CIMAC preview: Accelleron at forefront of combustion innovation for decarbonisation

With MEPC 83 meeting to discuss ways to decarbonise the maritime industry this month, one oft-cited solution is the move to lower carbon fuels such as ammonia, hydrogen and methanol. The main problem with this according to Rofka is the scarcity of these fuels.

“The industry is developing and ramping up technologies capable of burning methanol and ammonia ahead of the availability of these fuels,” says Rofka.

The solution is dual-fuel engines, something Roger Holm, vice president of Wartsila, predicted would be a feature of 2025 in The Motorship and looks to be spot on. Predictions for 2025: Hear from titans of industry | News | Motorship Or as Rofka says: “Engine designers are actively enhancing their engine portfolio with dual-fuel capabilities … They can burn diesel,and then they can burn an alternative fuel, like methanol or ammonia.”

Engine designers are therefore maintaining diesel operation as a baseline, developing dual-fuel systems that mimic diesel’s combustion characteristics, by using a small amount of diesel pilot fuel to ignite methanol or ammonia Although these fuels have different combustion properties and lower energy densities than diesel, the specific energy per engine cycle and the exhaust temperatures and flows stay within customary diesel ranges, so that turbocharging requirements also remain largely the same. This means engine performance can be maintained even when operating on methanol or ammonia, enabling ships to deliver the same power output and operational reliability as when using diesel.

If, in what is still a hypothetical situation, there is an abundance of green ammonia, allowing engine makers to produce mono-fuel ammonia engines, this would have significant implications for the design of turbochargers. Ammonia does not burn very quickly, so achieving the right air-to-fuel ratio, , would be a challenge, as there would need to be significantly less air. “That would be a development step for us,” states Rofka.

Alternatively, mono-fuel hydrogen engines with their extremely fast combustion would demand higher air-to-fuel ratios and turbochargers with higher pressure ratios to manage combustion dynamics. These two extremes would lead to divergent developments in turbocharging technology, possibly requiring entirely different approaches in air management systems.

Hydrogen, while gaining attention in some countries for domestic energy use and niche maritime applications like liquefied hydrogen carriers, is not considered a practical solution for mainstream marine propulsion. “Hydrogen will not be the fuel… Liquefying it is very complex… very energy intense… too complex, too expensive for ocean-going vessels,” Rofka explains.

Sequential solution

One promising innovation highlighted by Rofka is sequential turbocharging, which has been applied in both high- and low-speed engine segments. Sequential turbocharging involves using multiple smaller turbochargers that are activated or bypassed depending on engine load. At low loads, a single small turbocharger provides sufficient boost pressure to prevent the insufficient air-to-fuel ratios associated with large single units. As load increases, additional turbochargers come online sequentially, maintaining optimal matching throughout the engine’s operating range.

This approach helps prevent mismatches between the turbocharger and engine air pressures, and ensures adequate boost pressure even outside the standard design point for optimal efficiency and performance. By increasing boost pressure at low load — when one turbocharger handles all the exhaust flow — the engine benefits from improved air trapping, better combustion, higher efficiency and enhanced torque capability.

But sequential turbochargers are not a panacea. One potential issue regards efficiency. “Multiple small turbochargers may not always surpass the efficiency of a single large unit,” explains Rofka. He adds that efficiency losses occur because of scaling constraints, such as the inability to proportionally reduce tip clearances in smaller units, which affects airflow and increases internal losses. Larger turbochargers tend to be more efficient because of lower relative flow losses, from clearances, for example.

It seems that in practice, turbocharger design is a balancing act shaped by the engine’s operating profile. For high-speed engines with widely varying load demands, sequential setups offer crucial flexibility. In low-speed applications such as those found in large marine vessels, the motivation is primarily fuel efficiency. The ability to cut out one of two turbochargers under low load conditions creates higher boost pressure, and consequently higher peak cylinder pressure, which improves efficiency and reduces fuel consumption., Rofka says.

The evolution of turbocharging is set to play a vital role in enabling the engines of tomorrow but what that tomorrow will look like is the question.

