If I understand correctly, small planes like the Cessna 172 use 100 octane leaded gasoline with no ethanol. What would happen if you put regular unleaded car gas with ethanol in one?
First, it's important to note that the octane rating for aviation gas has nothing to do with the octane rating for car gas. This is because octane isn't calculated, it's measured. A specialized piston engine is run on the gas, and the compression ratio is increased until knocking occurs. The compression ratio is compared to a reference fuel, and that's the octane number. Because the Standard Test Engine is different for avgas compared to mogas, the octane number for those fuels isn't directly comparable.* It is generally true that a higher octane in an automotive context corresponds to a higher octane in an aviation context, but even that's not guaranteed.
Because of this, you can't know how well the fuel will resist knocking and pre-ignition when in an aviation engine. If the fuel just happens to have a high-enough anti-knock rating, it will most likely run just fine. But if it doesn't, then all the problems associated with knocking will occur, including damage to the pistons, piston rings, head gaskets, and even eventual destruction of the entire engine.
*Imagine trying to assign a single score to a bunch of runners, to decide which one is "best", based on a single test. Well, some people have good endurance without having a high top speed, and so would be good in a marathon but not at sprinting. So, the score those people would have is based on whether you run your test with a sprint or a marathon.
It depends on the engine type and the fuel the engine manufacturer has designed it for.
In the Cessna 172S PIM, it says:
Use of unapproved fuels may result in damage to the engine and fuel system components, resulting in possible engine failure
100 LL (Blue) and 100 (Green) are both approved for use on the 172. 100 fuel is used on some higher performance cars, so yes, the 172 can use "car gas", but only a specific type.
Now, if you use the "standard" car gas that you find at the gas station, you would most likely get detonation which is when the fuel-air mixture in the cylinder explodes instead of combusting normally. This is because the lower octane fuels are lighter molecules and vapourize a little more easily than higher octane fuels will. Because they vapourize more easily, they will combust/detonate much faster and possibly earlier than the higher octane fuel will.
There are also other engines that are designed to use diesel such as the Austro E-4 and Continental CD-230.
It depends. If the anti-knock rating is too low for the engine conditions then you will get detonation, which both limits power and causes major internal engine damage. ("octane" is a common term for one anti-knock rating system.)
Ethanol is highly polar and causes problems with moisture absorption that then turns into either ice crystals in the fuel lines or separation of the fuel in the tank into an aqueous layer, that will outright stop the engine, and a very low octane petroleum layer even worse for detonation than the fresh fuel. Water isn't attracted to 100LL and will settle out for easy removal without changing the chemistry of the fuel.
There is also a difference in the vapor pressure of the two fuels that can create bubbles in the fuel system and starve the engine.(Altitude, temperature, and fuel pressure dependent issues.)
Longer term. The seals and materials of the airplane fuel system may or may not be a material compatible with the ethanol formula. Ethanol is added as an oxygen carrier, along with the moisture problems this oxygenation greatly reduces storage life to only a few months in most climates and causes gums and gunk formation during storage; 100LL has anti-oxidants added and has a shelf life of a couple of years in storage. Ethanol is also highly correlated with metal corrosion of fuel systems parts during storage.
The small amount of lead in 100LL is needed to prevent damage to the exhaust valves of some engines. (Based on the alloys they used) This is due to micro-welding under heavy load [hot valves] which results in accelerated erosion of the valve seats, lead makes a protective coating to prevent this. This same coating will damage sensors and catalytic converters in many engines designed for unleaded fuel. The older high-lead green 100/130 fuel also used lead as an effective primary anti-knock ingredient.(allowing a cheaper base fuel)
The actual anti-knock rating needed depends on several factors, some C-172 used engines rated for 80/87 red avgas and they would likely be ok with the antiknock rating of highway ethanol 92, but others were made for or upgraded for 100/130 or 100LL which is approximately 105-110 on the highway anti-knock scale. (All USA rating systems, Europe is different.) And of course, you still have all those other hazards with ethanol-gasoline formulations. Pure ethanol is a different kettle of fish completely, the above info is for blends of petroleum and ethanol.