Piston vs. Gas Turbine Engines, Efficiency, Global View

The Global View  The issues of local and mid-range views of engine efficiency overlap the global view in the areas of maintenance and repair. 

Based on my experience, something that electronics and software engineers, working in the area of agricultural robotics, don't seem to appreciate is the high cost of machine maintenance.  Everyone is familiar with the high cost of auto repair these days, so why isn't the engineering community making the connection that robot repair is not going to be any less expensive?

Engine efficiency often comes at the expense of higher temperatures, higher pressures and greater mechanical complicity.  All of these are factors in decreased engine life, more stringent maintenance requirements and higher cost of repair.

Let's try some order-of-magnitude estimates, OME's, for the service costs of a robot harvester/tender and for the fuel budget savings for a 20% increase in engine thermal efficiency. 

First, an example repair situation...
   $100.00  2 hours labor
   $200.00  parts
   $100.00  tow-truck to pick-up/put-back our robot from/to the field
   $100.00  cost for a fill-in-for-the-day replacement robot
   $500.00  total cost to farmer for a typical repair  

Now look at the cost savings for a 20% increase in fuel efficiency.  Assume a fuel consumption of 2 gal/day.  At $4.00 a gallon, a 20% increase in fuel efficiency yields about 17% savings in fuel costs, which translates to about $1.35 a day.  At this rate it would take a year's worth of operation to balance out the cost of just one typical repair.   

From the point of view of the farmer's pocketbook, the bottom line is that reliability and low maintenance costs will always trump considerations of engine thermal efficiency .