Someone asked about compression ratios and engine efficiency. At the

time, I reccommended Wiki and Mr. Google not realizing they needed to

understand what to ask for . . . thee basics. So I replied:

I'm sorry, I should have been more specific about the effect of

compression ratio on engine efficiency. So let's start with this URL:

http://en.wikipedia.org/wiki/Gasoline/Petrol_engine
They don't really discuss compression ratio but this is the take away:

". . .

The thermodynamic limits assume that the engine is operating in ideal

conditions: a frictionless world, ideal gases, perfect insulators, and

operation at infinite time. The real world is substantially more

complex and all the complexities reduce the efficiency. In addition,

real engines run best at specific loads and rates as described by

their power band. For example, a car cruising on a highway is usually

operating significantly below its ideal load, because the engine is

designed for the higher loads desired for rapid acceleration. The

applications of engines are used as contributed drag on the total

system reducing overall efficiency, such as wind resistance designs

for vehicles. These and many other losses result in an engine's real-

world fuel economy that is usually measured in the units of miles per

gallon (or fuel consumption in liters per 100 kilometers) for

automobiles. The miles in miles per gallon represents a meaningful

amount of work and the volume of hydrocarbon implies a standard energy

content.

Most steel engines have a thermodynamic limit of 37%. Even when aided

with turbochargers and stock efficiency aids,

**most engines retain**

an average efficiency of about 18%-20% . . . ."

Practical, commonly sold engines have abysmal thermal dynamic

efficiencies. To be perfectly blunt, they are rubbish compared to what

they could (and should) achieve such as with the Prius Atkinson

cyycle. So lets look at two university web pages that go into details

addressing the effect of compression ratio:

http://web.mit.edu/16.unified/www/SP...es/node25.html
http://www.qrg.northwestern.edu/ther...otto/otto.html
Both papers show the math to derive the same efficiency formula for a

'perfect' Otto cycle engine:

efficiency = 1 - ( 1 / (r ** (k-1) ) )

r - compression ratio

k - specific heat ratio, a measure of energy in the fuel

For simplicity, here is the MIT chart:

http://web.mit.edu/16.unified/www/SP...nRatio_web.jpg
Here is the Northwestern chart:

http://www.qrg.northwestern.edu/ther...efficiency.gif
Note they have have different scales with the MIT showing the full

range and the Northwestern a more practical range we find today.

Now everything in these classical, Otto cycle lessons derives from the

P-v diagram:

http://www.qrg.northwestern.edu/ther...Pv-diagram.gif
The top curve coming from point 3 towards point 4 is the power stroke

curve and the longer it is, the more energy extracted. The x-axis is

the "v" expansion ratio. A greater compression ratio, the longer the

expansion stroke and more energy extracted. But higher compression can

lead to detonation and hammer the engine to pieces. The key to

efficiency is the longest possible expansion ratio to extract

mechanical energy.

The Prius Atkinson cycle changes the compression stroke so the fuel-

air charge is not compressed to ignition. Itt takes the line from 1 to

2 and breaks it into two sections:

1 to 1.5 - this is a flat line as the intake valve is kept open and

part of fuel-air mix goes back into the intake manifold to be sucked

in the next cylinder.

1.5 to 2 - this is the shortened compression stroke which being

smaller, also means less compression losses as well as avoiding

detonation or knock.

Qs - the energy added is less because there is less fuel-air to burn

Qout - is the same

4 - is moved to the right about 50% further (aka., 13 to 1 expansion

versus 9 to 1 typical compression ratio.) So the Prius has a much

longer power stroke to extract more energy.

I do not like this P-v chart but it is 'close enough:'

http://upload.wikimedia.org/wikipedi...nsonMiller.png
I do not like it because the segment 3-4 implies a substantial

increase in volume without work being done and that doesn't happen. If

you stretch 4 over to combine it with 3, you'll have an accurate,

Atkinson cycle P-v diagram.

Bob Wilson