August 2, 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 problem areas of (Figure 1) compression in the compressor and combustion, with its requirements on low Soot and NOx emissions. Now we look at how combustors are designed to achieve such low emissions.
In the combustor, the high-pressure air from the compressor burns the hydrocarbon fuel to CO2 while releasing substantial amounts of heat. The emission of CO2 is directly proportional to the amount of fuel used in the engine; thus, lowering CO2 emissions is about making the engine more efficient.
Unwanted emissions are NOx, unburnt hydrocarbons, CO, and Soot (which is unburned carbon). The design of the combustor shall reduce these emissions as close to zero as possible.
The initial combustor was the tube combustor (Figure 2), which later developed into an annular combustor. We will use the tube combustor to discuss the different methods to achieve efficient combustion and to deliver a clean, uniform, and acceptable-temperature gas to the turbine.
The air from the compressor is split 20% to the fuel injector (Figure 3) and 80% to the outside of the flame tube (see Figure 2 for names). The 20% forms a recirculating “smoke ring” area after the fuel nozzle, where the flame resides. The fuel is sprayed into the “smoke ring” center as droplets (the white areas in the flames) and vaporizes and burns in the Primary zone at air-to-fuel ratios of around 5 to 15 (stoichiometric burn is at 15), depending on engine thrust and at temperatures of more than 2,000°C.
In the dilution zone, the gas is diluted with air, which cools the gas before it hits the turbine nozzle at less than 1,600°C for single-aisle engines and 1,800°C for widebody engines when these run the hottest at takeoff thrust.
To create a more efficient combustor with smaller dimensions and more uniform turbine entry temperatures, the tube combustor was developed into an annular combustor, Figure 4.
Figure 4 shows the latest combustor from GE Aviation, developed for the GE9X. It uses a fuel injector that divides the fuel and airflow into a Pilot and Main flame area, Figure 5.
At engine start and low-power operation, only the rich burn Pilot part is in operation. At higher power, the outer ring of air and fuel (the Main part with its cyclonic air mixers and blue arrow fuel flows) cut in and dominates the burn with a lean burn process.
In total, about 70% of the combustor air passes through the Pilot and Main paths, where the dominant Main flow makes the whole combustor a lean combustor with low emissions at operational thrusts. By keeping the flame zone short with low top temperatures, the design delivers low NOx values.
The example shows GE Aviation’s solution to designing a low-emission combustor. Other engine OEMs solve the problem using similar rich and lean burn zones, but the mechanization differs.
There is constant development at the OEMs of ever-cleaner combustors with lower emissions of NOx, Soot, and CO.
We have now covered the combustor. The next station is the turbines, which we examine in the next Corner.
Will you also touch upon how the GE-Safrans hybrid electric core works, that have an electric generator/motor with electric power converter on each shaft, and hence likely can transfer power between the shafts, and what advantages that will have in ascend and level flight, and the resulting possible efficiency?
The fuel injectors normally inject fluid fuel that vaporize and hook up to oxygen molecules before its chemical combustion reactions. There are examples of pre injection fuel vaporisation. Turbulence just aft the “splash plates” helps this process but it also reduce pressure. So just enough turbulence in a short axial distance is best.
What are the potential environmental impacts of the proposed development project, and how can sustainable practices mitigate these effects, hence promoting ecological balance?
This is very interesting to me. My work of 30 the last 30 years was with Generators (mostly diesel but some Natural Gas sets). I did mechanics before that for myself and others and some engine work.
I watched the beginning of the Diesel emissions so am familiar with the various contaminants. It was funny to see EGR (exhaust gas reconciliation) become one of the main tools in diesels well after it was used in gasoline engines.
Unlike ground equipment (cars, big trucks, earth moving etc) jet engines don’t have any alternatives.
Natural gas was interesting as it was hugely cleaner than diesel. You could run oil in one of those sets for 5 years and not see the least bit of discoloration. It seems to me they should have implemented it wholesale.
Latest rockets have moved to Methane.
