Originally Posted by
Check Airman
why are the latest combusters difficult to light off (or light up)? .
Part of it is achieving a level of fuel atomization that can be ignited under cold conditions at a low, initial light up/off fuel flow rate with a nozzle system that has to achieve adequate atomization over a fuel flow range that varies over about a 50 to 1 ratio. For example, I remember one engine that had a light off fuel flow rate in the 700 lbs/hr range and takeoff flow rate in the 30,000 lbs/hour range. Even with a two stage nozzle system that is a difficult combustor and nozzle system design problem.
Another major factor is trying to balance what is called igniter immersion to be able to achieve light off in cold-soak starting conditions yet also to not burn up the igniter tip when operating at high power. Testing is performed to determine the positioning of the igniter tip relative to the spray pattern at lightup to just be able to ignite the edge of the spray pattern. If you just stick the igniter in farther to achieve easier light off, the igniter life may be measured in dozens of flight hours rather than hundreds or a few thousands of hours.
Regarding the term "light off," I have always worked in the Boeing Seattle Area world and did a lot of engine relight related work, and that is the term I always heard since the 1980s. It was probably a Boeing-ism. The Rolls-Royce engineers used different terms, but we tolerated each others' terminology differences (just kidding - I remember a bunch of great engineers from back in the 1980s from RR - unlike the the pilots, they were mostly named Ian rather than Nigel ;-). They said light-up and we said light-off. They said auto-relight (which eventually became standard) and we said autoignition, etc.)