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I've wondered this for a while, and the usual "diesels don't need it" is not a real answer. I did some google-fu this morning, and thought about it, and wrote a layman's explanation. Then, as usual, I kept crawling down the rabbit hole. Anyway, it was useful to me. Someone let me know if I'm out of whack. Hopefully it A) is right and B) helps a fellow idiot.
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Short Answer:

Diesels do not care about A/F ratio, as excess air does not significantly effect combustion. Torque production is controlled solely by controlling fuel delivery. Therefore, an air throttle is not needed.

Gasoline engines must maintain a narrow AFR to operate, and therefore must control both air and fuel. However, the carburetor or FI handles the fuel automatically, so from the operator's perspective output is controlled by air throttle (the butterfly valve).
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Long Answer:

The operator doesn't directly control engine speed. You indirectly control engine torque production, which is countered by intentionally applied load, and unavoidable internal frictional losses. The engine's speed simply rests wherever those torques balance. And, you indirectly control torque by controlling the size/energy of the combustion in the cylinders (the "boom"). This "boom" is the rapid volumetric expansion of the fuel/air mixture in the cylinder as it combusts, which releases heat and generates new gases. In a traditional ICE, no matter the fuel, the combustion of fuel/air mixture is a "deflagration," a relatively slow and controlled progression of flames.

Gasoline engines are spark ignited. In this case, the air/fuel mixture is already present in the cylinder, but not hot enough to burn. Combustion begins when the spark plug zaps (the electric arc itself is very very hot). This "premix flame" progresses outward from the ignition source, as the heat from one gasoline molecule's combustion heats the next to it's point of ignition, like a chain of dominoes. Though of course even a single molecule of gasoline in air will combust if brought to temp, it is possible for the mixture to be so lean or so rich that it cannot support this natural propagation of flames, and the domino chain breaks or does not begin at all. (A scientific measure of these characteristics, albeit under specific conditions, are the substance's Upper and Lower Flammability Limits.) Because of this and various mechanical limitations (temperature, power, fouling or burning of plugs, etc.) gasoline engines only operate in a rather small window of AFR around 11-16:1. Therefore, controlling the "boom" technically requires controlling both air and fuel. However, half of that process is taken care of for us by the carburetor or ECU, which delivers the appropriate amount of fuel based on incoming air. Overall: the operator changes airflow by turning a butterfly valve, an automated system supplies the appropriate amount of fuel, the "boom" changes, and engine torque production changes.

Diesel engines are compression ignited. They work because physics says that compressing a gas generates heat. (That's why your garage air compressor gets so hot when running.) Most diesels have compression ratios around 17-22:1, and the air is hot enough just from being squeezed, even at idle speeds, to successfully ignite any normal amount of diesel fuel in the cylinder, from a tiny spritz to a good squirt. (Technically, the compressed air is hot, and the fuel is ambient-ish until injected in the cylinder, where it heats up by drawing heat from the compressed air, and eventually becomes hot enough and ignition occurs.) Diesel fuel still has a UFL and LFL, but because the air all around is hot enough to cause ignition, all the diesel exposed to air immediately begins to combust, with only unvaporized fuel having to wait in a domino line. That domino line takes the form of a "diffusion flame" which travels inward with respect to the surface of liquid streams or droplets, as the outer molecules combust and expose the inner molecules to hot oxygen. The extra unburnt air molecules do not hamper the ignition event, and are simply pushed out with the rest on the exhaust stroke. The result is that a diesel engine can successfully operate at very high AFR, up to 160:1 on some turbocharged motors at idle. Seemingly the limit is more about control over such small amounts of fuel delivery, and the minimum force needed to turn over the motor. Therefore, to change the "boom," all you need to do is change the amount of fuel. Overall: the operator directly commands change in fuel delivery (the pedal either pushes part of a mechanical injection system, or tells the computer to add fuel), the "boom" changes, and engine torque production changes.
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Extra Information:

"Subsonic" means traveling slower than the speed of sound [~767mph].
"Supersonic" means traveling faster than the speed of sound [~767mph].
"Exothermic" means a chemical process that releases/generates heat.
"Pyrolyzed" means the material has been subjected to high heat in an inert environment, breaking down chemical bonds without actual 'burning.'
"Charcoal" is pyrolyzed wood product (i.e. the black chunks in your fire pit, or compressed stuff bought in a bag for grilling, although with added chemicals). What most people call 'coals' are actually these.
"Coal" is a solid fossil fuel dug from the ground (i.e. coal mine).
"Coke" is pyrolyzed coal.

