Deposits in Engines and Fuel Additives (Ch.3. Fuel/Engine Interactions)

Transcription

1 Lecture 7 Deposits in Engines and Fuel Additives (Ch.3. Fuel/Engine Interactions) Gautam Kalghatgi Fuel/Engine Interactions Ch. 3 Kalghatgi, G.T., " A Study of Inlet System Detergency in a Gasoline Engine using an Optical Method", SAE Paper No , SAE Transactions Journal of Fuels and Lubricants, Vol.101, 1992 Kalghatgi, G.T., " Combustion Chamber deposits and Knock in a spark-ignition engine - some additive and fuel effects", SAE Paper No , SAE Transactions Journal of Fuels and Lubricants, Vol.105, 1996 Kalghatgi, G.T. and Price, R.J., Combustion chamber deposit flaking, SAE Paper # , Journal of Fuels and Lubricants, Vol.109, 2000 Kalghatgi, G.T., Combustion chamber deposit flaking Studies using a road test procedure, SAE Paper # , Journal of Engines, Vol. 111, 2002

2 Deposits in SI Engines Deposits form on all internal surfaces of the engine with use.they can affect engine performance significantly Nature and formation Effect on engine operation Deposit control Test procedures employed Part I on fuel and inlet system deposits, Part II on Combustion Chamber Deposits - 2

3 Engine Deposits SI Engines Fuel derived deposits form at several places in engine and usually degrade its performance Carburettor Injector (esp for direct injection) Inlet port, inlet valves Spark plug Combustion chamber head and piston top

4 Injector Deposits Clean Slit injector from Toyota DISI Avensis 4

5 Injector Deposits Slit injector from Toyota DISI Avensis, after miles 5

6 Variation in additive dose rate can lead to different levels of inlet valve cleanliness. Optimax is formulated to give clean valves such as the one on the left. 6

7 Inlet system deposits and engine performance Flow of fuel / air impaired Fuel becomes absorbed Impact of deposit formation: Power loss Slow acceleration Poor driveability Poor cold start Increased emissions 3000 x magnification 7

8 8 Combustion Chamber Deposits Piston Top Cylinder #1 Cylinder #2

9 General Points about Deposits in Engines Deposit level reflects balance between deposit formation and removal mechanisms. These mechanisms are different in different parts of the engine e.g. in injectors, inlet system and combustion chamber Deposits in different parts of the engine affect engine performance in different ways and different degrees The mechanisms involved are usually not very well understood because they are very complex Test procedures used are long, expensive and often don t yield clear-cut results. 9

10 10 General Formation Mechanisms MECHANISMS ARE NOT WELL UNDERSTOOD CHEMICAL Precursors (long chain hydrocarbons containing N and O from partial reaction of fuel and oil components) Thermal decomposition PHYSICAL Products Oxidation Molecular rearrangement Recombination Deposits Blowback Liquid Fractions Condensation Temperature

11 Injector Deposits Fuel System Deposits Can plug injectors, upset fuel spray pattern cause driveability problems In port fuel injection (PFI) engines, injector deposits cannot be formed if the engine is run continuously. Need hot soak when injector tip temperature is high and remnants of fuel cook. Not so in Direct Injection (DI) engines. Olefins and di-olefins in the fuel - bad. Can be controlled by detergent additives more difficult in DI engines. (Carburettor deposits can increase fuel consumption, CO and HC emissions and cause rough idling and stalling. Can be easily controlled using additives) 11

12 Intake Valve and Port Deposits Can absorb liquid fuel, upset airflow pattern (important in DI engines) and cause driveability problems. Also increase part-throttle NOx. In the worst cases restrict engine breathing. Formation originated by reaction of fuel and lubricant with hot EGR (Exhaust Gas Recirculation), PCV (Positive Crank case Ventilation) and blow-by gases. Levels - DI engines > Carburettor Engines > PFI engines because of fuel washing off deposit precursors Olefins, di-olefin content of fuel, oil flowing down the valve stem, surface temperature affect formation. Can be controlled by detergent additives In the U.S. mandatory to use minimum level of detergent 12

