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8.9 How serious is valve seat recession on older vehicles? The amount of exhaust valve seat recession is very dependent on the load on the engine. There have been several major studies on valve seat recession, and they conclude that most damage occurs under high-speed, high-power conditions. Engine load is not a primary factor in valve seat wear for moderate operating conditions, and low to medium speed engines under moderate loads do not suffer rapid recession, as has been demonstrated on fuels such as CNG and LPG. Under severe conditions, damage occurs rapidly, however there are significant cylinder-to-cylinder variations on the same engine. A 1970 engine operated at 70 mph conditions exhibited an average 1.5mm of seat recession in 12,000km. The difference between cylinders has been attributed to different rates of valve rotation, and experiments have confirmed that more rotation does increase the recession rate . The mechanism of valve seat wear is a mixture of two major mechanisms. Iron oxide from the combustion chamber surfaces adheres to the valve face and becomes embedded. These hard particles then allow the valve act as a grinding wheel and cut into the valve seat . The significance of valve seat recession is that should it occur to the extent that the valve does not seat, serious engine damage can result from the localised hot spot. There are a range of additives, usually based on potassium, sodium or phosphorus that can be added to the gasoline to combat valve seat recession. As phosphorus has adverse effects on exhaust catalysts, it is seldom used. The best long term solution is to induction harden the seats or install inserts, usually when the head is removed for other work, however additives are routinely and successfully used during transition periods. ------------------------------ Section: 9. Alternative Fuels and Additives 9.1 Do fuel additives work? Most aftermarket fuel additives are not cost-effective. These include the octane-enhancer solutions discussed in section 6.18. There are various other pills, tablets, magnets, filters, etc. that all claim to improve either fuel economy or performance. Some of these have perfectly sound scientific mechanisms, unfortunately they are not cost-effective. Some do not even have sound scientific mechanisms. Because the same model production vehicles can vary significantly, it's expensive to unambiguously demonstrate these additives are not cost-effective. If you wish to try them, remember the biggest gain is likely to be caused by the lower mass of your wallet/purse. There is one aftermarket additive that may be cost-effective, the lubricity additive used with unleaded gasolines to combat exhaust valve seat recession on engines that do not have seat inserts. This additive may be routinely added during the first few years of unleaded by the gasoline producers, but in the US this could not occur because they did not have EPA waivers, and also may be incompatible with 2-stroke engine oil additives and form a gel that blocks filters. The amount of recession is very dependent on the engine design and driving style. The long-term solution is to install inserts, or have the seats hardened, at the next top overhaul. Some other fuel additives work, especially those that are carefully formulated into the gasoline by the manufacturer at the refinery, and have often been subjected to decades-long evaluation and use [12,13]. A typical gasoline may contain [16,27,32,38,111]:- * Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm to prevent its misuse as an industrial solvent, and now also used to identify grades of product. * Antioxidants, typically phenylene diamines or hindered phenols, are added to prevent oxidation of unsaturated hydrocarbons. * Metal Deactivators, typically about 10ppm of chelating agent such as N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper, which can rapidly catalyze oxidation of unsaturated hydrocarbons. * Corrosion Inhibitors, about 5ppm of oil-soluble surfactants are added to prevent corrosion caused either by water condensing from cooling, water-saturated gasoline, or from condensation from air onto the walls of almost-empty gasoline tanks that drop below the dew point. If your gasoline travels along a pipeline, it's possible the pipeline owner will add additional corrosion inhibitor to the fuel. * Anti-icing Additives, used mainly with carburetted cars, and usually either a surfactant, alcohol or glycol. * Anti-wear Additives, these are used to control wear in the upper cylinder and piston ring area that the gasoline contacts, and are usually very light hydrocarbon oils. Phosphorus additives can also be used on engines without exhaust catalyst systems. * Deposit-modifying Additives, usually surfactants. 