Part 3
6.16 What happens if I use the wrong octane fuel?
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.
6.17 Can I tune the engine to use another octane fuel?
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.
6.18 How can I increase the fuel octane?
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.
6.19 Are aviation gasoline octane numbers comparable?
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.
6.20 Can mothballs increase octane?
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.
