db2integra

I have already posted this elsewhere but I thought that it would be appreciated more here; sorry for the double post.


How is it that so few people have an understanding of the OTTO cycle engine? Lets look at a basic example (this is impossible, just for explanation). In engine 1 we have a 10:1 compression ratio, and it is running no boost (atmospheric pressure, 101kPa). Since this is an adiabatic compression (the process is fast enough that the heat transfer relative to the total internal energy of the gas is negligable), the air temperature increases as the pressure increases. Let us review the formulas needed to describe adiabatic ideal gas compression:


Since both nitrogen and oxygen components of atmospheric gas are diatomic, a value of gamma=1.4 can be used thus [PV^(1.4)] is constant. Calculating this out, we actually come up with a cylinder pressure around 369psi. This is obviously a little higher than it really should be becuase there is some heat lost, and there is also some overlap/leakdown. An additional factor is manifold vaccum, and vaccum present in cylinder when it is at the bottom of the stroke.

Now we will evaluate an engine using half the compression but twice the pressure; 5:1 CR and 202kPa MAP (about 14.7 psi of boost). If the calculation is done exactly the same, we should find that the the final pressure is around 278.8 psi. Notice that the first example is almost 100psi higher, while the SAME AMOUNT OF POWER IS PRODUCED. This is becuase the same actual amount of air has been compressed in each example, however the temperature of the first example is higher becuase it was compressed a greater amount. The important detail is the statement "if the calcluation is done exactly the same". This assumes perfect intercooler efficiancy, which is beond impossible. Still, the use of an intercooler does allow greater power to be achieved at the same or lower cylinder pressures due to lower charge temps.

Beond that information, there is an additional component of the problem to consider, the gas expansion. After the mixture is combusted, the higher compression engine achieves higher cylinder pressure due to the higher initial pressure (and higher temperature). The higher compression engine also sees higher combustion temperatures. Things change very quickly as the piston moves away from TDC however... because the high compression motor had a lower initial combustion volume, it's cylinder pressure drops twice as fast. By the time both systems reach the bottom of the combustion stroke, the high compression cylinder has half the initial/final pressure ratio. Becuase work is calculated by the integral of P*dV, this means that it produces less work; even if both engines had the same cylinder pressure and temperature initially, the low compression motor would produce significantly more torque at a given RPM.

It is thus safe to say that reducing the compression and increasing the boost will in any situation yeild more power than the opposite. The high boost setup is of course less tolerable in traffic because it takes more time to respond to throttle inputs. If higher compression and lower boost were more effective, what would be the point of a turbocharger anyway?

-Mike Rayno, ASMET, currently working on BSME

db2integra

more correctly, the process is isentropic (look it up) this should yeild similar results however. It should also be noted that the thermal efficiancy of an otto cycle engine increases with compression ratio, because the combustion pressure dissipates more quickly resulting in a more complete transfer of work from the gas. Saab is working on a variable compression motor with a 15:1 to 8:1 compression ratio variance. At 8:1, the engine recieves 40psi of boost from a supercharger (inefficient, but powerful). While at partial throttle, the CR increases and the boost decreases. High compression is good for efficiancy, while boost is good for power.

db2integra

No one cares?

EKhatch

Make me feel dumb.........what is "OTTO"?

Is that just some fancy way of saying 4-stroke?

fsp31

I care.

That's why I have a turbo.

I'd like to know how Saab plans to build a reliable variable compression motor. Especially since they seem to have enough trouble with reliable "set" compression motors..

VanHonda

I was just considering looking into this explanation for all of the boost vs. compression posts. It is notable however, that the internal combustion engine that we are dealing with is not a true otto cycle, but close enough for a comparison like this.

Good Post

Craig

RAWB

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EKhatch &raquo;</TD></TR><TR><TD CLASS="quote">Make me feel dumb.........what is "OTTO"?

Is that just some fancy way of saying 4-stroke? </TD></TR></TABLE>

Yes

Mr. Anderson

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by db2integra &raquo;</TD></TR><TR><TD CLASS="quote"> (inefficient, but powerful).</TD></TR></TABLE>
my car is an inefficient bitch

EKhatch

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RAWB &raquo;</TD></TR><TR><TD CLASS="quote">

Yes </TD></TR></TABLE>

So what does it stand for?

Egg-weezy

There's alot of other variable that go into the whole caculation when it comes to the effectiveness of a turbo setup with different compressions. If it only had to do with motor relations due to inner cylinder compression than yeah it doesn't make sense to have high comp. with low boost. But it all depends on what your end goal is going to be. Do you want a faster rpm ripping machine(high comp/low boost) or are you trying to get a fast *** drag car that is gonna pull 8secs(low comp/high boost)? Are you trying to get it setup for AutoX, Street racing, gear drops, 1/4mile, ect....Piont is, you can't just say that high boost/low comp is "Better" than high comp/low boost...it depends on your goals.

What about higher compression/higher boost levels compared to lower compression/higher boost levels???

VanHonda

otto is the guy who came up the the power cycle that the internal combustion engine is based on. Our engines technically aren't any cycle, but the otto cycle is the closest. In particular it is isometric (constant volume) heating and cooling, and isentropic compression and expansion in the engine cycle.

Craig

db2integra

Our engines are best described by the otto cycle, however the otto cycle does not describe all internal combustion engines. The diesel cycle is a good example of a higher compression cycle producing more efficiancy; while gasoline (otto) engines generally operate with a CR of 8:1 to 12:1, a diesel cycle engine may operate from 14:1 to as high as 25:1 CR. Some turbodiesel motors have absolutely tremendous effective compression.

db2integra

VanHonda&lt;--- It says I cannot use instant messaging... I believe this is because I am a trial user. Hit me up chaosisl33t=AIM