EP3947608A1

EP3947608A1 - Method for reducing low speed pre-ignition

Info

Publication number: EP3947608A1
Application number: EP20715818.9A
Authority: EP
Inventors: Abhishek Kar; Arjun Prakash; Allen Ambwere Aradi; Roger Francis Cracknell
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2019-04-01
Filing date: 2020-03-27
Publication date: 2022-02-09
Anticipated expiration: 2040-03-27
Also published as: WO2020201104A1; CN113710778A; US20220356409A1; CN113710778B; EP3947608B1; JP2022526585A; JP7558966B2

Abstract

Use of a gasoline fuel composition for reducing the occurrence of Low Speed Pre-Ignition (LSPI) in a spark-ignition internal combustion engine, wherein the gasoline fuel composition comprises a gasoline base fuel and has a PM Index of 1.4 or less.

Description

METHOD FOR REDUCING LOW SPEED PRE-IGNITION

Field of the Invention

The present invention relates to a method for reducing low speed pre-ignition in a spark-ignition internal combustion engine.

Background of the Invention

Under ideal conditions, normal combustion in a conventional spark-ignited engine occurs when a mixture of fuel and air is ignited within the combustion chamber inside the cylinder by the production of a spark

originating from a spark plug. Such normal combustion is generally characterized by the expansion of the flame front across the combustion chamber in an orderly and controlled manner.

However, in some instances, the fuel/air mixture may ignite prematurely prior to the spark plug firing, or after it fires and the ensuing flame front compressing and heating unburned end gases, thereby resulting in a phenomenon known as pre-ignition. Pre-ignition is undesirable as it typically results in the presence of greatly increased temperatures and pressures within the combustion chamber, which may have a significant, negative impact on the overall efficiency and performance of an engine. Pre-ignition may lead to "mega-knock" events which can cause damage to the cylinders, pistons and valves in the engine and in some instances may even culminate in engine failure.

Recently, low-speed pre-ignition ("LSPI") has been recognized amongst many original equipment manufacturers ("OEMs") as a potential problem for highly boosted, down sized spark-ignition engines, in particular high compression ratio direct injection spark-ignition engines. Contrary to the pre-ignition phenomenon observed in the late 50' s at high speeds, LSPI typically occurs at low speeds and high loads. LSPI is a

constraint that restricts improvements in torque at low engine speeds, which could impact fuel economy and drivability. The occurrence of LSPI may ultimately lead to so-called "monster knock" or "mega-knock" where potentially devastating pressure waves can result in severe damage to the piston and/or cylinder. As such, any technology that can mitigate the risk of pre

ignition, including LSPI, would be highly desirable.

There are multiple mechanisms leading to LSPI events discussed in the literature. One of those mechanisms involves ignition of the flaked-off deposits present inside the combustion chamber (e.g. around the piston crevice region or on the injector and cooler regions behind the spark plug) leading to LSPI events while another mechanism is based on the ignition of oil droplets inside the combustion chamber. It could be a combination of these two mechanisms (deposits and oil droplets) that results in LSPI or a yet to be determined mechanism .

It has been found that LSPI is more common in engines, such as modern downsized turbocharged spark ignition engines, that operate using an engine oil with high calcium content and a market-average gasoline fuel. Most commercial engine oils currently available in the market have high calcium content, generally ranging from 1200 ppm to 3000 ppm. Typically, as mentioned above, this LSPI phenomenon is common in the high torque, low speed operating conditions. Most Original Equipment Manufacturers (OEMs) calibrate their engine management systems to restrict engine operation in these regimes to prevent LSPI from occurring. However, operating in these regimes would potentially give the OEMs additional opportunity to decrease fuel consumption.

One solution to the problem of LSPI is to formulate engine oils such that they have a new composition.

Examples of those methods can be found in

W02015/171978A1, W02016/ 087379A1 , WO2015/042341A1. One such solution is to formulate engine oils having a very low calcium content (<500ppm) . The effects of lower calcium content in the engine oils in reducing LSPI occurrences have been described in SAE 2016-01-2275.

Such a formulation potentially modifies the chemical pathways in terms of the oil droplets that lead to LSPI. However, most current commercial engine oils have medium to high calcium content and therefore it would be

desirable to come up with an alternative solution for the problem of LSPI without having to reformulate the engine oil formulation.

