1 Definition of pre-ignition and phenomenology Pre-ignition is a self-ignition phenomenon in highly supercharged gasoline engines, which is characterized by incipient combustion even before the external ignition source (e.g. electric ignition spark). Because of the early combustion, strong knocking usually occurs subsequently. Pre-ignition occurs spontaneously primarily at high mean pressures and low engine speeds and is stochastically distributed. Pre-ignition events often occur in sequences if no countermeasures are taken. These sequences are thermodynamically driven. If PI events are caused by oil droplets or particles, they are usually single events, which are also distributed in the combustion chamber, stochastically. The energy conversion is rapid and leads to significantly higher combustion chamber pressures (see Graphic 1), which can reach 2 to 3 times the regular combustion chamber pressures. Acoustically, pre-ignition is audible as a distinct metallic-sounding noise (thump), which is due to the high pressure gradients. In the p-V diagram, pre-ignition can be clearly detected by the characteristic pressure rise (see Graphic 1). The occurring maximum pressures often exceed the measuring range of the pressure transducers, which is why usually only a qualitative statement about the events can be made. The consequence of these significant pressure overshoots is often damage to the piston. Typical damages are: • Fractures at ringland 1/ at the ringlands (see Graphic 2) • Fractures/ cracks/ washouts at the top land (due to blowby) • Fractures of the piston pin hub • Hot and cold tight piston rings • Piston ring fractures These damages may occur after single, violent events (force fractures) or may develop after several pre-ignition events (endurance/ fatigue fractures). In addition to the mechanical robustness of the piston, the magnitude of the pressure surge or the severity of the PI event is decisive. The further the heat release moves away from the ignition point towards the early combustion position, Aus Wissenschaft und Forschung 11 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 Pre-ignition phenomena in the tension field between operating agents and thermodynamic boundary conditions Thomas Emmrich, Klaus Herrmann, Michael Guenther* Against the background of EU legislation with regard to CO 2 emissions, a development trend towards higher geometric compression is emerging for gasoline engines. In principle, this leads - especially in combination with high mean pressure at low engine speed - to a higher pre-ignition tendency, well known as low speed pre-ignition (LSPI). The worldwide use of engine families with different fuel and oil quality represents an additional challenge, which has to be ensured within the scope of series development. IAV has extensive expertise and methodical approaches to minimize the risk of pre-ignition starting in the preliminary development through to series application and to avoid engine damage in the field. The definition and phenomenology of pre-ignition are presented. The presentation highlighted thermodynamic aspects as well as influences from operating agents and engine design. By using the IAV enthalpy approach, it is possible to evaluate designed engines objectively. This knowledge can also be used in the development of new engines concepts. Finally, a test method is presented which is used for the final assurance of the operational stability even in the case of stochastically occurring pre-ignition. Keywords LSPI, Pre-ignition, enthalpy-approach, endurancetest, oil impact, fuel impact, thermodynamics Abstract * Dr.-Ing. Thomas Emmrich Dipl.-Ing. Klaus Herrmann Dipl.-Ing. Michael Guenther IAV GmbH, Chemnitz, Germany 2 Influencing factors on pre-ignition 2.1 Dependence on operating point/ thermodynamics 2.1.1 Engine speed influence Generally, the risk of pre-ignition is greatest at the lowend torque (LET) point. This point marks the point of maximum mean pressure (= maximum engine torque) at minimum engine speed. In addition to the high load at low speed the knocking impact shifts the center of combustion heat release to a very late position and increases the time for generation of pre ignition conditions. Below the engine speed of the LET, the maximum mean pressure (proportional to the boost pressure) is not reached, or above this engine speed the time for the necessary chemical pre-reactions in the combustion chamber decreases. Both conditions therefore have a reducing effect on the risk of pre-ignition. 2.1.2 Mean effective pressure impact Assuming a stoichiometric fuel-air ratio (λ = 1), the increase in mean effective pressure (BMEP) is achieved by a higher degree of charging (increase in boost pressure) of the engine. Thus, the higher the boost pressure, the greater the internal energy (enthalpy) of the fresh gas at constant displacement. For a given compression ratio, the work of volume change during compression remains almost constant (neglecting wall heat exchange), meaning the enthalpy change of the working gas at TDC is proportional to the change in boost pressure. Graphic 4 shows this relationship for 2 operating points. 