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compression ratio

 
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aharris05
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1968 Chevrolet Camaro

PostPosted: Wed Aug 12, 2015 10:51 am    Post subject: compression ratio Reply with quote

I always read about how the high output motors run a high static compression ratio. I know that this also requires higher octane fuel, or less ignition advance to avoid knock. Can anyone explain to me how the CR affects power output? Is power improved throughout the whole range of RPM, or in a narrow band at the upper or lower end of range? Does it allow your motor to burn more fuel than a lower CR? If so, how?
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Big Dave
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PostPosted: Thu Aug 13, 2015 12:11 pm    Post subject: Reply with quote

"I know that this also requires higher octane fuel, or less ignition advance to avoid knock."

The first half is true. As your dynamic compression is increased the octane rating must also be increased. Static compression is one aspect of your dynamic compression (stroke and cam timing account for the other two factors). The effect of initial ignition timing and any additional or reduced ignition timing will not save a motor from detonation if your dynamic compression exceeds the octane rating of the fuel. In fact the loss of power from attempting to correct octane properties of the fuel isn't worth the effort.

"Can anyone explain to me how the CR affects power output?"

This is a question answered by the study of Thermodynamics. Your car runs on the Otto cycle (when speaking to some one many confuse this by assuming I am talking about an Auto cycle when in fact it is named after Nikolaus Otto a German engineer who built the first spark ignition engine back in 1876).

Here are a couple of thermodynamic diagrams that illustrate how the Otto cycle works.





These diagrams compare cylinder pressure and cylinder volume. Which from the Ideal Gas Law are directly related to each other on one side of the equation (with the temperature of the gas in the cylinder times some constants inversely proportional on the other side of the equation).

On the diagram the points four and six are the most important. It is the vertical distance between these two points that determines how much power the engine can theoretically make. The segment of the graph three and four represents the spark ignited burning of the fuel in the air trapped in the cylinder. That distance is basically fixed by the type of fuel burned. If you added Nitromethane it would jump up quiet a bit. Burn kerosene (as can be done in my 1947 John Deere tractor after heating the block up by starting it on gasoline first and it wouldn't get much lift at all but back when my John Deere was made gasoline was rationed and kerosene wasn't so you burned what was available.

The starting point of the burn can be raised higher by increasing the dynamic compression. The compression stroke starts at point six and can be pushed higher with a reduced volume combustion chamber or a dome on a piston or both. It can also be increased by closing the intake valve sooner, but then you choke off the amount of available air which could cause an over rich condition that could put out the flame front. Oldsmobile was the first motor car company to offer a "high compression" engine raising the static compression ratio from 7.5:1 to 10.25:1 back in the early sixties. Soon all of the car manufactures were offering a high compression high horsepower engine.

The actual power is the area out lined in yellow on the second graph. Anything you can do to move the lines further apart from each other will increase overall power. You can lower the bottom line by lowering your thermostat's rated temperature and increasing the capacity of your cooling system to shed heat at a faster rate (bigger radiator usually). You can raise the top line by increasing your static compression and changing the fuel you burn by adding chemical agents to oxidize the fuel better (or changing the fuel you burn to something else such as alcohol).

Where it states adiabatic process it means that energy stays in the system and can only leave it by doing work. In a gasoline powered engine the fuel burns at an explosive rate. As such you can imagine a tinny dwarf in your engine with a really big hammer that hits the top of the piston when the spark plug fires. All of the energy is instantly transferred to the piston that wants to go down faster than the piston is moving. In fact almost all of the combustion occurs at or near top dead center which differs from the Diesel cycle where the fuel burns from the top of the power stroke to the bottom. A diesel engine is more efficient in that the constant pressure on the cylinder is able to do more work than the sudden impact of the Otto cycle spark ignition.

Here is a PV Diesel graph on the left and a Temperature Entropy graph on the right side for comparison:



"Is power improved throughout the whole range of RPM"

Yes it does improve power across the full RPM range. High compression was outlawed by the EPA as high compression means high temperatures in the combustion chamber which results in Nitrogen being burnt that create a lot of nitrates that form acid in the air.

Does it allow your motor to burn more fuel than a lower CR? No

To burn more fuel you need to increase the displacement in a normally aspirated engine, or to add a turbo or a super charger to force in more air for a fixed displacement or both (a blown big block really sucks fuel in a hurry).

Big Dave
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10sec.et
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PostPosted: Thu Aug 13, 2015 10:31 pm    Post subject: Reply with quote

first thing that went through my mind was "where's Dave?" Very Happy
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af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

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SMOKEmUP
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PostPosted: Thu Aug 13, 2015 11:52 pm    Post subject: Reply with quote

I think what Dave is trying to say is, Yes.

More compression = more power, everywhere.

The problem is to figure out how much compression you can run without pre ignition.

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10sec.et
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PostPosted: Fri Aug 14, 2015 12:27 am    Post subject: Reply with quote

SMOKEmUP wrote:

The problem is to figure out how much compression you can run without pre ignition.


that's the beauty of EFI with timing control. you can get it on the ragged edge if you're tuning for max HP. lots of data logging.

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af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

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