US Serial No. 62/573723 relates to a method for reducing low speed pre-ignition by using a gasoline formulation which comprises a certain type of detergent additive package and/or certain detergent additive components, especially in the case when used in engines which are lubricated with engine oils having high levels of calcium.

SAE International Paper SAE-2010-01-2115 published 25 October 2010 relates to an investigation of the relationship between gasoline properties and vehicle particulate matter emissions. In the investigation described therein, various chemical species were

individually blended with an indolene base fuel and the solid particulate number (PN) emissions from each blend were measured over the New European Driving Cycle (NEDC) . A predictive model, termed the 'PM Index', was

constructed based on the weight fraction, vapour pressure and double bond equivalent (DBE) value of each component in the fuel. It was confirmed that the PM Index could accurately predict not only the total PN trend but also total particulate matter (PM) mass, regardless of engine type or test cycle.

It has now been found by the present inventors that by using a gasoline formulation which has a certain maximum Particulate Matter (PM) Index (as calculated according to the PM Index equation set out in SAE

International Paper 2010-01-2115), a surprising reduction in LSPI events can be achieved, especially in the case when used in engines which are lubricated with engine oils having high levels of calcium.

Summary of the Invention

According to the present invention there is provided the use of a gasoline fuel composition for reducing the occurrence of Low Speed Pre-Ignition (LSPI) in a spark- ignition internal combustion engine, wherein the gasoline fuel composition has a PM Index of 1.4 or less.

According to the present invention there is further provided a method for reducing the occurrence of Low Speed Pre-Ignition (LSPI) in a spark-ignition internal combustion engine, the method comprising supplying to the engine a gasoline fuel composition having a PM Index of 1.4 or less .

The features and advantages of the present invention will be apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.

Brief Description of the Drawings The drawings illustrate certain aspects of some of the embodiments of the invention, and should not be used to limit or define the invention.

Figure 1 illustrates the test procedure which was used for engine tests in the Examples hereinbelow.

Figure 2 is a plot of the results in Table 6 below.

Figure 3 is a plot of the results in Table 7 below. Detailed Description of the Invention

Fuel compositions for use herein generally comprise a gasoline base fuel and optionally one or more fuel additives. Fuel compositions comprising a gasoline base fuel are therefore gasoline fuel compositions. The gasoline fuel compositions herein have a maximum PM Index .

The PM Index of a gasoline fuel composition can be calculated herein using Equation (1) below (as disclosed in SAE-2010-01-2115) :

Equation (1)

In Equation (1), a number, or i, is assigned to each gasoline component in the gasoline composition, DBEi is the double bond equivalent value of component i,

V.P(443K)i is the vapour pressure of component i at 443K, and Wti is weight fraction of component i in the gasoline composition .

Further details of the origin of this equation for calculating the PM Index can be found in SAE paper SAE- 2010-01-2115, incorporated herein by reference in its entirety .

The gasoline fuel compositions for use in the present invention have a PM Index of 1.4 or less, preferably 1.3 or less, more preferably 1.2 or less, even more preferably 1.1 or less, especially 1.0 or less. In a preferred embodiment herein, the fuel compositions have a PM Index of 0.9 or less, preferably 0.8 or less, more preferably 0.7 or less, even more preferably 0.6 or less, especially 0.5 or less.

In one embodiment herein, the fuel compositions have a PM Index in the range of 0.4 to 1.4.

The level of occurrence of pre-ignition in a spark- ignited engine may be assessed using any suitable method. Such a method may involve running a spark-ignited engine using the relevant fuel and/or lubricant composition, and monitoring changes in engine pressure during its

combustion cycles, i.e., changes in pressure versus crank angle. A pre-ignition event will result in an increase in engine pressure before sparking, or even after

sparking where the flame front progressing across the cylinder excessively compresses and heats the unburned end gases to the point of spontaneous ignition: this may occur during some engine cycles but not others. Instead, or in addition to, crank angle location may be monitored, for example at an early burn cycle initiation before spark, or at the start of combustion (SOC) . Instead, or in addition to, changes in engine performance may be monitored, for example by maximum attainable brake torque, engine speed, intake pressure and/or exhaust gas temperature. Instead, or in addition to, a suitably experienced driver may test-drive a vehicle which is driven by the spark-ignited engine, to assess the effects of a particular fuel and/or lubricant composition on, for example, the degree of engine knock or other aspects of engine performance. Instead, or in addition to, levels of engine damage due to pre-ignition, for example due to the associated engine knock, may be monitored over a period of time during which the spark-ignited engine is running using the relevant fuel and/or lubricant

composition .

