The regulatory framework for engine manufactures is becoming increasingly tough as the issue of emissions joins noise as a key pressure point
Until now, noise has taken centre stage in the environmental debate, reaching a peak with the infamous transatlantic row over hushkits. While this was going on, the issue of emissions continued to creep up the agenda. At the sixth meeting of ICAO's Committee on Aviation Environmental Protection (CAEP 6) due in April next year the emissions issue is now set to take the same high profile as noise did at CAEP 5 back in 2001. Again, the Europeans are pushing the agenda, with Brussels supporting the idea of some form of environmental charge and promoting the idea of a further tightening of engine emission standards.
These emissions mainly fall into two categories. The first, carbon dioxide (CO2) is the main greenhouse gas, and is generally a high-altitude problem. The second type are various oxides of nitrogen (NOx), which are more of a problem in the immediate vicinity of airports - although it also has some effect on the upper atmosphere.
While CAEP 5 was dominated by the noise issue, it also carried out some important work on emissions. The Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC), that has still to be ratified by many countries, requires developed nations to reduce CO2, and requested that ICAO, as the international body for aviation, set out a framework for this. However, CAEP 5 decided not to develop a standard for CO2 as these emissions are directly related to fuel consumption, and CAEP decided there was already significant economic incentive in this field. But there is a more fundamental problem - the difficulty in agreeing an acceptable standard. Fuel consumptions varies not only by aircraft/engine type, but also depends on the stage length flow, making hard limits tough to set.
"There is a danger that if we develop a standard, designers will then design for that particular condition, even though it may not reflect what is really happening. It could even be detrimental," says John Crayston, coordinator of air transport and environmental programmes at ICAO.
Focus shift
Against this background, the focus for agreeing emission standards has moved on to NOx, although CO2 is still very much in the environmental firing line. The pressure for reductions in NOx emissions has tended to come from those concerned about air quality in and around airports, and, as a result, the limit for NOx emissions are set around a landing and take-off (LTO) cycle below an altitude of 3,000ft (915m). The original NOx standard was adopted back in 1981, with ICAO stipulating a 20% reduction in 1993 for newly certified engines, with a production cut-off from 1999.
In the same year, ICAO set a further 16% reduction for aircraft certified after 2003, and one of the key tasks for CAEP 6 will be to set a possible production cut- off date to follow this up - as well as setting a timetable for a yet more stringent standard for newly certified engines.
While the exact regulatory framework may be somewhat vague at the moment, engine manufacturers realise that they are under pressure to perform. For instance, Pratt & Whitney (P&W) estimates that the regulatory requirements for the next generation of engines will demand around 25% less fuel burn, 70% less NOx and a noise reduction of 40dB. P&W's latest engines have been developed with new combustors to reduce NOx and the manufacturer is working on improvements such as low emission combustors, advanced gear drive systems and swept hollow fan blades.
In the USA, NASA's ultra-efficient engine technology (UEET) programme is targeting a reduction in fuel consumption/CO2 of around 15%, and a 70% drop in NOx during the LTO cycle. NASA is working in partnership with a number of different aerospace companies, including P&W and GE. The latter is tackling the problem of increased NOx at high combustion temperatures. As a general rule, the higher the pressure ratio in engines, the better the fuel efficiency (which means less CO2) but the higher the NOx emissions per kg of fuel burnt. This problem is becoming more of an issue as manufacturers are increasingly under pressure to reduce both CO2 and NOx - not to mention noise.
Tony Houseman, attach‚ for environment and policy issues at the European Association of Aerospace Industries, says: "Increasingly, we are encouraging people to look at the design trade-off issue. Generally, CO2 has been seen as the main greenhouse gas, but NOx is becoming an issue in terms of local air quality." He warns that NOx could even become more of an issue than noise over time.
Juggling act
All this leaves engine manufacturers with a juggling act. "We are telling people that if we go in a certain direction, we may have to limit reductions in another area," warns Houseman.
GE is attempting to tackle this problem with a ceramic-mix composite combustor that will sustain combustion temperatures of 1,480°C - crucial for reducing NOx emissions. GE is meanwhile developing aspirating seals to reduce fuel consumption.
In Europe, aerospace companies have also teamed up on two programmes, one aimed at the near term and one targeting the year 2020. The first of these programmes - affordable near-term low emissions (ANTLE) - is aiming at a 12% reduction in fuel consumption compared with 1990s technology and a 60% reduction in NOx compared with ICAO's standards of the mid-1990s. A test bed with a Rolls-Royce (R-R) Trent 500 is being used for the ANTLE project. R-R's German Aerospace Research Programme is looking at reducing NOx through lean-burning technology. The aim of avoid the high temperatures where NOx is formed by pre-mixing the fuel and air prior to combustion.
