Air force

It may be a quirk of nature, or it may just be a symptom of how engineering development works: separate teams tackling the same problem come up with different solutions and the technologies have to compete in the market to see which will be successful. It has happened again and again, from communication protocols for wireless sensors, to the more familiar examples from our homes: Mac versus PC and VHS versus Betamax.

The latest of these competitions is set to begin in the air, as aero-engine manufacturers tussle for orders for the new generation of cleaner engines.

The challenge is to boost the efficiency of the jet engine for commercial aircraft, with geared turbofans and open rotors the competing technologies. Both work by maximising the amount of air pulled through the engine while minimising the amount of air mixed with fuel — a key ratio in jet-engine technology. But open-rotor engines are still in development, while geared turbofans are about to hit the market.

US company Pratt & Whitneyhas just introduced the PurePower PW1000G, the first commercial geared turbofan. The PurePower is a mid-range engine that already has two customers — Mitsubishi, which will use it on its Regional Jet, and Bombardier, which will use it to power its CSeries. ‘This new engine will redefine flight,’ claimed Bob Saia, vice-president of the Next Generation Product Family at Pratt & Whitney. ‘It will change everything.’

Geared turbofans look quite similar to the standard turbofan engine familiar from current airliners, except they are wider and shorter. They are a response to a natural limit of turbofan engines, which is a result of the way the engine components spin: the turbine at the heart of the engine burns fuel to create thrust, but it also drives the big fan on the front of the engine, which draws the air into the turbine.

The fan pulls a large amount of air into the engine but only a small amount of this is compressed and mixed with fuel to be burned in the turbine. To improve fuel efficiency, aerospace engineers maximise the amount of air pulled in, which is known as bypass air, while minimising the amount of fuel needed to drive the turbine. The ratio of the volume of the bypass air to the air mixed with the fuel is known as the bypass ratio; the higher this is, the more efficient the engine.

This is why successive generations of turbofan engines have had bigger and bigger fans, but this is also where the natural limit of the technology comes in. The turbine and the fan are both attached to the shaft that runs through the middle of the engine. As the fan gets bigger and heavier, the turbine has to generate more power to spin it. Conventional turbofan engines have now reached the point where the penalty of the weight of the fan cancels out the fuel efficiency of increasing the bypass ratio.

The geared-turbofan solution solves this problem by breaking the direct connection between turbine and fan. The drive-shaft from the turbine is connected to a series of gears in a reduction gearbox that allows the fan to turn slower than the turbine. This means the turbine can be very small and very fast, while the fan can be much larger and slower. Both of these are inherently efficient configurations, said Pratt & Whitney. There is another advantage — it is the fan that produces most of the noise of the engine, and big, slow fans are quieter than smaller, faster ones.

Saia listed some of the advantages of the engine over the most advanced conventional turbofans: in terms of fuel, the largest single running cost for any airline, the PurePower engine uses 12-15 per cent less, equivalent to 530 litres less fuel per trip, and 1.15 million litres less per year. With a fuel price of £0.46 pence per litre, this works out at an annual saving of more than £500,000 per engine. The fuel saving also equates to a reduction in CO₂ emissions of about 3,000 tonnes of CO₂per aircraft per year. ‘That’s equivalent to planting 700,000 trees,’ Saia claimed.

The engine produces half as much noise as a conventional turbofan and has a 72 per cent smaller noise footprint — the area around an airport affected by high noise levels. This would again save airlines money, explained Saia, as they have to pay ‘noise fees’, which would reduce by 2-3 percent. In all, he said, PurePower would save $1.5m (£1.04m) per aircraft per year.

Among the innovations in the engine is the fan itself, which has 18 wide blades made from a lightweight and classified metallic material. Most large-fan engines have more than 30 blades. The fan is driven at two-thirds of the speed of a conventional fan via, a gearbox developed by Italian specialist Avio, which has collaborated with Pratt & Whitney on many projects, including the engine for the F22 Raptor fighter jet.

At the heart of the engine is the combustor, where the fuel/air mixture is burned. This is the source of the engine’s CO2 emissions and where oxides of nitrogen are formed. These compounds are atmospheric pollutants resulting from the reaction between the oxygen and nitrogen in the air, and jet designers expend much effort on reducing them. The PurePower system is no exception, using the latest version of Pratt & Whitney’s TALON (Technology for Advanced Low Nitrogen Oxide) series of combustors.

The key to reducing NO_x production is to reduce the length of time that the burning gases are at their hottest point, as it is heat that promotes the reaction between oxygen and nitrogen. This entails controlling the proportions of fuel to air very carefully. A stoichiometric mixture — the ratio that provides the exact amount of oxygen needed to combust all the carbon in the fuel — produces the hottest flame and therefore the biggest amount of NO_x. Cooling the mixture down is therefore the priority.

The TALON combustors use a system that Pratt & Whitney calls rich/quench/lean, or RQL. At the front end of the combustor, the fuel/air mixture is rich — that is, it has more fuel in it than a stoichiometric mixture. In the middle, air is added to the mixture to quench or cool it so that at the exit point the mixture is lean and has the correct temperature for the turbine.

The combustor used in the PurePower engine, the TALON X, was designed by Pratt & Whitney and NASA, and uses techniques from industrial gas turbines, with a metallic liner and new methods for mixing the fuel with the air. Saia said that this halves NOx emissions, reducing them in ignition, cruise and landing mode.

The engine has been subjected to stringent tests, with all the separate components developed on dedicated rigs and the completed assembly run on both static and flying tests. ‘One of a kind demonstrator engines such as this are typically designed to test for approximately 100 hours,’ Saia said. ‘It’s extraordinary that the PW1000G demonstrator engine completed 406 hours of testing, including 120 hours in flight.’ The engine was tested at high altitude and subjected to 2G accelerations, he added.

The data will be used in the design phase of the project, which will start around the middle of the year, prior to the engine going into production.