Otto over Diesel for retrofit solutions

One of the transformative deals Accelleron completed in 2023 was the acquisition of fuel injection specialist Officine Meccaniche Torino (OMT), which made them partners in combustion. Put simply, turbocharging takes care of the air, injection looks after the fuel.

What made the deal transformative is that OMT had expertise in fuel injection for the Diesel cycle and for Otto engines..

Rofka explains that both principles are important for the uptake of dual-fuel engines, although the Otto cycle has proven increasingly relevant for retrofitted engines.

“We are developing technologies that support port fuel injection, which is used in the Otto cycle. So, you have a pre-mix of air and fuel that goes into the cylinder, and then it’s ignited by pilot fuel. It’s not the same as the Diesel cycle, where you inject the fuel at high pressure, and then burn it immediately.”

Diesel’s direct injection into the combustion chamber is followed by almost immediate ignition, due to the high pressure and temperature, whereas port injection pre-mixes the fuel with air, before it enters the cylinder. This mixture is then ignited, using a small amount of pilot fuel. The challenge here is that this air-fuel mixture is more sensitive: too early an ignition risks knocking while too late or uneven mixing can result in misfiring or incomplete combustion. Precise atomisation and vaporisation of the methanol or ammonia in the intake port are essential.

Both conventional diesel injectors and port injection systems demand exact control of the spray pattern and droplet size to ensure a homogenous mix, but port fuel injectors can only obtain this result through a much lower fuel pressure, which poses additional challenges. Rofka explains that if the mixture isn’t homogeneous—if, for instance, pockets of rich fuel form—these can ignite prematurely or fail to ignite at all, leading to engine instability and inefficiency. This has driven development of specialised injector technologies that atomise these alternative fuels into a mist-like spray, improving mixing and combustion quality.

Accelleron has simulated the air-to-fuel mixing process obtainable with port fuel injectors, using various spray patterns and injector orientations to identify the optimal parameters. Engine tests at a customer laboratory demonstrated that this innovation achieved high performance while reducing pilot fuel consumption to less than 5%. This approach is especially relevant as a retrofit solutions enabling existing fleets to run on low-carbon fuels without the need for entirely new engines.

Diesel still required

This system is still reliant on pilot fuel for ignition. In contrast, with pure diesel, there’s obviously no such requirement; the injection alone drives combustion.

In a dual-fuel methanol or ammonia setup, however, even as the engine runs primarily on these low-energy-density fuels, a diesel-based pilot fuel is still necessary for ignition. That involves high-pressure dual-injector setups, with two different high-pressure, direct injectors housed in a single casing—one for diesel and one for the alternative fuel—with increasingly precise injectors capable of delivering very small quantities of fuel with high accuracy. Rofka notes this is crucial, because the engines must ignite reliably, even at low loads.

In terms of pilot fuel proportion, the goal among engine designers appears to be 5% of total energy input, even under low load conditions, according to Rofka. This is an improvement over earlier systems that required 10% or more. The reduced amount of pilot fuel not only improves overall efficiency, but also supports decarbonisation goals, by limiting the amount of fossil-derived fuel used.

The complexity of these systems has led to packaging challenges, particularly with larger and more numerous injectors now required to accommodate the lower energy density of methanol and ammonia. Innovations in compact design and modularity are emerging as solutions, helping to fit these systems within existing engine geometries without significant redesign.

The development of two different viable approaches –– both high-pressure direct injection and port injection for dual-fuel engines—is yet another symptom of the maritime sector’s broader transition: accommodating sustainable fuels within the practical constraints of current fleets, while but keepingcombustion stable, efficient and emissions-compliant. With Accelleron’s merger with OMT, the company is ideally placed to lead this transition.

By combining its turbocharging expertise with OMT’s deep knowledge of advanced injection systems, Accelleron is uniquely positioned to deliver integrated combustion solutions for a multi-fuel future. Whether it’s enabling cleaner retrofits for existing fleets or advancing next-generation engine platforms, Accelleron is shaping how sustainable fuels are introduced into marine propulsion.