Precision ignition

Rocket propulsion could benefit from a laser-based ignition technique originally designed for clean and lean-burn gas turbines.

Rocket propulsion could benefit from a laser-based ignition technique originally designed for clean and lean-burn gas turbines.

A US-based turbine project ended in failure last month, but the principle of using a laser within advanced propulsion systems is still of interest. Matthew Thomas, director of special projects at CFD Research Corporation in the US, said there were several on-going projects. ‘We have evaluated the laser for cold gas-turbine ignition, bipropellant rocket ignition and instability damping, scramjet flame stabilisation, gun propellant initiation and cold diesel starting.’

Work on the rocket ignition showed most promise. Testing is under way on old Saturn V engines that mix gaseous oxygen and RP-1, a kerosene-like hydrocarbon fuel. To get the rocket going during the first few seconds these usually require a large torch or a ‘hypergolic’ chemical reaction involving two chemicals that ignite on contact with each other. Neither is especially safe or reliable.

Thomas said that recently declassified nuclear weapons technology had allowed them to develop a high-powered laser ‘spark’ to ignite and stabilise the combustion of the rocket propellants. Exploiting a laser in this way would give engineers more control over the ignition process than with the other methods. They could adjust the spark’s intensity more easily and direct it to the right place. Providing extra energy boosts at will would also help prevent the combustion reaction from fizzling out.

Having shown that the technique was viable, Thomas said the team was trying to model the ignition process to understand it better. The idea’s lack of success in gas turbines may, however, have set back the quest to increase their efficiency and reduce their emissions.

Since October 2001 a 12-month project had been under way at the US government’s National Energy Technology Laboratory in conjunction with Los Alamos National Laboratory, CFD and Caterpillar subsidiary Solar Turbines. Initially a laser was set up in a natural gas piston engine to investigate in principle whether it could be an alternative to a conventional spark plug. Pulses lasting 10 nanoseconds were fired through the spark plug port and focused with lenses and mirrors. The idea was to create a ‘plasma pool’ to help keep the flame going.

The team had thought that using laser sparks in this way could reduce unwanted NOx emissions to under five parts per million without needing to treat the exhaust itself or use catalysts in the engine cycle. However, one NETL source explained that it didn’t work as expected.

In fact, the creation of several hot spots within the engine may have contributed to the production of NOx, more or less cancelling out the benefits of a lean fuel-air mixture.

Thomas, on the other hand, believes that the wrong approach was taken. ‘You need to change the combustor itself to help the laser to work,’ he said. He is confident that with further funding and research the technique could work.

Sidebar:Fewer NOx-ious emissions

In a drive to increase efficiency and lower emissions, major gas turbine manufacturers are racing to develop the ultimate lean-burn engine. Gas turbines are found not only in aero-engines but in applications such as offshore power generators and naval propulsion systems.

They generally work with a particular mix of air to fuel, but when full power is not required the air content can be increased and the amount of fuel reduced.

This lean-burn technique improves efficiency and reduces the quantity of fossil fuels consumed. It also lessens environmental pollution, the main culprit of which is NOx emissions originating from the burnt air. The lower combustion temperatures in a lean-burn system reduce the amount of NOx formed.

However, more energy than usual is required to get the process going, and conventional spark plugs aren’t up to the job of igniting the fuel-air mixture when the air content is so high. They cannot reliably deliver sufficient energy and break down more quickly with overuse, presenting a problem for engineers.

Further projects are under way intended to improve the lean-burn cycle itself. In the UK Rolls-Royce has announced that it will try to reduce the average fuel burn in its turbines by 10 per cent over the next 10 years, and bring the amount of NOx its engines emit to half of that allowed under current legislation. One initiative involves injecting two or three times as much air into the turbine’s primary zone near the engine’s nozzle. With the reduction of temperature this should considerably decrease NOx emissions.

Pratt & Whitney also has a ‘green engine’ research programme. One initiative is the TALON (Technology for Advanced Low NOx) project, whose latest versions are already being incorporated into new Airbus A330 engines.

The company said the TALON combustor can direct cooling air only to where it is needed, thus saving the amount of superfluous air required. This means more is available to mix with fuel for lean burning.