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bob ward
04-15-2010, 10:11 AM
I inhabit a couple of old car forums, and every now and then someone mutters about modern petrol (gas) burning a lot hotter than say 1930s formulations.

My gut feeling is that that's not right, or if they are there's not much in it. Google wasn't much help on this one, has anyone got the full story?

billtr96sn
04-15-2010, 10:15 AM
Lets say soemone was born in 1930, they would be 80 now, so how the h*ll would they know about the calorific content of the fuel?

Hupp31
04-15-2010, 10:55 AM
Well, I can't speak to your question specifically, but I used to have a 1931 Hupmobile. There was a data plate inside, on the driver's side, just above the windshield as well as on the metal spark plug loom under the hood.

It read, "Use Ethyl Gasoline - Minimum 74 Octane".

JCHannum
04-15-2010, 11:17 AM
Lets say soemone was born in 1930, they would be 80 now, so how the h*ll would they know about the calorific content of the fuel?

Do people automatically become stupid at some age? If so, what is that age? Why would an 80 year old not be qualified to comment on the caloric content of 1930's gasoline?

As to the original question, don't confuse heat content and octane rating, they are not the same. I would suspect that refining in the 30's would be cruder than today's resulting in gasoline closer to kerosene and diesel fuels and actually have a higher caloric content, but a lower octane rating.

Evan
04-15-2010, 01:12 PM
Since diesel fuel doesn't contain octane it doesn't have an octane rating. It has a cetane rating, not the same thing. Diesel has a lot lower autoignition temperature than gasoline which is why it can be used in a compression ignition engine.

"Burning hotter" and the total amount of heat produced are not the same thing either. Heat is the amount of kinetic energy contained by a volume of matter while temperature is the velocity of the molecules regardless of the amount of heat.

Old fuels contained a significant amount of benzene which is no longer permitted in gasoline above very small amounts. The makeup of gasoline now is specified by it's combustion properties rather than by what chemical compounds it contains. Gasoline is not a single chemical, it is a blend of as many as 10 to 20 different chemicals most of which are in the Alkane group.

The total heat content will vary slightly but not by a great amount. Even the difference between gasoline and diesel on average is just around 5 to 10 percent depending on the specific gravity of the fuels. That is entirely overwhelmed by the actual efficiency of the engine that burns the fuel and a diesel engine is more efficient than a gas engine in large part because of the reduction in pumping losses.

boslab
04-15-2010, 01:29 PM
Not to mention a good ol helping of Tetra-ethyl lead, or Ethyl as they preferred to call it, Octane ranking [RON]changed when this additive was used, an increase as preignition was not an issue.
Invented By the same Nutter who later gave us CFCs, Thomas Midgley Jr, an asset to the environment
mark

MuellerNick
04-15-2010, 01:43 PM
Diesel has a lot lower autoignition temperature than gasoline which is why it can be used in a compression ignition engine.

Octane and cetane has about the same ignition temperature, which is at about 210C. In fact, octane is a bit lower.
You could well use gasoline in a Diesel engine. Problem is, it's burning too fast.
And gasoline is also self igniting, it's called knocking. It doesn't take to much to get knocking, modern engines are very close to it. That's why they do have knock sensors.


Nick

MotorradMike
04-15-2010, 01:44 PM
There is a lot of misinformation around about "Octane number". The fuel companies use "Hi-test", "Premium", and "Super" which adds to the confusion.

Octane number is a measure of 'Resistance to ignition' and is based on percentage of Octane in an Octane/Heptane mixture. So 80% Octane/20% Heptane is the standard used to determine 80 Octane fuel. There is no need for the actual fuel to contain any Octane or Heptane in order to have an octane number. Diesel for example, is roughly 50. The value for diesel is of no use but it has a number none the less. The only reason to use High octane fuel is to prevent ignition prior to the spark which is bad.