13 Combustion Chamber Deposits DEPOSIT GROWTH OVERVIEW Reflects balance between formation and removal Hence is a dynamic and to an extent, reversible process With normal fuels, in modern engines operating normally, primarily formed by fuel though oil also contributes Starts by condensation of deposit precursors on to the surface which is also the limiting factor. Surface temperature single most important parameter. It increases as deposits grow because deposits are very good insulators. Each fuel has limiting surface temperature above which it does not form deposits around 310 C for gasoline 13

14 Combustion Chamber Deposits FACTORS THAT AFFECT DEPOSIT GROWTH Changes in engine operating parameters that increase surface temperature reduce deposit growth Deposit growth is reduced by Increase in speed / load / coolant temperature Making the mixture strength richer than stoichiometric Using materials of low conductivity like ceramics 14

15 15 Combustion Chamber Deposits FACTORS THAT AFFECT DEPOSIT GROWTH Fuel and lubricant factors Boiling point is the most important fuel property higher the boiling point, greater the deposit formation Paraffins, olefins and aromatics rank in that order for increasing deposit formation. But in practical fuels this is difficult to separate from boiling point effects. Fuel detergent additive packages usually cause increase in deposit growth. But large variation between different additives and even for same additive between different engines and fuels.

16 16 Combustion Chamber Deposits PROPERTIES OF COMBUSTION CHAMBER DEPOSITS Large variation in properties across the combustion chamber Compared to fuel and lubricant, deposits have much higher concentrations of oxygen (16%-25%) and nitrogen (1%-2.5%) Heat capacity 0.8 to 1.9 kj/kgc Density 0.35 to 1.5 g/cc Thermal diffusivity around 3.5x10-7 m 2 /s Lower thickness in hotter regions Varying porosity

17 17 Combustion Chamber Deposits EFFECTS ON ENGINE PERFORMANCE - OVERVIEW Increase charge temperature and flame propagation rate Reduce heat lost to coolant and volumetric efficiency May affect final phase of combustion through chemical means Could flake and come off the surface.. And cause Octane Requirement Increase (ORI) Increase in NOx and less certainly, HC Improvement in (part-throttle) fuel economy Reduction in power Driveability problems / failure to start

18 18 Combustion Chamber Deposits EFFECTS ON ENGINE PERFORMANCE Octane Requirement Increase (ORI) MECHANISMS Volumetric occupy space, increase compression ratio < 10% Thermal incoming charge is heated Chemical not fully understood SOME OBSERVATIONS ORI does not simply correlate with deposit weights Piston- top deposits have little, end-gas deposits have most effect. Different additives have different effects ORI testing is long, costly, has poor rpeatability and reporoducibility. Hence difficult to come to sound conclusions.

19 19 Combustion Chamber Deposits EFFECTS ON ENGINE PERFORMANCE HC Emissions Increasing surface temperature should reduce HC Plugging of crevices might reduce HC (no direct evidence) Absorption of unburnt fuel and release later should increase HC Removal of combustion chamber deposits reduces HC in some tests but not in others NOx Emissions Increase in bulk gas temperature increases NOx Removal of combustion chamber deposits reduces NOx Piston- top deposits have little, end-gas deposits have most effect. CO Emissions No credible mechanism or convincing evidence on the effect of combustion chamber deposits

20 20 Combustion Chamber Deposits EFFECTS ON ENGINE PERFORMANCE Fuel Economy Heat lost to coolant is reduced. Faster flame development Improvement ranging from 2% to 15% reported in single/multicylinder engine tests, dynamometer tests and road tests Maximum Engine Power Volumetric efficiency reduced because of charge heating All reported studies are pre 1971, many with leaded gasolines Reduction depends on chamber design least for most compact design Loss in power decreased from 6% to 3% when CR was increased from 8 to 12 in one study Piston-top deposits have relatively small effect

21 21 Combustion Chamber Deposits EFFECTS ON ENGINE PERFORMANCE Combustion Chamber Deposit Flaking New Field Problem Starting problem following short drive and long park Deposit flakes formed when the engine is not running get trapped between valve and its seat while starting the engine and the engine fails to start Will the problem become more easily recognised as OBD systems become more wide-spread?

