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Gasoline FAQ - Part 3 of 4
Section - 7. What parameters determine octane requirement?

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7.1  What is the Octane Number Requirement of a Vehicle?

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].

7.2  What is the effect of Compression ratio?

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 for 
gasoline fuels in an SI engine [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.

7.3  What is the effect of changing the air-fuel ratio?

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.

7.4  What is the effect of changing the ignition timing?

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.  

7.5  What is the effect of engine management systems?

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. 

7.6  What is the effect of temperature and load?  

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]. 

7.7  What is the effect of engine speed?.

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.
          
7.8  What is the effect of engine deposits?

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.  

7.9  What is the Road Octane Number of a Fuel?
  
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:-
1. 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
   the ignition timing [28].
2. 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].

7.10  What is the effect of air temperature?
          
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.

7.11  What is the effect of altitude?

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.

7.12  What is the effect of humidity?.

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].

7.13  What does water injection achieve?.

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 [113].

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% [114]. 
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 [110].

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.

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