Ed:

There is no throttle in a diesel -- output is regulated by varying the amount of fuel injected rather than the amount of air/fuel mixture admitted to the the engine.

A naturally aspirated diesel (non-turbocharged) has maximum air flow at fast idle, with flow dropping off as rpms increase due to pumping losses (restriction of flow due to having to go through valves, etc).

A turbo will have the greatest air flow at highest load and fuel delivery.

Maximum efficiency is usually a bit below full fuel delivery, at the point just before the black smoke appears. More power is available at the cost of less specific horsepower as more fuel is added past that point -- black smoke is unburned fuel, and that fuel, being unburned, didn't produce as much power as would have been available if burned.

Another note of interest -- all diesel engines have a governor -- this adds some extra wrinkles to fuel delivery and boost control.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!