We have a lively shop discussion of this question. The answer is not obvious to any of us.
Can any of you guys shed light?

 

Otto cycle - Wikipedia, the free encyclopedia Have a read here.

 

I hope you work in a wood shop.....

 

Thank you, Lazy K. I hace read the article twice but I can't find any answer.
Here is my best attempt:
Increasing compression ratio decreases the amount of left-over exhaust gases in the cylinder and therefore allows for a higher percentage of good stuff to come in on the intake stroke, therefore increasing the energy in the cylinder.
No, post fiend, I don't work in a wood shop. If you have a common sense explanation, please put it up. I did not care for the veiled insult.

 

The math in that article is beyond me but as I understand it, the more you compress the fuel/air mixture before ignition, the more efficiently it will burn.
Apparently you get max efficiency at 17:1 and no increase after that. The big problem, of course, is preventing the fuel/air mixture igniting before the spark as you raise the compression ratio. Even more difficult with modern junk gas.

 

We understand the pre-ignition problem and the need for higher octane slower burning fuels to slow down the explosion rate.
We are old motorheads who have put high compression aluminum heads on flatheads and done everything in between. But the question still hangs.
Do we have any rocket scientists out there who can translate the physics into something we can understand?
Barroom arguments hang on this question.
Thank you, Lazy. We have you scratching your head and thinking.

Semper Fi

 

This reference is a lot simpler -- but it's pretty vague.

Compression ratio - Wikipedia, the free encyclopedia

Basically seems to say that the mixture burns better -- until it knocks.

hj

 

It was noted once by a friends dad that you will get more of a bang out of a drop of gas in a thimble than you will in a bathtub. I think it's that simple, hence my prickish response. However, it is pretty complicated in the final analysis, and I read through this and looked at their extreme example (15:1 vs 2:1) and found it simplifies it a bit over the previous link-at least at first.

Engine Compression Ratio - Tech - Popular Hot Rodding Magazine

 

It is the same principle as to why diesels are more efficient. With the higher compression, the pressure seen by the ignition are higher, and produce higher return on mechanical energy. You are taking more volume and squeezing it into a smaller area , which then gives the effect of more power and more complete burn of the fuel in the tighter area. You start already with an elevated pressure, and a smaller area. then the pressure spikes, and since it is in a smaller area, it pushes harder on the piston. If you have more area, the pressure spike max reading is lower, so less push on the piston.

 

Thank you, Fellro. But saying you have higher pressure in the combustion chamber- therefore more power on the power cycle does't do it- because you had to put in more energy compressing the gas in the first place.
Let's try this explanation:
Take a motor with 2:1 compression. At the top of the exhaust stroke, 50% of the residual exhaust gas is trapped in the combustion chamber meaning there is a 50-50 mix of exhaust gas and good fuel-air mix at the bottom of the intake stroke.
Take a 10:1 motor. At the top of the exhaust stroke, about 90% of the exhaust gases are expelled meaning that the crappy-do to good stuff mix at the bottom of the intake stroke is 90-10.
So, higher compression motors suck in more energy.
Make sense?

 

I'll try and dig out my old thermodynamics textbooks from 35 years ago. The equations were not that complicated, for an engineer. The basic is this, as compression goes up, so does efficiency. So, a higher compression engine gets more power output for a given input of fuel.

 

Pressure = force x area. If the pressure is higher, the force must be higher to keep the equation balanced. The area of the piston is not changing, so the force has to. This is why larger bores also produce more power as well. Sure, you draw in more fuel, but you have more area to distribute the force across.

In regards to the force from higher compression, you are packing the air and fuel molecules tighter together. If you were to take your 2:1 compression illustration, you have the molecules twice as close as they were before compression. Take it to 10:1, the molecules are 10 times closer together. The closeness already exerts some force, then you add the expanding gasses of the chemical reaction (ignition) and the force increases as a result. With the molecules being so tightly packed together, they are mixed better, and get a more complete burn, which increases the force of the expanding gasses due to more heat generation. In essence, more complete burn=more efficient, higher compression=more complete burn, so then higher compression=more efficient. Higher compression=tighter packed, tighter packed=higher temperatures, higher temperatures=more power, so more power=higher compression.

Otherwise, I will get my more modern thermo book out and get their explanations out... it is only a few years old.

Basically, the relationship is higher peak pressure =higher efficiency and power. The key is the closeness. You generate heat by simply compressing the air, which helps the burn. With lower compression, the closeness doesn't generate as much heat, so it takes more to get the fuel to burn, and much is wasted in simply getting ignition of all of the fuel.

With higher compression also comes higher Nitrous Oxides emissions, which are bad, that is why typically the compression is limited. The temperatures are high enough to get nitrogen and oxygen to combine instead of oxygen and carbon, and oxygen and hydrogen the normal byproducts of full combustion.

 

Combustion is a chemical reaction (duh, I know, stick with me). When you combine gasoline with oxygen, you are breaking down the gas molecule and combing the carbon and hydrogen atoms in the gas molecule with oxygen, producing water and carbon dioxide. When you break the bonds that are holding the gas molecule together, you release energy. More specifically, you are converting chemical energy into thermal energy and kinetic (physical) energy.

The kinetic energy comes from the fact that the carbon and hydrogen atoms are more densely packed together when they are part of a gas molecule than when they are part of H2O and CO2 molecules. As the reaction (combustion) occurs, these new H2O and CO2 molecules want to spread out and move away from the reaction. It's the same reason the wind blows. More gas molecules in one area are going to move to where there are less gas molecules, until the pressure is equalized. This movement is what creates the pressure inside the chamber. The reason the piston is pushed down at all is because there is less pressure on the underside of it. So the more pressure that is created inside the chamber, the faster that piston is going to be pushed down.

So basically, the more gasoline and oxygen molecules you stuff into a chamber, the more exhaust gasses are produced. More gasses produced equals higher pressure. More pressure equals more torque.

I hope that makes sense.

 

Good. Thank you, guys. Good discussion.
We buy the idea that packing the air/gasoline molecules tighter in the combustion chamber makes for more efficient combustion. But the business of having more pressure in the combustion chamber- therefore more power- doesn't. The piston has to be given more energy from somewhere to compress and that comes from robbing energy from the other cylinders or flywheel storage.
And the idea that my 10:1 motor expels more exhaust compared to the 2:1 motor seems like a common sense approach.
Keep it going.

 

Engine Compression Ratio - Tech - Popular Hot Rodding Magazine