Newsgroups: rec.autos.tech
Subject: Oxygenated Compounds in Gasoline
From: srgxbhh@grace.cri.nz
Date: Sun, 10 Jan 93 03:35:28 GMT
Organization: Industrial Research Ltd., New Zealand.
NNTP-Posting-Host: grv.grace.cri.nz


I apologise for posting this, however the followup function is
not functioning correctly on this system. 

>From: jgd@dixie.com (John De Armond)

>Doesn't matter what you'd call an oxygenate.  The term is used in
>the context of hydrocarbon fuels to refer to any molecule containing
>one or more oxygen atoms.  The alcohols, MTBE, ETBE, TAME and a other
>ethers fit the category.  If you think that ANY of these fuels can
>burn without oxygen and thus are hard to extinguish OR you believe
>that potassium perchlorate or any other oxidizer is blended into
>pump gas, then, well... I have some nice seafront property to sell...

Well I do think that ALL of these fuels can burn without oxygen.
Clue - Another element starting with F can replace Oxygen as the
oxidant. I am not saying it is used today as a transportation
fuel oxidant, but that these fuels " can burn without oxygen ".

In general , if people would realise that the oxygenated fuel
additives are really just partially burned hydrocarbons, they
would realise they are paying for oxygen that can not provide
energy. On a mass basis one molecule of oxygen is equivalent
to 1.33 carbons or 16 hydrogens, so significant amounts of
potential fuel energy are lost.

However the oxygenates can help manufacturers utilise
hydrocarbon feedstocks with inferior combustion characteristics
(eg low octane numbers ) as gasolines. The oxygenates have
significantly different combustion characteristics to the
common components of gasoline. Consequently, if correctly
formulated into gasolines the overall emissions may be reduced
for the targetted engines. Problems may arise if non-targetted
engine designs can not correctly respond to the different
combustion characteristics of fuels incorporating oxygenates
( or other unusual components ). In general most oxygenates were
originally developed as octane enhancers, as they could enable
gasoline manufacturers to provide high octane ( lead and manganese
free ) product with relatively minimal refinery reconfiguration.

The advent of reformulated gasoline in the US may change the role
of oxygenated additives, as refinerys have to be reconfigured to
produce the reformulated gasoline anyway.


>>1.) Fuel economy and performance generally go hand in hand, an increase
>>in one causes a decrease in the other. Both of mine decreased.

>No they don't.  If you change fuels so the mixture is lean under most
>conditions, you will make less power because of less than optimum
>power AND your economy will decrease because you open the throttle
>more to compensate for the loss of power.  If you don't believe that,
>look in any elementary engine text and observe a brake specific fuel
>consumption vs mixture graph.

He was discussing " performance ", you are discussing " power ".
You have also assumed his throttle opening compensation actually
consumed more fuel, which is not necessarily true, as YOU decided
to make " the mixture lean under most conditions ".

For any one vehicle on the same fuel, I would agree they "generally
go hand in hand " under the same conditions ( no. of passengers,
ambient conditions, road etc. etc. ), what I thought he was saying
was that he lost both performance and fuel economy, and inferring
the only difference was the oxygenated fuel.  If the engine, fuel,
vehicle, or conditions are modified then they may not go hand in
hand, however the discussion is then usually in absolutes, rather
than comparisons, mainly to identify the relevant parameters being
investigated. When my neighbour discusses "fuel economy" and 
"performance" ,I assume he is talking about the same vehicle UNLESS
he states otherwise.  Maybe I'm easily confused.

>>2.) I believe the specific energy of methanol is lower than gasoline,
>>requiring more to be burned for same horsepower, resulting in lower
>>fuel economy.
>
>Specific energy is almost the same for alcohol and gasoline:
>
>Gasoline    2.92
>methanol    3.08
>Ethanol     3.00

I'm not certain of the US definition of "specific energy" but here
it's defined in MJ/kg of the fuel. Typical values for fuels are
 Gasoline    43.8 MJ/kg     32.4 MJ/l
 Methanol    19.9           15.8
 Ethanol     26.8           21.1

I suspect your numbers may be in MJ/kg of stoichiometric mix, which
is not what I understood him to be discussing.

>The issues are
>oxygen content:
>[ numbers deleted ]
>Heating value
>Gasoline    42.7 MJ/kg
>Methanol    19.9
>Ethanol        26.8

Well these look the same as mine - we agree on something!

However these are only a few of the issues, as many other factors
also come into play when fuels are used in existing internal
combustion engines, eg Heat of vapourisation, Research Octane Number,
Motor Octane Number, Boiling Range, Road Octane Number etc. etc.

>Stoichiometric ratio (derived from oxygen content)
>[ numbers deleted ]
>Reference: "Automotive Fuels Handbook", Owens & Coley, Tbl 12.3, P267

Well if they have the "Specific Energy" of the fuel defined as the
"MJ/kg of stoichiometric mix", then they differ from my texts.

[ text on propane deleted ]

Bruce Hamilton    SRGXBHH@grv.grace.cri.nz
Newsgroups: rec.autos.tech