Re: Re: Re: Benefits of higher compression?.

Posted by Neil on June 27, 2004 at 00:04:40 from (138.88.15.38):

In Reply to: Re: Re: Benefits of higher compression?. posted by Additionally...Jerry/MT on June 26, 2004 at 18:08:08:

Cool! I like it when we get scientific. Assuming the same displacement, it is indeed the increased cylinder pressures throughout the power stroke that cause the increase in power with increased compression. As you pointed out an engine with 8:1 compression will, theoretically have 50% more peak pressure (around top dead center) than an engine with 6:1 compression ratio. As the piston progresses down the cylinder the difference between the pressures decreases until they are virtually identical at bottom dead center.

In order to determine how much more power will be made we need to determine how much higher the pressure in the higher compression engine is on average during the power stroke. Which is the same thing as saying we need to determine how much greater the thermodynamic efficiency of the higher compression engine is.

Unfortunately, it seems I threw out my thermo book the last time I moved. Fortunately, the internet is a wonderful place and you can find an explanation of compression ratio and thermodynamic efficiency at the following website –

http://www.qrg.northwestern.edu/thermo/design-library/otto/otto.html

As you can see there the equation for thermodynamic efficiency is:

1 – 1/(r^(k-1))

where:

r = compression ratio

k = Cp/Cv (which = 1.4 (you’ll just have to trust me on this one (thanks for the assist Jerry, I wouldn’t have remembered what it was otherwise))

Plugging and chugging, we arrive at the following theoretical thermodynamic efficiencies for various compression ratios:

4:1 – 42.6%

5:1 – 47.5%

6:1 – 51.2%

7:1 – 54.1%

8:1 – 56.5%

9:1 – 58.5%

10:1 – 60.2%

11:1 – 61.7%

12:1 – 63.0%

13:1 – 64.2%

14:1 – 65.2%

This is also shown graphically on the above-listed website.

Now, to find the theoretical increase in power from one compression ratio to another, one divides the thermodynamic efficiency of the higher compression ratio by the thermodynamic efficiency of the lower compression ratio. For example, to figure out the theoretical increase in power from a 6:1 compression ratio to a 8:1 compression ratio, one divides the thermodynamic efficiency at 8:1 (56.5%) by the thermodynamic efficiency at 6:1 (51.2%).

56.5/51.2 = 110.4%

Thus, the theoretical increase in power by going from 6:1 compression ratio to an 8:1 compression ratio is 10.4%.

The theoretical power gains for each of the 1 point jumps is listed below

4:1 – 5:1 = 11.5% power increase

5:1 – 6:1 = 7.8% power increase

6:1 – 7:1 = 5.7% power increase

7:1 – 8:1 = 4.4% power increase

8:1 – 9:1 = 3.5% power increase

9:1 – 10:1 = 2.9% power increase

10:1 – 11:1 = 2.5% power increase

11:1 – 12:1 = 2.1% power increase

12:1 – 13:1 = 1.9% power increase

13:1 – 14:1 = 1.6% power increase

It should be noted that the theoretical power increase is also the theoretical torque increase.

It is also worth mentioning that the equation for the theoretical increase in torque for a displacement increase combined with the necessary head and cam work to fill those extra cubes is considerably simpler - it is the new displacement divided by the old displacement:

New Displacement/Old Displacement

Thus, while a 33% increase in compression ratio (from 6:1 to 8:1) will yield a 10.4% increase in torque, a 33% increase in displacement coupled with the appropriate intake work will yield a 33% increase in torque.

Follow Ups:

• Re: Re: Re: Re: Benefits of higher compression?. Ron 03:05:35 06/27/04 (2)
  • Re: Re: Re: Re: Re: Benefits of higher compression Jerry/MT 20:10:15 06/27/04 (0)
  • Re: Re: Re: Re: Re: Benefits of higher compression Neil 08:46:27 06/27/04 (0)

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