Diesel emissions were referred to as Tiers. I think we are on Tier 4 or 5 here and Europe has Euro equivalent. I only worked with one Tier 2 engine (standby generators were not heavily regulated due to their minimum run times – most of my work was with that type.
Now its various strategies of Diesel Cats, series injections, swirl, Soot Filters and Urea Injection., Ultra Low Sulfur Diesel.
One of the big diesel busts was when Navistar insisted they did not have to use EGR, they were a few months away from the change over when management twigged to the reality that their engineer department was lying to them. Phew. The wound up buying diesel engines from Cummins as well as emission systems from them to put on their own engine to get through, lost billions.
Long way around of wondering if at some point any of those strategies apply to jet engines. I don’t see how but I never though I would see diesel engines with emissions systems on them either.
Natural gas has density problems, Propane is better, it does not seem like those would have what an aircraft needs.
I have seen some ships gone to LNG for power (or alternative in some marine zones) and those tanks take up huge volumes.
SAF fuel should help a little bit to cleaner emissions as it is a more pure fuel mainly reducing contrails, new jet engines have very low NOX compared to regulations requirements. So the main issue is CO2. The easiest way is to burn LH2 in high bypass ratio engines from boil off H2. GE power gas turbines have been burning natural gas with high H2 content for years, the first jet engine in Germany was started with hydrogen, so it is just careful engineering work to get jet engines and its fuel system to work and be certified with hydrogen fuel. Getting green/white hydrogen to airports is another task. Most likely thru pipelines mixed with Natural gas and LH2 processing plants. Governments can see this as a way to create fuel monopolies at airports and tax heavily creating a mess and delays for years.
From what Bjron has written and my own experience with NG and Propane, Hydrogen has the density issue and range would be a huge drop. Regional at best.
I ran into the issue in my work. I like the Natrual Gas sets as teh were clean but we did not have an indpendt fuel source. A quake of other sever of the natrual gas and no power.
Diesel powered generators of course did not have that issue. Ironic that we had a Fire Pump house with 6 diesel driven fire pumps and a natural gas genera or. 2000 gallons of diesel sitting there doing no good in an emergency though we could have used it for other diesel generators. We had a 8000 gallon tank of that though.
One answer was to convert the engine to propane. A tank would hold a week or so of running at 20 KW.
Back in the 80s it was common to have motor homes with dual fuel option of gasoline and of propane they had for the fridge and stove. Lost a lot of power but on the flats it was fine.
Aircraft have the tyranny of weight being the major build factor. That drives everything. Hydrogen does not play well.
I have been amazed that some big ships are fully LNG powered. The LNG carriers it makes sense as they can use boil off but a container ship or bulk carrier has to take a huge space in the middle if its purely LNG powered.
Back to the density issue.
I am a big proponents of offsets to allow jets and SAF or Kerosene as best they can but I sure don’t drive that train.
I watched Mentour pilot using a battery powered trainer. Where I learned to fly it was a good 20 minutes to a training area. The one he was flying had 45 minutes. 2 hours was pretty much a minimum from our flights and even a rural airport 45 minutes is not a lot of time.
As I recall a C-150/152 had a good 4-5 hours endurance so you never had to hedge on a return. Flight school from memory did two flights a day and fueled in late afternoon t after the 2nd flight
That factors in cost and utilization. You could fly half tanks and get in a good training flight but then they had to fuel in between.
A lot of offsets to try and get to work on fuel density.
Yes, that is why LH2 is the top contender for increased range very low emissions aircrafts. There were a good test of electric + fuel cell hydrogen flight test recently by Joby, https://www.jobyaviation.com/news/joby-demonstrates-potential-regional-journeys-landmark-hydrogen-electric-flight/
I get that part but the density of a Hy7drogen applies to a fuel cell as well.
The size of a container and location requirement for a Hydrogen container is still one of the issues.
It has to be a shaped tank and the only place to put it is in the fuselage.
It has to be large due to the fuel density problem.
Just a silly comment on a term used. Sot in Scandinavian translate to soot in English.
Thank you, I thought it was a typo but not worth calling out.
Thanks. Fixed, Bjorn