Saturday, September 10, 2011

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.

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