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Q: Diesel and Gasoline ( No Answer,   6 Comments )
Subject: Diesel and Gasoline
Category: Science > Earth Sciences
Asked by: stephen13-ga
List Price: $50.00
Posted: 17 Aug 2004 10:56 PDT
Expires: 19 Aug 2004 10:11 PDT
Question ID: 389036
how does diesel fuel differ from gasoline, how are they each refined
and how were they disocvered.  For Diesel start with Rudolph Diesel
and the origins of his invention.  How are the two differnt fule
combusted and work in engines and list any pros and cons of each type.

Request for Question Clarification by kriswrite-ga on 18 Aug 2004 10:13 PDT
Hi Stephen13~

Did touf's comment help? Or are you still interested in a documented Answer?


Clarification of Question by stephen13-ga on 18 Aug 2004 11:26 PDT
They are but i am testing the Google Answers service out and want to
see what I cna get for $50 answer.  I aksed a question that I am
fairly knowlegable just to see the quality of the service prior to
asking real work related quesitons.  thanks
There is no answer at this time.

Subject: Re: Diesel and Gasoline
From: touf-ga on 18 Aug 2004 10:04 PDT
Let's start with some basics.

Diesel fuel and Gasoline are both hydrocarbons, refined from crude oil.  

In the United States, there are typically three types of Diesel fuel;
D1, D2, and D4.  For most commercial and highway applications, you
really only see D1 and D2.

Gasoline, on the other hand, is rated according to an octane rating. 
Typical ratings range from 85 to 93.  By definition, pure isooctane
(to be defined later) has an octane rating of 100, while octane itself
has an octane rating of zero.  Using two methods, research and motor,
an equivalent octane rating is obtained for a given sample of
gasoline.  Increased octane does not improve performance; rather, it
simply retards knocking in your engine.  Many high end sport cars and
high-performance machines have high compression ratios, which require
higher octane.  However, throwing 93 octane gas into your '77 Datsun
will do nothing for you other than burn a hole in your wallet.  Since
it's based on a theoretical definition, you can see octane ratings
over 100.

When crude oil is refined/distilled, the crude oil is heated to a
given temperature.  This temperature happens to be the boiling point
of an individual components in the crude oil (for instance gasoline). 
These boiled off components (now in the gas phase) are cooled until
they condense into a liquid and collected.

For gasoline, this temperature is approximately 30 - 200 degrees C.
For diesel, this temperature is approximately 350 degrees C.

Gasoline is mostly octane; C8H18, however, the gasoline we use in
autos is isooctane, also called 2,2,4-trimethylpentane.  The reason
for this is that octane has a tendency to cause knocking in engines,
while isooctane does not.

Diesel is a mix of hydrocarbons, typically in the C9H20 to C12H26
range.  Obviously, diesel has a higher molecular weight than gasoline.

Diesel fuel was not discovered by Rudolph Diesel; he invented the Diesel engine.

The main difference between diesel and gasoline is in fact, how they are combusted.

- The Classic Four Stroke Gasoline Engine:

In this engine, there are four strokes:  The intake stroke;
compression stroke; combustion stroke; exhaust stroke.  In a typical
modern car with electronic fuel injection, your cylinder intakes
gasoline and air, mixed to the proper proportions thanks to your
onboard computer.  With older cars, this is done through the use of a
carbeurator.  This is the first stroke.  The second stroke involves
compressing this mixture.  At this point, this mixture is
theoretically homogeneous, meaning it's completely mixed at the
molecular level.  Next, your spark plug fires and combusts the mixture
causing an explosion of gas, which drives your cylinder outwards. 
This is where you get your power.  Finally, the fourth stroke, your
exhaust stroke, when your cylinder gets rid of all the already mixed
gases.  And the cycle starts over again...

