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IC Engine query
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Posted by: vijayguru

11/13/2006, 11:28:24

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Dear all,

What are all the factors needs to be considered, before fixing the compression ratio of an IC Engine.

thanks in advance

Modified by vijayguru at Mon, Nov 13, 2006, 11:32:15

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Re: IC Engine query
Re: IC Engine query -- vijayguru Post Reply Top of thread Forum
Posted by: randykimball
11/13/2006, 17:55:01

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First, let's clearify your question.

I am not saying you are using the wrong meaning, however many people do confuse two different things as compression ratio.

Then, for the benifit of all, not excludding those who get them confused, I will touch on both.

The amount of space that the gasses are compressed into at top dead center caliculated against the amount of space the gasses occupy at bottom dead center of a piston cycle is the compression ratio.

The amount of fuel mix in the atmospheric air being placed into the cumbustion chamber is fuel to air ratio, richness. Many people confuse these two items. I suspect someone taught it incorrectly somewhere for some time.

1st, .. Combustion ratio:
(Bottom Dead Center Volume/Top Dead Center Volume) to 1

2nd, .. to determine the desired combustion ratio of a not yet designed IC engine:
First you need to decide what fuel you will burn; what temp you want to burn at; what sized intake and exhaust valves or ports will move how much air at what RPM; what RPM you wish to run most effient at; what cam lift you will use (a lot to figure here); what exhaust and intake pressures and drags/ back pressure/ lag; what HP and torque you are trying to develop; what kind of fuel atomatization will be provided; and more.

3rd, .. to determine the best fuel to air ratio:
You need to know the compression ratio; ideal RPM range; valve openings; intake and exhaust lag and back pressures; altitude; super charged, turbocharged or normally asperated; efficiency/performance target; and more ... then tweak the settings for the many different RPM ranges in whatever bracketed divisions you decide are small enough. Then there are even more things like environmental requirements to meet.

Does any of this help?

These are the areas where a modern engine designer earns their wages most. These are the areas that make the difference between an effiecent, clean burning, powerful engine and a "lawn mower"/throw away engine.

The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by randykimball at Mon, Nov 13, 2006, 18:14:26

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