Gas Turbine or Reciprocating Engine: Where Each One Actually Wins

The answer is a capacity band, a duty cycle and an ambient temperature, not a technology preference.

Below roughly 50 MW, a modern lean-burn gas engine delivers 46 to 49 percent simple cycle efficiency against 38 to 42 percent for an aeroderivative turbine and 33 to 36 percent for an older heavy frame. Above roughly 400 MW on continuous duty, combined cycle turbines win decisively.

Between those two points, three things decide it. How often the plant runs at part load. How hot and how high the site is. And whether land, water and unit count are constraints.

One figure captures the ambient effect better than any argument. To deliver 100 MW firm at 48°C, a gas turbine plant must be built from turbines totalling 125 MW of ISO nameplate. A gas engine plant needs 105 MW.

This guide covers every axis of the comparison with the figures attached, including a rating convention that makes the two nameplates non-comparable at face value.

How the Two Machines Work

A gas turbine burns fuel continuously. A reciprocating engine burns it in discrete events inside cylinders. Every difference between them follows from that.

A gas turbine compresses air in an axial or centrifugal compressor, mixes it with fuel in a combustion chamber where combustion is continuous and automatic, and expands the hot gas through turbine stages that drive both the compressor and the generator. The thermodynamic cycle is the Brayton cycle, and efficiency depends on pressure ratio and firing temperature.

A reciprocating internal combustion engine draws air and fuel into a cylinder, compresses it, ignites it, and uses the expanding gas to drive a piston. Power generation engines are typically four-stroke, and the highest-efficiency machines are lean-burn gas engines, which run with excess air to lower combustion temperature and reduce NOx formation.

What follows from the difference

• A turbine is one machine with a fixed compressor, which makes it efficient at its design point and progressively less efficient away from it.
• An engine plant is many machines. A 20 MW requirement is met by two 10 MW engines or four 5 MW engines, and each can run at full load or not at all.
• That single structural difference drives the part-load behaviour, the redundancy, the maintenance model and the footprint.

Efficiency: Engines Win Simple Cycle, Turbines Win Combined Cycle

Lean-burn gas reciprocating engines have the highest simple cycle electrical efficiency of any combustion technology in their size class.

Below roughly 50 MW in simple cycle, a modern reciprocating engine typically achieves around 42 to 48 percent electrical efficiency against about 30 to 40 percent for a simple cycle gas turbine. In the 20 MW size range specifically, a simple cycle turbine runs between 30 and 35 percent.

Where the ranking reverses

Combined cycle. Adding a heat recovery steam generator and a steam turbine lifts gas turbine plant efficiency to 55 to 60 percent, and the latest heavy duty machines exceed 64 percent.

That is the single strongest argument for turbines, and it only applies where running hours justify the capital and the complexity of a steam cycle.

High-utilization plants above roughly 400 MW favor combined cycle turbines. Cycling plants below that threshold favour engines, and projects below 400 MW are precisely the range requiring objective technology comparison.

Heat rate

Heat rate is the fuel energy required per unit of electrical output, in Btu/kWh, and lower is better. The engine advantage in simple cycle is roughly 1,100 to 2,900 Btu/kWh depending on which turbine class you compare against.

On a plant running 4,000 hours a year, that difference compounds into the largest single line in the operating cost model.

The ISO Reference Trap

Gas turbines and reciprocating engines are rated at different reference conditions, which means their published nameplates are not directly comparable.

The reference ambient temperature for a gas turbine is 15°C. The reference ambient temperature for a reciprocating engine is 35°C.

That is a 20 degree difference in the starting point. A turbine quoted at 15°C and an engine quoted at 35°C are being measured under conditions that are 20 degrees apart, and the turbine is the one whose output falls fastest with temperature.

What it means in procurement

Comparing a turbine nameplate against an engine nameplate directly understates the turbine's real-world disadvantage on any warm site.

Require both to be quoted at your actual site conditions, with correction curves annexed, before any comparison is made.