Diesel Engine:

In a Diesel engine, your intake stroke is just air.  You only intake
air.  You compress the air, thereby heating it.  (Law of physics -
when you compress a gas, its temperature increases).  Then, the fuel
is sprayed into the cylinder, in the form of tiny droplets, in order
to maximize the surface area to volume ratio of each droplet.  This
mixture IS NOT mixed at the molecular level, or at any other level,
for that matter.  These droplets ignite, combust, and explode.  You
get power, then you have exhaust, just like a gasoline engine.

So, the main difference between how these fuels combust is the fact
that one is a premixed mixture which ignites.  The other is not

Pros and cons of each:

Typically, in a diesel engine, you compress the air a lot!  Typical
compression ratios are two to three times that of gasoline engines. 
So, you get more power per unit diesel fuel, meaning higher
efficiency.  Drawbacks, though, are that these droplets don't combust
quickly/completely compared to a premixed charge, with a sudden change
of engine speed.  Therefore, you often have a lot of unburned
hydrocarbons and soot in a diesel engine's exhaust while driving from
a standstill, for example.  Watch a bus/tractor trailer take off from
a red light, and you'll know what I mean.  Diesel engines are great
when used at a constant rpm, and typically have a lot of low end
torque.  They are often used in transportation devices which maintain
a constant rpm:  semi trucks, trains, etc, and are not typically used
in autos which take on a lot of stop and go traffic.  Diesel fuel also
is much safer than gasoline, and does not explode.  This is of great
concern for places where Diesel is used in confined spaces, such as on
a boat.

Gasoline engines are great in engines which do require stop and go
traffic, because the premixed gases combust much more easily than in
the case of a Diesel engine.  However, they have to use lower
combustion ratios because gasoline tends to knock rather easily. 
Knocking is basically an uncontrolled explosion of fuel in a cylinder.
 It damages the cylinder over time.  Lower combustion ratios means
lower efficiency.  Gasoline fuel is also prone to being highly
flammable, and can be dangerous is mishandled.  Gasoline burns cleaner
than Diesel, however, and releases less crud into the air because it
burns more complete.

Note that you do not necessarily need crude oil based Diesel fuel to
run an engine.  The principle of the Diesel engine can be used for any
heavy fuel with a relatively high flash point.  For instance, many
people sell biodiesel conversion kits, which allow you to make a
standard Diesel engine run on soybean oil.  With the current price of
fuel, it may be cheaper to do so...last I checked at a restaurant
supply store, a 5 gallon vat of soybean oil went for something like 7
Subject: Re: Diesel and Gasoline
From: touf-ga on 18 Aug 2004 10:06 PDT
One thign I forgot to mention is that in a Diesel engine, you do not
have a spark plug.  The hot air (heated from compression) is what
ignites the fuel.
Subject: Re: Diesel and Gasoline
From: touf-ga on 18 Aug 2004 13:05 PDT
So, if you know about this, then how'd I do?

I am actually pretty knowledgeable about this as well -- have done
extensive research in combustion.  I document myself...

Whaddya think?
Subject: Re: Diesel and Gasoline
From: stephen13-ga on 18 Aug 2004 14:09 PDT
You did very well.  I learned about D4, I only knew about D1/D2. 
Learned about the molecular makeup of each and the tempratures for
boiling point.  I only knew Diesel was higher than gas.  Also that
octane is theorhetical and octane itself carries no value.  I would
take that to mean an octane rating is closer to being like anhydrous
(without water) meaning that the higher the octane rating the less
octane is presnet that causes knocks?  bad analogy but it seems like
the rating is inverse of what it does.

The information led me to some more questions.  When refining crude do
you get gasoline when the gas is stripped off at its boiling point and
then get diesel when you take the gases at the higher poiling point,
meaning the crude produce both fuels from the same batch?

Also what was Diesel fuel called prior to Rudolph Diesel making the
first Diesel engine?

