I find it funny that at gas stations, generally you can purchase "regular" gasoline (in various octane levels) or diesel. At airports, generally you can purchase 100LL avgas or Jet-A.
Can you compare diesel to Jet-A, and "regular" gasoline to avgas? If so, what are the differences? Why are there only two classes of fuel - are there others that I don't know about?
Broadly speaking, there are three families of motor fuel that you're probably interested in: Diesels, Kerosenes, and Gasolines.
The difference between the families mainly has to do with the molecular weight (and thus boiling point / vapor pressure) of the fuel components - in order above from heaviest to lightest. The differences within each family are related to refining, blending, and additives, as well as process controls imposed on the fuel.
In the diesel family we have things like Highway Diesel and the various grades of home heating oil.
For transportation purposes these fuels are used in compression-ignition engines ("diesel engines"), where a high temperature and pressure will cause the fuel to ignite and burn, producing power.
For other purposes (like heating) we basically light the stuff on fire and are happy it burns.
The Kerosene family are close cousins of the Diesels, including K-1 and K-2 Kerosene (obviously) and Jet-A.
Jet-A is a high-purity kerosene based fuel manufactured under a specific ASTM standard (D1655), with specific physical properties, and is used in jet turbine engines. Fuel not meeting the Jet-A specification is generally recycled in the production chain for other purposes (for example it may be used in blending kerosene for heating, or blended into highway diesel).
Jet-A is suited for turbine engines, but can also be burned in other compression-ignition piston engines like diesel can (there are a wide variety of aerodiesel engines available from several manufacturers).
Other grades of Kerosene are used for motor fuels, cooking fuel ("camp stove fuel" is typically kerosene) heating, lighting, etc. depending on their refining and purity.
In the gasoline family we have Avgas, Autogas, and "Gasohol", all designed for use in spark-ignition engines.
Avgas actually comes in several grades, all manufactured to an ASTM standard (D910), with specific physical properties and specific permitted and required additives (such as Tetra-Ethyl Lead - TEL).
The grade (octane rating) of aviation gasoline is identified by colored dyes added to the blend.
The colors used in the US are:
Generally 100LL is the most widely available aviation fuel because it works in the widest range of engines.
Automotive gasoline comes in multiple octane grades, but generally it's all varying blends of "regular" (87 octane) and "premium" (93 octane or higher).
Automotive gasoline is manufactured under a different ASTM standard from Aviation gasoline (there are several applicable standards, D4814 is commonly used), which permits different additives including ethanol (usually up to 10%).
Gasohol is a slang term for automotive gasoline with any amount of ethanol blended into it, but these days it's often used to refer to blends like E85 (85% Ethanol, 15% gasoline stock).
Flex-fuel car engines and a few other specialty engines are designed to burn this fuel.
Why don't we burn highway diesel or heating kerosene in a 747?
In a word, purity. The additional controls imposed on Jet-A fuel by the associated standard produce a product with well-known properties. When you fuel a plane with Jet-A you can be confident that fuel won't freeze in the tanks at altitude (at least not unless you cold-soak it below -40 degrees). You also know the fuel will be "clean" and won't gunk up fuel filters or leave deposits inside the combustion core of the engine that can cause problems later.
On the other hand we don't burn Jet-A in a Mack Truck because it would be prohibitively expensive: Highway diesel doesn't have to meet the same strict tolerances as jet fuel, and theres no reason to burn a more tightly specified (and thus more expensive) fuel when a less expensive one is readily available.
The difference between Aviation Gasoline and Automotive Gasoline is a bit more subtle: the core blend stocks are extremely similar, but the additive packages are very different.
The biggest concern for aviation users is usually the ethanol commonly blended into automotive gasoline: Rubber components in aircraft fuel systems were not designed to handle alcohols, and can be damaged/degraded by ethanol.
For cars the concern is the lead found in aviation gasoline. Any "modern" car (with a catalytic converter and oxygen sensors in the exhaust system) can't burn leaded fuel as the lead byproducts in the exhaust will foul both the catalyst matrix and the oxygen sensors (though a 1950s Plymouth Belvedere would have no trouble burning the stuff).
Work on an unleaded aviation fuel is ongoing - theoretically the fuel that comes out of that process could be equally suitable as a road fuel and an aviation fuel, however the chances of it making its way to your corner gas station are relatively slim, except possibly as a blend stock from batches that don't meet the final specification for a 100-octane unleaded aviation fuel, but are suitable for recycling into automotive gasoline blends.
There have been several fuels used historically through aviation. I'll start with the common fuels here, and then cover fuels that are purely of historical interest.
100LL -- The garden-variety "avgas" used by most piston-engine aircraft. Similar to a superpremium gasoline in formulation (lots of high-octane alkylate!) with a small amount of tetra-ethyl lead (TEL) added to it (the LL stands for "low lead"). Despite its formulation, it plays an analogous role in the aviation (esp. GA) world to the gasoline we put in our cars.