A reduction in the occurrence of pre-ignition may be a reduction in the number of engine cycles at which pre ignition events occur or a reduction in the rate at which pre-ignition events occur within the engine, and/or in the severity of the pre-ignition events which occur (for example, the degree of pressure change which they cause) . It may be manifested by a reduction in one or more of the effects which pre-ignition can have on engine

performance, for example impairment of brake torque or inhibition of engine speed. It may be manifested by a reduction in the amount or severity of engine knock, in particular by a reduction in, or elimination of, "mega knock". Preferably, in the present invention, a

reduction in the occurrence of pre-ignition is a

reduction in the number of engine cycles in which pre ignition events occur.

Since pre-ignition, particularly if it occurs frequently and leads to "mega-knock", can cause

significant engine damage, the fuel compositions

disclosed herein may also be used for the purpose of reducing engine damage and/or for the purpose of

increasing engine longevity.

The uses and methods of the present invention may be used to achieve any degree of reduction in the occurrence of pre-ignition in the engine, including reduction to zero (i.e., eliminating pre-ignition) . It may be used to achieve any degree of reduction in a side effect of pre ignition, for example engine damage. It may be used for the purpose of achieving a desired target level of occurrence or side effect. The method and use herein preferably achieves a 5% reduction or more in the

occurrence of pre-ignition in the engine, more preferably a 10% reduction or more in the occurrence of pre-ignition in the engine, even more preferably a 15% reduction or more in the occurrence of pre-ignition in the engine, and especially a 30% reduction or more in the occurrence of pre-ignition in the engine. In an especially preferred embodiment, the method and use herein achieves a 50% reduction or more in the occurrence of pre-ignition in the engine. In another especially preferred embodiment, the method and use herein completely removes the

occurrence of pre-ignition in the engine.

Examples of suitable methods for measuring Low Speed Pre-Ignition events can be found in the following SAE papers: SAE 2014-01-1226, SAE 2011-01-0340, SAE 2011-01- 0339 and SAE 2011-01-0342. Another example of a suitable method for measuring Low Speed Pre-Ignition events is the test method described in the Examples hereinbelow.

The gasoline fuel compositions herein comprise a gasoline base fuel. The gasoline base fuel may be any gasoline base fuel suitable for use in an internal combustion engine of the spark-ignition (gasoline) type known in the art, including automotive engines as well as in other types of engine such as, for example, off road and aviation engines. The gasoline used as the base fuel in the liquid fuel composition of the present invention may conveniently also be referred to as 'base gasoline' .

Gasolines typically comprise mixtures of

hydrocarbons boiling in the range from 25 to 230°C (EN- ISO 3405), the optimal ranges and distillation curves typically varying according to climate and season of the year. The hydrocarbons in a gasoline may be derived by any means known in the art, conveniently the hydrocarbons may be derived in any known manner from straight-run gasoline, synthetically-produced aromatic hydrocarbon mixtures, thermally or catalytically cracked

hydrocarbons, hydro-cracked, hydro-isomerized petroleum fractions, catalytically reformed hydrocarbons or

mixtures of these. Sulfur and nitrogen levels in the final gasoline should be minimized by, for example, judicious hydro-treating to within the regulated

specifications for the respective regional market. All of these gasoline components may be derived from fossil carbon or renewables.

The specific distillation curve, hydrocarbon

composition, research octane number (RON) and motor octane number (MON) of the gasoline are not critical for the present invention.

Conveniently, the research octane number (RON) of the gasoline may be at least 80, for instance in the range of from 80 to 110, preferably the RON of the gasoline will be at least 90, for instance in the range of from 90 to 110, more preferably the RON of the

gasoline will be at least 91, for instance in the range of from 91 to 105, even more preferably the RON of the gasoline will be at least 92, for instance in the range of from 92 to 103, even more preferably the RON of the gasoline will be at least 93, for instance in the range of from 93 to 102, and most preferably the RON of the gasoline will be at least 94, for instance in the range of from 94 to 100 (EN 25164) ; the motor octane number (MON) of the gasoline may conveniently be at least 70, for instance in the range of from 70 to 110, preferably the MON of the gasoline will be at least 75, for instance in the range of from 75 to 105, more preferably the MON of the gasoline will be at least 80, for instance in the range of from 80 to 100, most preferably the MON of the gasoline will be at least 82, for instance in the range of from 82 to 95 (EN 25163) .

