To obtain maximum use of air’s internal energy (ambient heat), do not allow the air that goes to the first power stage in a multiple stage expansion to rise in temperature above ambient. Maximum practical cold production can be reached by means of first stage expansion with early cutoff, then heat can be added. Some heat (toward ambient but not above) should be absorbed just before first stage intake, such as electric heating pads on elbows where cold is naturally produced, and other places where expansion takes place as the air approaches the engine.

Add heat to engine air whenever possible; first absorb ambient heat, then recover compression heat, then add purposely generated heat, if any.
Reheating compressed air to raise pressure just prior to engine intake is much cheaper in energy cost than an equivalent pressure increase attained by compressing more air.
Regenerative braking works well with air cars, and compression braking saves brakes and heats the air in the tanks.

It takes less work to increase the pressure of a volume of air from 100 to 200 psi than it would take to increase the same air from 0 to 100 psi. This can be verified by looking at any air compressor power consumption chart ever published. It is the rationale behind the closed cycle pneumatic power plant, which can do more work with smaller machinery.

Any chance to boost the pressure of already compressed air instead of compressing atmosphere will lower the relative size of the machinery needed to do that task, because more energy per unit volume of compressed air is handled by a booster than by a normal atmosphere compressor with the same displacement.

It is possible to put low pressure air into a high pressure tank against very little resistance by taking advantage of the Bernoulli Effect, which is the answer to the 1870 physics riddle known as Maxwell’s Demon. Potential and kinetic energy can be caused to trade places so that each is used for what it does best, and neither gets in the way of the goal, which is to keep the tank full as cheaply as possible.

All compression work is lost as heat. This little-known fact is straight out of the textbooks.
The energy that pushes pistons is heat, not pressure, so if we arrange to use solar-source heat to run an air car, then the air car is a self-fueling solar air car.
The longer cold, partially expanded air stays in the engine, the more free heat it will absorb from its surroundings. To keep it moving slow, behaving more like a heat sponge, try the following: lower rpm, multiple-stage (compound) expansion, heat exchangers (no bends in piping) between stages.

Extra pressure needed for any reason as a part of the power process should be generated only as needed so that compression heat can be used immediately and storage pressure can be kept to a minimum. The more air you store in a given space, the higher the maximum storage pressure becomes.

Find ways to use compressed air at its full pressure, such as jet pumps and other pressure exchangers. Design around this concept, rather than using regulators to lower the air's pressure to that desired.