Moscow Airport Bombing

[Swatopluk]

I'd have to try to find the info somewhere. I believe I learned that in an out-of-curriculum lecture at the university but that is by now some years ago.
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The 'nitrogen-enriched' could also simply mean that Shell put some bio compound in there (more common in diesel though). That would not actually boost the engine.

Edit: According to this the new fuel introduced between the Battle of France and the Battle of Britain meant a switch from 87 octane to 100 octane but does not say anything about the fuel chemistry.

Edit 2: It seems that there was not one 100 octane fuel but several competing blends (on of them by Shell) using different methods to get the octane number up. But the one website that seems to go into details provokes my computer to warn me that visiting it could harm my system (no details).

[Bob in a quantum-state-of-faith]

Interesting, Swato.

Back in the day, when I was still a chemistry major, I did a stint in organic chem.   Obviously, to keep interest, we discussed at length, petroleum, one of the most useful organic chemicals humans use on a regular basis (aside from actual food, obviously).

I was told then, that "octane" once was a measure of the percentage of 8-carbon hydrocarbon molecules, hence the name "octane".   Gasoline is a blend of various lengths, ranging from 10-12 carbon to as few as 4 or 5 (although those being light, tend to evaporate away).  In addition, there are often benzine (ring molecules) and benzine-like chemicals as well.  But those are seen as undesirable, as benzine is a known carcinogenic.  

Then there is a plethoria of non-organic additives.  The original king, was tetraethyl lead.  The "lead" in leaded gasoline.  It was added strictly as a lubricant-- it did not burn up in the combustion process, but vaporized, and coated the exhaust valves improving engine life.   The vaporous chemical went straight out the tailpipe, and into the nearby environment-- it typically condensed rather quickly, and even now, pick a random stretch of high-traffic roadway, if you test the soil in the ditches, you'll find high concentrations of lead....  especially in the plants... (why it is never a good idea to give highway hay [grass mown from roadway right-of-ways] to food animals...).   But this compound had no effect on the "octane rating".   Fortunately, it was replaced with other non-lead chemicals in the late 70's and finally phased out completely.

Nowadays, the "octane" rating has little to do with the actual percentage of 8-carbon hydrocarbons, but instead is a measure of gasoline's resistance to pre-detonation.  Since gasoline is specifically formulated to be ignited by a spark (in contrast to diesels), you don't want the fuel spontaneously burning from hot engine parts.  Octane is a measure of the fuel's resistance to this sort of pre-detonation.  The higher the octane, the more resistant it is to unwanted ignition.

Contrary to popular misconception, high octane gasoline has no more energy than low octane-- the energy content is the same.  But high octane, with it's additional stabilizing chemistry, can operate in an engine with very high compression (and thus, more efficiency) without igniting prematurely.   In long-stroke airoplane engines, I could see how high octane would be a must.  The higher compression would also offset some of the high-altitude problems, and help with powerloss in thinner atmospheres.  But the usual answer for that, was supercharging-- and at higher altitudes, you'd up the speed of your supercharger to compensate.

Which, again, brings us to a need for higher octane-- supercharged engines typically operate at much higher cylinder compressions than naturally aspirated engines do.

So why do some people complain that low octane gasoline gives poorer fuel economy, that the more expensive high octane blends?  

Easy:  modern engines have a knock sensor.  Whenever it detects engine knock (pre-detonation), it adjusts the spark timing to compensate, which reduces efficiency.  It does this, by firing the plug a fraction of a second earlier (spark advance), igniting the fuel mixture earlier on the power-stroke than would give optimum economy.   So by switching to a higher octane, the spark can be retarded dramatically, improving economy.  

Not because of more "fire" in the gasoline...



One of the more interesting additives in gasoline, is detergent.  This stuff is chemically similar to your dishwasher's soap or modern laundry cleaning agents.   It has a variety of useful benefits, the most obvious one is that it permits the various chemicals in the gasoline to stay combined, and not separate out.  That it also picks up dirt (and holds it) from the fuel system is just a side effect.   But adding detergents also means they have to add in anti-foaming agents, to keep the gasoline from forming soapsuds in the tank, when sloshed about.  You can't pump soapsuds with a liquid pump...

