An increase of absolute humidity of 1.0 g water/kg of dry air lowers the
octane requirement of an engine by 0.25 - 0.32 MON [27,28,38].
Water injection, as a separate liquid or emulsion with gasoline, or as a
vapour, has been thoroughly researched. If engines can calibrated to operate
with small amounts of water, knock can be suppressed, hydrocarbon emissions
will slightly increase, NOx emissions will decrease, CO does not change
significantly, and fuel and energy consumption are increased .
Water injection was used in WWII aviation engine to provide a large increase
in available power for very short periods. The injection of water does
decrease the dew point of the exhaust gases. This has potential corrosion
problems. The very high specific heat and heat of vaporisation of water
means that the combustion temperature will decrease. It has been shown that
a 10% water addition to methanol reduces the power and efficiency by about
3%, and doubles the unburnt fuel emissions, but does reduce NOx by 25% .
A decrease in combustion temperature will reduce the theoretical maximum
possible efficiency of an otto cycle engine that is operating correctly,
but may improve efficiency in engines that are experiencing abnormal
combustion on existing fuels.
Some aviation SI engines still use boost fluids. The water-methanol mixtures
are used to provide increased power for short periods, up to 40% more -
assuming adequate mechanical strength of the engine. The 40/60 or 45/55
water-methanol mixtures are used as boost fluids for aviation engines because
water would freeze. Methanol is just "preburnt" methane, consequently it only
has about half the energy content of gasoline, but it does have a higher heat
of vaporisation, which has a significant cooling effect on the charge.
Water-methanol blends are more cost-effective than gasoline for combustion
cooling. The high Sensitivity of alcohol fuels has to be considered in the
engine design and settings.
Boost fluids are used because they are far more economical than using the
fuel. When a supercharged engine has to be operated at high boost, the
mixture has to be enriched to keep the engine operating without knock. The
extra fuel cools the cylinder walls and the charge, thus delaying the onset
of knock which would otherwise occur at the associated higher temperatures.
The overall effect of boost fluid injection is to permit a considerable
increase in knock-free engine power for the same combustion chamber
temperature. The power increase is obtained from the higher allowable boost.
In practice, the fuel mixture is usually weakened when using boost fluid
injection, and the ratio of the two fuel fluids is approximately 100 parts
of avgas to 25 parts of boost fluid. With that ratio, the resulting
performance corresponds to an effective uprating of the fuel of about 25%,
irrespective of its original value. Trying to increase power boosting above
40% is difficult, as the engine can drown because of excessive liquid .
Note that for water injection to provide useful power gains, the engine
management and fuel systems must be able to monitor the knock and adjust
both stoichiometry and ignition to obtain significant benefits. Aviation
engines are designed to accommodate water injection, most automobile engines
are not. Returns on investment are usually harder to achieve on engines that
do not normal extend their performance envelope into those regions. Water
injection has been used by some engine manufacturers - usually as an
expedient way to maintain acceptable power after regulatory emissions
baggage was added to the engine, but usually the manufacturer quickly
produces a modified engine that does not require water injection.
- You forgot to refill it.
- Your friendly neighbourhood thief "borrowed" the gasoline -
the unfriendly one took the vehicle.
- The fuel tank leaked.
- Your darling child/wife/husband/partner/mother/father used the car.
- The most likely reason is that your local garage switched to an oxygenated
gasoline, and the engine management system compensated for the oxygen
content, causing the fuel consumption to increase (although the effect on
well tuned engines is only 2-4%).
No. Many of the abnormal combustion problems are induced by the same
conditions, and so one can lead to another.
Preignition occurs when the air-fuel mixture is ignited prematurely by
glowing deposits or hot surfaces - such as exhaust valves and spark plugs.
If it continues, it can increase in severity and become Run-away Surface
Ignition (RSI) which prevents the combustion heat being converted into
mechanical energy, thus rapidly melting pistons. The Ricardo method uses an
electrically-heated wire in the engine to measure preignition tendency. The
scale uses iso-octane as 100 and cyclohexane as 0.
Some common fuel components:
There is no direct correlation between antiknock ability and preignition
tendency, however high combustion chamber temperatures favour both, and so
one may lead to the other. An engine knocking during high-speed operation
will increase in temperature and that can induce preignition, and conversely
any preignition will result in higher temperatures than may induce knock.
Misfire is commonly caused by either a failure in the ignition system, or
fouling of the spark plug by deposits. The most common cause of deposits
was the alkyl lead additives in gasoline, and the yellow glaze of various
lead salts was used by mechanics to assess engine tune. From the upper
recess to the tip, the composition changed, but typical compounds ( going
from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4; 3Pb3(PO4)2.PbClBr.
Run-on is the tendency of an engine to continue running after the ignition
has been switched off. It is usually caused by the spontaneous ignition of
the fuel-air mixture, rather than by surface ignition from hotspots or
deposits, as commonly believed. The narrow range of conditions for
spontaneous ignition of the fuel-air mixture ( engine speed, charge
temperature, cylinder pressure ) may be created when the engine is switched
off. The engine may refire, thus taking the conditions out of the critical
range for a couple of cycles, and then refire again, until overall cooling
of the engine drops it out of the critical region. The octane rating of the
fuel is the appropriate parameter, and it is not rare for an engine to
require a higher Octane fuel to prevent run-on than to avoid knock [27,28].