"Combustion, or burning" is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. [Wikipedia] << This is the fundamental process, and may or may not include fire or flames! >>

"Smoldering" is the slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. [Wikipedia] << Glowing red embers in your charcoal BBQ grill. >>

"Fire" is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. [Wikipedia] << This also may or may not include flames. There actually CAN be fire without flames! >>

A "flame" is the visible, gaseous part of a fire. [Wikipedia] << If the fuel was already a vapor or gas, then burning it would immediately make a flame. But, for liquid or solid fuels (like wood), if the burning process is hot enough, it will vaporize the fuel closest to it, which creates more flame, and more vaporizing, etc, causing a continuous flame. >> << Interestingly, coke (see above) has no gas product because of the pyrolysis; it's all been baked out. Therefore, while it does burn - and very hotly! - it does NOT produce flame. >>

"Deflagration" is subsonic combustion propagating through heat transfer: hot burning material heats the next layer of cold material and ignites it. Most 'fires' found in daily life, from flames to explosions such as from black powder, are deflagrations. [Wikipedia]

An "explosion" is a rapid expansion in volume associated with an extremely vigorous outward release of energy, usually with the generation of high temperatures and release of high-pressure gases... Subsonic explosions [can be] created by low explosives through a slower combustion process known as deflagration. [Wikipedia] << Note that an explosion is NOT a type of combustion, but is a physical effect that can be generated in many ways, the most common of which is high speed combustion. Because of this common relation, some types of quickly-combusting materials are categorized as 'explosives.' >> << The precise distinction between explosion and non-explosion seems unclear to me, and I have not found a source to clearly define it. However, in keeping with most of the comments and general attitudes I have seen in technical explanations, I believe the expansion rate and energy release rate of air/fuel mix in a gasoline motor does NOT qualify it as an explosion. >>

"Detonation" is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it... Supersonic explosions created by high explosives are known as detonations and travel through shock waves. [Wikipedia]

"Ignition" I couldn't really find a specific definition/explanation for, so I will interpret it to mean the initiation/beginning of any combustion process, regardless of means.
 

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I like your write up, nice job. I'd like to add something, and possibly simplify the matter a bit.

A gasoline engine needs the butterfly valve in the intake to create a negative pressure, or vacuum in the intake. Gasoline is sucked into a cylinder, along with the air, through the intake valve. ( Be it from a carburetor, throttle body injector, or injector located in the intake manifold.) Then this mixture is compressed and ignited by spark.

Now a diesel, relies on atmospheric pressure* to fill the cylinder with air, and fuel is delivered directly into the combustion chamber, where it is compressed to the point where it detonates.

I know it leaves a lot out about fuel trim and timing and what-have-you, but it's basic, and yes there are variations and caveats to this explanation.

*Excluding diesels such as a Detroit which require forced air to run.

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Both require oxygen to run. Fuel. Air, and heat or spark. Without 1 of the 3 you just flood the cylinder and hydrolock it


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Dunc, you might be interested to know that some diesels back in the day did have devices that looked like "throttle butterfly valves". Toyota had a line of six cylinder diesel engines in their LandCruisers sold in Oz and NZ back in the '80s (maybe elsewhere as well as Japan) and the "throttle" was directly linked to the gas pedal, and worked in conjunction with a pneumatic governor, that controlled the injection pump. Obviously nothing to do with actual fuel handling but the first time I saw it, I had a "what the???" reaction : ) Seemed to work pretty well - one could feather the throttle quite nicely and it weirdly made the engine feel like it was a gas engine. If it wasn't for the typical diesel rattle, one could have been fooled into thinking it was
 

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Both require oxygen to run. Fuel. Air, and heat or spark. Without 1 of the 3 you just flood the cylinder and hydrolock it
without fuel you won't flood the cylinder ;-)
 
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