Engine trivia pop quiz

James Snover

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In a four stroke engine typically found in cars, if you raise the compression ratio, all else being equal, your engine now makes more power.

But think about it for a moment: to compress air and fuel takes energy. While you are on the compression stroke your engine is not producing power from that cylinder. It is, in fact, consuming power to be able to compress the intake charge. And if you raise the compression ratio, say from 9:1 to 12:1, it now takes a lot more power from the engine to compress the intake charge. So ... how do you get more power from raising the compression ratio?

I'll toss the answer up tomorrow, if no one gets it.
 

Black02GT

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Thermal efficiency and the expansion event increasing at a higher rate than the compression event fuel allowing. More initial force, greater twist. Not really sure what you're going for but looking forward to your answer, I'm sure it'll be long and well thought out.

Is E. Schaider on here to give us a technical beat down or is he only on the other side?
 
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MG0h3

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Interesting discussion.

One of the first things you notice with increased compression is more low end torque. Example: diesel.

The amplified explosion forcing one piston down is transmitted through the crank and rods to all the other cylinders.

As stated above, I’m sure your answer will be informative.


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thomas91169

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Yeah but at the same time you are compressing, another cylinder is on its power stroke. The same amount of energy that is pushing the power stroke cylinder down is being used to compress the next a/f charge.
 

Black02GT

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Yeah but at the same time you are compressing, another cylinder is on its power stroke. The same amount of energy that is pushing the power stroke cylinder down is being used to compress the next a/f charge.

Think I misinterpreted where you were going with it. Same amount threw me off.
 

Silver Talon

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You end up packing the air molecules much tighter for a more efficient flame front across the piston. The loss in power in higher compression is no different than running a supercharger, it takes power to make power. But still results in a net gain.
 

SecondhandSnake

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It's pretty basic engineering stuff. You're talking an Otto cycle engine. The net work developed over the cycle is represented by the area contained within the P-v curve. By increasing your compression ratio, you increase P2, or the pressure after the compression stroke (represented by 1-2.) Even if you assume that the energy transferred to the cylinder by combustion is the same between ratios, you end up with a greater area contained within the curve. Basically even though you put more work in on the compression stroke, your overall net work is greater, owing to the fact that P2 and P3 move up higher, resulting on an even higher work out during the power stroke as well.

otto%2Bcycle.jpg


Cliff's: Work is area inside the curve. Compression ratio goes up, P2 and P3 are higher. Area is bigger. Work is higher.
 

SID297

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Basic chemistry/physics. By creating a smaller space for the air/fuel charge you are bringing the constituent components closer together allowing for more complete and efficient combustion event. Hence a greater amount expansion gases are at play. That increased energy release is greater than the work required to compress the charge.
 
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DaleM

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Because potential energy and a spark?

OK Bozos, stop clowning around.
 

derklug

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The greater the compression, the more dense the propellent charge. The denser the charge, the faster the rate of burn. Faster rate of burn gives more pressure. If you burn gunpowder in your hand you get a nasty burn. Wrap the gunpowder tightly and you get a stub.
 

James Snover

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The answer is: all other things being equal, you now get more net power from the engine due to the increased "Expansion Ratio," a term we never hear much of, these days. With the same valve timing, when you install 12:1 compression ratio pistons in your engine, you now also have a 12:1 expansion ratio. That is where the benefit of the higher compression ratio is found.

The thermodynamic answers were partly correct, too. Didn't see anything wrong in them, but the big picture is the expansion ratio. In most engines, the combustion event takes place several degrees before it ever hits TDC and the max 12:1 compression. So, the thermodynamic effects of increased compression ratio, while playing a part, aren't the whole enchilada.

SID297 came very close, however: the total expansion volume isn't increased. To get the higher compression ratio, the piston sits higher in the bore, so there is actually a little less over-all volume. But close!

secondhandsnake gets extra internet points for the graph, which is dead-on. Study it well, everyone. It's what it's all about. Understand the graph, go faster.
 

SID297

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The answer is: all other things being equal, you now get more net power from the engine due to the increased "Expansion Ratio," a term we never hear much of, these days. With the same valve timing, when you install 12:1 compression ratio pistons in your engine, you now also have a 12:1 expansion ratio. That is where the benefit of the higher compression ratio is found.

The thermodynamic answers were partly correct, too. Didn't see anything wrong in them, but the big picture is the expansion ratio. In most engines, the combustion event takes place several degrees before it ever hits TDC and the max 12:1 compression. So, the thermodynamic effects of increased compression ratio, while playing a part, aren't the whole enchilada.

SID297 came very close, however: the total expansion volume isn't increased. To get the higher compression ratio, the piston sits higher in the bore, so there is actually a little less over-all volume. But close!

secondhandsnake gets extra internet points for the graph, which is dead-on. Study it well, everyone. It's what it's all about. Understand the graph, go faster.

I didn't mean overall cylinder volume increases. Updated for clarity.
 

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