2.1.3 Charge air temperature impact The temperature characterizes the internal kinetic energy of a substance, i.e. a high charge air temperature indica- Aus Wissenschaft und Forschung 12 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 the higher the pressure peaks. This results from the combination of the pressure increase due to energy release with simultaneous compression as a result of the volume reduction caused by the upward movement of the piston (compression) and the high-frequency pressure components from knocking combustion. Damage to the cylinder bore of the affected cylinder can remain minor if detected in time. Such pre-ignition damage can be detected by measuring the blowby or monitoring the crankcase pressure. Slight or incipient damage can only be reliably detected with high sensitivity of the measuring section and a great deal of experience. Since the resulting increase in crankcase pressure or blowby is only very slight or lies within the permissible tolerance, very narrow monitoring limits on the engine test bench are essential. In field operation without monitoring, PI damage is usually only detectable in the event of severe damage to the piston as a result of the loss of power that occurs. In multi-cylinder engines, compression measurement can be used to determine the affected cylinder. Graphic 1: Pressure curve for PI at n = 1500 rpm and BMEP = 22 bar Graphic 2: Typical damage pattern after several pre-ignitions Graphic 3: Schematic representation of the preignition risk in the engine map tes a higher enthalpy state. The charge air temperature characterizes the starting conditions of the thermodynamic process. If we disregard the wall heat losses during compression, we find the enthalpy increase due to the increase of the charge air temperature at TDC (end of compression) (Graphic 5). accelerates the propagation speed of this flame front and minimizes the risk of unwanted self-ignition (knocking), which is why the Otto combustion process is referred to as a turbulence-driven combustion process. As already stated in (Dahns, Han, & Magar, 2009), a low-knock combustion process also results in a lower risk of preignition. DI/ MPI mixture preparation: The advantage of the duct-injected MPI process is the excellent homogenization of the fuel-air mixture in the intake manifold and the avoidance of lubricant film wash-off and wall wetting when the engine is warm (Brandt, et al., 2010). In addition to using fuel evaporation enthalpy for internal cooling, the DI process still offers the possibility of optimizing the effect of internal cooling via different injection patterns. The effect of these mechanisms is explained in more detail in section 3. 2.2.2 Influence of the residual gas fraction Because the combustion gases are not completely expelled from the combustion chamber, the mixing temperature of the fresh gas charge is raised as a function of the residual gas content. High fresh gas temperatures in the cylinder lead to an increased tendency to pre-ignition. In addition, depending on the mixture quality of the fuel grade and the oil composition, there are radicals in the exhaust gas, which increase the tendency to self-ignition. Especially at critical operating points, high residual gas contents in the cylinder and unfavorable residual gas composition due to incomplete combustion must be avoided. In particular, acetone and high NOx contents in the residual gas can have a catalytic effect on spontaneous combustion (Hoffmeyer, et al., 2009). 2.2.3 Constructive design for oil balance Since the number of possible influencing factors is very large, some typical causes are listed, but they do not claim to be exhaustive: • Washing off of liner (fuel wall buildup) • Entry into fresh gas (blowby recirculation in intake tract, ATL storage) • Storage in crevices or deposits (oil carbon in the top land, oil carbon, combustion residues) • Cylinder wall temperature (depending on coolant temperature) • Pockets/ sinks in the intake section, where oil emulsion can accumulate • Lubricant film thickness (e.g. depending on the tangential force of the piston rings) Aus Wissenschaft und Forschung 13 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 Graphic 4: Mean effective pressure influence Graphic 5: charge air temperature impact 2.2 Dependence on the designed engine 2.2.1 Constructional design - combustion process Internal cylinder flow: Starting from an external initialization of combustion, e.g. by an ignition spark, combustion progresses concentrically around this starting point in a flame front. The combustion causes the pressure and temperature in the combustion chamber to rise sharply, which shifts the thermodynamic framework conditions in the combustion chamber toward the self-ignition limit of the fresh gas. However, the chemical reactions are time-dependent, which is why pre-ignition only occurs at low engine speeds. Turbulence (kinetic energy of the particles) Market-specific quality criteria for fuels require adapted test cycles to ensure the operational reliability of the engine. PI-fuels are used specifically for the calibration of pre-ignition protection functions in the control unit and their escalation stages. On the other hand, the mechanical robustness of the engine with regard to pre-ignition can be checked within a reasonable time frame. Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 In Graphic 6, the influence of crankcase pressure is shown as an example of these influencing factors. The increase in pressure was achieved externally via a pressure source and corresponds approximately to the doubling of the blowby value in the test engine (simulation lifetime). The 50 % change in PI frequency shows the relationship well. An increase in crankcase pressure can occur on the engine practically due to several causes: • Natural wear on the engine (wear of the piston rings - reduction of the tangential force) • Breakage of one or more piston rings • Wear on the liner • Fault in the crankcase ventilation system 2.3 Influence of operating agents 2.3.1 Influence of fuel Fuel quality has a very great influence on pre-ignition behavior. Within the EU, fuel quality is regulated in DIN EN 228 (Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit, 2020). This standardization prescribes, according to the fuel quality, a certain minimum octane number, which characterizes the knocking behavior. As mentioned in (Spicher & Rothe, 2007), conclusions about the pre-ignition behavior can be drawn from the octane number. Fuels with a high octane number cause a lower risk of pre-ignition than fuels with a low octane number. The admixture of diesel fuel, which reduces the octane number and thus the knock resistance and increases the ignition readiness, relies on this effect. Various mineral oil companies offer special fuels (referred to here as PI-fuel) that are specifically formulated to increase the tendency to self-ignition several times over compared with standard fuels (e.g. RON 95). A comparison of the components of PI-fuel and standard fuel RON 95 shows the differences with the same antiknock properties (see Graphic 7). The PI-fuel used for the tests contains approx. 1/ 3 more aromatics. Particularly long-chain and thus high-boiling aromatics are knock-resistant and thus realize the same octane number as in the standard fuel RON 95, but participate only to a limited extent in the mixture formation. The comparison of the boiling lines (see Graphic 9) for both fuels reflects this fact. The fuel components that really participate in mixture formation exhibit lower knock resistance and thus cause the significant increase in pre-ignition frequency (see Graphic 8). Evaporation behavior of the fuel is closely related to the fuel spray. The higher boiling point of PI-fuel prolongs the liquid phase of the spray. This increases the risk of wall wetting resulting in fuel ingress into the oil. Graphic 6: Influence of crankcase pressure Graphic 8: Influence of fuel quality on pre-ignition behavior Graphic 7: Composition of PI-fuel and standard fuel RON 95 E10 2.3.2 Lubricating oil influence Engine oil is a formulation of long-chain hydrocarbons of mineral or synthetic origin. Additives are added to the basic formulation depending on the manufacturer and properties, which makes it very difficult to make a generalized statement regarding the influence of pre-ignition. In principle, however, engine oil has a high boiling point due to its components and thus poor evaporation properties. However, oil vapor mixed with air is very ignitable. Parallels can be drawn here with diesel fuel. It has been shown that the pre-ignition effect caused by the use of a particular oil can be engine-specific. In general, the focus is on oil consumption and certain additive components (Yasueda, Takasaki, & Tajima, 2011). From the large number of possible factors influencing engine oil on pre-ignition frequency, 4 typical representatives are presented: • In (Takeuchi, Fujimoto, Hirano, & Yamashita, 2012), the influence of calcium salt compounds as a typical D/ D additive component on pre-ignition tendency is presented. Particles and combustion residues bind to this salt and are thus kept in suspension. This property prevents the deposition of these particles in the engine and thus has a positive influence on cleanliness and mechanical function. However, as the content of this salt compound in the oil increases, so does the tendency of the engine to pre-ignite. In Graphic 10, this influence is illustrated on a turbocharged 2-liter gasoline engine. Calcium was added in the form of calcium stearate. The experiment showed that doubling the calcium content from 1000 ppm to 2000 ppm led to a doubling of the pre-ignition frequency. The reason given by (Takeuchi, Fujimoto, Hirano, & Yamashita, 2012) for this behavior is that oil droplets/ oil vapor enter the combustion chamber through the wiping action of the piston