Typically, gasolines comprise components selected from one or more of the following groups; saturated hydrocarbons, olefinic hydrocarbons, aromatic

hydrocarbons, and oxygenated hydrocarbons. Conveniently, the gasoline may comprise a mixture of saturated

hydrocarbons, olefinic hydrocarbons, aromatic

hydrocarbons, and, optionally, oxygenated hydrocarbons.

Typically, the olefinic hydrocarbon content of the gasoline is in the range of from 0 to 40 percent by volume based on the gasoline (ASTM D1319); preferably, the olefinic hydrocarbon content of the gasoline is in the range of from 0 to 30 percent by volume based on the gasoline, more preferably, the olefinic hydrocarbon content of the gasoline is in the range of from 0 to 20 percent by volume based on the gasoline.

Typically, the aromatic hydrocarbon content of the gasoline is in the range of from 0 to 70 percent by volume based on the gasoline (ASTM D1319), for instance the aromatic hydrocarbon content of the gasoline is in the range of from 10 to 60 percent by volume based on the gasoline; preferably, the aromatic hydrocarbon content of the gasoline is in the range of from 0 to 50 percent by volume based on the gasoline, for instance the aromatic hydrocarbon content of the gasoline is in the range of from 10 to 50 percent by volume based on the gasoline.

The benzene content of the gasoline is at most 2 percent by volume, more preferably at most 1 percent by volume based on the gasoline.

The gasoline preferably has a low or ultra low sulphur content, for instance at most 1000 ppmw (parts per million by weight), preferably no more than 500 ppmw, more preferably no more than 100, even more preferably no more than 50 and most preferably no more than even 10 ppmw .

The gasoline also preferably has a low total lead content, such as at most 0.005 g/1, most preferably being lead free - having no lead compounds added thereto (i.e. unleaded) .

When the gasoline comprises oxygenated hydrocarbons, at least a portion of non-oxygenated hydrocarbons will be substituted for oxygenated hydrocarbons. The oxygen content of the gasoline may be up to 35 percent by weight (EN 1601) (e.g. ethanol per se (i.e. pure anhydrous ethanol)) based on the gasoline. For example, the oxygen content of the gasoline may be up to 25 percent by weight, preferably up to 10 percent by weight.

Conveniently, the oxygenate concentration will have a minimum concentration selected from any one of 0 and 5 percent by weight, and a maximum concentration selected from any one of 30, 20, 10 percent by weight.

Preferably, the oxygenate concentration herein is 5 to 15 percent by weight.

Examples of oxygenated hydrocarbons that may be incorporated into the gasoline include alcohols, ethers, esters, ketones, aldehydes, carboxylic acids and their derivatives, and oxygen containing heterocyclic

compounds. All of the above oxygenates may contain saturated and/or unsaturated hydrocarbon backbones, as well as aromatic moieties. Preferably, the oxygenated hydrocarbons that may be incorporated into the gasoline are selected from alcohols (such as methanol, ethanol, propanol, 2-propanol, butanol, tert-butanol , iso-butanol, prenol, isoprenol and 2-butanol), ethers (preferably ethers containing 5 or more carbon atoms per molecule, e.g., methyl tert-butyl ether and ethyl tert-butyl ether) and esters (preferably esters containing 5 or more carbon atoms per molecule) ; a particularly preferred oxygenated hydrocarbon is ethanol.

When oxygenated hydrocarbons are present in the gasoline, the amount of oxygenated hydrocarbons in the gasoline may vary over a wide range. For example, gasolines comprising a major proportion of oxygenated hydrocarbons are currently commercially available in countries such as Brazil and U.S.A., e.g. ethanol per se and E85, as well as gasolines comprising a minor

proportion of oxygenated hydrocarbons, e.g. E10 and E5. Therefore, the gasoline may contain up to 100 percent by volume oxygenated hydrocarbons. E100 fuels as used in Brazil are also included herein. Preferably, the amount of oxygenated hydrocarbons present in the gasoline is selected from one of the following amounts: up to 85 percent by volume; up to 70 percent by volume; up to 65 percent by volume; up to 30 percent by volume; up to 20 percent by volume; up to 15 percent by volume; and, up to

10 percent by volume, depending upon the desired final formulation of the gasoline. Conveniently, the gasoline may contain at least 0.5, 1.0 or 2.0 percent by volume oxygenated hydrocarbons .

