Dave,
The concept of having the expansion volume being greater than the compression volume is imperative to gaining the most power per pound of fuel used, that is lowering the B.S.F.C.
I once did some mathematics/physics calculations on this.
I started by using a unit volume of air (14.7 mol of air) at a chosen under engine bonnet temperature, at atmospheric pressure and compressed it mathematically at a ratio 10:1.
10 to 1 being the compression ratio I chose.
I then used the Boyle's and Charles gas laws to calculate the new pressure and temperature of the compressed gas.
I then added one mol of petrol to the mixture, (to get the stoichiometric ratio or chemically correct ratio)
I then looked up the energy in 1 mol of petrol and then used it in a change in temperature formula to calculate the new temperature of the burnt fuel mixture if the piston did not move from T.D.C
Given that I knew the volume and the new temperature of the burnt mixture at the compression ratio at 10:1 at T.D.C. I was able to again use the Boyle's and Charles gas laws to calculate the final pressure.
Now what I wanted to know was how far would the piston have to travel down the bore (or should I say how much would the volume have to increase) to get to the point that the pressure on top of the piston was at atmospheric pressure again.
I also assumed that no energy (heat) was lost to the surrounding engine components (piston, cylinder wall, head etc,) such as in a ceramic engine.
And there was no frictional loss.
This would mean that all the chemical energy in the burn would have had to be transferred in to rotational mechanical energy in my theoretical internal combustion engine.
Now my calculations indicated an expansion ratio of about 30-33 times the compressed volume.
Unfortunately I have lost my original working so are only going by memory here..
What this implies is a petrol engine would need a compression ratio of about 10:1 and an expansion ratio of about 30:1 ( in theory).
Now to see if this had any credibility I divided the 10:1 by the 33:1 and the answered is about a 1/3 or about 30% which just so happens to be very close to the thermal efficiency on the typical 10:1 petrol/gasoline engine.
( I better put a special little note here to say the above calculations relate to a pre-emissions engine and that injecting an inert gas like burnt exhaust gas via the EGR valve will change the above expansion ratio and make it smaller and is a topic on it's own.)
So how does this relate to the real world........?
From my observations and experience an engine having an expansion volume greater than the compression ratio does not give any great horse power increase, but it does give a massive drop in fuel consumption.
That is, I played with something along these lines and increased the fuel economy of my 63 falcon from 30mpg (imperial gallon) to 40mpg.
Which is about a 30% increase.
Now I did not use any custom made or exotic parts, they where just what I scrounged up at the auto wreckers at the time.
(yes it's been a long time since anyone could buy 63 Falcon parts at your local auto wrecker)
So that makes me think that the ideas and patents present here are overly complicated and overly exotic.
So how did I do it?
I raised the mechanical compression ratio way beyond the octane rating of the fuel, lowered the volumetric efficiency so as the true compression ratio suited the fuel octane. This gave me a low compression volume and a high expansion volume. But having such a low volumetric efficiency loses power, so to compensate I enlarged the cubic capacity of the engine.
In a nut shell I put a 144 inline falcon cylinder head and carburettor on a 170 engine. I did a little more than this but hopefully you get the idea.
The compression ratio went from 8.7:1 to 10.3:1
At first I was using high octane leaded fuel and the engine was prone to over heating, it had a noisy exhaust (very noisy in the cabin), it suffered detonation, and the exhaust manifold was extremely hot. The exhaust manifold is under the clutch pedal and the floor was too hot for my feet. I was getting about 30mpg.
I then experimented with the then new low octane unleaded fuel.
With out any other modification the car when from 30mpg to 40mpg.
The exhaust when cold, there was no detonation, no over heating, and the engine run extremely quiet and smooth. So quiet that one would have to open the bonnet to see if the engine fan was rotating to know if the engine was running. And some times I tried to start the engine only to get excessive noise from the starter because the engine was already running.
The torque curve was a little lower in the rev's, but I just compensated by changing from 4:1 to a 3.5:1 diff ratio.
When doing too much city driving the plugs tended to foul up, but cleared up on long country runs.
It was about as powerful as the 144 engine it replaced and it drove about the same, but got exceptional fuel economy.
Later I played around with the idea of using a vacuum secondary carburettor to control (limit) volumetric efficiency, but today it would be better to let the computer management system control volumetric efficiency so that the engine could not enter detonation.
So in my opinion, the internal combustion engine has still got plenty of developmental life in it and if I can increase fuel efficiency by 30%, it's going to be around for a lot longer yet.
Peter
