Pressure Gain Combustion: The Retrofit Play That Could Unlock Efficiency Gains the Brayton Cycle Can't

Gas turbine engineers have been squeezing the Brayton cycle for decades, and the returns are getting thin. Modern compressor and turbine component efficiencies are already above 90%, leaving little room for meaningful improvement through conventional means. The next step-change in simple-cycle efficiency almost certainly has to come from somewhere else - and a Finnish startup called Finno Exergy is betting it comes from the combustor itself.

The company's pressure gain combustion (PGC) concept is the subject of a new analysis in Modern Power Systems, which flags its retrofit potential for GE's Frame 3 and Frame 5 machines^[1]. That's a specific and interesting target. It's not a pitch for next-generation H-class turbines. It's a pitch for the installed base.

Photo: Joao / Unsplash

What Pressure Gain Combustion Actually Does

Conventional gas turbines burn fuel at constant pressure - the Brayton cycle's defining constraint. The combustor adds heat but doesn't add pressure; in fact, it loses a little through friction. Pressure gain combustion replaces that constant-pressure heat addition with a process - typically pulsed or rotating detonation - that generates a net increase in stagnation pressure across the combustor. That pressure rise is then available to do additional work in the turbine, which is why the thermodynamic gains can be substantial.

Pressure gain combustion, through the use of rotating detonation combustors, promises to bring about a step change in thermodynamic efficiency for propulsion and power generation gas turbines. The academic literature is broadly consistent on the magnitude: if overall system pressure gain can be achieved, detonation-based cycles can improve thermal efficiency by 5% to 10% over the traditional constant-pressure combustor - an improvement substantially higher than any other technological advancement in the propulsion and power generation industry.

Finno Exergy's approach is specifically pulsed deflagration combustion rather than rotating detonation - a distinction that matters for integration with existing hardware. The company's innovation utilizes pulsed deflagration combustion compatible with existing gas turbine technology, which has yielded significant improvement in efficiency. Two generations of test rigs have demonstrated up to 40% pressure gain, while cutting-edge CFD models guide optimization and industrial integration.

The Frame 3 and Frame 5 Angle

The retrofit target matters as much as the technology. There are thousands of aging GE Frame 3, 5, 6, and 7 gas turbines in operation around the world, and they remain of high value to any grid - whether as a source of emergency standby power or as a means of bringing stability to the grid by acting as synchronous condensers.

The GE Frame 5 is a heavy-duty 26-30 MW turbine with approximately 2,800 units installed worldwide. The Frame 3 dates to GE's first commercial power-generating gas unit, installed in 1949, and machines in that lineage are still running on US gas pipelines today. These are not cutting-edge machines. Their simple-cycle efficiencies are modest by modern standards. That's exactly the point: the efficiency headroom for a PGC retrofit is larger on an older, lower-pressure-ratio machine than on a modern H-class unit already operating near its thermodynamic ceiling.

Finno Exergy's PGC system has been developed to make the best use of carbon-neutral fuels in gas turbine applications, and this unique retrofitting solution will reduce fuel consumption and enable the transition to carbon neutrality in hard-to-decarbonize sectors.

The fuel flexibility claim is worth noting separately. Since 2015, Finno Exergy has been developing a novel pressure gain combustion system for gas turbines, with the technology having the potential to reduce fuel consumption and enable the retrofitted gas turbine to operate with any mixture of hydrogen and methane. That's a meaningful operational benefit for operators who want to blend hydrogen into their fuel supply without a full combustor replacement.

The Efficiency Numbers - and the Caveats

The headline figure from Finno Exergy's Shell GameChanger collaboration is striking: results showed that replacing the traditional combustor with the pressure gain combustion system has the potential of reducing fuel consumption by almost 40% in small gas turbine applications.

That figure needs context. It comes from test rig results, not from a commissioned unit running at commercial load. PGC has recently emerged as a promising approach to achieve major performance improvements in current gas turbines operating on the Joule cycle, where the potential for major performance upgrades has nearly plateaued - but the inherently unsteady and periodic nature of this combustion technology presents challenges in modelling the PGC process and understanding its impact on the cycle.

The gap between test rig performance and installed system performance is where most combustion technology programs have historically lost ground. The pressure gain measured at the combustor exit is not the same as the efficiency gain delivered at the generator terminals - there are losses at the turbine interface, in the transition ducting, and in the control system managing the pulsed combustion event. A PGC combustor with optimistic losses recovers more exergy than a conventional combustor due to pressure gain, but the word "optimistic" is doing real work in that sentence.

What Finno Exergy Has - and What It Doesn't

This is where the project sits right now, as best as can be determined from public information:

• Technology validation: Completed, through Shell's GameChanger program and Business Finland grant funding.
• Test rig results: Two generations of rigs, up to 40% pressure gain demonstrated.
• OEM cooperation: First cooperation with an OEM confirmed, with retrofit cost estimation for a 10 MW turbine completed.
• Commercial deployment: Not yet. No signed offtake agreement, no permitted retrofit installation, no commissioned unit.

Finno Exergy claims to be the only company in the world that can offer a working pressure gain combustion system for gas turbine applications, capable of unlimited continuous operation while generating a remarkable amount of pressure gain. That's a strong claim. It may well be accurate at the test rig scale. The question is whether it holds at the scale of a Frame 5 running at commercial load, 8,000 hours a year, with the combustion dynamics that entails.

The company is at the stage where the physics look right and the engineering is advancing. It is not at the stage where a plant operator can sign a contract and expect a commissioned unit. The path from here to there requires a full-scale demonstration on an operating turbine, regulatory sign-off on the modified combustion system, and a commercial partner willing to take the first-mover risk on their machine.

Why This Matters Now

The timing is not accidental. GE Vernova's gas turbine backlog climbed to 116 GW in Q2 2026, with the company now taking reservations for 2031 deliveries. New large gas turbines are effectively sold out for years. That creates a structural incentive to extend the life and improve the performance of existing machines - which is precisely the market Finno Exergy is targeting.

The Frame 3 and Frame 5 installed base is aging, but it isn't going away. Grid operators in emerging markets, industrial users running mechanical drive applications, and peaking plant operators in developed markets all have reasons to keep these machines running. If a combustor retrofit can credibly deliver a double-digit reduction in fuel consumption while adding hydrogen-blend capability, the economics of keeping an older turbine in service improve materially.

The GE Frame 5, with approximately 2,800 units installed worldwide, represents the most well-known turbine in its 26-30 MW range. Even a fraction of that installed base represents a substantial commercial opportunity - if the technology can be demonstrated at full scale.

The thermodynamics of pressure gain combustion are not in serious dispute. The engineering challenge of integrating an unsteady, pulsed combustion process with a turbine designed for steady-state isobaric flow is real and not yet fully resolved. Finno Exergy has made genuine progress. What it needs next is a commissioned demonstration - with a named turbine, a named site, and an operator willing to put their machine on the line. Until that exists, this remains a technology to watch rather than a project to count.