1. Carburettor Deposits, additives to prevent these were required when crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controls were introduced. Some fuel components reacted with these gas streams to form deposits on the throat and throttle plate of carburettors. 2. Fuel Injector tips operate about 100C, and deposits form in the annulus during hot soak, mainly from the oxidation and polymerisation of the larger unsaturated hydrocarbons. The additives that prevent and unclog these tips are usually polybutene succinimides or polyether amines. 3. Intake Valve Deposits caused major problems in the mid-1980s when some engines had reduced driveability when fully warmed, even though the amount of deposit was below previously acceptable limits. It is believed that the new fuels and engine designs were producing a more absorbent deposit that grabbed some passing fuel vapour, causing lean hesitation. Intake valves operate about 300C, and if the valve is kept wet, deposits tend not to form, thus intermittent injectors tend to promote deposits. Oil leaking through the valve guides can be either harmful or beneficial, depending on the type and quantity. Gasoline factors implicated in these deposits include unsaturates and alcohols. Additives to prevent these deposits contain a detergent and/or dispersant in a higher molecular weight solvent or light oil whose low volatility keeps the valve surface wetted [46,47,48]. 4. Combustion Chamber Deposits have been targeted in the 1990s, as they are responsible for significant increases in emissions. Recent detergent-dispersant additives have the ability to function in both the liquid and vapour phases to remove existing deposits that have resulted from the use of other additives, and prevent deposit formation. Note that these additives can not remove all deposits, just those resulting from the use of additives. * Octane Enhancers, these are usually formulated blends of alkyl lead or MMT compounds in a solvent such as toluene, and added at the 100-1000 ppm levels. They have been replaced by hydrocarbons with higher octanes such as aromatics and olefins. These hydrocarbons are now being replaced by a mixture of saturated hydrocarbons and and oxygenates. If you wish to play with different fuels and additives, be aware that some parts of your engine management systems, such as the oxygen sensor, can be confused by different exhaust gas compositions. An example is increased quantities of hydrogen from methanol combustion. 9.2 Can a quality fuel help a sick engine? It depends on the ailment. Nothing can compensate for poor tuning and wear. If the problem is caused by deposits or combustion quality, then modern premium quality gasolines have been shown to improve engine performance significantly. The new generation of additive packages for gasolines include components that will dissolve existing carbon deposits, and have been shown to improve fuel economy, NOx emissions, and driveability [49,50,111]. While there may be some disputes amongst the various producers about relative merits, it is quite clear that premium quality fuels do have superior additive packages that help to maintain engine condition [16,28,111], 9.3 What are the advantages of alcohols and ethers? This section discusses only the use of high ( >80% ) alcohol or ether fuels. Alcohol fuels can be made from sources other than imported crude oil, and the nations that have researched/used alcohol fuels have mainly based their choice on import substitution. Alcohol fuels can burn more efficiently, and can reduce photochemically-active emissions. Most vehicle manufacturers favoured the use of liquid fuels over compressed or liquified gases. The alcohol fuels have high research octane ratings, but also high sensitivity and high latent heats [8,27,80,116]. Methanol Ethanol Unleaded Gasoline RON 106 107 92 - 98 MON 92 89 80 - 90 Heat of Vaporisation (MJ/kg) 1.154 0.913 0.3044 Nett Heating Value (MJ/kg) 19.95 26.68 42 - 44 Vapour Pressure @ 38C (kPa) 31.9 16.0 48 - 108 Flame Temperature ( C ) 1870 1920 2030 Stoich. Flame Speed. ( m/s ) 0.43 - 0.34 Minimum Ignition Energy ( mJ ) 0.14 - 0.29 Lower Flammable Limit ( vol% ) 6.7 3.3 1.3 Upper Flammable Limit ( vol% ) 36.0 19.0 7.1 Autoignition Temperature ( C ) 460 360 260 - 460 Flash Point ( C ) 11 13 -43 - -39 The major advantages are gained when pure fuels ( M100, and E100 ) are used, as the addition of hydrocarbons to overcome the cold start problems also significantly reduces, if not totally eliminates, any emission benefits. Methanol will produce significant amounts of formaldehyde, a suspected human carcinogen, until the exhaust catalyst reaches operating temperature. Ethanol produces acetaldehyde. The cold-start problems have been addressed, and alcohol fuels are technically viable, however with crude oil at <$30/bbl they are not economically viable, especially as the demand for then as precursors for gasoline oxygenates has elevated the world prices. Methanol almost doubled in price during 1994. There have also been trials of pure MTBE as a fuel, however there are no unique or significant advantages that would outweigh the poor economic viability . 