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You control the power an engine produces by regulating the air delivery, which in turn regulates the timing and fuel delivery, except in Diesels which have no air delivery restriction.
Engines are most efficient when they have maximum air delivery at fairly low RPM, without full load enrichment. I remember a Dyno test of a 2.5 L GM engine. At 1500 RPM producing 20 HP the engine required a specific volume of fuel. When the load was increased to 50 HP (same RPM), the fuel requirement rose by 50%, while the power produced rose by 150%. That is the essence of best BSFC, you get more work out of the same amount of fuel by making the engine work harder but not at maxumum for that RPM.
This is the essence of pulse and glide. Long before the term "Hypermiler" was ever adopted this principle was known. Using the "cheaper" (fuel wise) higher load capabilities of engines people who were not constrained by traffic and speed used P&G to extend the range of their cars. It mostly started in WW2 (but was done before that) Use the higher efficiency load to store energy in your car as inertia, then kill the engine and coast.
80% load is the ideal load range, becasue it does not use enrichment. Lower RPM does not encounter the much higher friction losses of high RPM.
Why does it use more fuel to accelerate at higher rates?
You are doing more work to increase the inertial state of the vehicle.
My Insight would use 12 times the fuel at maximum acceleration than it did at lower constant speeds. The secret is to minimise the fuel used in acceleration then use the additional inertia to travel considerable distance with little or no fuel consumption.
regards
Mech
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