One last question are you paid to add comments, are you just a member
of GA and look for quesitons to answer?  This is faily complete reply
for a user group.  Thanks -steve
Subject: Re: Diesel and Gasoline
From: touf-ga on 18 Aug 2004 16:47 PDT
Regarding octane, not exactly.  I apologize, but in my haste, I gave
you incorrect info.  By definition, n-heptane has an octane rating of
zero.  Isooctane, however, still has an octane rating of 100.

Technically, the definition of 87 octane gasoline is 87 parts
iso-octane to 13 parts heptane...well, not really.  As I mentioned
before, gasoline is mostly isooctane.  However, there are tons of
additives, detergents, and other stuff the respective gas companies
put into the fuel.  Some are required by law, others, like Techron by
Chevron, claim to help clean your engine.  To determine the octane
rating of a given fuel, researchers use two methods; the research
method and the motor method.  The basic idea for both involves
changing the compression ratio of a cylinder until the engine starts
to knock.  They do a curve fit based on some preset calibration and
obtain an equivalent octane rating.

The difference between the Research and Motor methods is that the
research method is typically obtained under fairly easy engine
conditions; low speed, no load, etc.  For the Motor method, the engine
is presented with a more rigorous environment.  It has load; speeds
are higher; temperatures are higher, etc.

Gas companies take an average of these two numbers (on the yellow
stickers you see at the pump, you often see R+M/2) and obtain an
equivalent octane rating.

When refining gas, the refineries start with lower temperatures and go
up.  By doing this, they distill gasoline first; then Diesel.  (BTW,
between gasoline and diesel is kerosene)

Before Rudolph Diesel invented the Diesel engine, I don't believe
"Diesel" fuel had a name.  Back in the 1800s, refining techniques were
not all that great, and dependance on liquid fossil fuels was not that
high.  Recall that coal was the main power source back then.

Diesel got his idea for the Diesel engine when he was traveling in
North Africa in the late 1800s.  He saw people starting fires in an
interesting way: they placed a small bit of wood in the bottom of a
tube and then rammed a plunger into it. As I mentioned before,
compressing a gas increases its temperature, so this ramming action
caused the wood to heat up, smolder, and burn.

Using this same principle, Diesel worked on more efficient engine than
what was available at the time (gasoline and steam/coal)  Diesel's
ultimate idea was to design an engine for farm use that could run
using fuel grown by the farmer himself, and used peanut oil as his
fuel of choice.  (This is the biodiesel thing I was talking about
earlier) So I guess to answer your question, it was called peanut oil
before it was called Diesel fuel (sorry...)

Diesel revealed his invention at the 1900 World's Fair in Paris, and
the first use for his engine was by the military.  It slowly moved to
the commercial sector and finally, in 1936, to the consumer line. 
That's when Mercedes-Benz debuted the world's first diesel passenger
car, the 260D.

The reason we use Diesel fuel now is that, well, it's basically a
waste product from the refining/distillation process.  We really had
no other use for the heavy stuff that was too heavy to use in gasoline
engines, not quite heavy enough to use for heating
applications...Thus, the modern Diesel engine (and its fuel) was born.

As for your last question, no, I am not a paid answerer.  I wish I
were, but alas, I wasn't able to get in on the action early enough and
always see "Google is not accepting applications at this time", so I
just go around and answer what I know.  It's the "bored at my real
job" approach.  That's actually the great thing about Google Answers,
is that often, you get a few freebies in there.  But if you want to
clear your conscience and send me a check, my address is.... :-)
Subject: Re: Diesel and Gasoline
From: kriswrite-ga on 19 Aug 2004 08:57 PDT

I just wanted to let you know why I did not complete an official
Answer to your question. I was surprised to discover that I couldn't
find adequate sources to back up the information I knew about Diesel
vs. Gas. As Researchers, we strive to always give links or additional
sources for you to check the facts and further your knowledge. Given
that I couldn't find such resources, I didn't feel I could improve
upon Touf's comment :)


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