rings and can oxidize there with oxygen from the fresh gas. This reaction provides the ignition source for pre-ignition. • As a 2 nd influencing factor, oil dilution was investigated. This process is found in vehicles which are primarily operated in urban and short-distance traffic. In the case of under-stoichiometric engine operation as a result of cold-start enrichment, a certain amount of fuel is introduced into the engine oil via the wall wetting, depending on the engine. When the engine is at operating temperature, most of this added fuel evaporates again. If the engine is not at operating temperature, higher molecular weight compounds remain in the engine oil and cause the engine oil level to rise. Due to lower viscosity and the lower boiling point of fuel, a risk potential can be derived here. In the test, 20 % fuel was added directly to the fresh engine oil. However, the test shows that oil dilution has no demonstrable influence on the pre-ignition risk. However, the high-boiling components that end up in the engine oil via the wall wetting due to a higher boiling point cannot increase the pre-ignition risk either, because these components have a high knock resistance (see section 2.3.1). Aus Wissenschaft und Forschung 15 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 Graphic 9: Boiling line comparison of PI-fuel and standard fuel RON 95 E10 Graphic 11: Influence of oil dilution with fuel Graphic 10: Effect of calcium compounds on preignition behavior • Graphic 12 shows that the additive packages generally do not have a negative influence. A near-series oil quality and its Group IV base oil are compared here. Despite an increase in the aromatics content and thus in the reactivity to auto-ignition, the pre-ignition index drops significantly. In this particular case, this is possibly due to the reduced lubricating oil concentration as a result of lubricant film height (reduction of surface tension by surfactants) or positively acting oil-fuel interactions. • The different influence of the reaction kinetics of different base oil formulations is confirmed by Graphic 13. Group V base oils are suspected of being more reactive than Group III and IV oils. The polyethylene glycol used here reduces the tendency to pre-ignition by more than 50 %. Even if the hydroscopically acting glycol is not used for lubrication in the basic engine, it clearly shows the influence of the lack of reaction kinetics. The reason for this can be, among other things, the lubricating film thickness with improved sealing effect as well as the non-existent fluctuation of residual gas nests. 2.4 Classification of pre-ignitions According to the theories formulated in public releases and the influences presented, the following classification of influences on pre-ignition (see Graphic 14) is made: 3 Evaluation of pre-ignition behavior with the IAV enthalpy approach As mentioned in section 1, to date there is no clear theory on the origin and mechanisms of pre-ignition. Neverthe- Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 Graphic 13: Pre-ignition frequency of additivated engine oil and glycol in comparison Graphic 12: Influence of additives in oil on preignition frequency Graphic 14: Main factors influencing pre-ignition behavior less, new engines with higher mean effective pressures are being built all the time. Here, it is important to have an objective evaluation criterion for the pre-ignition problem at hand. For the characterization of thermodynamically based pre-ignition, the internal energy of the charge is a suitable criterion, as described in section 2.1.2. The IAV model approach (Günther, Tröger, Kratzsch, & Zwahr, 2011) is based on the consideration of the operating point-dependent variables of charge pressure, charge temperature and air ratio. Here, the enthalpy H describes the energy contained in the working gas, which is formed by • the enthalpy of the fresh gas • the enthalpy of the residual gas • the enthalpy of the fuel at the time ‘inlet closes’. For the ‘inlet closes’ to ‘FTDC’ interval, energy is supplied to the fresh gas by the volume change work, an energy exchange occurs via the wall heat transfer, and energy is supplied via the injected fuel (DI) and removed again by its evaporation. The calculated enthalpy at TDC is proportional to the reaction kinetics. Relating the enthalpy to the displacement makes it possible to compare different engines. All corresponding formulas are presented in Graphic 15. The enthalpy approach is based on purely thermodynamic factors. For the evaluation, the thermodynamic parameter enthalpy H at a specific operating point is assigned to the associated number of pre-ignitions, which is a result of all factors influencing the pre-ignition. The limit enthalpy derived from this represents the pre-ignition limit (tolerable statistical frequency of pre-ignitions at a specific operating point, e.g. x/ 10000 ASP or x/ h). This limit enthalpy is to be determined in the engine test by parameter variation. The enthalpy curve for a charge air temperature variation on a 4-cylinder engine is shown in Graphic 16. If one pre-ignition per cylinder and hour is declared permissible, the limit enthalpy can be read from this diagram. 