A dual fuel engine burns two different fuels simultaneously, typically using a small injection of diesel to ignite a larger charge of a gaseous or alternative fuel such as natural gas, hydrogen, or methanol. The concept dates back to a patent filed by Rudolf Diesel himself in 1898, but it has found renewed urgency as industries look for ways to cut carbon emissions from heavy transport and marine shipping without scrapping entire fleets of existing engines.
How the Two Fuels Work Together
In the most common arrangement, a gaseous fuel like natural gas is mixed with intake air and drawn into the cylinder during the intake stroke, just as air alone would be in a conventional diesel engine. Near the end of the compression stroke, a small quantity of diesel, called the pilot fuel, is injected directly into the cylinder. That pilot charge auto-ignites under the high pressure and temperature of compression, creating multiple flame kernels that then spread through the surrounding gas-air mixture and burn it.1Energy Conversion and Management. Theoretical study of the effects of pilot fuel quantity and its injection timing on the performance and emissions of a dual fuel diesel engine The gaseous fuel is the primary energy source and controls the engine’s power output, while the diesel pilot acts purely as a reliable ignition trigger.
This design neatly solves a practical problem. Natural gas, hydrogen, and ammonia all have characteristics that make them difficult to ignite reliably in a compression-ignition engine on their own. Natural gas has a high auto-ignition temperature; ammonia has a very low flame speed and narrow flammability range; hydrogen ignites easily but is hard to meter safely at high pressures. By keeping the proven diesel compression-ignition cycle and simply shrinking the diesel dose to a small pilot, engine designers get the ignition reliability of diesel with the emissions or cost benefits of the alternative fuel.
From Rudolf Diesel’s Patent to Modern Marine Giants
Rudolf Diesel filed a patent application on April 6, 1898, describing a method in which a fuel mixture is compressed below its own ignition point but above the ignition point of a second “auxiliary combustible.” Injecting that second fuel would trigger ignition of the primary mixture. The patent, titled “Method of Igniting and Regulating Combustion for Internal Combustion Engines,” was granted in 1901 and is widely considered the origin of dual fuel combustion.2IntechOpen. Dual-Fuel Combustion Despite the elegance of the idea, the mechanical complexity and rough running caused by auto-ignition and knocking kept it out of commercial use for decades. The first commercial dual fuel engine was produced in 1939 by the National Gas and Oil Engine Company in Great Britain.
World War II gave the concept a push. Shortages of liquid fuels drew interest from engineers in Britain, Germany, and Italy, who experimented with substituting gaseous fuels in diesel engines. After the war, economic and environmental motivations drove continued development. By 1949, an engineer at the Cooper-Bessemer Corporation reported a dual fuel engine achieving about 40 percent thermal efficiency at full load, described at the time as the most efficient engine known.3IntechOpen. Dual-Fuel Combustion Today, dual fuel engines are used in stationary power generation, long-haul trucking, and increasingly in large marine vessels where liquefied natural gas is available as a fuel.
The Emissions Trade-Off
The environmental case for dual fuel engines is real but complicated. Running on natural gas and diesel in dual fuel mode significantly lowers nitrogen oxide (NOx), carbon dioxide (COâ‚‚), and particulate matter (PM) emissions compared to straight diesel. That is the good news. The bad news is that hydrocarbon (HC) and carbon monoxide (CO) emissions can increase by several times, or in some conditions more than a hundredfold, compared to normal diesel combustion. There is also a persistent trade-off between NOx and HC: tuning the engine to push one down tends to push the other up.4Fuel Processing Technology. A review on natural gas/diesel dual fuel combustion, emissions and performance
This trade-off is not a small technicality. The elevated HC emissions come partly from unburned methane escaping the combustion process, a phenomenon known as methane slip. And methane is a far more potent greenhouse gas than COâ‚‚ over shorter time horizons, so the COâ‚‚ savings from burning natural gas can be partially or fully offset by methane that sneaks through unburned.