Examples of suitable gasolines include gasolines which have an olefinic hydrocarbon content of from 0 to 20 percent by volume (ASTM D1319), an oxygen content of from 0 to 5 percent by weight (EN 1601) , an aromatic hydrocarbon content of from 0 to 50 percent by volume (ASTM D1319) and a benzene content of at most 1 percent by volume. Also suitable for use herein are gasoline blending components which can be derived from a biological source. Examples of such gasoline blending components can be found in W02009/077606, W02010 / 028206 , W02010/000761, European patent application nos. 09160983.4, 09176879.6, 09180904.6, and US patent application serial no.

61/312307.

Whilst not critical to the present invention, the base gasoline or the gasoline composition of the present invention may conveniently include one or more optional fuel additives. The concentration and nature of the optional fuel additive (s) that may be included in the base gasoline or the gasoline composition used in the present invention is not critical. Non-limiting examples of suitable types of fuel additives that can be included in the base gasoline or the gasoline composition used in the present invention include anti-oxidants, corrosion inhibitors, antiwear additives or surface modifiers, flame speed additives, detergents, dehazers, antiknock additives, metal deactivators, valve-seat recession protectant compounds, dyes, solvents, carrier fluids, diluents and markers. Examples of suitable such

additives are described generally in US Patent No.

5,855,629. Suitable detergent/dispersants to minimize engine and fuel delivery system deposits can be selected from derivatives of PIB-Amines, Mannichs, Polyether

Amines, Succinimides , and mixtures thereof.

Conveniently, the fuel additives can be blended with one or more solvents to form an additive concentrate, the additive concentrate can then be admixed with the base gasoline or the gasoline composition of the present invention .

The (active matter) concentration of any optional additives present in the base gasoline or the gasoline composition of the present invention is preferably up to 1 percent by weight, more preferably in the range from 5 to 2000 ppmw, advantageously in the range of from 300 to 1500 ppmw, such as from 300 to 1000 ppmw.

The fuel compositions may be conveniently prepared using conventional formulation techniques by admixing one or more base fuels with one or more performance additive packages and/or one or more additive components.

Lubricant compositions for use in the spark ignition engines described herein generally comprise a base oil and one or more performance additives, and should be suitable for use in a spark-ignited internal combustion engine. In some embodiments, the lubricant compositions described herein may be particularly useful in a

turbocharged spark-ignited engine, more particularly a turbocharged spark-ignited engine which operates, or may operate, or is intended to operate, with an inlet

pressure of at least 1 bar.

High calcium content in the oil is frequently found to exacerbate Low Speed Pre-Ignition, and hence the present invention has been found to be particularly useful in high calcium engine oil environments, but the present invention will be useful in any circumstances in which the engine is prone to Low Speed Pre-Ignition, regardless of oil calcium content. Hence, the lubricant compositions for use herein can have a calcium content of Oppmw or greater, preferably 500ppmw or greater, more preferably lOOOppmw or greater, even more preferably 1200ppmw or greater, yet more preferably 1500ppmw or greater, especially 2000ppmw or greater, as measured according to ASTM D5185.

In one embodiment of the invention, the lubricating composition comprises from 1200ppmw to 3000ppmw, on the basis of the total lubricating composition. In another embodiment herein, the lubricant compositions have a calcium content from 1500 ppmw to 2800 ppmw, preferably from 2000 ppmw to 2800 ppmw, more preferably from 2500 ppmw to 2800 ppmw, on the basis of the total lubricating composition, as measured according to ASTM D5185.

Optional lubricant additives which may be included in the lubricating composition herein include anti-wear agents, anti-foam agents, detergents, dispersants, corrosion inhibitors, anti-rust additives, anti-oxidants, extreme pressure additives, friction modifiers, viscosity index improvers, pour point depressants, and the like.

The lubricant composition herein preferably has a magnesium content of from 1 to lOOOppmw, preferably from 200 to 800 ppmw, based on the total lubricant

composition .

A preferred additive for use in the lubricant composition herein is a zinc-based anti-wear additive, such as a zinc dithiophosphate compound. Zinc-based anti-wear additives are well known in the art of

lubricating compositions. It is preferred that the level of zinc present in the lubricant composition is in the range of 0 to 1200 ppmw, preferably in the range from 600 to 1200 ppmw, based on the total lubricant composition.

