I see the disconnect here. We are basically saying the same thing, from two different perspectives using very similar terminology with different practical understandings of their meanings.
You are talking about the physical mechanics of diesel engines you have worked on. I am talking about the chemical reaction taking place inside the engine. Your diesel engine might not care about the mixture range
it was designed to operate within, but the combustion reaction occurring in each cylinder absolutely cares about mixture and the engine designer has taken care of that in the fueling system specs. Mixture is controlled, but just not to the stoichiometric mixture. I don't know what your definition of "stoichiometric" is, but it doesn't seem to align with the definition commonly used by scientists and engineers in the field.
There is a range of air-fuel mixtures that can support a combustion reaction for any given fuel. For diesels, the air-fuel flammability limit is pretty wide...ranging from about 3:1 to 42:1. Stoichiometric is about 14.7:1. Compression ignition engines are assumed to be constant-pressure and typically have a stratified air-fuel mixture. As such, they typically operate with excess air (that's not the same as "lean"), but combustion always occurs at
near stoichiometric conditions, because the fuel will only combust when sufficient oxygen is present at the flame front. And the fuel will continue to burn as long conditions are suitable within the combustion region.
Any air/fuel mixture ratio above the stoichiometric value is lean. And because we have already established that the mixture limits are pretty wide, and combustion occurs near stoichiometric conditions, it's pretty easy to conclude that diesels can and do operate with more oxygen than required for the mass of fuel. It is not always desirable to operate at stoich, many times (and this is true for diesels) maximum power and or torque are achieved at some mixture other than stoich.
And to be clear: my understanding of mixture is not tainted by the function of gasoline engines. I don't care about plugs, or fouling, etc. Gasoline engines operate as the constant-volume Otto cycle with homogenous mixtures. Totally different beasts. But, at the same time, governed by the same rules of chemistry and physics when it comes to the air/fuel mixture discussion regarding combustion reactions inside the cylinders.
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I'm not sure that's completely true, seems like in a gasoline engine with a throttle plate at the intake, air ingested into the engine at idle with throttle plates closed would be considerably less than part or wide open throttle. I'm having a hard time getting that to register
Yup, because the engine is spinning slower. The swept volume of the piston is the same for every rotation of the engine. It is fixed, and that fixed volume always gets filled with air/fuel mixture. But as RPM increases, the number of times that fixed volume gets swept in a set amount of time goes up. Which means the flow rate through the engine increases. Thinking about it another way: the throttle plate doesn't control how much air is in a cylinder. It controls how much air goes
through the engine per minute. And if that value goes up, RPM must go up because that's the only way the engine can "pump" more through it's fixed volume.
