Gasoline has a limited shelf life for several reasons.
Loss of volatiles - many gasolines have a lot of octane in the front
end ( very volatile fraction ), losing the volatiles lowers the fuel
octane. Loss of volatile front end octane can damage older engines
with carbs and no knock sensors, especially under intense
acceleration. It's not such a problem with modern auto engines,
but other small engine might have problems. Loss of volatiles also
cause hard starting on cold engines, and only gasoline vapour will
burn, not liquid.
Instability of the hydrocarbon fraction, particularly olefins (
alkenes, arenes ), the fuel stability treatment is designed to be
stable for 4- 6 months. As retail fuels in many parts of the world are
seaonally adjusted, diesel for cold flow, gasoline for volatility (
more volatile in winter to help starting, and less volatile in summer
to prevent vapour lock in older carbs, small engines, etc. ), there's
no reason to go for longer stability.
The olefin fraction is the major cause of gums, and also deposits
buidling up in the engine manifolds and back of valves. Engines with
fuel injection often recirculate the fuel, and the fuel can get quite
warm, so stability is still an issue for many modern FI engines .
Instability in the additives. Oil companies add enough additives to
meet the stability and performance requirements at the time of
manufacture and expected retail life. Some of those additives are
expected to react with exposed metals, rogue fuel breakdown products,
etc etc, and may form insoluble precipitates or gums. Other additives,
eg lead, oxygenates, MMT etc, are added to meet octance requirements,
and may also have other effects, such as changing the fuel lifetime.
Gum and deposit formation occurs rapidly once inhibitors have be used
up, and may be catalysed by traces of fine rust or bare metal
surfaces, warm temperatures, light, presence of water, etc. etc..
Storing gasoline is a very hit and miss exercise, even when
additional storage stabilizers have been added, because it depends
on the original formulation and additives as well as the storage
conditions. Once gums have formed, they do not redissolve into
a fresh tank of fuel, so prevention is preferred.
Oil companies do not like retail outlets to have low turnover of
any fuel, and may require outlets to maintain a turnover rate in each
fuel tank so they can provide seasonal fuels as required.
>> > Ok, so what kind of shelf life is it reasonable to expect?
>> Figure six months, max. Maybe a little more if you add stabiliser.
Depends on the ambient temperature, and the additive package of the
fuel, storage life in winter is usually longer than in summer, Six
months is a sensible maximum for winter, summer should be 3-4 months
maximum. For engines with recirculating gasoline and fuel injection, I
would reduce those times to 3 months maximum. Sure the engine will
adjust as best it can for low octane fuel, but why bother when it's
relatively easy to avoid.
>I once had a car laid up for 4 years. Made no special effort to preserve the
>fuel. On restarting it ran fine on the stuff that had lain in the tank all
>that time. New cars are often stored for months without a fuel problem.
So, feel happy, but I've dealt with customer complaints where their
fuel formed gums after 3 months, and the gums blocked the carb
jets. Not certain about the US, But new cars used to be given just a
couple of gallons of fuel with that had extra stabilisers to ensure
all active surfaces in the fuel system, especially those subjected to
vapour and condensation were inhibited and didn't corrode. Military
fuels and tropical fuels also have more stringent stability
reuirements, and often have additives to prolong storage stability.
OE fuel was also expected to last for 6 months storage regardless of
ambient May not be used now with polymeric fuel systems and fuel
injection systems, but I suspect that some OE fills still have extra
stability if the manufacturer expects to stockpile or ship the
vehicles long distances..