8.1  What causes an empty fuel tank?

* 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% ).

8.2  Is knock the only abnormal combustion problem?                  

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:-
             paraffins       50-100
             benzene           26  
             toluene           93
             xylene          >100
             cyclopentane      70
             di-isobutylene    64
             hexene-2         -26

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 thus ( if set correctly ) prevents run-on.   
            
8.3  Can I prevent carburetter icing?
          
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 [28].

Two types of additive are added to gasoline to inhibit icing:- 
- 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%.

If you have icing problems, the addition of 100-200mls of alcohol 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 anhydrous isopropyl alcohol as the first treatment, and isopropyl 
alcohol is also preferred for more severe conditions. Oxygenated gasolines 
using alcohols can also be used. It's important to ensure the alcohol is
anhydrous, as some grades contain up to 30% water.
   
8.4  Should I store fuel to avoid the oxygenate season?

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. 
          
8.5  Can I improve fuel economy by using quality gasolines?

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 [49]. 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 [50]. 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.         

8.6  What is "stale" fuel, and should I use it?

"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. 
               
8.7  How can I remove water in the fuel tank?

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 
poor driveability. 

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.

8.8  Can I used unleaded on older vehicles?

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 
unleaded gasolines. If premium unleaded petrol containing relatively
high levels of aromatics is used, some carburetted engines from the 1960s
may experience spark plug fouling, however most vehicle manufacturers
have guides to ensure careful engine tuning will eliminate most of the
problem.