9.4 Why are CNG and LPG considered "cleaner" fuels. CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20% ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to butane. The fuel has a high octane and usually only trace quantities of unsaturates. The emissions from CNG have lower concentrations of the hydrocarbons responsible for photochemical smog, reduced CO, SOx, and NOx, and the lean misfire limit is extended . There are no technical disadvantages, providing the installation is performed correctly. The major disadvantage of compressed gas is the reduced range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120 litre capacity), and they usually represent about 50% of the gasoline range. As natural gas pipelines do not go everywhere, most conversions are dual-fuel with gasoline. The ignition timing and stoichiometry are significantly different, but good conversions will provide about 85% of the gasoline power over the full operating range, with easy switching between the two fuels . Concerns about the safety of CNG have proved to be unfounded [119,120]. CNG has been extensively used in Italy and New Zealand ( NZ had 130,000 dual-fuelled vehicles with 380 refuelling stations in 1987 ). The conversion costs are usually around US$1000, so the economics are very dependent on the natural gas price. The typical 15% power loss means that driveability of retrofitted CNG-fuelled vehicles is easily impaired, consequently it is not recommended for vehicles of less than 1.5l engine capacity, or retrofitted onto engine/vehicle combinations that have marginal driveability on gasoline. The low price of crude oil, along with installation and ongoing CNG tank-testing costs, have reduced the number of CNG vehicles in NZ. The US CNG fleet continues to increase in size ( 60,000 in 1994 ). LPG ( Liquified Petroleum Gas ) is predominantly propane with iso-butane and n-butane. It has one major advantage over CNG, the tanks do not have to be high pressure, and the fuel is stored as a liquid. The fuel offers most of the environmental benefits of CNG, including high octane. Approximately 20-25% more fuel is required, unless the engine is optimised ( CR 12:1 ) for LPG, in which case there is no decrease in power or increase in fuel consumption [27,118]. There have been several studies that have compared the relative advantages of CNG and LPG, and often LPG has been found to be a more suitable transportation fuel [118,120]. methane propane iso-octane RON 120 112 100 MON 120 97 100 Heat of Vaporisation (MJ/kg) 0.5094 0.4253 0.2712 Net Heating Value (MJ/kg) 50.0 46.2 44.2 Vapour Pressure @ 38C ( kPa ) - - 11.8 Flame Temperature ( C ) 1950 1925 1980 Stoich. Flame Speed. ( m/s ) 0.45 0.45 0.31 Minimum Ignition Energy ( mJ ) 0.30 0.26 - Lower Flammable Limit ( vol% ) 5.0 2.1 0.95 Upper Flammable Limit ( vol% ) 15.0 9.5 6.0 Autoignition Temperature ( C ) 540 - 630 450 415 9.5 Why are hydrogen-powered cars not available? The Hindenburg. The technology to operate IC engines on hydrogen has been investigated in depth since before the turn of the century. One attraction was to use the hydrogen in airships to fuel the engines instead of venting it. Hydrogen has a very high flame speed ( 3.24 - 4.40 m/s ), wide flammability limits ( 4.0 - 75 vol% ), low ignition energy ( 0.017 mJ ), high autoignition temperature ( 520C ), and flame temperature of 2050 C. Hydrogen has a very high specific energy ( 120.0 MJ/kg ), making it very desirable as a transportation fuel. The problem has been to develop a storage system that will pass all safety concerns, and yet still be light enough for automotive use. Although hydrogen can be mixed with oxygen and combusted more efficiently, most proposals use air [114,119,121-124]. Unfortunately the flame temperature is sufficiently high to dissociate atmospheric nitrogen and form undesirable NOx emissions. The high flame speeds mean that ignition timing is at TDC, except when running lean, when the ignition timing is advanced 10 degrees. The high flame speed, coupled with a very small quenching distance mean that the flame can sneak past narrow inlet valve openings and cause backflash. This can be mitigated by the induction of fine mist of water, which also