Zero carbon combustion: effect on lubricant aging and its impact on emissions
Scientifc problem
Today, the energy and transport sectors are going through a period of profound change, which includes in particular the awareness of the impact of emissions on the environment and health. In particular, whether for road/maritime transport or for the production of electricity, the use of "Zero-Carbon" compounds, such as hydrogen (H2) and ammonia (NH3), is an avenue to limit emissions not only due to the absence of carbon in their structure and therefore the absence of associated carbon emissions (CO2, CO, HC and soot), but also on the prospect of production processes based on renewable resources .
However, these compounds exhibit unusual combustion properties compared to conventional hydrocarbons. For example, H2 generates a high heat release rate (HRR) (and therefore a more intense maximum pressure in the chamber), with in addition a significant formation of water as a combustion product or even non-flammable NOx emissions negligible due to its very high combustion temperature. As far as NH3 is concerned, it has unfavorable combustion properties: narrow flammability limits and a low laminar combustion rate. In addition, its alkaline character and the presence of nitrogen in its structure respectively affect the compatibility of materials and the formation of nitrogen oxides within the combustion system.
These unconventional combustion characteristics can impact the lubricant properties through potentially greater thermal and chemical stress and increased contamination with oxidizing components such as nitrogen oxides. In addition, extreme thermal stresses can cause deterioration of viscosity properties or the formation of deposits and thus increase the oil's contribution to particulate emissions. The evolution of lubricant performance over time is highly dependent on the lubricant's resistance to oxidation and aging in the liquid phase.
Historically, widely studied conventional heat engine applications have highlighted aging problem in the liquid phase of hydrocarbons. The latter can lead to the formation of unstable multiphase systems, often linked to the formation of deposits or the addition of insolubles from the product environment of. These mechanisms, still poorly understood, are responsible for vehicles major problems (abnormal combustion, fouling, etc.). Figure 1 schematically represents the overall chemical process relating to the stability of a lubricant. Thanks to the "blow-by" mechanism, the combustion gases, including oxygen and also other oxidizing agents such as NOx, interact with the oil constituents in the crankcase and lead to the formation of hydro-peroxides . The latter, by interacting with each other, lead to the formation of soluble and insoluble species, which results in an alteration of the oil properties and in deposits formation.
The contribution of engine oil to the formation of carbonaceous particles is now well known in the literature. Thanks to the "reverse blow-by" mechanism or the simple effect of the rise on the walls of the cylinder by the piston rings, the lubricating oil enters the combustion chamber. Through its own combustion, the lubricant thus contributes to the formation of particles by increasing the quantity of semi-volatile hydrocarbon species available for the formation of soot, thus impacting not only exhaust emissions but also the occurrence abnormal combustion phenomena (low-speed pre-ignition, 'LSPI') as such. In addition, metals and other chemical species used as additives can promote the semi-volatile compounds nucleation resulting from combustion and participate in metal-rich nanoparticles formation.
In this context, the objective of our thesis subject is centered on understanding the lubricant aging and its impact on particulate emissions in the case of engine combustion with “Zero-Carbon” fuels. The experimental approach that will be pursued aims to investigate the lubricant physico-chemical properties degradation mechanism descriptors in relation to particles formation. The expectations of this thesis will provide elements of understanding and essential data for lubricant formulation and abnormal combustion knowledge necessary for the 3D calculations as well as on air quality. A better understanding of this issue will make it possible to continue to optimize the oil/combustion system match and to reduce the innovative combustion systems with zero carbon fuels impact on the environment.
Thesis positioning
In the transport sector, the issue of aging fluids remains an important research theme regularly impacted by changes in technology for both fluids and engines. Added to this are increasingly strict regulatory constraints.
Particulate emissions are strictly regulated because of their impact on air pollution and health. Different studies, mainly carried out in diesel engines, have shown how the particulate emissions are impacted by the lubricant but even for conventional fuels, the impact of the lubricant chemistry is not clear. It is difficult to conclude on a universal trend according to the oil type (synthetic, semi-synthetic or mineral). The sulfur content, the viscosity, the volatility, as well as the content of certain metallic compounds present in the additives, such as calcium, seem to be the factors to be further investigated.
In addition, the lubricant aging seems to have a significant impact on particulate emissions. Some studies point to the fact that new oils seem to emit more particles in mass than aged oils during rolling, due to the presence of aromatics. In addition, the engine oil aging also directly affects the combustion process. Various studies have shown how lubricant aging can increase the propensity for LSPI due to oil and additive degradation and wear metal contamination, particularly Fe and Cu.
The thermo-oxidative degradation is at the engine oil aging origin, which leads to alter the product quality, which can thus limit the system efficiency or even lead to failures. Figure 2b reports the different spectral signature of nitration and oxidation between new oil and used oil, proving their existence. Indeed, the reactive species presence, such as oxygen in a high temperature environment can trigger the radical mechanism of oxidation in the liquid phase. Similarly, NOx from combustion can come into contact with the lubricant through the presence of an oil film on the cylinder liner or the piston grooves, or via the transport of NOx to the crankcase (blowing mechanism). by, Figure 2a). In the crankcase, the NOx begin to react with the oil with a radical mechanism which leads to hydro-peroxides and ester-nitrates formation. This mechanism competes with oxidation, all being a function of temperature (Figure 2c). NOx (NO and NO2) can promote the engine oil aging, in particular through the nitro-oxidation process but also sludge and varnish formation as an increase in viscosity. However, the relative impact of ester-nitrates versus peroxides on the overall oxidation process is understudied, and therefore the importance that should be attached to the accumulation of these products in motor oil is to be further investigated.
The use of "Zero-carbon" fuels, given their more extreme combustion characteristics than conventional fuels, can lead to potentially greater thermal and chemical stress on the lubricant and increased dilution of oxidizing components and nitrogen oxides. In addition, engine oil is composed of a set of additives including in particular inorganic compounds which can also affect the lubricant aging in particular conditions such as during H2 or NH3 combustion, still very few studies exist on the subject.
The complexity and different parameters involved in the oil/combustion system match clearly show the challenge that remains behind the need to know and control these phenomena which come into play in the lubricant role, in particular for systems including new fuels.
This thesis interest is therefore to contribute to improving the knowledge, inconsistent until now, on the oil interaction with Zero-Carbon fuels combustion. The thesis subject original interest is based on the lubricant aging characterization for these new combustions and its role in polluting emissions formation .
Thesis supervisor : Christine ROUSSELLE ⇒ christine.rousselle@univ-orleans.fr
Thesis promoter : Lucia GIARRACCA ⇒ lucia.giarracca@ifpen.fr
Thesis promoter : Perrine COLOGON ⇒ perrine.cologon@ifpen.fr