Are reciprocating engines or gas turbines better for an oil-fired power plant?
Where heavy fuel oil is the fuel, medium-speed reciprocating engines are usually the better fit, with higher efficiency, tolerance of heat and altitude, and good part-load performance. Gas turbines suit lighter liquid fuels, larger single units and sites expecting to switch to natural gas soon, but burning heavy oil in a turbine needs extensive fuel treatment.
Why the Fuel Drives the Decision
Heavy fuel oil is cheap per unit of energy and easy to ship and store, which is why so many island grids, mines and emerging-market utilities rely on it. It is also viscous, dirty and chemically awkward. It contains sulphur, vanadium, sodium, ash and sludge-forming compounds that attack hot components and clog fuel systems.
Medium-speed four-stroke engines were developed around fuels like this. Their combustion tolerates contaminants that would quickly damage a turbine’s hot section. Gas turbines can burn heavy oil, but only with fuel washing and fuel treatment that remove sodium and inhibit vanadium corrosion, and with more frequent hot-section maintenance. Many turbine owners therefore burn light distillate rather than heavy oil, at a higher fuel cost.
Side by Side on the Factors That Matter
The table compares the two technologies as they are usually applied on oil-fired plants. Specific models vary, so treat it as a starting point for a study, not a conclusion.
| Factor | Medium-speed HFO engines | Gas turbines on liquid fuel |
|---|---|---|
| Fuel flexibility | Designed for heavy oil; most also run on diesel, and many can be converted to gas | Best on distillate; heavy oil needs washing and fuel treatment |
| Simple-cycle efficiency | High, and holds up well at part load | Lower for most sizes, and drops faster at part load |
| Hot climate and altitude | Modest derate | Significant derate as air temperature rises |
| Unit size | Small to mid-sized units; plants are built from many engines | Larger single units; fewer machines per plant |
| Start and load-up | Fast, with good step-load response | Aeroderivatives fast; heavy-duty frames slower |
| Maintenance pattern | Regular, frequent work on cylinders and fuel equipment | Fewer interventions, but costly hot-section inspections |
| Water use | Low, with radiator cooling | May need water injection for emissions on liquid fuel |
USP&E’s broader comparison of turbines and reciprocating engines covers the same question for natural gas, where the balance shifts toward turbines in some applications.
Fuel Treatment: The Hidden Plant
Both technologies need a fuel treatment plant on heavy oil, but the scale differs. An engine plant heats the fuel, separates water and solids in centrifugal separators, and filters it before injection. A turbine burning the same fuel adds fuel washing to remove water-soluble sodium and potassium salts, chemical dosing to inhibit vanadium attack, and tighter limits on what reaches the combustor.
That extra treatment brings equipment, chemicals, water and operator skill. On remote sites the burden is real. Our guide to HFO fuel systems for stations describes the engine-plant fuel train, and the article on heavy fuel oil quality and handling explains what each contaminant does.
Heat, Altitude and Part Load in Practice
Gas turbine output depends heavily on the mass of air passing through it. On a hot afternoon, or at a high-altitude mine, a turbine can lose a large share of its rated output, which is often just when demand peaks. Inlet cooling helps, but adds equipment and water use. Reciprocating engines lose much less, which is one reason they dominate in hot, high places.
Part-load behaviour matters on plants that follow demand. An engine plant switches units on and off to keep each engine near its best efficiency. A turbine plant with fewer, larger units has to run them at partial output, where fuel use per unit of electricity rises. For small and medium grids with large daily swings, engines usually win on fuel.
Maintenance and Skills
Engine plants need frequent, predictable work: cylinder head overhauls, injector changes, separator cleaning and turbocharger service. The skills are widely available, spares are modular and work can be scheduled engine by engine without taking the whole plant down.
