Gasoline FAQ
Contents:
If you use a fuel with an octane rating below the requirement of the engine,
the management system may move the engine settings into an area of less
efficient combustion, resulting in reduced power and reduced fuel economy.
You will be losing both money and driveability. If you use a fuel with an
octane rating higher than what the engine can use, you are just wasting
money by paying for octane that you can not utilise. The additive packages
are matched to the engines using the fuel, for example intake valve deposit
control additive concentrations may be increased in the premium octane grade.
If your vehicle does not have a knock sensor, then using a fuel with an
octane rating significantly below the octane requirement of the engine means
that the little men with hammers will gleefully pummel your engine to pieces.
You should initially be guided by the vehicle manufacturer's recommendations,
however you can experiment, as the variations in vehicle tolerances can
mean that Octane Number Requirement for a given vehicle model can range
over 6 Octane Numbers. Caution should be used, and remember to compensate
if the conditions change, such as carrying more people or driving in
different ambient conditions. You can often reduce the octane of the fuel
you use in winter because the temperature decrease and possible humidity
changes may significantly reduce the octane requirement of the engine.
Use the octane that provides cost-effective driveability and performance,
using anything more is waste of money, and anything less could result in
an unscheduled, expensive visit to your mechanic.
In general, modern engine management systems will compensate for fuel octane,
and once you have satisfied the optimum octane requirement, you are at the
optimum overall performance area of the engine map. Tuning changes to obtain
more power will probably adversely affect both fuel economy and emissions.
Unless you have access to good diagnostic equipment that can ensure
regulatory limits are complied with, it is likely that adjustments may be
regarded as illegal tampering by your local regulation enforcers. If you are
skilled, you will be able to legally wring slightly more performance from
your engine by using a dynamometer in conjunction with engine and exhaust gas
analyzers and a well-designed, retrofitted, performance engine management
chip.
Not simply, you can purchase additives, however these are not cost-effective
and a survey in 1989 showed the cost of increasing the octane rating of one
US gallon by one unit ranged from 10 cents ( methanol ), 50 cents (MMT),
$1.00 ( TEL ), to $3.25 ( xylenes ) [108]. Refer to section 6.20 for a
discussion on naphthalene ( mothballs ). It is preferable to purchase a
higher octane fuel such as racing fuel, aviation gasolines, or methanol.
Sadly, the price of chemical grade methanol has almost doubled during 1994.
If you plan to use alcohol blends, ensure your fuel handling system is
compatible, and that you only use dry gasoline by filling up early in the
morning when the storage tanks are cool. Also ensure that the service station
storage tank has not been refilled recently. Retailers are supposed to wait
several hours before bringing a refilled tank online, to allow suspended
undissolved water to settle out, but they do not always wait the full period.
Aviation gasolines were all highly leaded and graded using two numbers, with
common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is
the Aviation rating ( aka Lean Mixture rating ), and the second number is the
Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade,
100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to
replace the 80/87 and 100/130. Soon after the introduction, there was a
spate of plug fouling, and high cylinder head temperatures resulting in
cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a
limited scale. The Aviation Rating is determined using the automotive Motor
Octane test procedure, and then converted to an Aviation Number using a
table in the method. Aviation Numbers below 100 are Octane numbers, while
numbers above 100 are Performance numbers. There is usually only 1 - 2
Octane units different to the Motor value up to 100, but Performance numbers
varies significantly above that eg 110 MON = 128 Performance number.
The second Avgas number is the Rich Mixture method Performance Number ( PN
- they are not commonly called octane numbers when they are above 100 ), and
is determined on a supercharged version of the CFR engine which has a fixed
compression ratio. The method determines the dependence of the highest
permissible power ( in terms of indicated mean effective pressure ) on
mixture strength and boost for a specific light knocking setting. The
Performance Number indicates the maximum knock-free power obtainable from a
fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine
designed to utilise the fuel can obtain 150% of the knock-limited power of
iso-octane at the same mixture ratio. This is an arbitrary scale based on
iso-octane + varying amounts of TEL, derived from a survey of engines
performed decades ago. Aviation gasoline PNs are rated using variations of
mixture strength to obtain the maximum knock-limited power in a supercharged
engine. This can be extended to provide mixture response curves which define
the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost
( weak about 16:1 stoichiometry ) before knock [110].
The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.
The legend of mothballs as an octane enhancer arose well before WWII when
naphthalene was used as the active ingredient. Today, the majority of
mothballs use para-dichlorobenzene in place of naphthalene, so choose
carefully if you wish to experiment :-). There have been some concerns about
the toxicity of para-dichlorobenzene, and naphthalene mothballs have again
become popular. In the 1920s, typical gasoline octane ratings were 40-60
[11], and during the 1930s and 40s, the ratings increased by approximately 20
units as alkyl leads and improved refining processes became widespread [12].