Methane Slip and How Engineers Are Tackling It
Methane slip is one of the biggest headaches in natural gas dual fuel engines, especially at low and medium loads where the flame does not always propagate completely through the lean gas-air mixture. The unburned methane exits through the exhaust, reducing engine efficiency and undermining the greenhouse gas advantage of switching from diesel to natural gas in the first place.5SAE International. A Study on the Use of Intake Flow Path Modification to Reduce Methane Slip of a Natural Gas-Diesel Dual-Fuel Engine
Several approaches are being tested to reduce methane slip. One involves physically modifying the intake manifold with an insert that changes the airflow pattern entering the cylinder. In testing, this kind of insert cut methane slip by roughly 28 percent and CO emissions by about 25 percent, but at a cost: NOx emissions rose by about 23 percent and thermal efficiency dipped by around 1.7 percentage points.6SAE International. A Study on the Use of Intake Flow Path Modification to Reduce Methane Slip of a Natural Gas-Diesel Dual-Fuel Engine That NOx-versus-methane seesaw, again.
Another strategy involves blending hydrogen into the natural gas supply. In simulations of large two-stroke marine engines, increasing the hydrogen molar fraction to 40 percent reduced methane slip by about 27 percent while simultaneously cutting equivalent COâ‚‚ emissions by roughly 294 grams per kilowatt-hour. The downside was a slight increase in fuel consumption and a modest drop in power output.7Fuel. Numerical simulation of methane slip from marine dual-fuel engine based on hydrogen-blended natural gas strategy The hydrogen essentially burns faster and more completely, sweeping up methane that would otherwise escape. Whether this makes economic sense depends heavily on the cost of hydrogen, which remains a moving target.
The Low-Load Efficiency Problem
Dual fuel engines tend to shine at high loads, where there is enough fuel in the cylinder to sustain vigorous combustion. At low loads, the story changes. A natural gas/diesel dual fuel engine running at low load with a conventional diesel injection strategy typically has lower thermal efficiency and produces higher greenhouse gas emissions than a straight diesel engine doing the same work.8Applied Energy. On greenhouse gas emissions and thermal efficiency of natural gas/diesel dual-fuel engine at low load conditions The lean gas-air mixture simply does not burn as cleanly or completely when the engine is loafing, and the pilot diesel dose is too small to create enough ignition energy to drive thorough combustion.
Engineers are addressing this through injection strategies like split injection, where the diesel pilot is divided into two pulses rather than delivered all at once. They are also experimenting with rail pressure tuning, variable injection timing, and higher compression ratios. In large marine two-stroke engines, increasing scavenging air pressure from 3 to 3.75 bar boosted thermal efficiency from roughly 49 percent to about 58 percent by packing more air into the cylinder and speeding up combustion.9Scientific Reports. The impact of scavenging air state on the combustion and emission performance of marine two-stroke dual-fuel engine The underlying challenge, though, is that dual fuel engines need careful operating-condition management to avoid working in regimes where they are worse than the diesel engines they are supposed to improve upon.
Knock at High Loads
If low load is where efficiency suffers, high load is where knock becomes a threat. Engine knock occurs when the unburned gas-air mixture ahead of the advancing flame front auto-ignites spontaneously, creating violent pressure oscillations that can damage pistons and bearings. Marine dual fuel engines are especially vulnerable because their large bore diameters mean the flame has to travel farther, giving the unburned end-gas more time to reach auto-ignition conditions.10Case Studies in Thermal Engineering. Multi-load knock detection of a marine micro-ignition diesel (DI) -natural gas (PFI) dual-fuel engine based on in-cylinder pressure signals
Higher compression ratios improve efficiency but also raise the probability of knocking, because they drive cylinder pressure and temperature closer to the auto-ignition threshold. Advancing diesel injection timing, another route to better efficiency, makes the knock problem worse at high load. Split diesel injection has shown promise as a countermeasure. By splitting the diesel pilot into two pulses, the flame kernels do not propagate as far or as fast into the end-gas region, slowing the pressure and temperature rise rate and reducing knock intensity. The early partial burning of the premixed natural gas in the squish region also dilutes the remaining unburned gas, making it more resistant to spontaneous ignition.11Applied Energy. Split diesel injection effect on knocking of natural gas/diesel dual-fuel engine at high load conditions
Hydrogen as the Primary Fuel
Natural gas has dominated dual fuel research, but hydrogen is gaining ground as a primary fuel paired with diesel ignition. Because hydrogen contains no carbon, burning it produces zero COâ‚‚ at the point of combustion. In one study of a heavy-duty diesel truck fitted with a hydrogen-diesel dual fuel system using pre-manifold injection, COâ‚‚ reductions scaled roughly in proportion to the hydrogen energy fraction, which ranged from 10 to 30 percent. The system was installed on a commercial class-8 truck without modifying the original diesel injection system or engine control unit, showing that hydrogen dual fuel can work as a bolt-on addition to existing hardware.12International Journal of Hydrogen Energy. Characterizing hydrogen-diesel dual-fuel performance and emissions in a commercial heavy-duty diesel truck
Push the hydrogen fraction much higher and the results get more dramatic. At low loads, researchers have run a hydrogen-diesel dual fuel engine with hydrogen providing up to 98 percent of the energy, achieving simultaneous reductions of over 90 percent in both carbon emissions and NOx, with soot dropping by 85 percent compared to diesel-only operation. At medium loads, however, NOx emissions climb sharply because of the high combustion temperatures that hydrogen’s energy density produces.13International Journal of Hydrogen Energy. Combustion and emission characteristics of a hydrogen-diesel dual-fuel engine The load-dependent behavior is a recurring theme in dual fuel engines: no single calibration works perfectly across the entire operating range.