Turbines need less routine attention but more specialised intervention. Hot-section inspections and major overhauls require trained teams, special tools and sometimes a factory visit for parts. Burning heavy oil shortens the intervals between them. Owners should weigh not only the cost but the availability of skilled people where the plant is located. USP&E’s O&M teams maintain both technologies, which helps when a site runs a mixed fleet.
Planning for Natural Gas Later
Many oil-fired plants are built with an eye on gas arriving later, by pipeline or as liquefied natural gas. That prospect changes the comparison. Gas turbines are naturally suited to gas and become more attractive once it arrives. Many medium-speed engines can be converted to dual-fuel or pure gas operation, keeping the original investment working.
- Estimate when gas is likely to arrive and how firm that date is.
- Compare the fuel cost of running on heavy oil or distillate until then.
- Check the conversion path for the chosen engines, or the gas readiness of the chosen turbines.
- Leave space and connections in the layout for gas reception and treatment.
Our article on transitioning an HFO plant to gas turbines looks at phased approaches, and units from USP&E’s gas turbine inventory can be added beside existing engines when gas becomes available.
Emissions and Permitting Considerations
Heavy fuel oil contains sulphur, ash and metals that end up in the exhaust, so oil-fired plants face scrutiny from environmental regulators and lenders. Reciprocating engines and gas turbines handle this differently. Engines burning HFO usually need attention to sulphur oxides, particulates and nitrogen oxides, sometimes with flue gas treatment. Gas turbines running on heavy oil need fuel treatment to remove corrosive metals and may need water injection to limit nitrogen oxides.
Permitting rules vary from country to country, and development finance lenders often apply their own environmental and social standards on top. Stack height, dispersion modelling, noise limits and waste oil handling all form part of the permit. Planning these early avoids redesign after the equipment has been chosen.
Questions to Settle Before Choosing
A short list of questions narrows the choice quickly.
- What fuel will be available for the life of the plant, and could gas arrive later?
- Will the plant run at steady baseload or follow a changing load?
- How hot is the site, and how high above sea level?
- What skills are available locally to operate and maintain each technology?
- How quickly is the power needed, and what equipment is in stock?
- What emissions limits apply, and what do lenders require?
Answers to these questions usually point clearly to one technology. Where they do not, a short feasibility study comparing lifetime fuel, maintenance and availability settles the matter.
When Each Technology Wins
Experience across many oil-fired projects points to a few patterns.
- Engines: heavy oil as the long-term fuel, hot or high sites, variable demand, small and medium grids, modular growth.
- Turbines: distillate or gas as the main fuel, large steady loads, limited space per megawatt, gas arrival expected soon.
- Mixed plants: engines for flexibility and peak following, a turbine for steady base output once gas arrives.
A short feasibility study comparing fuel cost, availability, derate and maintenance over the expected life usually settles the question for a given site.
HFO engines and gas turbines: questions
Can a gas turbine burn heavy fuel oil?
Some heavy-duty turbines can, but only with fuel washing to remove sodium and potassium, fuel treatment to inhibit vanadium corrosion and more frequent hot-section maintenance. Many owners use light distillate instead.
Why do HFO engines perform better in hot climates?
Turbine output depends strongly on air density, which falls as temperature rises. Reciprocating engines are far less sensitive, so they lose less output on hot days and at altitude.
Which is more efficient on oil, an engine or a turbine?
In simple-cycle operation, medium-speed engines are generally more efficient than gas turbines of similar size and hold their efficiency better at part load.
Can an HFO engine plant be converted to natural gas?
Many medium-speed engine models have a conversion route to dual-fuel or pure gas running. The maker’s conversion path should be checked before purchase.
Is maintenance cheaper on engines or turbines?
It depends on fuel and duty. Engines need more frequent routine work; turbines need fewer but costlier interventions, and heavy oil shortens turbine inspection intervals.
Compare the Options for Your Site
USP&E builds and runs both engine and turbine plants and has 25,000 MW contracted since 2002. For a technology comparison based on your fuel, climate and load, email info@uspeglobal.com or call +27 10 822 2324.
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