22 22 Combustion Chamber Deposits -Flaking Cyl #6, Car A. After 3000 miles After spraying with water

23 23 Combustion Chamber Deposits -Flaking Cyl #4, CAR B. Afetr 3000 miles After spraying with water

24 Combustion Chamber Deposits -Flaking Studies using engine and road tests ( SAE , SAE , SAE ) - results in line with field experience - quantification of flaking and test variability established - wide variation between fuels - thermal environment of deposit formation important - detergent additive packages suppress flaking - Flaking directly linked to loss of compression and startability problem FURTHER READING ON TOPICS COVERED IN THESE SLIDES By G.T. Kalghatgi SAE Combustion chamber deposits in spark-ignition engines a literature review SAE Deposits in gasoline engines a literature review 24

25 Diesel Injector Deposits Known to occur in the field Reduce maximum power by reducing the fuelling rate Internal deposits occurring on surfaces not seeing combustion products, can lead to needle sticking and serious driveability problems. Can be controlled by detergent additives Test methods difficult and expensive Mechanisms of formation not understood Effects on engine operation need to be understood better 25

26 26 Deposits in Diesel Engines but deposits can also occur on the outside of the injector, beside the hole

27 Diesel DI Injector Fouling Electron micrographs of the combustion chamber ends of nozzle holes NEW INJECTOR FOULED INJECTOR

28 Injector Spray Patterns CLEAN INJECTOR similar sprays INJECTORS dissimilar spray patterns, one spray missing DIRTY

29 Deposits in Diesel Injectors Primary concern is injector deposits Deposits in nozzle internal surfaces restrict fuel flow and reduce power Deposits build up with base fuel Deposits are controlled by additives 29

30 30 Deposits in Diesel Engines - Internal Deposits that restrict needle movement can hold the injector open or shut Internal deposits

31 Fuel Additives Chemicals added in relatively small quantities ( < 0.5%v) either to improve fuel performance or to correct a deficiency Usually added as a package of more than one chemical along with a solvent/diluent like xylene or gasoline Typical injection rates are 1000 ppm to 1500 ppm. Upto half the package could be solvent Usually added at fuel distribution depots by individual oil companies Usually manufactured by specialist additive/chemical companies 31

32 PERFORMANCE Gasoline Fuel Additives (contd) Deposit Control very common Combustion modifiers anti-knocks Friction Modifiers reduce friction STORAGE and DISTRIBUTION Dehazers in gasolines Biocides Markers and dyes Antioxidants MANY OF THESE ADDITIVES ARE MULTIFUNCTIONAL 32

33 Gasoline Fuel Additives Detergent Packages DETERGENT Surface active material. Hydrocarbon soluble polymer connected through a suitable linking group to a polar head CARRIER FLUID High boiling point thermally stable solvent DEHAZER prevents suspension of entrained water (emulsion) The polar head attaches itself to deposit precursors which is then swept away by the fuel Sometimes package components can interact with fuel and cause increased deposits Testing done in industry standard bench and road tests 33

34 34 Diesel Fuel Additives Ignition Improvers (also added at the refinery to meet fuel spec) Cold Flow Additives (usually added at the refinery to meet fuel spec) Lubricity Improvers Detergent Reodorants (unusual) Antifoam Dehazers) Corrosion Inhibitors } Typical Dose Rates (ppm) Hundreds } Tens } Less Performance packages Injector cleanliness Steel corrosion protection Foam control (+ further functionalities) than ten

35 35 Commercial Additive Package Development RESEARCH DEVELOPMENT TESTING MARKET TRIAL COLLECTING EVIDENCE OF BENEFITS FOR PRODUCT CLAIMS CHECKING FOR SIDE EFFECTS Inlet Valve Sticking Catalyst Durability Toxicological Aspects Component Compatibility Handling and Distribution Lubricant Degradation Paint Staining Spark Plug Fouling.