The above is good enough for cocktail parties where people sometimes claim they run Premium every 5th tank to keep their engine new or some such rubbish.

I'm sure some of you can poke holes in it but for most of us, I think it's close enough to the truth so as to be used to counter grossly incorrect statements about "Hi octane".

lynnl
04-15-2010, 01:46 PM
..... because of the reduction in pumping losses.

"Pumping losses", what's that? Are we just saying that diesels, as a rule, turn at lower rpm's?

MuellerNick
04-15-2010, 02:01 PM
and a diesel engine is more efficient than a gas engine in large part because of the reduction in pumping losses.

That is almost dead wrong.
Diesel's efficency comes most part from the high compression and the (ideally) isobaric expansion cycle.


Nick

bob ward
04-15-2010, 02:23 PM
Thanks for the input so far guys.

My thoughts are that a litre/gallon of 1930's petrol/gasoline type hydrocarbons are basically similar to todays hydrocarbons and would contain more or less the same number of BTUs

While todays petrol is more highly refined, and contains a lot more volatiles then it used to because we mainly run sealed fuel systems today, I don't know that those factors have increased the BTU capacity of the fuel or indeed raised combustion chamber temperatures by a significant amount.

thedieter
04-15-2010, 02:29 PM
I know that gas burns my hands a lot more now than in the '30s :D

I thought diesel was more effecient because it contains more BTUs than gasoline.

Best regards, Jack

Willy
04-15-2010, 03:02 PM
1 Gal Propane = 91,600 Btu's

Gal Gasoline (mid grade) = 125,000 Btu's

1 Gal of #2 Fuel Oil or Diesel = 139,000 Btu's

Cetane number refers to a diesel fuels ignition delay, or how quickly it ignites after it is injected into the combustion chamber.

Nick and Evan are both partially right.
Running a constant vacuum requires a significant amount of pumping energy. Energy that is wasted. And as Nick said, the high compression of a diesel engine is also responsible for it's higher fuel efficiency compared to a gasoline fueled engine. These two factors plus the high btu content of diesel fuel are all responsible for a diesel engine's ability to deliver lower fuel consumption figures compared to that of a gasoline.

I too will go with what JC Hannum said about the gasoline of the 30's having a very slightly higher btu content because of the fuel was not as highly refined then as it is now.

MuellerNick
04-15-2010, 03:12 PM
Running a constant vacuum requires a significant amount of pumping energy.

That would imply, that the Miller cycle, or the Atkinson-cycle (the modern one, not the initial one) is much more efficient. But it aint.

Also, the pumping vacuum has the wanted side-effect of cooling down the inhaled air. Temperature-difference is what counts in efficency.


Nick

Willy
04-15-2010, 03:28 PM
Nick, the engine's pumping loses of generating a vacuum consumes significantly more energy than the very slight gains from the cooling effect of the vacuum.
I will agree though that the high compression ratio of a diesel engine is probably the greatest factor contributing to it's efficiency, but pumping loses and fuel btu content are very definite contributors to the efficiency increase over that of a gasoline engine.


Edited for spelling

JCHannum
04-15-2010, 03:43 PM
I don't know how accurately those BTU figures reflect what the actual BTU content of gasoline at the pump is in today's world. Algore additives plus 5%, 10% or more ethanol can significantly lower those numbers. Ethanol is quoted at a BTU content in the 80,000 BTU/gallon range depending on the source.

MuellerNick
04-15-2010, 03:46 PM
but pumping loses ...

Then a gasoline engine at full throttle should be as efficient as a Diesel.


and fuel btu content are very definite contributors to the efficiency increase over that of a gasoline engine.

Efficiency is not defined by how many gallons it consumes, but how much of the chemical energy of the fuel is converted to mechanical output.


Nick

Willy
04-15-2010, 04:30 PM
Then a gasoline engine at full throttle should be as efficient as a Diesel.