R-R reports: "Intermediate-pressure testing has demonstrated the ability to meet targets with pre-mixed solutions." The company states that, although there has been encouraging progress towards its goal of reducing NOx by 50% from 1990s standards by 2010, "a considerable challenge remains to demonstrate all the technical attributes required for a flightworthy combustion system". R-R is also involved with the European-led component validation for low emissions aero engines (CLEAN) project that aims for a 20% reduction in CO2 and 80% reduction in NOx by 2020.
All this development is taking place against a background of the possible imposition of an environmental charge, in Europe if not the rest of the world. Last year, the European Commission (EC) called for a study from Dutch consultancy CE Delft into possible "economic incentives" to mitigate greenhouse gas emissions. The report is basically a feasibility study into the two options - an environmental charge or a performance standard incentive (PSI) charge. The latter would be revenue neutral, whereas the former would incur a charge in proportion to the volume of emissions and would thus be revenue raising.
The report argues that although in the past high fuel prices have led carriers to consider - and sometimes implement - re-engining programmes, this is unlikely to be an option in response to environmental penalties, as such programmes are too costly - especially if the measures are carried out on a regional basis.
In the longer term, CE Delft notes that ultra-high bypass engines offer possibilities for CO2 reduction, while noting the problem of increased NOx. In the shorter term, CE Delft concludes that a move towards charging for CO2 emissions would help to reduce NOx. The report argues that this is likely to be the case for three key reasons. First, lower fuel consumption anyway tends to lead to lower NOx. Second, there is little chance that the existing fleet will be retrofitted with high NOx but low CO2 engines. Finally, aircraft that introduce winglets or riblets to reduce fuel burn will also produce less NOx. However, the report notes that, as its analysis is aimed at the year 2010, it has not considered the possible long-term impacts of incentives on engine development.
Re-engining uneconomic
CE Delft looked at the possibility of re-engining the Boeing 737-400 configured with a CFM56-3 B-2 in response to emission penalties, and came to the conclusion that even under the harshest regime it did not make economic sense. The report is based around three potential charging levels. That starts, at the high end with €50/t ($54/t) for CO2 and €6/kg ($6.5/2.2lb). The low end sits at €10/t CO2 and €1.2/kg of NOx. A midpoint of €30/t of CO2 and €3.6/kg of NOx is also considered.
Based on the high end charge, the cost of new engines for a 737-400 would come to $5 million, plus the expense of making adjustments to the avionics and main landing gear as well as new engine cowlings to accommodate the larger fan diameter. That comes nowhere near the $2.67 million needed to balance out the penalties over the aircraft's useful life.
The study also looked at the Boeing 747-400, and again concluded that re-engining was unlikely. The most popular engine choice, the GE CF6-80C2B1F, is the most environmentally friendly for combined LTO CO2 and NOx. The only other possibility for reducing NOx emissions is the Pratt & Whitney PW4x58 engine. However, this has 4% higher LTO specific fuel consumption, which means that any gains will be either minimal or non-existent. The study also looked at the possibility of replacing a R-R RB211-524G-T engine with a CF6-C2B-1F, but again concluded that this was unlikely to be economically feasible unless the whole flight was within the European charging zone. This is clearly unlikely for any long-haul aircraft. With a mid-range charge and presuming that 20% of the flight is within Europe, the break-even investment of $24.9 million is similar to the estimated $24.4 million cost of the new engines, before other factors are taken into account.
Rather than engine replacement, the main effect of a charge system is likely to be a reduction in the average fleet age. That will vary by airline type, ranging from low-cost carriers with the youngest but most heavily utilisted fleets, through to express freight operators which tend to have very old fleets. However, the imposition of an environmental charge is likely to lead to accelerated fleet renewal.
CE Delft estimates that the "lowest cost age" of an aircraft - the age where total fuel, maintenance and capital costs are lowest - is around 10-12, years assuming average sector lengths and utilisation levels. This is lowered by a year at the mid-range charge level, while imposing a charge at the high end would move the mark by two years. The report estimates that with an average historic fuel efficiency improvement of 1.7%, each year of accelerated fleet renewal will reduce CO2 by 1.7%, but admits that the impact on NOx is less certain.
Noting the tendency for recent engines to have higher pressure ratios and therefore higher NOx emissions, the report predicts an overall NOx reduction of around 1.2% per year, with the reduced fuel burn compensating for the 0.5% increase in NOx per kg of fuel burnt.
CE Delft point outs that this analysis should be treated with caution. Of course, there is no figure at the moment for a potential charge. The report bases its charge figures on what it sees as the external costs of the climatic impact of aviation in order to give some idea of the possible parameters. "It is to be stressed that the ultimate choice of level is a political issue," the report emphasises. However, it may contain some key clues as to the future regulations that airlines and engine manufacturers may face.
REPORT BY COLIN BAKER IN LONDON
Source: Airline Business