The governing standards

• ISO 3977 defines the reference conditions for gas turbine ratings: 15°C, 1.013 bar at sea level, 60 percent relative humidity.
• ISO 3046 defines reciprocating internal combustion engine performance and its reference conditions.

A specification that names one and not the other has only half a comparison. For full detail on gas turbine site rating and derate, see our guide to gas turbine power stations and ISO conditions.

Part Load: Where the Argument Is Really Decided

• Reciprocating engines maintain 85 to 90 percent of peak efficiency at 25 percent load. Gas turbine efficiency falls off sharply below about 75 to 80 percent of rating.
• The reason is structural rather than a design weakness.
• A gas turbine is a single machine with a fixed compressor. Reducing output means reducing airflow, which lowers combustion temperature and moves the machine away from its design point. Efficiency falls as a consequence.

A reciprocating plant is not one machine. It meets partial demand by running fewer units at full load, so plant-level efficiency stays nearly flat across a wide load range.

The numbers

• Engines start in 2 minutes, turn down to 25 percent and hold part-load efficiency across that range. At 80 percent load a lean-burn gas engine still delivers around 41 percent efficiency.
• A turbine at 50 percent load is operating well off its design point, with efficiency and heat rate both degraded.

Why this decides more projects than the headline efficiency does

Very few plants run at 100 percent load continuously. A plant following load, firming renewables or serving a variable industrial process spends most of its hours at part load.

At that point the engine's part-load advantage compounds on top of its simple cycle advantage, and the comparison is no longer close.

If the plant genuinely runs flat out at high utilization, the turbine's combined cycle efficiency reverses that, which is why the duty cycle has to be established before any technology discussion begins.

Ambient Temperature and Altitude

Gas turbine output is set by compressor mass flow, so capacity and efficiency fall predictably as air density falls with temperature and altitude. Engines are far less affected.

The firm capacity arithmetic

To deliver 100 MW firm at 48°C, the gas turbine plant must be built from turbines totaling 125 MW of ISO nameplate. The gas engine plant needs only 105 MW of ISO nameplate.

That is a 20 percent turbine derate against roughly 5 percent for engines, approximately a quarter of the turbine's percentage loss.

You are buying 20 percent more turbine to deliver the same firm megawatts, and paying for it in capital, fuel and maintenance across the asset life.

Efficiency at high ambient

Gas engines hold 44 to 46 percent simple cycle efficiency at high ambient temperature. The derated gas turbine plant runs at 30 to 33 percent.

The efficiency gap widens on a hot site rather than narrowing, because the turbine loses both output and efficiency while the engine loses relatively little of either.

Altitude

A 10 MWe gas turbine plant loses 18 percent of its power at an altitude of 1,500 metres above sea level.

Modern lean-burn reciprocating engines undergo no derating due to site altitude up to 1,500 metres.

The derating has an immediate effect on investment cost. An equivalent gas turbine plant at 1,500 metres costs 18 percent more than the same plant at sea level, which makes engines a strong default for high-altitude projects.

What to do about it

Turbine derate can be partly recovered with inlet air cooling, which adds capital and parasitic load and works best in hot dry conditions. It does not close the gap entirely and it does not help with altitude.

On a hot or high site, model both technologies at site conditions before comparing anything.

Capital Cost

Gas turbines cost substantially less per installed kilowatt than reciprocating engines.

The US Energy Information Administration's 2024 generator construction cost survey recorded combustion turbines at 1,418 MW of installed capacity averaging $841 per kW, and reciprocating engines at 480 MW averaging $1,490 per kW.

That is roughly 77 percent more per kW for engines, and it is the clearest single argument on the turbine side of the comparison.

How to read it against the rest

Capital cost is one line in a lifecycle model, and the engine recovers part of the gap through fuel.

On a plant running substantial hours, the simple cycle efficiency difference of 8 to 13 percentage points is worth more over twenty years than the capital difference. On a plant running a few hundred hours a year, it is not.

Running hours is the variable that decides which number dominates, and it should be modelled rather than assumed.

