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Gasoline FAQ - Part 4 of 4

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Archive-name: autos/gasoline-faq/part4
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Last-modified: 17 November 1996
Version: 1.12

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 [29]. 
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 [115]. 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 [15]. 

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 [117]. 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 [118]. 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 [124]. 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 [124]. 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 [124], 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 [128].
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 [114]. 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 [130], 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 [116]:-                          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 [28].

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 [131]. 
              
                               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]. 

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