July 26, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.
To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.
We have covered the compression in the compressor (Figure 1) and now go on to combustion in the combustor.
In the combustor, the high-pressure air from the compressor burns the injected fuel to produce a high-volume combustion gas that drives the turbines. The gas ultimately exits in the nozzle and produces jet thrust.
The combustion process is made complicated by several factors:
Figure 2. NOx limits versus the CAEP levels and the NOx emissions for engines of different generations and OPRs. Source: EASA.
The above makes combustion chamber design particularly hard as:
Figure 3. The formation and consumption of Soot (i.e., smoke) and NOx in a modern combustor. Source: Cumpsty, Jet Propulsion.
The requirement for very stable and efficient combustion, close to stochiometric level ( = where all air Oxygen is combined with the fuel’s Carbon to CO and CO2) at all engine power levels/altitude/speeds and ever lower emissions while delivering ever higher levels of heated gas in the smallest possible volumes (to keep engine mass and size low) has made combustor design a real “Rocket Science.”
We will look at the history of combustor design and where it is now in the next Corner.
Often you divide the combustor into different zones. First the diffuser from the compressor exit and the dome that splits the airflow into the cooling air outside the burner liner and the airflow into the primary zone for mixing with fuel for hot and quick burn, then you want to quench the flame as the fuel is consumed and lower its temperature below NOx generation for finally mix with more cooling air to get desired temp profile into the turbine nozzle. The chemical reactions during combustion are complex and fast to finally ideally only produce H2O and CO2. The air coming into combustion can contain sand and salt, that recrystallize in different areas downstream.