January 31, 2020, ©. Leeham News: We now look at ways to increase the fuel efficiency of our airliner and by it, improve the CO2 situation for our environment.
Let’s start with understanding where we are with the efficiency of our present air transport system. To get a feel for where we are we will compare it to our road transport system.
The largest gains in airliner fuel efficiency was achieved on the engine side over the last decades. To understand how we can make an airliner gas turbine engine even more efficient we need to understand how it’s run today and where inefficiencies occur.
The best way to get a feel for how our airliner engines are designed and run is to compare them to other transport engines. This gives us a perspective on the task at hand.
An airliner turbofan is a rotational air pump (Figure 1) using a continuous process of compression (by the fan/compressors), combustion (in the combustor) and work extracting decompression in the turbines. The power of the turbines drives the compressors but also the fan, which is the key component for pumping the air faster out the back of the engine than its entry speed.
The efficiency of the different process stages is measured in % of achieved output power to input power. Typical transfer efficiencies on today’s best engines from e.g. axle power from the turbines to achieve compression in the compressors are just north of 90%.
But the designer only has this efficiency available in a sweet spot. The compressors/turbines work with rotating aerodynamic wings (blades/stators in engine speak), and these only have maximum efficiency in a narrow airspeed band (mass flow in engine speak). Figure 2 shows a typical compressor efficiency diagram and how the working point of the compressor varies between maximum power extraction at the takeoff/climb, then to the long cruise phase and finally to descent.
The diagram looks busy, but it’s not difficult to decode. On the Y-axis we have achieved compression work as an output pressure to input pressure ratio. On the X-axis we have the amount of air flowing through the compressor at different compressor RPMs. The lines with fractions on them like 1.02 are the engine RPM lines with 1.02 being 102% compressor (N2) RPM.
Think of the graph as a mountain map with the height rings as iso-efficiency lines. The top is at 92% efficiency and as you either climb or descend on either side you loose efficiency.
The table to the left is the legend for a mission simulation with the different simulation points for a typical airliner mission. TOC is top of climb, the most stressing point for the engine aerodynamically (the highest normalized flow), CR is cruise, TO is takeoff with its V speeds, CL climb and then Descent.
An airliner engine designer makes sure all the components in the chain align the maximum efficiency sweet spot with the phase of flight that contributes the most to the overall efficiency of the aircraft, the cruise phase. The core efficiency of the best turbofans is at 55% to 60% for this cruise sweet spot. It also means the engine has less efficiency at takeoff, climb, and descent power. The turbofan is then working outside its efficiency sweet spot.
This designing the engine to work at its efficiency sweet spot for cruise shall be contrasted with how our road transport engines are designed. In the last Corner, I wrote about our best car engines having up to 30% efficiency. This is when they are running at their efficiency sweet spot, at around 75% power.
Because our egos like overly strong engines we use a fraction of the car’s engine power in our daily cruise. Our car engines are far too big, we use about 10% to 20% of our car engine’s power when we cruise at the highway at 60mph/100kph. This is if we have a mid-size car with the smallest engine option. In our daily commute, our use of the fuel’s energy is worse. Figure 3 shows how the fuel’s inherent energy is typically used in our cars.
The useful transport work for our cars is when we overcome the Aerodynamic drag and Rolling resistance of the car. In city driving, this amounts to 7% of the energy input. In highway driving 18%. The rest is losses.
It’s no wonder one can make electric or hybrid cars which can improve on our road car’s efficiency! They are scandalously inefficient (part of which is our love of large cars with large engines)! A hybrid can run its combustion engine at the optimal 75% power when it charges the car’s batteries to extend the car’s range. Also, the braking can partly be done by reversing the electric motor to regenerate potential energy to battery energy when we coast for the stoplights.
So the added complexity/weight of a hybrid system is motivated by the outrageous inefficiency of our road car designs. For markets where large cars with large engines are popular (think SUVs) the situation is even worse than in Figure 3.
The situation for trucks is better as their engines are operating closer to their peak efficiency, as do diesel trains and ships.
I go through this so we can understand the world of difference between our road transport system, why hybrids are a good alternative there, and our air transport system where a hybrid system has a hard time to improve an already efficient system.
When we look at concepts to improve our airliner engines we must remember they are flown at their peak efficiency at 80% to 90% of the flight time and that about 80% of the developed thermal power is transferred to propulsive power through our high bypass ratio turbofans.
With these examples, we realize we must rid ourselves of the ideas from the road car industry on how to improve the efficiency further. They simply don’t apply. All ideas must be viewed in the context of the total difference in base efficiencies and operating profiles between the transport systems.
With this in our baggage, we can now look at ways to lower the fuel burn and by it the CO2 emissions for air transport.