has the benefit of increasing thermal efficiency ( although the water lowers the combustion temperature, the phase change creases voluminous gases that increase pressure ), and reducing NOx . An alternative technique is to use direct cylinder induction, which injects hydrogen once the cylinder has filled with an air charge, and because the volume required is so large, modern engines have two inlet valves, one for hydrogen and one for air . The advantage of a wide range of mixture strengths and high thermal efficiencies are matched by the disadvantages of pre-ignition and knock unless weak mixtures, clean engines, and cool operation are used. Interested readers are referred to the group sci.energy.hydrogen and the " Hydrogen Energy" monograph in the Kirk Othmer Encyclopedia of Chemical Technology , for recent information about this fuel. 9.6 What are "fuel cells" ? Fuel cells are electrochemical cells that directly oxidise the fuel at electrodes producing electrical and thermal energy. The oxidant is usually oxygen from the air and the fuel is usually gaseous, with hydrogen preferred. There has, so far, been little success using low temperature fuel cells ( < 200C ) to perform the direct oxidation of hydrocarbon-based liquids or gases. Methanol can be used as a source for the hydrogen by adding an on-board reformer. The main advantage of fuel cells is their high fuel-to- electricity efficiency of about 40-60% of the nett calorific value of the fuel. As fuel cells also produce heat that can be used for vehicle climate control, fuel cells are the most likely candidate to replace the IC engine as a primary energy source. Fuel cells are quiet and produce virtually no toxic emissions, but they do require a clean fuel ( no halogens, CO, S, or ammonia ) to avoid poisoning. They currently are expensive to produce, and have a short operational lifetime, when compared to an IC engine [125-127]. 9.7 What is a "hybrid" vehicle? A hybrid vehicle has three major systems . 1. A primary power source, either an IC engine driven generator where the IC engine only operates in the most efficient part of it's performance map, or alternatives such as fuel cells and turbines. 2. A power storage unit, which can be a flywheel, battery, or ultracapacitor. 3. A drive unit, almost always now an electric motor that can used as a generator during braking. Regenerative braking may increase the operational range about 8-13%. Battery technology has not yet advanced sufficiently to economically substitute for an IC engine, while retaining the carrying capacity, range, performance, and driveability of the vehicle. Hybrid vehicles may enable this problem to be at least partially overcome, but they remain expensive, and the current ZEV proposals exclude fuel cells and hybrids systems, but this is being re-evaluated. 9.8 What about other alternative fuels? 9.8.1 Ammonia (NH3) Anhydrous ammonia has been researched because it does not contain any carbon, and so would not release any CO2. The high heat of vaporisation requires a pre-vaporisation step, preferably also with high jacket temperatures ( 180C ) to assist decomposition. Power outputs of about 70% of that of gasoline under the same conditions have been achieved . Ammonia fuel also produces copious quantities of undesirable oxides of nitrogen (NOx) emissions. 9.8.2 Water As water-gasoline fuels have been extensively investigated [113,129], interested potential investors may wish to refer to those papers for some background. Mr.Gunnerman advocates hydrocarbon/water emulsion fuels and promoted his A-55 fuel before the new A-21. A recent article claims a 29% gain in fuel economy , and he claims that mixing water with naphtha can provide as much power from an IC engine as the same flow rate of gasoline. He claims the increased efficiency is from catalysed dissociation of A-21 into H2 in the engine, because the combustion chamber of the test engines contain a "non-reactive" catalyst. For his fuel to provide power increases, he has to utilise heat energy that is normally lost. A-21 is just naphtha ( effectively unleaded gasoline without oxygenates ) and water ( about 55% ), with small amouts of winterizing and anti-corrosive additives. If the magic catalyst is not present, conventional IC engines will not perform as efficiently, and may possibly be damaged if A-21 is used. The only modification is a new set of spark plugs, and it is also claimed that the fuel can replace both diesel and gasoline. It has been claimed that test results of A-21 fuel emissions have shown significant reductions in CO2 ( 50% claimed - who is surprised when the fuel is 55% water? :-) ), CO, HCs, NOx and a 70% reduction in diesel particulates and smoke. It's claimed that 70% of the exhaust stream consists of water vapour. He has formed a joint venture company with Caterpillar called Advanced Fuels. U.S. patent #5,156,114 ( Aqueous Fuel for Internal Combustion Engines and Combustion Method ) was granted to Mr.Gunnerman in 1992. 