What is the difference between a Motor and an Engine?

This next post was intended to be about engine efficiencies.  But in the process of thinking about the subject of efficiency, it occurred to me that there is no clear engineering definition of what an "engine" is in the first place.  The reason this is a problem is the following: a measure of efficiency only has meaning when applied to a well-defined system.  And by well-defined, it is meant that there exists a boundary that unambiguously separates all relevant factors into those that are included in the system and those that are not. 

So before I start a discussion of engine efficiencies, I thought I should do a post on just what exactly an “engine” is in the first place.  And a good exercise to start with seemed to be contrasting our usage of the term "engine" with our usage of the word "motor".

If you put the words "engine versus motor" into your favorite Internet search engine, you'll find any number of discussions addressing the various meanings given to these two words.  It would appear from the web-discussions I've looked at, that for most cases the terms motor or engine can be used interchangeably.  But, while saying "rocket motor" or "rocket engine" is equally acceptable, one never says "electric engine", only "electric motor".  This would seem to imply that there is some distinction between these terms. 

It appears from use, that motor is a more general term than engine.  A motor is something that drives/runs a machine/mechanism; while an engine is a particular kind of motor.  But if you then look at all of the various devices that get referred to as "engines", there doesn't seem to be any consistent pattern as to what is or isn't one.  At which point, one's quest to find a useful distinction between the terms motor and engine ends empty-handed.

The reason this is important for a discussion of robotic power sources is that how one defines "efficiency" depends on the nature of the power source one is using.  If one starts with the general notion that a motor is a device that takes energy from some source and converts it into mechanical work, then "efficiency" is a measure of the "completeness" of that conversion process. 

But different laws of physics are going to kick in depending on the nature of a motor's conversion process.  So one always needs to be mindful when comparing efficiency of one choice for a robot's power source versus another, that one isn't making the mistake of comparing apples to oranges.  That is, one always needs to make sure that one's comparison is being made between systems that encompass the same thermodynamic universe.

A perfect example from current events is the fact that an electric motor can always be made more efficient in converting electrical energy into mechanical work than an internal combustion engine can ever be at converting the chemical energy of its fuel into mechanical work.

Those marketing "green technology" use this fact to advertise electric cars as being far more efficient than a comparable sized gas-powered car.  But a true apples-to-apples comparison would have to include into the electric motor's thermodynamic universe the efficiency of the power plant generating the electricity, any losses in the power grid's network, any losses in the battery charger circuit, and etc.

So, in an odd turn around, even though "motor" is used in a more general sense than "engine", the term engine encompasses a much broader system-wide view of the total energy conversion process between the original energy source and the final mechanical work output.

The moral of this tale is that, from an engineering point of view, the term "engine" is ambiguous, so any discussions regarding engine efficiencies will always be required to also include a well-defined description of the thermodynamic universe encompassing one's arena of comparison.

Piston vs. Gas Turbine Engines: Reliability and Maintenance

Reliability:  In my recent searching through a number of aircraft engine related web sites, I came across numerous comments to the effect that gas turbines were more reliable in operation and went longer between routine scheduled maintenance cycles than piston engines.  I have no background in this area, so I only have this anecdotal information to go on.  But it does make sense, since a gas turbine has only one main moving part versus the dozens found in a piston engine.  Also, a gas turbine runs in a continuous smooth manner versus the intense internal pounding that a piston engine is subject to. 

Maintenance:  Any service and maintenance requirements for a 'bot's power source will be inherited from those same requirements for the harvester/tender as a whole.  So with this in mind... 

It would be a violation of the WID Rule  having to add an additional workforce to service the special repair/maintenance needs of a team of robots when there are already farm machinery mechanics on the payroll.

WID Rule, Corollary 2: Regarding operation, service and maintenance, K.I.S.S., since the workforce that will be responsible for these jobs in the future will be the same people that now operate, service and maintain the existing farm equipment.

First, the most likely place for a robot harvester/tender to break down will be out working in the middle of a field.  For this reason, field repairability is a must.  The way to meet this design constraint is to break a 'bot's design into modules that can be easily swapped in/out in the field by someone with a few hand tools and minimal experience.

Second, a field supervisor has enough to worry about already, making sure the crop is picked, packed and moved out of the field on time  The added responsibility of dealing with daily 'bot break-downs is not something they need.  Given that there might be anywhere from 10 to 20 'bots in the field at any one time, the mean-time-between-failure for a 'bot needs to be on the order of 200-500 hours, so that a field supervisor can expect to go, at least day-or-two, without having to deal with yet another 'bot breakdown.   

Lastly, most crops are seasonal.  The ideal situation is for the length of the farming season to coincide with the time for a harvester/tender's major services.  This way, the winter off-season, when most 'bots would be out of service anyway, can be used to do the tear-down and rebuild maintenance necessary to get the 'bots ready for the next season.  This implies a 2,000 to 3,000 hours run time for our 'bot's power source.  

In the end, I don't know if any of these requirements are accessible to either a piston or gas turbine engine.  So I have some homework to do for the future.