Maintenance: The Genuinely Contested Axis

Maintenance is where the two sides of this comparison disagree most, and both have data.

The turbine case

GE Vernova's comparison of aeroderivative against reciprocating engines reports aeroderivative gas turbines needing maintenance only once a year, or at 4,000 operating hours, against high-speed reciprocating engines requiring 50 times more maintenance events per year.

The same analysis puts the saving at more than 13,000 man-hours over a three-year maintenance cycle, and notes that an LM2500 in combined cycle consumes about 2 mL/MWh of lube oil per year, 200 times less than a reciprocating engine, which can translate to savings of more than $1 million per year on a 100 MW plant.

The engine case

Engines require more frequent scheduled maintenance than gas turbines, at lower cost per event, and the comparison turns on running hours rather than on either technology being inherently cheaper.

Crucially, engine maintenance happens one unit at a time while the rest of the plant keeps running. Turbine maintenance stops the unit.

The field data

Power Engineering's analysis of fluctuating-demand plants found that eight years of reciprocating engine plant data ranked one through eight for highest O&M cost against the field of aeroderivative gas turbine plants.

That is a significant result and it favors turbines. It is also drawn from a specific plant set and a specific duty, and it sits alongside the engine's fuel advantage rather than cancelling it.

How to resolve it for your project

Model total cost of ownership with your running hours, your fuel price and your labor rate. The maintenance comparison genuinely inverts depending on all three, and no general answer survives contact with a specific project.

Staffing

An engine plant needs more maintenance labor more often. A turbine plant needs specialist intervention less often, and the intervention is larger.

Footprint, Unit Count and Scale

• Gas turbines put far more power into far less space, and at large scale that becomes a civil and electrical problem rather than a mechanical one.
• Aeroderivative gas turbines deliver about 22 times more power output per unit than comparable high-speed diesel reciprocating engines.

What that means at scale

A 500 MW engine plant means 200-plus engines, each with its own foundation, exhaust, cooling, gas train, lube system and switchgear cell, plus the gas piping, cabling and land to connect them all.

The mechanical comparison is only part of the picture at that size. Civil works, electrical distribution and land acquisition scale with unit count rather than with megawatts.

Where the engine's unit count is an advantage

Below the scale where unit count becomes unmanageable, modularity is a benefit rather than a cost.

A multi-unit engine plant can have installed capacity ranging from 2 to 50 MW with incremental addition of units as demand grows. Capacity is added in phases, matched to actual demand, without overbuilding at the outset.

Water

• Gas turbines frequently require water for inlet cooling or NOx control through water or steam injection. Engines use less.
• On a water-constrained site this is a permitting question rather than an operating cost question, and it can be decisive.

Reliability and Redundancy

• An N+1 engine plant loses one unit of capacity for a service outage. A single-turbine plant loses everything.
• That is the structural reliability argument for engines, and it is not about either machine being more or less reliable individually.
• A plant built from eight 5 MW engines running at N+1 loses 12.5 percent of capacity when one unit is serviced. A plant built from one 40 MW turbine loses 100 percent.

Where turbines recover the argument

• Fewer units means fewer things to fail. The aeroderivative maintenance interval of once a year or 4,000 hours means fewer planned outages to schedule around.
• And a multi-turbine plant has the same N+1 logic available, at a coarser granularity.

Insurance

Insurers assess configuration, redundancy and maintenance regime when pricing machinery breakdown risk. A documented N+1 arrangement with recorded maintenance is underwriting evidence regardless of prime mover.

Start Time and Standby

Reciprocating engines reach full output in about two minutes. Aeroderivative gas turbines reach full power in under ten minutes, and combined cycle plants take far longer.

For fast-start duty both technologies are credible, and the engine is faster.

The standby cost nobody mentions

Reciprocating engine cylinders must be hot before starting quickly. The energy needed to maintain ready-to-start conditions can be over five times the standby power needed for an industrial gas turbine of the same size.

On a plant held ready for long periods and started rarely, that standby parasitic load is a real operating cost and it works against the engine's fuel advantage.