Naphthalene has a blending motor octane number of 90 [52], so the addition of
a significant amount of mothballs could increase the octane, and they were
soluble in gasoline. The amount usually required to appreciably increase the
octane also had some adverse effects. The most obvious was due to the high
melting point ( 80C ), when the fuel evaporated the naphthalene would
precipitate out, blocking jets and filters. With modern gasolines,
naphthalene is more likely to reduce the octane rating, and the amount
required for low octane fuels will also create operational and emissions
problems.
The actual octane requirement of a vehicle is called the Octane Number
Requirement (ONR), and is determined by using series of standard octane fuels
that can be blends of iso-octane and normal heptane ( primary reference ),
or commercial gasolines ( full-boiling reference ). In Europe, delta RON
(100C) fuels are also used, but seldom in the USA. The vehicle is tested
under a wide range of conditions and loads, using decreasing octane fuels
from each series until trace knock is detected. The conditions that require
maximum octane are not consistent, but often are full-throttle acceleration
from low starting speeds using the highest gear available. They can even be
at constant speed conditions, which are usually performed on chassis
dynamometers [27,28,111]. Engine management systems that adjust the octane
requirement may also reduce the power output on low octane fuel, resulting
in increased fuel consumption, and adaptive learning systems have to be
preconditioned prior to testing. The maximum ONR is of most interest, as that
usually defines the recommended fuel, however it is recognised that the
general public seldom drive as severely as the testers, and so may be
satisfied by a lower octane fuel [28].
Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation
Efficiency = 1 - (1/compression ratio)^gamma-1
where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 [23].
The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.
Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel
ratio, but was very engine specific. Modern engines, with engine management
systems, now have their maximum octane requirement near to 14.5:1. For a
given engine using gasoline, the relationship between thermal efficiency,
air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel
ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum
power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal
efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio
is controlled at part throttle by a closed loop system using the oxygen sensor
in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio
is used during full throttle operation to reduce knocking while providing
better driveability [38]. An average increase of 2 (R+M)/2 ON is required
for each 1.0 increase (leaning) of the air-fuel ratio [111]. If the mixture
is weakened, the flame speed is reduced, consequently less heat is converted
to mechanical energy, leaving heat in the cylinder walls and head,
potentially inducing knock. It is possible to weaken the mixture sufficiently
that the flame is still present when the inlet valve opens again, resulting
in backfiring.
The tendency to knock increases as spark advance is increased. For an engine
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93
octane fuel, retarding the spark 4 degrees lowers the octane requirement to
91, whereas advancing it 8 degrees requires 96 octane fuel [27]. It should
be noted this requirement depends on engine design. If you advance the spark,
the flame front starts earlier, and the end gases start forming earlier in
the cycle, providing more time for the autoigniting species to form before
the piston reaches the optimum position for power delivery, as determined by
the normal flame front propagation. It becomes a race between the flame front
and decomposition of the increasingly-squashed end gases. High octane fuels
produce end gases that take longer to autoignite, so the good flame front
reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine is
intended to use. The timing of the spark is advanced sufficiently to ensure
that the fuel-air mixture burns in such a way that maximum pressure of the
burning charge is about 15-20 degree after TDC. Knock will occur before
this point, usually in the late compression - early power stroke period.
The engine management system uses ignition timing as one of the major
variables that is adjusted if knock is detected. If very low octane fuels
are used ( several octane numbers below the vehicle's requirement at optimal
settings ), both performance and fuel economy will decrease.
The actual Octane Number Requirement depends on the engine design, but for
some 1978 vehicles using standard fuels, the following (R+M)/2 Octane
Requirements were measured. "Standard" is the recommended ignition timing
for the engine, probably a few degrees BTDC [38].
Basic Ignition Timing
Vehicle Retarded 5 degrees Standard Advanced 5 degrees
A 88 91 93
B 86 90.5 94.5
C 85.5 88 90
D 84 87.5 91
E 82.5 87 90
The actual ignition timing to achieve the maximum pressure from normal
combustion of gasoline will depend mainly on the speed of the engine and the
flame propagation rates in the engine. Knock increases the rate of the
pressure rise, thus superimposing additional pressure on the normal
combustion pressure rise. The knock actually rapidly resonates around the
chamber, creating a series of abnormal sharp spikes on the pressure diagram.
The normal flame speed is fairly consistent for most gasoline HCs, regardless
of octane rating, but the flame speed is affected by stoichiometry. Note that
the flame speeds in this FAQ are not the actual engine flame speeds. A 12:1
CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s,
and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds
are also very dependent on stoichiometry.