Another preferred lubricant additive for use herein is a molybdenum-based friction-reducing additive, such as molybdenum dithiocarbamate . Molybdeum-based friction- reducing additives are well known in the art of

lubricating compositions. It is preferred that the level of molybdenum present in the lubricant composition herein is in the range of 0 to 1000 ppmw, preferably in the range from 0 to 900ppmw, more preferably from 0 to 500ppmw, based on the total lubricant composition.

To facilitate a better understanding of the present invention, the following examples of certain aspects of some embodiments are given. In no way should the

following examples be read to limit, or define, the entire scope of the invention.

Examples

Three different fuels were used in the present examples (Fuel A, Fuel B and Fuel C) . The chemical compositions and the properties of these fuels are shown in Table 1 below. All fuels were blended to have the same RON, MON and ethanol content, and Fuel B and C were blended to have the same aromatic content. The PM Index of each of the fuels was calculated according to Equation (1) above (as published in SAE 2010-01-2115 published

25^th October 2010) .

Table 1

The lubricant type used in the present Examples was a GF-5 certified high calcium containing lubricant of 5W 30 viscosity grade having a calcium content of 2763 ppm as measured according to ASTM D5185. Table 2 below sets out the chemical and physical properties of the

lubricant .

Table 2

Fuels A, B and C were subjected to the following test method for measuring LSPI events and the frequency thereof .

Test Method for Measuring LSPI

The test protocol used for measuring LSPI events in the present examples is described below. The engine used was the GEM-T4 engine.

The commonly used variables for LSPI detection are:

(1) Crank angle location at an early burn cycle initiation before spark, i.e. 2% Mass Fraction Burned (MFB) .

(2) the peak pressure during pre-ignition and combustion (at or beyond 100 MPa or greater than the sum of mean peak pressure and 4.7 times the standard peak pressure .

(3) Crank angle locations at the start of combustion

(SOC) through post-processing software from FEV which uses an LSPI detection algorithm (further details of which can be found in Haenel et al, SAE Int. J. Fuels Lubr., Volume 10, Issue 1 (April 2017) entitled

'Influence of Ethanol Blends on Low Speed Pre-Ignition in Turbocharged, Direct-Injection Gasoline Engines; SAE Paper 2019-01-0256 entitled 'Analysis of the Impact of Production Lubricant Composition and Fuel Dilution on Stochastic Pre-Ignition in Turbocharged, Direct-Injection Gasoline Engines'; US9869262B2 and US10208691B2 ) . The pressure levels are implicit to the LSPI detection algorithm and they need to be outside the normal

combustion pressure conditions.

The variable used for LSPI detection in the present method is crank angle location at the start of combustion (method no. (3) above) .

In summary, the step-by-step approach to detecting LSPI was :

Calculation of the average combustion cycle without pre-ignition to determine pressure trace and SOC.

Definition of cycle SOC: +/- 2% pressure above average (represented by Pmax in the Figure) before a spark sets the trigger taking into account the burn delay .

Calculation of LSPI and knock characteristics based on the input from the above two factors, and continuously saving the pressure traces.

Multi-engine dynamometer was used for these

experiments. The steady-state, i.e. constant speed and constant load, test procedure in Figure la was used for engine tests herein unless another test procedure is explicitly mentioned. Steady-state tests consisted of operating the egine for 160,000 cycles for a total duration of 4 hours and 30 minutes with a 2 minutes interval of coasting at the same speed but a lower load to let the engine cool down to ambient conditions after each of the 4000 engine cycles. Ten repeats of the cycle shown in Figure 1 make up one engine test. The LSPI events were counted initially for 160,000 cycles and then scaled to a million cycles to finally report LSPI events in parts per million (ppm) unit (or events per million (epm) ) .

Transient state tests were incorporated into the test procedure to reflect real-life driving conditions.

A load-step method was incorporated into the long steady-state test procedure whenever applicable. Figure lb and Figure lc display the load-step method which acted as a quick 'screener' for the response of various lubricant and calibration changes at very high loads (usually more than 21 bar BMEP) . The test procedure involved operating the steady-state LSPI test at each load point for half the number of engine cycles (i.e. 80,000 cycles) and then moving onto a higher load. Such a process helped determine impact from changes in engine conditions or operating fluid to LSPI response in a relatively shorter amount of time without putting the engine through stress at very high loads where LSPI events can result in high and potentially damaging in cylinder pressure values (P_max) · The load-step procedure was used when the objective was to explore the maximum BMEP achievable under certain engine conditions with a minimum amount of LSPI events. Few tests were also performed under transient conditions in order to

understand the responsiveness of the engine to rapid fluctuations in speed-load operating strategies (i.e. close to real-life driving conditions) . As shown in Figure Id, these conditions involved a rapid increase in load for few seconds followed by coasting. These cycles were repeated for about 5-10 seconds for a total of 50, 000 cycles .

