Engine cylinder temperature problems

[hlaija51]

Hello,

My name is Jason, I am a senior high school student (new at using Physics forums), and am currently looking into alternative fuels, specifically a substitute for the popular leaded aviation fuel in use today, 100LL.
100LL contains lead to prevent engine knocking, (pre-detonation of fuel in the cylinder before the spark plug fires), and can with stand higher compression ratios than normal gasoline. It's auto ignition temperature is around 250 degrees centigrade (523 K). I am trying to calculate the temperature of the gasses in the engine cylinder, assuming the piston is flat, and there is no leakage of gasses around the piston ring, when the piston is at top dead centre (right before the spark plug ignites the air fuel mixture.

Many high compression aircraft engines can reach compression pressures around 15-20:1. As some of the bio and alternative fuels I am looking into have various autoignition temperatures, I would like to calculate the temperatures of the lowest compression to highest. This explains the low specs bellow.

The current engine I am trying to calculate is:
a 235 cubic inch
4 cylinder
5.00:1 compression ratio.

Every time I try to calculate this using the combined gas law or by using P1/T1=P2/T2 my answer is ridiculously high, something in the range of 1300 K.


Could I please have some help, trying to figure out the air/fuel vapour temperature in the engine cylinder during the most compression. And could I please get a formula, and see the steps to solving this problem. please only use metric numbers, and STP data


Thanks
Much appreciated.
Jason

 

[ldeasterday7]

im not really qualified to answer this question, hopefully someone else can help you out more. I do know that at high pressures the ideal gas laws break down. Maybe you could try using van der Waal's equation for real gases.

good luck

 

[hlaija51]

Ok thanks, everything in that formula looks manageable and seems to make sense, accept the a and b values, How does one go about calculating thouse?

Thanks very much for the lead
Jason Hlaing

 

[256bits]

halaija51

You will note that P/T is not a constant for your situation, and I have no idea how you have misinterpreted the ideal gas law, PV=nRT. In any case PV/T = a constant, since n nor R change but P, V and T change.

You know P1, V1 and T1 at bottom dead centre of the stroke, assuming the air is not heated by the walls of the hot engine. You know V2 at top dead centre of the compression stroke, but not P2 or T2. so how do you determine those two values.

First of all you need to know the thermodynamic cycle of your engine. A gasoline engine operates under what is called the Otto cycle. See this site for more information ( as usual a Wiki... site )
http://en.wikipedia.org/wiki/Otto_cycle

There it tells you that the compression ( and expansion or power ) stroke follows an isentropic compression for an idealized Otto cycle. This means the stroke is operating adiabatically ( no heat flow from the walls of the engine to the gas during compression ) and
is reversible ( no change in entropy ) You may look up these terms on your understanding.

An example of an adaibatic revesible compression is given somewhat halfway down this site.
http://en.wikipedia.org/wiki/Adiabatic_process

Follow the calculations and substitute your data with your own values to find the P2 and T2 at the top of the compression stroke.