight.

This minimizes the amounts of exhaust gas which can spill over the edges of the turbine blades without doing the useful work of turning the blades to drive the high-pressure shaft, driving the high-speed compressor stages, and the low-pressure shaft driving the fan.

The greater the amount of exhaust gas (composed of an ignited mixture of fuel and compressed air) which does useful work turning the turbine blades, the more efficient the engine is, and the less fuel it burns overall.

Modulated cooling (of the high-pressure turbine blades): relies on a simple, but valid, premise that the temperature of the exhaust gas in which the blades are turning is higher when the engine is working hardest — during take-off, the first-stage climb, and just before landing — and lower when the engine is working less hard, during cruise and in descent idle mode. This ensures the turbine blades don’t require the same amounts of cooling air — during all phases of flight.

In some of the latest turbofan engines, FADEC controls allow the amounts of parasitic turbine cooling air bled from the compressor stages to be varied automatically in ways that optimize the cooling flows throughout every phase of flight, reducing the overall amounts of parasitic cooling air needing to be used and making the engine more efficient.

Digital Monitoring of Engine Performance and Condition

Beyond digital engine control, use of digital engine performance monitoring opens up vast opportunities for engine condition-reporting to the OEM and the operator’s maintenance organization in near-real time.

With the aircraft’s engine control computer and health monitoring unit constantly measuring and transmitting as many as hundreds — and in the most modern BizAv turbofan engines, thousands — of engine performance parameters, maintenance engineers on the ground can use predictive analytics algorithms to assess each engine’s maintenance condition proactively, even before the aircraft lands.

“They can then make all necessary arrangements for required forthcoming maintenance to be made in ways which minimize any downtime the aircraft or engine will have to undergo,” says James Hoare, Senior Director of Engineering in Honeywell Aerospace’s Propulsion Engineering Group.

“In many cases ground engineers will be able to detect the increasing likelihood of future maintenance actions well before any alert light on the aircraft illuminates to indicate an abnormal condition, or the potential failure of a part.”

Even more significantly, says Hoare, because engine health monitoring units in modern BizAv engines measure so many of the engine’s operating and internal performance parameters, maintenance technicians will be able to assess each engine’s condition, not only as regards soon-to-be-required maintenance, but also the aircraft’s environmental performance.

If engine operating parameters, such as exhaust gas temperature margin or HPT stage 1 temperature margin, are deteriorating to the point where the engine is performing or will soon perform sub-optimally, then the engineers can pre-emptively pull the engine for maintenance to restore its performance to peak environmental efficiency.

According to Hoare, for operators of Honeywell engines the data tools it makes available in the marketplace (such as its Forge software platform) provide them with the ability to predictively analyze, understand and act proactively on engine performance data.

The Rolls-Royce Pearl provides an excellent example of a modern engine which, through its engine vibration health monitoring unit (EVHMU), can monitor more than 10,000 different performance parameters.

According to Colm Golden, Senior Vice President of the Rolls-Royce Pearl Program, this and the ability of the aircraft to transmit the data to the manufacturer in real time has given Rolls-Royce the ability for the first time to aggregate engine performance data from the entire fleet of Pearl engines as they are being operated in the air.

This will allow Rolls-Royce to understand more about how pilots use the engines in flight, and also the engines’ behavior in different flight conditions.

“This will allow us to optimize engine cycles and ratings for weather conditions, and optimize fuel flow for each phase of flight,” Golden says. “In the past Rolls-Royce had to rely only on ground tests of engines (which by definition could not be precise) to get an idea of how its engines would operate when flying in varying climatic conditions and different phases of flight.”

Engine Integration and Aircraft Mission Profile

There is a couple of other important considerations for BizAv engine designers when seeking to improve engine efficiency and reduce environmental emissions.

Engine Integration: First is the importance for overall engine operational efficiency of the aerodynamic and structural integration of the engine with the airframe on which it is to be mounted. “For both turbofans and turboprops, engine integration and installation on the aircraft play a key role in optimizing engine performance across all phases of flight,” says Hoskin.

Integration of the engine and nacelle includes the position in which the engines are mounted on the aircraft, particularly for business jets designed to cruise at high subsonic speeds and at high altitudes, according to Hoare.

Business jets typically are configured to have their engines mounted at the back of the aircraft, high on the fuselage, near the vertical stabilizer, in order to minimize the amount of engine noise experienced in the cabin. For different aircraft types, “How does that impact the system and the mission?” he asks.

Mission Profile: Designers also have to optimize each engine type for the typical mission profile envisaged by the manufacturers of the aircraft the engine is intended to power — a decision which often influences their choice whether to offer an airframe manufacturer a turboprop engine or a turbofan engine for a given aircraft type.

(Nobody, for example, would think of powering the Gulfstream G700 with a turboprop engine, nor install Pearl engines on a Twin Otter!)

“Turboprops inherently have high bypass ratios,” (in the 50-to-1 to 100-to-1 range), says Hoare. “They are very efficient,” their large propellers producing enough bypass air to ensure that turboprops operate at nearly the theoretical limit of their propulsive efficiency throughout each flight.

“Turboprops are ideal for operating at lower altitudes, on smaller aircraft, and they’re best suited to short hops,” he says. The exhaust gases from turboprop engines produce little if any measurable contribution to the powerplant’s overall thrust.

Turbofans, on the other hand, produce some of their thrust from the hot gases exhausting from their cores, though in today’s BizAv turbofan engines most of the thrust is produced as cold bypass air by the large fan at the front of the engine.

Turbofan engines are suited to operating at faster cruise speeds and higher cruise altitudes, but offer less propulsive efficiency than turboprops. Both categories of engine can realize performance gains from improved thermal efficiencies.

In Summary

Today’s business aviation engines, turbofan or turboprop, are technological marvels which incorporate cutting-edge advances in many fields of science and technology.  Neither the turboprop nor the turbofan is inherently superior in operation to the other type of engine — both are classes of gas turbine engine, optimized for different jobs, and often for different wallets.

A third class of turbine aero engine, the turboshaft, is optimized for another different mission, powering rotorcraft.

The day is likely to dawn within the next 20 years when a new, fourth class of turbine aero engine — the open fan or open rotor — will marry the high propulsive efficiency of the turboprop to the high thermal efficiency of the to create a more environmentally friendly engine capable of efficiently powering business aircraft and commercial aircraft alike.

Today this is typified in the Open Fan design being developed in CFM International’s Revolutionary Innovation for Sustainable Engines (RISE) R&D program to study potential next-generation turbine engine configurations, propulsion methods and fuel sources. The Open Fan could offer bypass ratios as high as 75-to-1 (CFM’s goal for the Open Fan design) while offering thrust levels capable of powering the largest business and commercial aircraft. 

More information from:

Honeywell Aerospace – https://aerospace.honeywell.com
Pratt & Whitney Canada – www.pwc.ca
Rolls-Royce – www.rolls-royce.com

Read the previous article from Chris Kjelgaard - How BizAv’s Top Engine OEMs are Cutting Emissions