An important aspect of the test method is also the oil flushing procedure consisting of four oil changes and filter changes interrupted by 30 minutes of engine operation to circulate the flushing oil.

LSPI Measurement Procedure

LSPI events are in general followed by large

'aftershock' (or following) events which could be both pre-ignition events induced by hot spots or knock events. However, these aftershock events cannot generally be considered as distinct LSPI events since they originate due to pressure wave reflections in the cylinder caused by the initial pre-ignition event. To differentiate between these events from the LSPI cycles, aftershock events are defined as pre-ignition events within three cycles after the leading pre-ignition event. If the following phenomenon occurs within three cycles, the window for the second following event is again three cycles after the first following event, etc. Independent events need therefore to be minimum four cycles apart.

Table 3 gives an example of how each LSPI events are reported in the present experiments.

Table 3: Example of LSPI Counts in 17 Combustion Cycles

The engine specification used in the present

examples is set out in Table 4 below:

Table 4

The test conditions sensitive to PM/PN formation and LSPI for this engine are shown in Table 5 below. A AVL

Microsoot sensor was used for recording PM/PN.

Table 5

Table 6 below shows the Particulate Number (PN) , number of LSPI events and the PM Index (as determined according to SAE Paper SAE-2010-01-2115) for each of the Fuels A-C. Figure 2 is a plot of the results in Table 6.

Table 6

Table 7 below sets out the number of LSPI events per test for Fuels A, B and C, as well as the PM (as defined in SAE-2010-01-2115) and the PM Index for each of Fuels A-C. Figure 3 is a plot of the results shown in Table 7. Table 7

Discussion

The results in Tables 6 and 7 and in Figures 2 and 3 show that the fuel having the highest PM Index (Fuel C) also has the highest number of LSPI events. Further, the fuel having the lowest PM Index (Fuel A) has the lowest number of LSPI events. Fuel C, which has a PM Index of 2.83, exhibits significantly higher level of LSPI events compared with Fuel B and Fuel A which have a PM Index of 1.36 and 0.49, respectively.

Claims

C L A I M S

1. Use of a gasoline fuel composition for reducing the occurrence of Low Speed Pre-Ignition (LSPI) in a spark- ignition internal combustion engine, wherein the gasoline fuel composition comprises a gasoline base fuel and has a PM Index of 1.4 or less.

2. Use according to Claim 1 wherein the spark-ignition internal combustion engine is a direct injection spark- ignition internal combustion engine.

3. Use according to Claim 1 or 2 wherein the gasoline fuel composition has a PM Index of 1.0 or less.

4. Use according to any of Claims 1 to 3 wherein the gasoline fuel composition has a PM Index of 0.8 or less.

5. Use according to any of Claims 1 to 4 wherein the spark-ignition internal combustion engine is lubricated with a lubricant composition comprising 500ppmw of calcium or greater, based on the total lubricant

composition .

6. Use according to Claim 5 wherein the lubricant composition comprises from lOOOppmw of calcium or

greater, based on the total lubricant composition.

7. Use according to Claims 5 or 6 wherein the lubricant composition comprises 1500ppmw of calcium or greater, based on the total lubricant composition.

8. Use according to any of Claims 5 to 7 wherein the lubricant composition comprises lOOOppm or magnesium or less, based on the total lubricant composition.

9. Use according to any of Claims 5 to 8 wherein the lubricant composition comprises a zinc-based anti-wear additive in an amount of 1200ppmw or less, based on the total lubricant composition.

10. Use according to any of Claims 5 to 9 wherein the lubricant composition comprises a molybdenum-based friction reducing agent at a level of lOOOppmw or less, based on the total lubricant compostion.

11. Method for reducing the occurrence of Low Speed Pre-

Ignition in a spark ignition internal combustion engine, the method comprising supplying to the engine a gasoline fuel composition comprising a gasoline base fuel and having a PM Index of 1.4 or less.