Model it where the duty is standby or emergency rather than load-following.

Combined cycle start

A combined cycle plant must bring the steam cycle up behind the gas turbine, which requires purging procedures and gradual ramp to full output. A ten-minute gas turbine start does not produce a ten-minute plant start.

Fuel Flexibility

Both technologies run on multiple fuels, and reciprocating engines generally offer broader liquid fuel capability.

Gas turbines run on natural gas, liquid fuels including diesel, and increasingly hydrogen blends, with dual-fuel configurations able to change over during operation.

Reciprocating engines run on natural gas, diesel, heavy fuel oil and biogas, and some can switch between liquid and gaseous fuels during operation. Heavy fuel oil capability is an engine advantage with no turbine equivalent at this scale.

Heavy fuel oil is the lowest-cost fuel available on many remote and island projects, and the engine's ability to burn it changes the economics entirely where it is available.

Dual-fuel capability on either technology adds a fuel system, storage and a changeover function that must be tested rather than assumed.

Emissions

• Both technologies require NOx control, and the applicable US regulation differs by technology rather than by fuel.
• Gas turbines use dry low NOx combustion, which premixes fuel and air to lower flame temperature, or water and steam injection, with selective catalytic reduction where limits require it.
• Lean-burn gas engines run with excess air, which lowers combustion temperature and reduces NOx formation without aftertreatment in many applications. Selective catalytic reduction and oxidation catalysts are used where limits are tighter.

40 CFR Part 60 Subpart KKKKa applies to stationary combustion turbines constructed, modified or reconstructed after 13 December 2024. Stationary reciprocating internal combustion engines fall under a different subpart with its own limits and testing requirements.

Confirm which applies before comparing emissions control costs, because they are not the same regulation.

Governing Standards

Four standards frames govern the specification and rating of the two technologies, and a comparison that cites only one is incomplete.

The duty rating classes

ISO 8528 defines four duty ratings for engine generating sets. Emergency Standby Power (ESP) for emergency supply at variable load with no overload. Limited Time Running Power (LTP) for a restricted number of hours per year. Prime Running Power (PRP) for unlimited hours at variable load. Continuous Operating Power (COP) for unlimited hours at constant load.

Specifying the wrong class is one of the most common and most expensive errors in engine procurement, because an engine rated for standby duty and operated continuously will not reach its design life.

Name the class in the specification and match it to the actual duty profile.

Outside the United States

ISO and ASME standards apply internationally. EPA subparts are US federal regulation, and other jurisdictions apply their own emissions frameworks and grid codes, which can influence prime mover choice as much as the technical comparison does.

Selection by Application

Seven applications, and the binding requirement differs in each.

• Utility peaking and reserve. Both work. Engines start faster and hold part-load efficiency; turbines offer larger single-unit blocks and lower capital per kW. Duty hours decide.
• Utility base load above 400 MW. Combined cycle turbines, decisively, because combined cycle efficiency above 60 percent outweighs everything else at high utilisation.
• Industrial captive power. Usually engines below 50 MW, because the load is variable, the site is space-constrained less often than assumed, and the comparison is against a retail tariff rather than a wholesale price.
• Oil and gas. Turbines dominate mechanical drive for compression. For electrical generation on remote sites, engines frequently win on fuel flexibility and derate behavior.
• Mining and remote sites. Engines, generally. High altitude, high ambient, modular phasing and heavy fuel oil capability all favor them, and the 18 percent turbine derate at 1,500 metres is often decisive. For mine power load profiles, see our guide to powering a mine.
• Data centres. Contested and currently active. Engines offer efficiency and part-load behavior; turbines offer power density, fewer maintenance events and low NOx without aftertreatment. 
• Island and microgrid systems. Engines, usually, because delivered fuel cost dominates and part-load efficiency matters on a small variable load. Battery storage changes the calculation by covering the seconds before a unit reaches load; see our guide to battery energy storage systems.

