2003 Accord losing MPG on highway

No... No, No, No.

Assume the idealized hypothetical where the full volume of a cylinder
full of fuel-air mixtures is at 1 atmosphere pressure as the valves
close and the piston begins its compression stroke; the compression
ratio is determined by dividing the volume of the cylinder at TDC
(say 40cc) by the volume of the cylinder at BDC (say 400cc).

Now assume a circumstance where a turbo charger or supercharger has
filled the same cylinder, just as the valves close and the piston
begins its compression stroke, with fuel-air mixture at 2 atmospheres
pressure; the compression ratio is still determined by dividing the
volume of the cylinder at TDC (still 40cc) by the volume of the
cylinder at BDC (still 400cc).

No change in compression ratio, no mysterious "units".

- So why more power from turbo/super charging?

Basic physical chemistry: At constant volume and temperature there
will be twice as much fuel and air in the cylinder at twice the
pressure. (The Ideal Gas Law: PV=nRT, where P,V,T, and n are
Pressure, Volume, Temperature, and amount of the gas with R as the gas
constant.)
&
Simple thermodynamics: Twice the fuel-air - "twice" the power.

- Why does increased compression ratio yield increased power?

Basic combustion science: The greater the density and temperature of
the fuel-air charge, the more efficiently it burns.

 

<snip>

The reason why I said "units" is because you could use any method of
measurement you like; English or Metric. Most use metric, so you would use
milliliters (or cubic centimeters; 1mL = 1cc) of fuel & air. But if someone
wanted to use cubic inches, the same mathematical formula would be used to
calculate the ratio and/or pressure.

 

Yeah... I'm not buying it.

You claimed a turbocharger will double the number of "units" of volume
in a cylinder: That is patently false and incorrect. Whether
measured in cc, ci, cubic fish eyes or ping pong balls; the swept
volume of a cylinder, the volume of the combustion chamber, and
thereby the compression ratio, all are rigidly fixed in any production
automobile and cannot be varied without major mechanical modification.

A turbo/supercharger only increases the pressure of the fuel-air
mixture in the quite fixed maximum volume of a cylinder. By
increasing the pressure of the air-fuel mixture the density is
increased, resulting in an increase in the amount of fuel and air
available for combustion.

PV=nRT. It's perhaps the primary physical law.

The swept volume stays quite constant, the combustion chamber volume
stays quite constant, the compression ratio stays quite constant:

It is NOT rocket surgery.

 

You keep missing that word "effective".

 

I'm afraid you've got the wrong person...I never said anything about a
turbocharger or supercharger in any of my postings. If you'll scroll back
up to the first few responses to this message, you'll find that it was "Jim
Yanik" who mentioned turbos and superchargers.

Jonathan

P.S. What I said is correct. For example, 1.5 Liters = 1500cc = 91.5 cubic
inches. So regardless of whether you use liters, cubic centimeters, or
cubic inches as your standard for measuring volume, the compression ratio
for the engine is still figured the same way; not by the volume of air/fuel
in the cylinder.

 

[Addition to my last message]

Furthermore, after reading your post about how the compression ratio
is determined by dividing BDC volume by TDC volume, I believe that is
completely accurate.

 

Jim Yanik wrote:

The term you need to use is pressure. "Effective Compression Ratio"
(Usually called Dynamic Compression Ratio), refers to something else.
Specifically, the intake valve does not close at bottom dead center, it
closes later (typically). Thus during the first part of the piston's
upward movement, little or no compression takes place. Knowing the
geometry of the engine and the valve timing, one can calculate the
dynamic (or effective) compression ratio. Turbo boost (or throttle
restriction) does not factor into the calculation of effective
compression ratio.

 

And you were responding to, and inclusively referencing, the previous
post referring to turbo/supercharging.

What you've said is incorrect: because you are invoking units in two
non-consistent purposes:

"Say your compression ratio is 10:1, you put ten "units" of air/fuel
mixture in the cylinder. It will be compressed to 1/10 of it's
original "size" (volume) upon combustion. If you put in 20 "units",
it will be compressed to a 20:2 ratio, which reduces back to 10:1.
30:3 reduces to 10:1, and so forth. More "units" just causes a more
powerful combustion, creating more pressure to push the piston down
more forcefully."

NO.

Your equations only work to transpose actual units: A cylinder which
measures at 10ci will measure 167cc, a combustion chamber which
measures 1ci will measure 1.67cc, as the units drop out the
compression ratio will remain the same "unit"-less 10/1.

Your last sentence embodies the confusion: "More "units" just causes
a more powerful combustion, creating more pressure to push the piston
down more forcefully."

NO

A 167cc cylinder would _not_ create more power than an identical
cylinder measured in the English system at 10ci: Cc's of air-fuel are
not more energetic than ci's of air-fuel, even if "there are more of
them".

One cannot fill a cylinder of X units of _volume_, let us use
"fuckwits", completely for comic effect, imagine a cylinder with a
swept volume of 10"fuckwits", or 10"fckwt"...

....One cannot in any way fill a cylinder of a swept volume of
10"fckwt", with 20"fckwt" worth of volume.

Hypothetically: One can fill a cylinder of a swept volume of
10"fckwt" with 10"fckwt" of a gas at a density of 100g/l, at 1atm.

In the case of supercharging, one can fill that same cylinder with
10"fckwt" of the same gas at a density of 200g/l, at 2 atm pressure.

"Twice" as much fuel, "twice" as much oxidant: "Twice" the energy
release. (Twice the charge pressure greater combustion efficiency.)

....SAME VOLUME.

If the combustion chamber is 1"fckwt" in volume the compression ratio
is 10"fckwt"/1"fckwt" or 10/1, a completely "unit"-less quantity,
regardless of the internal pressure of the cylinder.

So, you see: "Units" of volume don't really enter into it at all.

Perhaps an illustrating aside: Intake tuning can result in increased
power as the pressure waves formed in the intake system are exploited
to pressurize the intake charge to some appreciable level over NA.

....Still no change in "units" or volume.

You will notice that the pressure of the compressed charge will
increase quite dramatically as boost is increased, which is why most
factory turbocharged street engines have reduced compression ratios,
compared to their NA cousins.

 

You keep misusing the word "effective".

 

<snip>

OK, last time of explaining what I meant, and then I give up. I know and
understand that RATIO is "unit-less". What DOES have units is volume; i.e.
the displacement of an engine. (1.5L, 1500cc, and 91.5ci are ALL EQUAL TO
EACH OTHER, so _of course_ one isn't going to generate more power than the
others) When I mentioned "units" before, I couldn't remember g/l, which, as
you said, is the UNIT for density. Also, as per your info, you can increase
the amount of fuel/air density, (the UNIT for that is "g/l") and the amount
of pressure, (the UNIT for that is "atm") but the VOLUME (L, cc, ci) is
fixed. I got that part. So my "units" on the compression ratio were the
g/l : atm. So in essence, what I said is just like what you said with the
'fckwts': 20:2 & 30:3 reducing to 10:1 compression ratio; 20g/l at 2 atm &
30 g/l at 3 atm. I guess my wording was misleading. But, I do understand
completely what you are saying, and it all makes perfect sense to me. So
you don't need to elaborate or expand on it any further. ;-)

Jonathan

P.S. So do you now understand that my intention was for the "units" to be
"g/l" and "atm" when I gave my examples? I just couldn't remember them.
(I've been out of high school for 7 years now and I hated chemistry class)
;-)