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Author Topic: Engine Theory Part 1 - Compression Ratio  (Read 6732 times)

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Marks DTM Calib

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Engine Theory Part 1 - Compression Ratio
« on: 30 September 2008, 08:59:57 »

Been wanting to put something like this together for some time in order to give those that are interested a little extra insight in the workings and theory of an Otto cycle engine.

Ok, lost you already?

Right, what is an Otto cycle engine well, put simply, its the 4 stroke spark ignition engine (a Diesel follows the diesel cycle) we know and love and have lived with for many years now and gets its name from Nicolaus Otto (in collaboration with Gottlieb Daimler and Wilhelm Maybach) who produced the first working 4 stroke spark ignition engine prototype.

Right, I am going to start with compression ratio and its affects on power and engine efficiency.

Compression ratio is the ratio between the volume of fuel air mix above the piston at the bottom of its stroke to the volume of the fuel air mix when the piston has squashed it all at the top of the stroke.

Now, engine efficiency is determined by;

Compression Ratio (CR)
Ratio of specific heats (k which is approx 1.4)

The formula being;


Where Eff is the efficiency.

So, an X30XE (which has a 10.8:1 compression ratio) has the potential to be 63% efficiency.....ow how we wish!

So, why isn't it, well many things affect this and I hope to cover some of them in greater detail over the next few weeks but, in simple terms its down to losses in the system.

Right, back on topic, clearly if we increase the compression ratio we improve the efficiency and if we improve the efficiency then we must be getting more power from the same amount of fuel and air.

We also know that the bigger the pressure on the engines piston then the greater the power output.

So, thats easy then!

Well, not quite because if we increase the compression ratio we reach a point where the compression itself lights our fuel (to early!) and not our precisely timed spark and its at this point we might as well buy a diesel!

To make matters worse, the efficiency improvement is minimal once you get beyond a 9:1 compression ratio!

So, why are modern engines so high anyway then?

The answer is flame speed. A higher compression ratio also means the air/fuel mixture is quite dense (the fuel molecules are closer to the oxygen ones!) and the speed of the combustion increases with the density of the mixture. Low density means low combustion speed.

This explains why we advance the timing (automatically on modern engines and by vacuum advance on older dizzys!). A high vacuum means a lean, less dense mixture and a slow combustion speed. As a result the mixture must be ignited earlier.

With a higher CR you've got a higher average effective pressure on the piston, thus because of the faster combustion speed the available fuel can be burned more efficiently.

So, to summarise, high compression ratio is good for power and efficiency but, take it to high and you get pre-ignition problems so we have to compromise!
« Last Edit: 30 September 2008, 09:03:44 by Mark »
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