9.8.3 Propylene Oxide Propylene oxide ( CH3CH(O)CH2 = 1,2 epoxypropane ) has apparently been used in racing fuels, and some racers erroneously claim that it behaves like nitrous oxide. It is a fuel that has very desirable volatility, flammability and autoignition properties. When used in engines tuned for power ( typically slightly rich ), it will move the air-fuel ratio closer to stoichiometric, and the high volatility, high autoignition temperature ( high octane ), and slightly faster flamespeed may improve engine efficiency with hydrocarbon fuels, resulting in increased power without major engine modifications. This power increase is, in part, due to the increase in volumetric efficiency from the requirement for less oxygen ( air ) in the charge. PO is a suspected carcinogen, and so should be handled with extreme care. Relevant properties include :- Avgas Propylene Oxide 100/130 115/145 Density (g/ml) 0.828 0.72 0.74 Boiling Point (C) 34 30-170 30-170 Stoichiometic Ratio (vol%) 4.97 2.4 2.2 Autoignition Temperature (C) 464 440 470 Lower Flammable Limit (vol%) 2.8 1.3 1.2 Upper Flammable Limit (vol%) 37 7.1 7.1 Minimum Ignition Energy (mJ) 0.14 0.2 0.2 Nett Heat of Combustion (MJ/kg) 31.2 43.5 44.0 Flame Temperature (C) 2087 2030 2030 Burning Velocity (m/s) 0.67 0.45 0.45 9.8.4 Nitromethane Nitromethane ( CH3NO2) - usually used as a mixture with methanol to reduce peak flame temperatures - also provides excellent increases in volumetric efficiency of IC engines - in part because of the lower stoichiometric air-fuel ratio (1.7:1 for CH3NO2) and relatively high heats of vaporisation ( 0.56 MJ/kg for CH3NO2) result in dramatic cooling of the incoming charge. 4CH3NO2 + 3O2 -> 4CO2 + 6H20 + 2N2 The nitromethane Specific Energy at stoichiometric ( heat of combustion divided by air-fuel ratio ) of 6.6, compared to 2.9 for iso-octane, indicates that the fuel energy delivered to the combustion chamber is 2.3 times that of iso-octane for the same mass of air. Coupled with the higher flame temperature ( 2400C ), and flame speed (0.5 m/s), it has been shown that a 50% blend in methanol will increase the power output by 45% over pure methanol, however knock also increased . 9.9 What about alternative oxidants? 9.9.1 Nitrous Oxide Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the combustion chamber is filled with less useless nitrogen. It is also metered in as a liquid, which can cool the incoming charge further, thus effectively increasing the charge density. With all that oxygen, a lot more fuel can be squashed into the combustion chamber. The advantage of nitrous oxide is that it has a flame speed, when burned with hydrocarbon and alcohol fuels, that can be handled by current IC engines, consequently the power is delivered in an orderly fashion, but rapidly. The same is not true for pure oxygen combustion with hydrocarbons, so leave that oxygen cylinder on the gas axe alone :-). Nitrous oxide has also been readily available at a reasonable price, and is popular as a fast way to increase power in racing engines. The following data are for common premixed flames . Temperature Flame Speed Fuel Oxidant ( C ) ( m/s ) Acetylene Air 2400 1.60 - 2.70 " Nitrous Oxide 2800 2.60 " Oxygen 3140 8.00 - 24.80 Hydrogen Air 2050 3.24 - 4.40 " Nitrous Oxide 2690 3.90 " Oxygen 2660 9.00 - 36.80 Propane Air 1925 0.45 Natural Gas Air 1950 0.39 Nitrous oxide is not yet routinely used on standard vehicles, but the technology is well understood. 9.9.2 Membrane Enrichment of Air Over the last two decades, extensive research has been performed on the use of membranes to enrich the oxygen content of air. Increasing the oxygen content can make combustion more efficient due to the higher flame temperature and less nitrogen. The optimum oxygen concentration for existing automotive engine materials is around 30 - 40%. There are several commercial membranes that can provide that level of enrichment. The problem is that the surface area required to produce the necessary amount of enriched air for an SI engine is very large. The membranes have to be laid close together, or wound in a spiral, and significant amounts of power are required to force the air along the membrane surface for sufficient enriched air to run a slightly modified engine. Most research to date has centred on CI engines, with their higher efficiencies. Several systems have been tried on research engines and vehicles, however the higher NOx emissions remain a problem [132,133].