... modern gasolines contain a whole bunch of chemicals which have nothing at all to do with combustion.   What is not known, is the effect of these chemicals on the environment, as most of them survive the combustion process, and are ejected as vapor. 

To my knowledge, nobody is testing for these things at the tailpipe...

... we really, really need to switch to either hydrogen power, or pure electric cars...

[Swatopluk]

The most 'simple' way (nothing simple in the technical details) is to inject water into the combustion chamber (the detergent works here as an improvement).
Superchargers were added to the airplane engines but not from the start.
There were experiments with diesel engines for airplanes.
It's remarkable how short the working life of military airplane engines was at the time. It even dropped duringt he war. Although the German jet engines were of a better design than their British counterparts they had to be built from inferior materials due to supply problems. Iirc they could be run safely for less than 20 hours in total. On the other hand the ramjets for the V1 were deliberately built so cheaply that they would not last much longer than the fuel in the tank..
As far as the energy is concerned, few people know that high explosives usually have a far lower energy content than the same amount of gasoline or coal.
It's not about the energy but the speed it can be released.

[Aggie]

As far as the energy is concerned, few people know that high explosives usually have a far lower energy content than the same amount of gasoline or coal.

IIRC, those spectacular fireball-type explosions one sees on action movies are made using a small amount of high explosive and a large amount of gasoline or other fuel.  The HE is just to disperse the fuel, which provides the 'boom' and the spectacle.

[Swatopluk]

gas explosions are even more powerful but look less spectacular

[Aggie]

Gotta love BLEVEs for the boom-factor.

[Swatopluk]

Mixing water into the burning oil mixture also makes the fireball far more impressive

[Aggie]

Are you trying to get men in dark suits to visit? 

I used to get some interesting flares full of tiny flames from a burning and boiling can of paraffin wax, by dropping in water or water-containing material.

[Swatopluk]

My experience comes from a safety lesson at the university. The effect is also know as: what happens, if you try to extinguish the fire in your frying pan by pouring water on it. (Answer: no fire in the pan anymore but that will be about the only thing in your kitchen not on fire)

[Sibling Zono (anon1mat0)]

You mean like this?

http://dsc.discovery.com/videos/mythbusters-water-and-oil.html

[Aggie]

Oh, yes - we used to get some good fireballs in the kitchen by deglazing pans with wine over gas burners.  We had an open kitchen at the front of the restaurant, with nearly floor-to-ceiling windows, so we'd get people stopping to watch.  Made me feel like a monkey in a zoo....

That was probably a couple of tablespoons of oil and wine; I think the flames were partially potentiated by the alcohol, but mostly it was the oil burning that made the flare.  I'm talking about 1 or 2 ft flames of very short duration, not much of a burn hazard.

[Bob in a quantum-state-of-faith]

As for those famous Hollywood orange-ball explosions for cars?

I've been told that it's a mixture of gasoline and diesel fuel.  The diesel by itself is hard to get to burn spectacularly, but mix in a bit of gasoline, to increase it's vapor pressure (willingness to vaporize).  And yes, a charge of black powder wrapped in foil or other tight package, is often the trigger-- it blows the diesel/gasoline mixture out into a large plume, and also ignites it.  Usually detonated by an electric trigger, so they can make it blow on cue.  After the cameras are rolling... of course.



It's a little known "Hollywood fact" that any car, when being filmed for a movie, if it goes over a cliff or off a bridge, will spontaneously explode shortly after leaving the edge of the cliff/bridge....

 

Another little known "Hollywood fact" is that if you shoot any car in it's gas tank, it will explode in a spectacular fireball.   But only after the final scene is caught on film...

 

[Aggie]

I was tempted to take a course on this once - I did my Display Fireworks training and apparently they offered a Special Effects course at the same time.  Never got around to applying for my final fireworks license, although I did enough shows to qualify.

[Swatopluk]

Even dragons use gasoline in the movies. When I saw Lindgren's The Brothers Lionheart for the first time, I almost laughed when Katla started to spew fire for the nature of it was so obvious. At least the nozzle of the flamethrower was not visible in the dragon's mouth as in e.g. At the Earth's Core.

[Sibling DavidH]

When I was a kid we occasionally built a small fire well away from anywhere and put a full can of gas lighter fuel on.  Sometimes they'd just split and whoosh, but mostly they made a huge, satisfying bang.