What to Specify

• A prime mover specification is comparable only if both technologies are quoted on the same basis.
• Output and heat rate at site conditions, not at either ISO reference, with correction curves annexed.
• The heating value basis, LHV or HHV, stated explicitly.
• The duty profile in hours per year and the expected load distribution, because part-load behavior cannot be evaluated without it.
• The duty rating class for engines, named per ISO 8528.
• Firm capacity required at design ambient, from which nameplate is derived rather than assumed.
• Performance test code and acceptance criteria, naming ASME PTC 22 for turbines.
• Emissions limits and the control technology required, with the applicable regulation named.

Ask for both technologies from the same adviser. A quotation from a turbine supplier and a quotation from an engine supplier are two sales documents, not a comparison.

What Prismecs Does

Prismecs supplies, installs, commissions, operates and maintains both gas turbine and reciprocating engine plant, and is OEM-agnostic across both.

Delivered project scope includes eight TM2500 dual-fuel units totaling 260 MW at Birr, Switzerland, online in six months with a new 220 kV interconnection; four TM2500 units totalling 110 MW at Duqm, Oman, kept grid-ready with resident O&M crews, CMMS and parts support; an LM2500XPRESS plant at Miaoli, Taiwan delivered in ten months; three LM6000PC units adding 150 MW of fast-start reserve; and an LM6000 fleet decommissioned in Norway, transported and recommissioned at a new site.

That record is aeroderivative gas turbine delivery. Stating it plainly matters on a comparison page, because an adviser's delivered experience shapes what they recommend, and you should know what ours is before weighing our view of the engine case.

Capability spans power generation asset services for both prime movers, technology and consulting for options analysis and modelling, EPCM services for delivery, I&C services for electrical systems and commissioning, O&M services for the operating phase, ready-to-ship equipment inventory including generators, and distributed energy solutions for microgrids and storage integration.

OEM-agnostic matters most on a decision like this one, because the party comparing the two technologies should not be selling one of them.

Apply this article's criteria to any proposal, including ours. Ask for both technologies quoted at your site conditions rather than at their respective ISO references. Ask for the duty profile to be modelled rather than assumed. Ask for total cost of ownership with your running hours and fuel price. Ask which ISO 8528 duty class the engine quotation assumes.

To discuss prime mover selection, capacity sizing or an operations and maintenance scope, send your required firm capacity, site location and elevation, design ambient temperature, expected load profile and running hours, and fuel position to sales@prismecs.com or call +1 (888) 774-7632.

Frequently Asked Questions

Which is more efficient, a gas turbine or a reciprocating engine?

In simple cycle, the engine. Lean-burn gas reciprocating engines achieve 46 to 49 percent at roughly 7,000 to 7,400 Btu/kWh, against 38 to 42 percent for aeroderivative turbines and 33 to 36 percent for older heavy frames in simple cycle. In combined cycle the ranking reverses, with gas turbine plants reaching 55 to 60 percent and the latest heavy duty machines exceeding 64 percent.

At what capacity does each technology win?

Below roughly 50 MW, reciprocating engines typically win on simple cycle efficiency, achieving 42 to 48 percent against 30 to 40 percent for a simple cycle turbine. High-utilisation plants above roughly 400 MW favour combined cycle turbines. Projects below 400 MW are precisely the range requiring objective technology comparison, because the answer depends on duty cycle, ambient conditions and running hours rather than on capacity alone.

Why are turbine and engine nameplates not directly comparable?

Because they are rated at different reference conditions. The reference ambient temperature for a gas turbine is 15°C under ISO 3977, while the reciprocating engine reference ambient under ISO 3046 is 35°C. A 20 degree difference in the starting point means a direct nameplate comparison understates the turbine's real-world disadvantage on any warm site. Require both quoted at your actual site conditions.

How much does a gas turbine derate in hot conditions?

Substantially more than an engine. To deliver 100 MW firm at 48°C, a gas turbine plant must be built from turbines totalling 125 MW of ISO nameplate, while a gas engine plant needs only 105 MW. That is a 20 percent turbine derate against roughly 5 percent for engines, approximately a quarter of the turbine's percentage loss, and it means buying 20 percent more turbine for the same firm output.