Obviously, engines with fuel injection systems do not have the problem, and
idle speed is an important factor. Later model carburettors have an idle
stop solenoid which partially closes the throttle blades when the ignition
key was off, and (if set correctly) thus prevents run-on.
Yes, carburettor icing is caused by the combination of highly volatile fuel,
high humidity and low ambient temperature. The extent of cooling, caused by
the latent heat of the vaporised gasoline in the carburettor, can be as much
as 20C, perhaps dropping below the dew point of the charge. If this happens,
water will condense on the cooler carburettor surfaces, and will freeze if
the temperature is low enough. The fuel volatility can not always be reduced
to eliminate icing, so anti-icing additives are used. In the US, anti-icing
additives are seldom required because of the widespread use heated intake
air and fuel injection .
Two types of additive are added to gasoline to inhibit icing:
If you have icing problems, the addition of 100-200mls of alcohols to a full
tank of dry gasoline will prevent icing under moderately-cold conditions.
If you believe there is a small amount of water in the fuel tank, add 500mls
of isopropyl alcohol as the first treatment, and isopropyl alcohol is also
preferred for more severe conditions. Oxygenated gasolines using alcohols
can also be used.
- Surfactants that form a monomolecular layer over the metal parts that
inhibits ice crystal formation. These are usually added at concentrations
of 30-150 ppm.
- Cryoscopic additives that depress the freezing point of the condensed water
so that it does not turn to ice. Alcohols ( methanol, iso-propyl alcohol,
etc. ) and glycols ( hexylene glycol, dipropylene glycol ) are used at
concentrations of 0.03% - 1%.
No. The fuel will be from a different season, and will have significantly
different volatility properties that may induce driveability problems. You
can tune your engine to perform on oxygenated gasoline as well as it did on
traditional gasoline, however you will have increased fuel consumption due
to the useless oxygen in the oxygenates. Some engines may not initially
perform well on some oxygenated fuels, usually because of the slightly
different volatility and combustion characteristics. A good mechanic should
be able to recover any lost performance or driveability, providing the engine
is in reasonable condition.
Yes, several manufacturers have demonstrated that their new gasoline additive
packages are more effective than traditional gasoline formulations. Texaco
claimed their new vapour-phase fuel additive can reduce existing deposits by
up to 30%, improve fuel economy, and reduce NOx tailpipe emissions by 15%,
when compared to other advanced liquid phase additives . The advertising
claims have been successfully disputed in court by Chevron - who demonstrated
that their existing fuel additive already offered similar benefits. Other
reputable gasoline manufacturers will have similar additive packages in their
premium quality gasolines . Quality gasolines, of whatever octane
ratings, will include a full range of gasoline additives designed to provide
consistent fuel quality.
Note that oxygenated gasolines must decrease fuel economy for the same power.
If your engine is initially well-tuned on hydrocarbon gasolines, the
stoichiometry will move to lean, and maximum power is slightly rich, so
either the management system ( if you have one ) or your mechanic has to
increase the fuel flow. The minor improvements in combustion efficiency that
oxygenates may provide, can not compensate for 2+% of oxygen in the fuel
that will not provide energy.
"Stale" fuel is caused by improper storage, and usually smells sour. The
gasoline has been allowed to get warm, thus catalysing olefin decomposition
reactions, and perhaps also losing volatile material in unsealed containers.
Such fuel will tend to rapidly form gums, and will usually have a significant
reduction in octane rating. The fuel can be used by blending with twice the
volume of new gasoline, but the blended fuel should be used immediately,
otherwise teh old fuel will catalyse rapid decomposition of the new,
resulting in even larger quantities of stale fuel. Some stale fuels can drop
several octane numbers, so be generous with the dilution.
If you only have a small quantity of water, then the addition of 500mls of
dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb the water,
and will not significantly affect combustion. Once you have mopped up the
water with IPA, small, regular doses of any anhydrous alcohol will help
keep the tank dry. This technique will not work if you have very large
amounts of water, and the addition of greater amounts of IPA may result in
Water in fuel tanks can be minimised by keeping the fuel tank near full, and
filling in the morning from a service station that allows storage tanks to
stand for several hours after refilling before using the fuel. Note that
oxygenated gasolines have greater water solubility, and should cope with
small quantities of water.
Yes, providing the octane is appropriate. There are some older engines that
cut the valve seats directly into the cylinder head ( eg BMC minis ). The
absence of lead, which lubricated the valve seat, causes the very hard
oxidation products of the exhaust valve to wear down the seat. This valve
seat recession is usually corrected by installing seat inserts, hardening
the seats, or use of specific valve seat recession protection additives
( such as Valvemaster ). Most other problems arise because the fuels have
different volatility, or the reduction of combustion chamber deposits.
These can usually be cured by reference to the vehicle manufacturer, who
will probably have a publication with the changes. Some vehicles will
perform as well on unleaded with a slightly lower octane than recommended
leaded fuel, due to the significant reduction in deposits from modern
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.
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
- 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.
- 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.
- 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
- 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
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].
- 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
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.
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]
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 .