There is still room to improve combustion processes in large-sized natural gas-fuelled engines, also in the sense of minimizing methane slip.

Developing technologies to improve combustion processes and minimizing methane slip in large natural gas-fuelled engines is the major goal of the project Advanced Combustion System using diesel and natural gas blends for Internal Combustion Engines applications minimizing methane slip, which integrates the project portfolio of the Research Centre for Gas Innovation (RCGI).

Coordinated by Professor Guenther C. Krieger Filho, from the Escola Politécnica (Engineering School) of the USP, the project focuses on engines generally used by vessels (including those carrying methane) and thermoelectric plants. “They are engines fuelled by diesel and bunker oil, which are heavy oils, and we want engines to operate with natural gas, which emits less CO2. Natural gas is a fuel which tends to detonate in internal combustion engines, though. If we make the same engines operate with natural gas, we will probably not achieve the power they would have with diesel,” advances Krieger.

He explains that large engines that use natural gas, instead of having an ignition spark plug, count on a pilot flame started by a diesel spray in a pre-combustion chamber. “Diesel is injected and, due to this pre-chamber temperature and pressure, an initial flame is formed, ensuring a more stable operation to the engine, due to the lack of detonation, or due to the lack of misfiring (failure in combustion cycles).”

The idea is to replace diesel or the heavy oil that moves the engine with natural gas, while keeping the pre-combustion chamber with diesel spray ignition. The researchers want to know how to control the two flames (that of gas in the main chamber and that of diesel in the pre-chamber), to have the expected power and stability in combustion. For this, they are building a combustion chamber with optical access (quartz windows) to study what occurs during the burning process with natural gas.

“There is a main chamber, filled with natural gas and air, with a small chamber in it, which we are going to fill with natural gas and air, and then inject a diesel spray inside it. Owing to the pressure and temperature, the diesel makes this ignition feasible. This mix with the gas and the air inside it starts to burn and the flame spreads, ensuring the combustion process.”

According to Krieger, there are already large engines fuelled with natural gas, but there is still room to improve the combustion processes, for example to minimize the methane slip, which occurs when the combustion process does not develop completely. “In one of the existing technologies, which does not penalise the engine power, natural gas is injected when the piston is at the end of the compressive process. However, in case it does not burn, when the exhaust valve opens, natural gas leaks from the cylinder and enters the atmosphere. This slip is what we want to avoid, since natural gas is basically composed of methane.”

The engineer remarks that, in this case, failures in the combustion system have a high cost to the environment. “Methane is 20 times more harmful to the environment as carbon dioxide. If it does not burn, that is, if this combustion cycle does not work and CH4 escapes into the atmosphere, it causes damages in terms of emissions. The technology we are studying ensures high power and safety with the engine operating with natural gas alone, and not only in the dual fuel system.”

In the first stage of the project, a combustion chamber (a vessel pressurised up to 100 bar) will be develop, with optical access allowing the passage of laser beams, so that the team can study the process of forming the ignition mix and the combustion process. “The idea is to deliver experimental data of high scientific value, obtained from ‘state-of-the-art’ combustion diagnosis technology. These data will help solve the problems that may occur in this combustion process.”

In the second stage, Krieger and his team, composed of three other engineers, will acquire a single cylinder, dual fuel (gas and diesel), research engine with optical access. “Its characteristics are close to those of a real engine. We can place a laser beam at the window and, when it is reflected or altered, we manage to understand the combustion process. It is then possible to make some measurements: the velocity field of the gases, where the drops of this diesel spray are from, their diameter and speed. We will be able to identify where the region of the flame is and how it was formed. All these data will feed a computational model, which we intend to extrapolate to large engines.”

Krieger states that, with the recent standards that provide increasingly lower emission rates for greenhouse gases (GHGs), large engines will have to turn to natural gas. “The process under study has to be better than that provided by the standards, since they are going to be increasingly stricter. And natural gas is a great alternative for reducing emissions.”

The whole experimental part will be developed at the Laboratory of Advanced Combustion Diagnosis; its assembling is the goal of one of the projects conducted within the RCGI ambit. “The laboratory will be a multi-user centre, that is, research teams having a demand for using it will be able to do it, even not being connected to the RCGI or to Poli, as long as they present their proposal and this is also interesting to us. We want more people to use it, the demands to appear.” The laboratory is being equipped with different types of laser diagnostics, which allow accurate analyses of the combustion processes studied.