As far as pumping loses are concerned, yes. Add 18:1-22:1 compression pistons and diesel fuel, and it would be just as efficient.
Pumping loses in a gasoline fueled engine are only a factor at part throttle...where we do most of our driving.

This is the reason a gasoline engine has more of an engine braking effect than a diesel engine. Because it creates no vacuum a diesel engine's braking effect must be augmented with either an exhaust brake, (essentially a restriction in the exhaust, much like the restriction of the throttle valve in the intake of a gasoline engine) or an engine brake that opens the exhaust valves at the top of the compression stroke, thereby wasting all energy used to compress the air in the cylinder.


Efficiency is not defined by how many gallons it consumes, but how much of the chemical energy of the fuel is converted to mechanical output.

Yes, the efficiency is usually listed as BSFC ( brake specific fuel consumption) usually expressed in pounds of fuel per brake horsepower hour, at least it is in North America, not sure if this standard applies elsewhere.

MuellerNick
04-15-2010, 04:41 PM
Yes, the efficiency is usually listed as BSFC ( brake specific fuel consumption) usually expressed in pounds of fuel per brake horsepower hour, at least it is in North America, not sure if this standard applies elsewhere.

This is consumption or specific consumption. It is not efficiency!

Edit:
I think I have to be more precise: Thermal efficiency.
If you talk about specific consumption, you always compare among the same system (Diesel, Otto) and the same fuel. Efficency also considers the energy content of the fuel.

Nick

MuellerNick
04-15-2010, 04:52 PM
As far as pumping loses are concerned, yes.

If you compare brake specific consumption diagrams of a Diesel and an Otto engine, you will be surprised! At 50% of maximum effective pressure, both have a specific consumption of about 115%. So, the pumping losses are only hot air. :-)
No really, they are far below what you think. I can't give you links, the site is down and I only found diagrams in Kraut-speak.


Nick

Evan
04-15-2010, 05:01 PM
Octane and cetane has about the same ignition temperature, which is at about 210C. In fact, octane is a bit lower.
You could well use gasoline in a Diesel engine. Problem is, it's burning too fast.
And gasoline is also self igniting, it's called knocking. It doesn't take to much to get knocking, modern engines are very close to it. That's why they do have knock sensors.


I didn't refer to the autoignition temperature of octane or cetane. I refered to the autoignition temperature of diesel fuel vs gasoline.

The autoignition temperature of gasoline is 536F (289C), while Diesel is is around 480 (250C). This will vary for both products depending on the available feed stocks and the time of year in northern climates.

Pumping losses at low rpms such as when driving around town can reduce the efficiency of a gasoline engine to half that of a diesel used in similar conditions.

MuellerNick
04-15-2010, 05:42 PM
I answered to that quote of yours:
"Diesel has a lot lower autoignition temperature than gasoline which is why it can be used in a compression ignition engine."

Now:

The autoignition temperature of gasoline is 536F (289C), while Diesel is is around 480 (250C). This will vary for both products depending on the available feed stocks and the time of year in northern climates.


This is not a lot (albeit, it supports your theory).
But if you take an Otto engine that knocks at a CR of 14:1, it wont be enough for a Diesel to ignite. A 100ccm Diesel would require at least a CR of 18:1 to run.
Now, what fuel has a lower auto-ignition temperature?

One source:
Diesel: 220 ... 300C
Gasoline: 220 ... 450C

Other source (Wiki):
Diesel: 220C
Gasoline: 200...300C


Nick

Evan
04-15-2010, 07:33 PM
It is a lot. What seems to be small difference makes a large difference to how easy something will ignite. In general with petroleum derived chemicals the more carbon in the molecule the lower the autoignition temperature. This is not related to vapour pressure or volatility but it does track with the density of the fuel. Although diesel is easier to light when vaporized it is a lot less volatile than gasoline and so seems to be harder to ignite.

