1.  Field of the Invention
The present disclosure relates generally to commercial aircraft, particularly passenger aircraft and, more particularly, to aircraft taxiing and auxiliary or secondary power systems for aircraft.  In particular, the present disclosure relates to a secondary on board propulsion system that provides the capability of taxiing an aircraft without having to start or use the main aircraft engine(s).  This taxi engine also functions as an APU and can provide thrust so that main engines can operate at less than rated thrust during takeoff and initial climb. 
2.  Description of the Related Art
In modern aircraft and particularly passenger aircraft, weight, space, and costs are exceedingly important.  It is known that up to 15% of the costs, and more recently due to increased fuel costs probably more than 15% of the costs, to operate a passenger aircraft are typically spent while the aircraft is on the ground.  A significant portion of these costs are for taxiing between airport locations (e.g., gate, hangar, maintenance areas and runway) with power typically supplied by the aircraft's flight engines. 
When flight engines are used for taxiing, they operate in a very inefficient region of their rated thrust and, accordingly, consume substantial quantities of fuel for each minute spent in this operational mode.  The resultant fuel costs can be especially substantial at busy airports where aircraft frequently spend extended times between a gate and the runways with the flight engine(s) running.  In addition, significant brake maintenance costs are incurred.  When the flight engines are used for taxiing, the brakes must be frequently engaged to counter the high thrust of these engines.  This significantly increases the frequency of brake repair and adjustment. 
Many passenger aircraft also include separate power systems that provide a number of ground services (e.g., cooling, heating, lighting, power for hydraulics, engine start, ground system checkout, and emergency power).  These additional systems are often referred to as auxiliary power units (APU) and emergency power units (EPU).  Because they add weight to the aircraft, they are another source of added fuel costs. 
Any reduction in parts, weight and complexity in aircraft systems is highly desirable.  System reliability and maintainability are also important issues, since they impact the availability of an aircraft and its overall costs.  Thus, a reduction in parts, weight and complexity in such systems is considered highly desirable.  Reliability and maintainability of aircraft systems are also very important issues, since they impact the availability of the aircraft and overall costs. 
Auxiliary power systems have been integrated into aircraft to meet the aforementioned requirements.  The integration of an auxiliary power unit (APU), emergency power unit (EPU), environmental control system (ECS) and engine start system (ESS) with reduced weight and size are known and are disclosed in a number of United States patents, such as U.S. Pat.  No.  4,684,081 (Cronin), U.S. Pat.  No.  5,235,812 (Klaass et al.),  U.S. Pat.  No.  5,309,029 (Gregory et al.),  U.S. Pat.  No.  5,408,821 (Romero et al.),  and U.S. Pat.  No.  5,490,645 (Woodhouse).  Such systems include the capabilities of providing power for ground check-out, ground cooling, main engine start, flight cooling, and emergency engine start.  However, these existing on board power systems, while providing many essential functions, do not provide the capability of taxiing the aircraft on the ground between the gate, hangar, or maintenance area to the runway and back without having to use the main engine(s).