87UL/E0 (similar fuels are called 82UL or 85UL) -- This is a tightly specified form of motor gasoline (mogas) -- without any ethanol or oxygenates in it, as many aircraft fuel systems aren't compatible with ethanol -- that is used in low-compression aviation piston engines mounted in airframes certified or STCed for operation on motor gasoline. Many light (SLSA) and ultralight aircraft, as well as some certificated types, can use this as it can be cheaper than 100LL; however, it's rarer to find it on the ramp, while 100LL is ubiquitous in the US and in other places where spark-ignition GA engines are common.
Jet-A/JP-5/JP-8 -- Jet fuel, or turbine fuel in some contexts. It is a highly refined kerosene that can be burned both in turbine engines and in diesel piston engines; spark-ignition engines can't use it though. Additive packages (Prist™ or generic fuel system icing inhibitor (FSII), static dissipative additives, biocides, and other additives) are commonly found in this fuel. It can be found anywhere commercial aviation is present, and thus is available worldwide. Role-wise, it is analogous to commercial diesel fuel, but furthermore can be substituted for diesel in some situations (the military runs diesel engines on JP-8 all the time, but many jet fuels require a lubricity additive to be used successfully in diesel engines not purpose-designed to run on jet fuel). JP-8 and JP-5 are the military kerosene-based jet-fuels (JP-5 was the original naval jet fuel, with JP-8 being a modern derivative suitable for tri-service use).
Jet-B/JP-4 -- This fuel is what is known as a "wide-cut" jet fuel, with properties akin to a mixture of gasoline and kerosene (it contains hydrocarbons from both fractions). It was used in early jet aviation as it is easier to refine and ignite than Jet-A, but it has poorer safety properties than Jet-A. It can be found in extreme northern climes where Jet-A begins to crystallize or gelatinize. JP-4 is an older military jet fuel type similar in character to Jet-B, but it was replaced by JP-8.
Propane/Liquefied Petroleum Gas (LPG) -- This is the same stuff you buy tanks of for your gas grill. It is the fuel of choice for hot-air balloons, as it burns cleanly and lights off reliably with only a low-power ignition system fitted (such as a piezo sparker).
80/87 -- A low-octane leaded gasoline, akin to the leaded motor gasoline of bygone days. Aircraft don't need this any longer -- you can always put 100LL into a plane that would otherwise use 80/87, and most low-compression engines nowadays burn 87UL instead.
100/130 -- The predecessor to 100LL, with twice the lead content -- this was a common fuel for high-compression piston engines in the past; all such engines burn 100LL now though.
115/145 -- This super-specialty leaded fuel is used for piston-engine air racing and other applications where maximum performance is needed from forced-draft, very-high-compression piston engines such as those found in WWII era fighter aircraft. As others have said, it's only seen in the occasional truckload headed to Reno.
JP-7 -- A super-high-flashpoint jet fuel formulated for the SR-71 Blackbird, but no longer made due to the discontinuance of the Blackbird program. It cannot be used in ordinary jet engines (it won't ignite reliably).
RP-1 -- An ultra-refined kerosene, used for rocketry. Can be substituted for jet fuel, if you don't mind it costing more than ancient French wine on gallonage basis!
In general, 100LL avgas compares to Regular Unleaded and Jet-A compares to Diesel.
There are other types available - 80/87 Avgas (tinted red) used to be the equivalent of Regular Leaded and 100/130 (tinted green) was Premium. These fuels are rather difficult to find now, and in some areas simply unavailable. 115/145 Avgas is like Formula-1 racing fuel. A refinery might make a few truckloads on special order.
Jet-A is basically clean Diesel, you can put it in your tractor or big truck with no harm to anything other than your wallet.
Jet-B is regular jet fuel mixed with gasoline. Starts better in very cold weather, but also burns really, really well if you crash.
JetA is indeed a kind of diesel fuel prepared for use in turbines. There are a number of other jet fuels, such as JP1, JP2, etc, but these are fairly rare, since they are usually used for military turbines.
100LL is fuel for piston engines and is formulated much like car gas, with some chemical differences and a higher octane (100). Some engines can use 87 "mogas" buy they are mostly small engines and some converted ones.
Even if 75 years old, this SAE paper 430113 contains useful information for those alien to the refining technology:
• A Refiner's Viewpoint on Motor Fuel Quality SAE Technical Paper 430113
W. M. Holaday, John Happel
Of the many gasoline characteristics in which the consumer is interested, a limited number are subject to close control by the refiner. The most important of these -antiknock quality, vapour pressure, and distillation- are controlled largely by the requirements of modern automotive engines as determined by road tests and by customer acceptance. The various processes which the refiner uses to control the properties of his gasoline involve to a considerable extent the complete rebuilding of crude oil molecules which form the starting point of the refining process. The refiner's problem includes not only the manufacture of gasoline to meet a variety of automotive requirements, but the production of sufficient quantities of fuel with the greatest economic efficiency.