One of the wild cards in the autoignition temperature of gasoline is the amount of benzene and/or alcohol. Benzene has a very high autoignition temperature so if it is present it forces up the temperature of the product. Alcohol is also a lot higher than the alkanes in gasoline and has a similar effect. In nearly all jurisdictions here benzene must be kept to an extremely low level and so has little to no effect on the ignition temperature.

Alcohol is added to some fuels here but not all.

Willy
04-15-2010, 08:02 PM
From Wiki: http://en.wikipedia.org/wiki/Engine_efficiency



Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. Engine efficiency improves considerably at open road speeds...


Some gasoline engines that are direct injected and have no throttle plate stuck in the incoming air stream do not suffer these pumping loses.

Again from Wiki: http://en.wikipedia.org/wiki/Gasoline_direct_injection



In addition, there are no throttling losses in some GDI engines, when compared to a conventional fuel injected (http://bbs.homeshopmachinist.net/wiki/Fuel_injection) or carbureted (http://bbs.homeshopmachinist.net/wiki/Carburetor) engine, which greatly improves efficiency, and reduces 'pumping losses' in engines without a throttle plate.


I agree thermal efficiency is a measure of overall efficiency, but BSFC is how industry compares the virtues of one engine design over another. An efficient automotive diesel engine consumes approximately .30-.34 lb./bhp/hr or about 41% thermal efficiency. Ship engines are much more efficient, MAN for example has two stroke marine engines that are as high as 54%-55% thermally efficient, haven't done the math but BSFC figures are in the low .20 lb./bhp/hr.
So yes, strictly semantics, but BSFC is a measure of efficiency just as much as thermal efficiency figures.

Edited for grammer

CCWKen
04-15-2010, 08:57 PM
Model As had it made. The gasoline of the 30s was much better than that of the pre-teens, teens and twenties. That was about the time lead compounds were added and you could buy antifreeze. Octane ran from 60-80 by the 30s. Early gas was around 40-45 octane and it was unleaded. The Model Ts (1909-27) had 4:1 compression. In the early years, you had to buy gas at the hardware or feed store and bring your own bucket. Gas stations were few and far between even in the major metropolitan areas. It wasn't uncommon to use coal oil mixed with a little gasoline or kerosene to extend your miles.

Oh yeah, the good ole days. :rolleyes:

J Tiers
04-15-2010, 09:08 PM
The whole "octane number" thing is a bit cracked anyway.... Tells you nearly nothing that you want to know. And higher numbers may be actually BAD from a value standpoint.

A standard engine which is used for the purpose, is adjusted to knock a specific amount when run on the fuel. Then the mixture of octane and heptane is used to run teh engine with same conditions, adjusted for the same results, and the number comes out. That is the "research" method. There is also a "motor" method with a variant of the procedure.

The number given on the pump is a combination of the two, usually a straight average ((R +M)/2), which is marked on the pump.

How accurate that is, well, who knows? I don't know what the tolerance is, but it is bound to be reasonably substantial.

In any case, it doesn't have to affect the energy content, but often does. It surely does affect the way the stuff works in an engine.

if there is a lot of alcohol in the fuel, the octane number goes UP, since alcohol has a research octane number around 130. but the energy content dives, because ethanol has considerably less weight per unit volume.

Volume is the stupidest possible way to sell fuel, from the consumer standpoint.

tlfamm
04-15-2010, 09:26 PM
Rudolf Diesel's early experiments used powered-coal as fuel: not surprisingly, erosion was a problem and he switched to kerosene, and later to oil.


And here's something new to me:

" ... while crossing the English Channel on the way to consult with the British Admiralty, Diesel disappeared at sea on September 30, 1913. His body was never recovered and suicide is considered to have been a possible reason for his death as he was known to be emotionally unstable and given to occasional breakdowns."

http://www.madehow.com/inventorbios/11/Rudolf-Christian-Karl-Diesel.html

tdmidget
04-16-2010, 12:41 AM
CCWKen: What is the difference between "coal oil" and "kerosene"? NONE!
This is a subject with more mythology than ancient Greek history.
Want 100% octane? here it is:
http://www.chemsavers.com/servlet/the-3216/Octane,-Reagent,-250ml/Detail
At roughly $200.00 /gallon, here it is. Gasoline , like all petroleum products , is based on performance , not content.

