Overall Pressure Ratio (OPR) is a fundamental factor in understanding a jet engine’s efficiency, power output, and overall performance. Overall pressure ratio (OPR) represents the ratio of the total pressure at the exit of the compressor stage to the total pressure at the inlet, indicating the level of compression the air undergoes as it passes through the engine.
Jet engines operate on the “Brayton” thermodynamic cycle. Simply, the cycle consists of compressing ambient air, mixing the air with fuel, then igniting the mixture, which expands, doing work. As air is compressed, its temperature rises. A higher OPR means a higher compression of the air before entering the combustion chamber. This leads to a more significant temperature increase during combustion, resulting in higher gas velocities and increased thrust or power output.
We often use OPR as a factor in the evaluation, analysis and due diligence of advanced jet engines for client projects. A propulsion manufacturers ability to push OPR levels will typically track along a path of increasing performance over time. Historically, (see Figure 1), engine OEM’s have progressed their turbofan and turboprop OPR incrementally, at a fixed rate per decade – essentially along an experienced-based, linear path. As a result, significant leaps in OPR from an OEMs trendline of experience should be carefully understood and evaluated to assess where challenges may arise. Technology Readiness Levels (TRL) and Manufacturing Readiness Levels (MRL) should also be assessed for a deeper assessment and evaluation of risk.
OEM OPR Improvements Follow an Experience-Based Trajectory
Turbofan OPR Growth vs Time
Turboprop/Turboshaft Engine OPR vs Time by OEM
Additional considerations in the evaluation of jet engine technology when targeted towards achieving high OPR levels, include:
Specific Fuel Consumption (SFC):
Higher overall pressure ratios contribute to lower SFC (the amount of fuel required to produce a unit of thrust or power), making the engine more fuel-efficient. Fuel costs range from 25-40% of an airline’s Direct Operating Cost, depending on the aircraft class, flight leg and current market fuel costs. As such, OEMs justify enormous investments to reduce fuel costs that will differentiate their products in the eyes of an airline or aircraft OEM. Comparing the relatively older CFM56 vs newer LEAP engines, the CFM56 has an OPR of 26:1 achieved in nine stages of compressor, while the LEAP achieves an OPR of 40:1 within a 10-stage compressor… so technology has yielded the ability to increase pressure rise per stage to very impressive levels.
Material and Durability Considerations:
The corollary to increased OPR is advancing material technologies that assure the system meets customer durability expectations. Maximizing thermal efficiency requires higher temperatures, which results in reduced material life and durability for a fixed material technology. The challenge for engine OEM’s is to simultaneously push the state of the art of jet engine aerodynamic efficiency and material capability, that is, requiring advanced aerodynamics that can achieve pressure rise efficiently, and material systems (base material, thermal coating technologies & heat transfer/cooling technologies) that can withstand the higher pressures and temperatures. As shown in Figure 2 for one OEM, turbine inlet temperature, and therefore hot-section capability has steadily increased at a relatively consistent rate along with introductions of new materials.
However, when the industry is pressed by competition to overcommit, risking too large a technology jump within a committed design, the operators and users risk multiple product upgrades shortly after entry-into-service. A good example of this was on the 787, where both engine manufacturers overcommitted vs their technology capability, and underwent multiple NPI upgrades over a ten-year period, with the result being upset customers.
Summary
As the industry has seen in the latest new engines from virtually all OEM’s, there is tremendous enthusiasm and demand for fuel efficiency advances but increasing frustration due to new engines being significantly less durable than their predecessors. At a time when production rates are ramping up, and supply chains are stressed, supporting new technology engines has been a huge issue for airlines, even predating the pandemic. Understanding new technologies being introduced, maturity levels, managing risk and the potential MRO requirements will be increasingly critical to satisfying operator and airline needs.