High-pressure direct injection of hydrogen, rather than port injection into the intake manifold, is an active area of development for heavy-duty applications. This approach injects hydrogen directly into the combustion chamber at high pressure, giving the engine more precise control over how and when the hydrogen burns.14Fuel. Performance and emissions of a novel high-pressure direct injection hydrogen dual-fuel engine Direct injection also avoids some safety concerns associated with having a hydrogen-air mixture sitting in the intake manifold.
Ammonia and Methanol Pairings
Ammonia is attracting serious attention as a carbon-free fuel for large marine engines. It can be produced from renewable electricity, stored as a liquid at relatively modest pressures, and carries no carbon atoms to form COâ‚‚. The catch is that ammonia is hard to ignite: it has a high auto-ignition temperature, needs substantial ignition energy, and burns slowly. In a dual fuel arrangement, diesel serves as the ignition source, creating a high-velocity jet flame in the main combustion chamber that then ignites the ammonia.15Scientific Reports. Combustion and emission characteristics of ammonia-diesel marine high pressure direct injection low-speed dual-fuel engine
The emissions picture for ammonia dual fuel is different from natural gas. There is no COâ‚‚ from burning ammonia itself, but ammonia contains nitrogen, and combustion can produce both NOx and nitrous oxide (Nâ‚‚O), which is a potent greenhouse gas. Research suggests that keeping the ammonia energy ratio below about 60 percent helps lower flame temperatures enough to reduce NOx, but increasing the ammonia share also raises Nâ‚‚O emissions, partially offsetting the carbon benefit. Unburned ammonia escaping through the exhaust is another concern, requiring effective aftertreatment systems.16Fuel Processing Technology. Ammonia as fuel for marine dual-fuel technology: A comprehensive review
Methanol is a simpler molecule and easier to handle than ammonia, which makes it appealing for retrofitting existing diesel engines. In one study examining a methanol/diesel direct injection engine, thermal efficiency peaked at about 41.5 percent when methanol provided half the energy, and both NOx and soot emissions fell consistently as the methanol share increased. However, HC and CO emissions followed the opposite pattern, rising at higher methanol fractions.17Fuel. Effects of methanol energy substitution ratio and diesel injection timing on a methanol/diesel dual-fuel direct injection engine Methanol can also introduce combustion instability at high substitution ratios, with cycle-to-cycle pressure variations growing as the methanol share increases.18Journal of the Energy Institute. Analysis of the nonlinear dynamic characteristics of the combustion instability in a methanol/diesel dual-fuel engine
Retrofitting Existing Engines
One of the most commercially significant aspects of dual fuel technology is that it can be retrofitted onto engines that were originally designed for straight diesel. Retrofit kits for natural gas typically add gas injectors to the intake runners, a gas supply and pressure regulation system, and an electronic controller that manages the substitution ratio, the proportion of energy provided by gas versus diesel.19SAE International. Characteristics of Exhaust Emissions from a Heavy-Duty Diesel Engine Retrofitted to Operate in Methane/Diesel Dual-Fuel Mode The original diesel injection system stays in place and continues to function; the kit simply reduces how much diesel is injected and makes up the difference with gas.
