Engine management based on cylinder pressure makes gaseous-fuel engines on ships more eﬃcient without any negative impact on nitrogen oxide emissions. Studies by IAV and manufacturers of large-bore engines show that this approach permits highly eﬃcient operation of the engine near to its mechanical load limit. Operating all cylinders on equal terms is also recommended in terms of performance.
The maritime sector has its equivalent to urban environmental zones. In the Emission Control Areas (ECAs), which can be found near coastlines or in the North and Baltic Sea, engine emissions of nitrogen oxide, sulfur and carbon dioxide must be kept to an absolute minimum. The sulfur comes from the heavy fuel oil used on vessels (with a share of up to 3.5 percent). When a captain enters a zone with stricter emission limits, he has to change over to normal diesel fuel or rely on an elaborate exhaust gas aftertreatment system. Together with adapted combustion methods, it is thus possible to keep nitrogen oxide emissions under control – but there are high costs involved.
Natural gas: clean, low-cost fuel
Fortunately, there is another alternative. ”Natural gas is a wonderful solution for keeping emissions below the permitted limits without too much input. Modern gaseous-fuel engines designed as lean-burn concepts offer high power density and high efficiency levels while complying with the emission regulations“, says Dr Andreas Broda, IAV team leader for engine testing and exhaust gas aftertreatment of gaseous-fuel engines for commercial vehicles. ”Intensive work is therefore in progress worldwide on marine gaseous-fuel or dual-fuel engines.“ Natural gas (as LNG) offers other advantages in addition to low emissions. It is available in large quantities at low prices, something that is not likely to change in future either. Furthermore, methane has a better ratio of hydrogen to carbon atoms and therefore generates less CO2in the combustion process.
While high-output stationary gaseous-fuel engines are fitted with prechambers with spark plugs, in marine engines diesel fuel can be added (1 to 5 percent) as a “chemical spark plug”. Intelligent engine management is crucial in any case to achieve the same efficiency as in diesel engines, while remaining reliably within the permitted operating limits. ”The engines run in a very narrow operating range between knocking and misfiring“, explains Benjamin Tilch, IAV team manager for performance & emission (P&E) concepts and functions in the Commercial Vehicle Powertrain division. ”If we always want to stay in a safe operating range under varying ambient conditions with fluctuating gas qualities, we have to keep a certain safety margin to the mechanically acceptable peak pressure. Ideally, this can also be used for greater efficiency.“
The best way to operate the engine as close as possible to its mechanical limits consists in closed-loop control based on cylinder pressure signals. This ensures that combustion takes place near the permissible peak pressure and thus with maximum efficiency. IAV has worked with manufacturers to examine this kind of engine control based on cylinder pressure in a medium-speed gaseous-fuel engine with cylinder-specific gas injection. The tests were carried out on a real-time-capable, angle-based engine model (software-in-the-loop) and a detailed engine model (model-in-the-loop). The IAV experts then implemented the control concepts and the analysis functions in a rapid prototyping engine control unit environment (IAV RP ECU) with a bypass connection to the engine control unit.
Initially, the cylinder pressure was fed into the real-time-capable indication system IAV Indicar where it was filtered, corrected and used to compute the released heat. This permitted online calculation of all relevant data such as combustion start, the main center of heat release and maximum cylinder pressure together with indications of engine knocking and misfiring. The data are then forwarded as input variables to the control unit. For comparing various control strategies, the team examined a range of reference variables, including air/fuel ratio, combustion duration, mean indicated cylinder pressure and ignition timing.
As efficient as a diesel engine
The investigation showed that cylinder-pressure-based engine control really was capable of minimizing the safety margin to the mechanical load limits, thus further enhancing the already high efficiency of the gaseous-fuel engine without exceeding the stipulated nitrogen oxide limits. It also transpired that all cylinders should be balanced. There is no point in operating them on different pressure levels to reduce NOxemissions. ”Engine control based on maximum cylinder pressure proved to be highly promising“, summarizes Broda. ”This permits fast, high-precision control of the engine in relation to load, despite its high thermal mass.“