Engine management systems are now an important part of the strategy to
reduce automotive pollution. The good news for the consumer is their ability
to maintain the efficiency of gasoline combustion, thus improving fuel
economy. The bad news is their tendency to hinder tuning for power. A very
basic modern engine system could monitor and control:- mass air flow, fuel
flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ), knock
( vibration sensor ), EGR, exhaust gas temperature, coolant temperature, and
intake air temperature. The knock sensor can be either a nonresonant type
installed in the engine block and capable of measuring a wide range of knock
vibrations ( 5-15 kHz ) with minimal change in frequency, or a resonant type
that has excellent signal-to-noise ratio between 1000 and 5000 rpm [112].
A modern engine management system can compensate for altitude, ambient air
temperature, and fuel octane. The management system will also control cold
start settings, and other operational parameters. There is a new requirement
that the engine management system also contain an on-board diagnostic
function that warns of malfunctions such as engine misfire, exhaust catalyst
failure, and evaporative emissions failure. The use of fuels with alcohols
such as methanol can confuse the engine management system as they generate
more hydrogen which can fool the oxygen sensor [76] .
The use of fuel of too low octane can actually result in both a loss of fuel
economy and power, as the management system may have to move the engine
settings to a less efficient part of the performance map. The system retards
the ignition timing until only trace knock is detected, as engine damage
from knock is of more consequence than power and fuel economy.
Increasing the engine temperature, particularly the air-fuel charge
temperature, increases the tendency to knock. The Sensitivity of a fuel can
indicate how it is affected by charge temperature variations. Increasing
load increases both the engine temperature, and the end-gas pressure, thus
the likelihood of knock increases as load increases. Increasing the water
jacket temperature from 71C to 82C, increases the (R+M)/2 ONR by two [111].
Faster engine speed means there is less time for the pre-flame reactions
in the end gases to occur, thus reducing the tendency to knock. On engines
with management systems, the ignition timing may be advanced with engine
speed and load, to obtain optimum efficiency at incipient knock. In such
cases, both high and low engines speeds may be critical.
A new engine may only require a fuel of 6-9 octane numbers lower than the
same engine after 25,000 km. This Octane Requirement Increase (ORI) is due to
the formation of a mixture of organic and inorganic deposits resulting from
both the fuel and the lubricant. They reach an equilibrium amount because
of flaking, however dramatic changes in driving styles can also result in
dramatic changes of the equilibrium position. When the engine starts to burn
more oil, the octane requirement can increase again. ORIs up to 12 are not
uncommon, depending on driving style [27,28,32,111]. The deposits produce
the ORI by several mechanisms:
- They reduce the combustion chamber volume, effectively increasing the
compression ratio.
- They also reduce thermal conductivity, thus increasing the combustion
chamber temperatures.
- They catalyse undesirable pre-flame reactions that produce end gases with
low autoignition temperatures.
The CFR octane rating engines do not reflect actual conditions in a vehicle,
consequently there are standard procedures for evaluating the performance
of the gasoline in an engine. The most common are:
- The Modified Uniontown Procedure
- Full throttle accelerations are made
from low speed using primary reference fuels. The ignition timing is
adjusted until trace knock is detected at some stage. Several reference
fuels are used, and a Road Octane Number v Basic Ignition timing graph is
obtained. The fuel sample is tested, and the trace knock ignition timing
setting is read from the graph to provide the Road Octane Number. This is
a rapid procedure but provides minimal information, and cars with engine
management systems require sophisticated electronic equipment to adjust
adjust the timimg [28].
- The Modified Borderline Knock Procedure
- The automatic spark advance is
disabled, and a manual adjustment facility added. Accelerations are
performed as in the Modified Uniontown Procedure, however trace knock is
maintained throughout the run by adjustment of the spark advance. A map
of ignition advance v engine speed is made for several reference fuels
and the sample fuels. This procedure can show the variation of road octane
with engine speed, however the technique is almost impossible to perform
on vehicles with modern management systems [28].
The Road Octane Number lies between the MON and RON, and the difference
between the RON and the Road Octane number is called 'depreciation" [111].
Because nominally-identical new vehicle models display octane requirements
that can range over seven numbers, a large number of vehicles have to be
tested [28,111].
An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]. When the combined effects
of air temperature and humidity are considered, it is often possible to use
one octane grade in summer, and use a lower octane rating in winter. The
Motor octane rating has a higher charge temperature, and increasing charge
temperature increases the tendency to knock, so fuels with low Sensitivity
( the difference between RON and MON numbers ) are less affected by air
temperature.
The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:
- Reduced air density provides lower combustion temperature and pressure.
- Fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement.
- Manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.