CCWKen
04-16-2010, 01:28 AM
Officially, this:

Kerosene
Kerosene, also known as paraffin oil, is refined by the distillation of cannel coal or crude oil. It was patented in 1855, and the first oil refineries produced kerosene.

Coal Oil
Although now used interchangeably with kerosene, coal oil is refined shale oil. It is produced by the distillation of oil shale, a sedimentary rock with kerogen, and has been used a a fuel and lubricant (mineral oil) for centuries.

In the teens, the term coal oil and coal tar was used in slang to describe the thick tar-like run-off from leaking oil derricks. It was basically crude oil and black; hence the "coal" reference. It was unrefined oil. Farmers use to collect it buckets and use it for fuel. The oldtimers know what I meant. I guess I should have said "crude oil" for you gen-x crowd. Or is it gen-y now? :cool:

wierdscience
04-16-2010, 01:35 AM
That is almost dead wrong.
Diesel's efficency comes most part from the high compression and the (ideally) isobaric expansion cycle.


Nick

And don't forget the greater energy density of the diesel fuel itself.

MuellerNick
04-16-2010, 05:49 AM
Rudolf Diesel's early experiments used powered-coal as fuel: not surprisingly, erosion was a problem and he switched to kerosene, and later to oil.

That's not true. Initially he worked with crude oil (mainly from Russia, they had a very light oil). He later tried it with coal dust, but that was after he got the Diesels running.

By far the very best book about the development of the Diesl engine is Lyle Cummin's book "Diesel's engine Vol. 1". I think it is out of print, and you'd have to pay $$$ to get a used one.


Nick

strokersix
04-16-2010, 08:38 AM
http://www.mechadyne-int.com/vva-reference/images/pv-diagram-for-a-throttled-si-engine.png

Here is a PV diagram for a throttled spark ignited (gasoline engine). Note the size of the pumping loop versus the size of the power loop. At WOT the pumping loop becomes much smaller and the power loop bigger.

MuellerNick
04-16-2010, 09:50 AM
Note the size of the pumping loop versus the size of the power loop.

That diagram is nice and shows, that there are pressure-differences. It doesn't show losses. For that, you have to compare specific consumption maps.

Finally, that site (http://www.motorlexikon.de/?I=2604) is up again. In the upper right, you find two typical maps. Top: Otto, bottom: Diesel.

Don't compare the absolute numbers, but the relation between full throttle and chocked. This way you can see how big (or small) pumping losses are.


Nick

Willy
04-16-2010, 11:51 AM
Creating a vacuum requires power, these are the pumping loses that I have been referring to. While all engines have a certain pumping lose associated with the movement of air, air throttled engines have a significant restriction in their intake tract at part throttle which inevitably requires a certain amount of horsepower to overcome.

These pumping loses are a parasitic drag that diesels do not have. Although you refuse to acknowledge these basic fundamentals Nick, they are very real. By working around these intake restrictions real improvements to overall efficiency can be realized.

From an article in 2004 in which Daimler/Chrysler, GM, Honda, and BMW do agree with the very real improvements to be gained be reducing or eliminating the pumping loses that you say are irrelevant.

http://findarticles.com/p/articles/mi_m3012/is_10_184/ai_n6261226/




Since diesels don't use a throttle to meter intake air, they avoid the pumping loses which diminish a gasoline engine's efficiency. Just this year, DaimlerChrysler, GM and Honda engineers began incorporating that very feature in their gas engines. Disabling half the cylinders during cruising necessitates a wider throttle opening to feed active cylinders with the air they need to maintain equivalent power. Reduced pumping losses can boost efficiency by ten or more percent.
BMW uses an alternate approach. With its Valvetronic concept, incoming air flow is regulated solely by varying the amount of intake-valve lift. Throttles are dispensed within BMW 6- and 8-cylinder gas engines to avoid the pumping losses they impose.



