Modular auxiliary power unit assembly for an electric vehicle

1. A propulsion system comprising:

an electric motor;

an energy storage unit configured to supply a primary power to the electric motor; and

a plurality of auxiliary power units (APUs) configured to supply a secondary power to at least one of the electric motor and the energy storage unit, each of the plurality of APUs comprising:

a free-piston engine configured to generate a mechanical output;

a linear generator configured to transform the mechanical output to an electrical power; and

a controller configured to:

receive a power command from the at least one of the electric motor and the energy storage unit;

determine an amount of secondary power needed to meet the power command; and

selectively activate a number of the plurality of APUs to generate the needed amount of secondary power, wherein selectively activating a number of the plurality of APUs comprises:

individually operating each of the plurality of APUs in one of an on condition or an off condition based on the needed amount of secondary power; and

operating each of the plurality of APUs in the on condition above a minimum efficiency threshold, including selectively operating each of the plurality of APUs in the on condition at various speeds at less than a full power-generating capacity and generating an equal fraction of the secondary power.

2. The propulsion system of claim 1 further comprising one of a rectifier and a bidirectional convertor electrically connected to the linear generator.

3. The propulsion system of claim 1 further comprising a DC link electrically connecting the electric motor and the energy storage unit to distribute power therebetween; and

wherein the secondary power supplied by the plurality of APUs is fed into the DC link to be distributed to at least one of the energy storage unit, the electric motor, and an auxiliary load.

4. The propulsion system of claim 1 wherein the free-piston engine comprises an internal combustion engine having a drive assembly.

5. The propulsion system of claim 4 wherein the drive assembly comprises:

a rod;

a first piston coupled to a first end of the rod; and

one of a returning spring and a second piston coupled to a second end of the rod.

6. The propulsion system of claim 1 wherein the controller is configured to determine a transition strategy for altering an operating efficiency of each of the plurality of APUs in the on condition.

7. The propulsion system of claim 1 wherein the controller is configured to receive one of a band-limited instantaneous power requirement of the electric motor and a recharging command for the energy storage device.

8. The propulsion system of claim 1 wherein each of the plurality of APUs further comprises at least one fuel injector configured to control firing of the free-piston engine; and

wherein the controller is configured to regulate a firing sequence of the at least one fuel injector of each of the selectively activated plurality of APUs to selectively manage at least one of a noise and vibration output of the selectively activated plurality of APUs and an operating efficiency of the propulsion system.

9. A propulsion system comprising:

an electric motor;

an energy storage device;

a DC link electrically connecting the electric motor and the energy storage device;

a plurality of crankshaft-less engine and linear generator combinations electrically connected to the DC link; and

a controller configured to:

determine a power requirement of the electric motor and energy storage device; and

selectively activate a number of the plurality of crankshaft-less engine and linear generator combinations to provide power to the DC link based on the power requirement;

wherein each of the selectively activated crankshaft-less engine and linear generator combinations is operated above a minimum efficiency threshold but at less than a maximum power-generating capacity at various speeds and generating an equal fraction of the power requirement.

10. The propulsion system of claim 9 wherein the controller is further configured to:

determine a charge requirement of the energy storage device; and

if the charge requirement is below a charge threshold, control the plurality of crankshaft-less engine and linear generator combinations to recharge the energy storage device.

11. The propulsion system of claim 9 wherein the controller is further configured to selectively operate a fuel-injection timing sequence for each of the activated crankshaft-less engine and linear generator combinations to minimize noise and vibration of each of the activated crankshaft-less engine and linear generator combinations.

12. The propulsion system of claim 9 wherein the controller is further configured to selectively operate a fuel-injection timing sequence for each of the activated crankshaft-less engine and linear generator combinations to operate at a frequency other than a natural frequency of each of the activated crankshaft-less engine and linear generator combinations.