4 Measuring pre-ignition, evaluation parameters and limit values In contrast to knock analysis, unfiltered signals are analyzed and the absolute peak pressure is used as a measure. Since the absolute peak pressure is composed of lowfrequency and high-frequency components, it is extremely important to record all components as unaltered as possible. The threshold of combustion chamber pressure, above which pre-ignition must be assumed, can be determined as follows: For the counting and evaluation of the pre-ignition phenomenon, only damage-relevant pre-ignitions are used, which exceed the designed reciprocating strength of the piston. This criterion is defined individually for each engine variant. In Graphic 17, an analysis of the pre-ignition tendency is shown as an example. Here, the lower pressure class _ = _ + 20 Pressure maximum of regular combustion incl. maximum cycle fluctuation Detection distance Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 Graphic 16: Determination of the enthalpy limit using the example of a charge air temperature variation Graphic 17: Statistical evaluation of pre-ignition with pressure classification Graphic 15: Collection of formulas for the IAV enthalpy approach Robustness test run: Objective: • Verification of mechanical robustness • Generation of many PI’s in a reasonable amount of time • Achievement of the engine-specific number of PI’s according to manufacturer’s specifications related to each cylinder or engine as a whole Fuel qualities: • PI-fuel Components: • Stationary test at LET speed + 100 rpm (safety reserve to prevent signs of aging during the test) • Dynamic engine speed change • Dynamic load change References Brandt, S., Knoll, G., Schlerege, F., Pischinger, S., Wittler, M., Stein, C., . . . Gohl, M. (2010). Beeinflussung der Schmierölemission durch die Gemischbildung im Brennraum von Verbrennungsmotoren. 19.Aachener Kolloquium Fahrzeug- und Motorentechnik, (S. 571-592). Aachen. Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit. (06 2020). Von https: / / www.bmu.de/ themen/ luft-laerm-verkehr/ verkehr/ kraftstoffe/ kraftstoffqualitaet/ abgerufen Dahns, C., Han, K.-M., & Magar, M. (2009). Vorentflammung bei Ottomotoren. Frankfurt/ M: FVV-Forschungsbericht zum Vorhaben 931. Günther, M., Tröger, R., Kratzsch, M., & Zwahr, S. (2011). Enthalpiebasierter Ansatz zur Quantifizierung und Vermeidung von Vorentflammungen. MTZ - Motortechnische Zeitschrift 72 (04), 296-301. Hoffmeyer, H., Montefrancesco, E., Beck, L., Willand, J., Ziebart, F., & Mauss, F. (2009). Catalytic Reformeated Exhaust Gases in Turbocharged DISI-Engines. SAE Paper 2009-01-0503. Spicher, U., & Rothe, M. (2007). Extremklopfer - Ursachenforschung nach schadensrelevanten klopfenden Arbeitsspielen. Frankfurt/ Main: FVV e.V., Abschlussbericht FVV-Vorhaben 816. Takeuchi, K., Fujimoto, K., Hirano, S., & Yamashita, M. (2012). Investigation of engine oil effect on abnormal combustion in turbochargesd direct injection - spark ignition engines. SAE Int. J. Fuel Lubr. 5 (3) 2012-01-1615. Yasueda, S., Takasaki, K., & Tajima, H. (2011). The abnormal combustion caused by lubrication oil on high BMEP gas engines. 13. Tagung ‘Der Arbeitsprozess des Verbrennungsmotors’, (S. 324 - 336). Graz. Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0026 for PI detection was set at 135 bar. The distribution of peak pressures detected for this fuel variation shows that the vast majority of PI events are above 215 bar peak pressure. This information can be used as an indication for the mechanical design of the piston. 5 Proposal of an endurance run for the evaluation of the application and mechanical robustness with regard to pre-ignition Defined acceptance methods are useful for the final acceptance of the engine, but also for the evaluation of asbuilt conditions. An endurance run covering all relevant load points that can occur in the field is particularly important for the pre-ignition problem. As already worked out, pre-ignition occurs in special load situations and is stochastically distributed. Therefore, these load points must be approached in a targeted manner in the endurance run. In order to set statistically stable conditions, PI-fuels are used in some cases. For this purpose, the following subdivision of the endurance program was made: Graphic 18: subdivision of endurance test run Calibration test run: Objective: • Verification of the PI protection function  Detection of PI  Response to PI → prevention of sequences • Synthetic replication of field-relevant extreme situations Fuel qualities: • Spec. Fuel (RON95/ 98) • PI-fuel Components: • Stationary test at LET speed + 100 rpm (safety reserve to prevent signs of aging during the test) • Dynamic engine speed change • Dynamic load change • Cold sooting followed by transient operation (low engine speed+high load)