Edited for spelling and grammar, Nick your English is better than mine!:D

Evan
04-16-2010, 01:31 PM
In city driving engine mechanical losses account for about 20% of the total losses in converting energy to motion. About 40% of the losses are unavoidable since they are a consequence of the Carnot limit which applies to all heat engines that don't operate at absolute zero.

At idle the losses are 100% due to engine losses since no other factors are operative. A diesel will idle while using less than half the fuel that a petrol engine uses. This is because a diesel does not require a premixed air/fuel mixture that is within the upper and lower explosive limits. Instead the fuel is sprayed into the hot compressed air and some of it will be within the correct proportions to ignite.

A gasoline engine must supply a charge to the cylinder that is pre mixed to the correct air/fuel mixture. This is controlled by restricting the airflow on the intake with a throttle plate. In order for the engine to run it must develop enough power to pull almost a full vacuum in order to obtain sufficient air to mix with the fuel.

This also applies to operation when moving with the engine operating at low throttle, such as coasting up to a traffic light, going down a hill or stuck in slow moving traffic. Under these conditions the overwhelming majority of the efficiency losses are engine losses and the majority of those engine losses are pumping losses in a gasoline engine.

If you look at the fuel consumption rating for city driving vs highway driving for a gasoline engine vs a similar sized diesel engine you will find that the diesel loses much less efficiency than a gasoline engine. This is seen in a smaller difference between city/highway numbers for the diesel. The reason is entirely due to engine losses and those are almost entirely accounted by pumping losses.

saltmine
04-16-2010, 02:30 PM
Rudolph Diesel experimented with coal dust, and gunpowder just like most early engine designers. Both are difficult to keep airborne, and cause inconsistent combustion and corrosion. His first "diesel" (compression ignition) engine was actually designed to run on peanut oil. But, due to the expense of peanut oil he later switched to kerosene, then to light crude oil (which later carried the title of "diesel fuel".

What I find surprising is the fact that almost no inventor, at the time, attempted to use LNG for fuel.

The book "Diesel's engine Vol.1" was written by Clessie Lyle Cummins, and yes, I believe it's out of print.

Rudolph Diesel was a victim of unscrupulous businessmen of the day. Much like many of the period's inventors, their patent rights were stolen and most of the time they were left penniless and despondent. Rumors of his suicide or murder probably had a lot to do with his business dealings.
Louis Chevrolet had a similar experience with the fledgling GM company, and lost the right to use his name on products. Chevrolet worked as a mechanic in the GM factory he founded for most of his remaining years, before succumbing to pneumonia. He also died penniless.

BTW, disabling the valve train to control the number of cylinders operating was tried, first by Ford, in the late 1970's, then GM in the early 1980's. Ford's design never got past the prototype stage, mainly due to loss of control of the valve train because of sticking actuators. The GM attempt did make it to production, much to GM's embarrassment, as the Cadillac 4-6-8. Truly an automotive disaster.

Seastar
04-16-2010, 03:53 PM
I am a pilot who flys a piston aircraft that requires 100 octane low lead gasoline.
The EPA has proposaed eliminating lead in aviation fuel as a result of a petition by the "Friends of the Earth" or some such.
As a Purdue Engineer I have been aware of and watching this development by some people at Purdue:
http://www.aopa.org/sunnfun/2010/100413swift.html?WT.mc_id=100413special&WT.mc_sect=sfn
Note that this biomass fuel is heavier than normal avgas and has 15% more energy content. It has a motor octane rating of 104.
I am trying to get a tankfull or two to use in my highly instrumented expermental 180 HP aircraft. My engine has a 9.2:1 compression ratio.
Bill