How does altitude affect each technology?

A 10 MWe gas turbine plant loses 18 percent of its power at 1,500 metres above sea level. Modern lean-burn reciprocating engines undergo no derating due to site altitude up to 1,500 metres. The derating has an immediate effect on investment cost, since an equivalent gas turbine plant at 1,500 metres costs 18 percent more than the same plant at sea level, which makes engines a strong default for high-altitude projects.

Which performs better at part load?

Reciprocating engines, clearly. They maintain 85 to 90 percent of peak efficiency at 25 percent load, while gas turbine efficiency falls off sharply below about 75 to 80 percent of rating. A turbine is a single machine with a fixed compressor, so reducing output means reducing airflow, lowering combustion temperature and moving off its design point. An engine plant runs fewer units at full load instead.

What does each technology cost per installed kilowatt?

The EIA's 2024 generator construction cost survey recorded combustion turbines at 1,418 MW of installed capacity averaging $841 per kW, and reciprocating engines at 480 MW averaging $1,490 per kW, roughly 77 percent more for engines. Capital cost is one line in a lifecycle model, and whether the engine's fuel advantage outweighs it depends entirely on annual running hours.

Which costs more to maintain?

Genuinely contested. GE Vernova reports aeroderivative needing maintenance once a year or at 4,000 hours against high-speed engines requiring 50 times more maintenance events, saving over 13,000 man-hours across a three-year cycle. Field data from Power Engineering found eight years of reciprocating engine plant data ranking one through eight for highest O&M cost against aeroderivative plants. Model it with your own running hours.

How much lube oil do the two technologies consume?

An LM2500 aeroderivative in combined cycle consumes about 2 mL/MWh of lube oil per year, which is 200 times less than a reciprocating engine, and can translate to savings of more than $1 million per year on a 100 MW plant. Lube oil consumption is one of the more significant and least discussed differences in the operating cost comparison between the two technologies.

How much space does each require?

Aeroderivative gas turbines deliver about 22 times more power output per unit than comparable high-speed diesel reciprocating engines. At scale that becomes a civil problem: a 500 MW engine plant means 200-plus engines, each with its own foundation, exhaust, cooling, gas train, lube system and switchgear cell, plus the gas piping, cabling and land to connect them. Civil and electrical works scale with unit count.

Which configuration is more resilient to a unit failure?

An N+1 engine plant loses one unit of capacity for a service outage; a single-turbine plant loses everything. A plant built from eight 5 MW engines loses 12.5 percent of capacity when one unit is serviced, while a plant built from one 40 MW turbine loses all of it. Turbines recover part of this argument through fewer units and longer maintenance intervals.

Which starts faster, and what does standby cost?

Engines reach full output in about two minutes; aeroderivative turbines reach full power in under ten. But reciprocating engine cylinders must be hot before starting quickly, and the energy needed to maintain ready-to-start conditions can be over five times the standby power needed for an industrial gas turbine of the same size. On standby duty held ready and started rarely, that parasitic load matters.

What are the ISO 8528 duty ratings?

Four classes for engine-driven generating sets. Emergency Standby Power for emergency supply at variable load with no overload capability. Limited Time Running Power for a restricted number of hours per year. Prime Running Power for unlimited hours at variable load. Continuous Operating Power for unlimited hours at constant load. Specifying the wrong class is a common and expensive error, because an engine rated for standby and run continuously will not reach design life.

Which standards govern the comparison?

ISO 3977 for gas turbine procurement and reference conditions, ISO 3046 for reciprocating internal combustion engine performance, ISO 8528 for engine-driven generating sets and their duty ratings, and ASME PTC 22 for gas turbine performance testing. On emissions, 40 CFR Part 60 Subpart KKKKa applies to stationary combustion turbines while stationary reciprocating engines fall under a different subpart entirely.

Tags: Gas Turbine Reciprocating Engine Prime Mover Selection Part Load Efficiency Power Plant Derate