In a gasser with a carburetor, the throttle plate varies the amount of air pulled through the venturi, and thus the amount of fuel and resulting fuel/air mixture at a given rpm. With fuel injection, the amount of air is varied, the mass of the air measured, and the correct amount of fuel is injected into either the throttle body or the intake manifold to achieve the proper fuel/air mixture.

'Light' throttle allows a small volume of air, full throttle allows the complete volume of the cylinder. Light throttle = less fuel = low torque, full throttle = max fuel = max torque. This is true at any rpm, and at any rpm there is a range of torque values that the engine can produce.

There is no air/fuel mixture taken into a diesel cylinder as such. Air, and only air (exhaust from the EGR, and manifold vapors don't really count) is drawn into the the intake manifold and through the intake valve.

Fuel is injected into the pre-combustion chamber. The amount of fuel injected varies at a given rpm, depending on the postition of the injector rack, which determines the quantity of fuel to be injected.

Now, in the case of a headwind, there is more force resisting the forward motion of the car at a given speed. If we ingnore the small 'slip' of the torque converter (modern transmissions have locking converters at highway speed) the rpm do not change but the 'load' on the engine increases. More fuel must be introduced to make more torque to overcome the load.

At any rpm there is an upper limit to the amount of fuel that can be injected and burned efficiently. If more fuel is injected, it is simply exhausted as incompletely burned fuel, or black smoke.

In a gasser too much fuel in the fuel/air mix will will show up as black smoke in the exhaust, but if the mixture is made even more 'rich' it will not ignite and combustions stops.

In a diesel, surplus fuel will also show up as black smoke, but it is almost impossible to inject enough fuel to stop the combustion process. Before that happens, huge amounts of smoke and even liquid raw fuel will run out of the exhaust...

Best Regards,
Jim