Pre-Lab

This lab is about practical stuff like power, and energy and what efficiency really is.

By the time this lab is over you should have a much better intuitive understanding of these things and of engines in general.

4.	Is the watt an acceptable way to measure mechanical power (as opposed to electrical power)?

5.	If process A is 50% efficient and it feeds into process B which is also 50% efficient what is the overall efficiency of A feeding into B and becoming a useful output at C?

6.	True or false, the correct way to obtain efficiency for a system is to average all the various efficiencies that make up the system?

9.	What circumstances can you think of that might make an inefficient process a good one?

10.	Calculate the Carnot efficiency of a heat engine with the hot end at 100 C. and the cold end at 23 C and also with the cold end at 0 C.

The engine that we are going to work with in the lab, the Smart Stirling Engine, is called a Stirling engine. It was invented in 1816 by a Scottish minister named Robert Stirling who didn't name it after himself. He called his engine a ???hot air engine with an economiser.??? About a hundred years later someone else decided that these engines should be called Stirling engines and the name stuck.

When you read the history of science it always seems that theory comes first then someone figures out how to make something useful out of it. But that???s not the way it usually works. For example, the Stirling engine was invented by Robert Stirling in 1816. About fifty years later a very good French scientist and engineer named Sadi Carnot came along a figured out an extremely useful formula for determining just how efficient any given heat engine could be.

The formula is as follows. Temperature of the hot side of the engine minus temperature of the cold side of the engine divided by the temperature of the hot side of the engine times 100 is the maximum efficiency possible for any engine operating between those temperature extremes. The formula is written (T(hot) ??? T(cold))/T(hot).

It???s important to remember to use absolute degrees such as Kelvin or Rankine. If you plug Fahrenheit or Celsius degrees into the Carnot formula all you will get is a meaningless number.

So here???s another question. Say you have an engine operating between a T(hot) of 100 degrees C (that would be 373 K) and a cold reservoir of room temperature (say about 22 degrees C or 295 K).

11.

Which will get you more efficiency (according to Carnot): raising the hot temperature 25 K or cooling the cold temperature 25 K? Please take this seriously as your boss (who doesn't have a clue about physics) is demanding that you make the Smart Stirling Engine more efficient or else he???s going to take away your company car and cancel your expense account. Your boss does understand expense accounts.

So if the Carnot formula determines the maximum efficiency of any heat engine, and we know that we can???t exceed this, then the question comes to mind, how much of the Carnot efficiency can we reasonably expect to achieve with any given engine?

Well if you have been extremely careful, and if you use the right type of engine (a Stirling engine) you can get up to 50% of Carnot efficiency. Perhaps this could be achieved with other cycles as well. Any heat engine that achieve 50% of Carnot is an extremely efficient engine!

With all this talk about efficiency, and the fact that Stirling engines have the most efficient engine cycle possible, you probably wonder if the Smart Stirling Engine is a 50% of Carnot engine? Not a chance. Is it a 25% of Carnot engine? No. Then why do we have one in the lab?

We have one here for several reasons. First, it is a good teaching tool to help you learn about engines in general. This engine wasn't designed to be efficient. It was designed to be a good teaching tool, to work well in a lab, and to be easy to understand. It was also designed to a specific price, because if a school can't afford to buy one, it doesn't do anyone any good.

In the lab you will find out what it???s real efficiency is compared to it???s Carnot efficiency.

12.	How much of the heat that is expended in heating these resistors, actually goes into the Stirling engine as opposed to simply heating the air around the engine? Here is a hint: measure the area of the rectangular resistors. How much of this area is in contact with the engine?