For methanol retrofits, the modifications are broadly similar. In one heavy-duty diesel engine conversion, the standard intake manifold was replaced with one accommodating six methanol injectors, with injection pressure adjustable between 3 and 10 bar. Timing and duration of methanol injection were managed by an open engine control unit, giving researchers fine-grained control over the substitution ratio at different operating points.20Fuel. Effects of port-fuel injected methanol distribution on cylinder-to-cylinder variations in a retrofitted heavy-duty diesel–methanol dual fuel engine In principle, the engine can revert to pure diesel operation if the alternative fuel supply runs out, a valuable fallback that battery-electric or fuel-cell conversions cannot easily offer.
Controlling the diesel substitution rate in real time is not trivial. Five key engine variables feed into the control algorithm: the diesel injection quantity per cycle, the gas injection quantity per cycle, engine speed, intake air flow, and the excess air coefficient. Getting the substitution rate wrong means either wasting diesel (too much pilot fuel) or risking incomplete combustion (too little).21Applied Thermal Engineering. A method to precisely control the diesel substitution rate of diesel-natural gas dual fuel engine
Safety and System Complexity
Adding a second fuel to an engine means adding fuel storage tanks, supply lines, pressure regulators, injectors, sensors, and control logic that a diesel-only engine does not need. In marine applications, where engine failure can have serious consequences, this extra complexity carries safety implications. Failure of a gas supply sensor or actuator, for instance, could lead the engine to inject the wrong amount of gas, potentially causing knock, misfires, or even a dangerous gas leak into the engine room.22SAGE Journals. Safety performance assessment of a marine dual fuel engine by integrating failure mode, effects and criticality analysis with simulation tools
Dual fuel marine engines require more sophisticated engine control and safety systems than conventional diesel engines, including gas detection in enclosed spaces, ventilation systems for double-walled piping, and emergency shutdown procedures for the gas supply. Classification societies such as Lloyd’s Register and DNV have developed specific rules governing dual fuel installations on ships. The added maintenance burden and crew training requirements are real operational costs that sit alongside the fuel savings.
What Hydrogen Does to Engine Oil and Wear
A less obvious consequence of dual fuel operation involves what happens to the lubrication system. In a hydrogen-diesel dual fuel engine, the presence of hydrogen in the combustion chamber produces significantly more water vapor than diesel combustion alone. That extra moisture contaminates the lubricating oil. One study found that hydrogen operation decreased oil viscosity by 26 percent and increased concentrations of metallic wear particles in the oil, with iron up about 18 percent, copper up 29 percent, aluminum up 22 percent, and chromium up 27 percent compared to diesel-only operation.23Lubricants. Investigation of Emission Characteristics and Lubrication Oil Properties in a Dual Diesel–Hydrogen Internal Combustion Engine Those metal particles come from accelerated wear of cylinder liners, bearings, and piston rings.
The implication is that oil change intervals may need to be shorter for hydrogen dual fuel engines, and the choice of lubricant may need to be different from what works in a conventional diesel. For fleet operators evaluating a switch to hydrogen dual fuel, the economic calculus has to include not just fuel costs and COâ‚‚ reductions but also increased maintenance spending on lubrication and potentially earlier overhaul of wear-critical components. This is the kind of hidden cost that rarely shows up in headline comparisons of fuel options but matters considerably over the life of an engine.
High-Pressure Direct Injection Spray Dynamics
Much of the cutting-edge work on dual fuel engines focuses on high-pressure direct injection, where both the diesel pilot and the gaseous fuel are injected directly into the combustion chamber at high pressure rather than premixing the gas with intake air. This approach gives the engine finer control over combustion timing and can reduce methane slip because the gas is not sitting in the intake manifold or cylinder during the compression stroke, where it might escape unburned into crevice volumes.
Optical experiments using a constant-volume test vessel have revealed that the diesel spray behaves differently in a dual fuel injection than when injected alone. The interaction occurs in two stages. Initially, the diesel spray penetration and spray area are smaller than in a single-diesel injection, because the co-injected methane jet disrupts the diesel spray’s development. In the second stage, both fuels continue penetrating forward together, and the diesel spray actually grows larger than it would alone. The diesel injection pressure has a direct linear influence on when the transition between these two stages occurs. Higher diesel pressure causes the sprays to meet earlier. The methane jet also makes the combined spray pattern asymmetric, shifting the diesel spray cone toward the gas jet side.24PubMed Central. Experimental Study of Dual-Fuel Diesel/Natural Gas High-Pressure Injection Understanding these spray dynamics matters because uneven fuel distribution in the combustion chamber leads to local pockets of rich or lean mixture, which translate directly into emissions and efficiency penalties.