Engine Compression vs. Exhaust Gas Temp? 1

[RobHughes]

Given the same motor, same fuel (high enough octane to be compatible with the higher C/R) and different compression ratios (let's say one is 8.5:1 and the other is 10.5:1). Will there be any difference in exhaust gas temperature?

FYI- I just "found" this site, after reading about 10 pages of posts, I'm hooked. Thanks to all who contribute and make the site what it is.

Rob

 

[MaxRaceSoftware]

the 10.5:1 CR engine will have "Lower EGTs"
than 8.5:1 CR

the lower the CR the higher the EGT

the higher the CR the lower the EGT


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[mburgess]

Larry,
Why is this?? Maybe due to the greater expansion of burnt gasses, or from the greater work done by burnt mixture ??
What would be the relative difference in the EGT. Enough to require a change in the length/dia of a set of tuned length headers??
Regards,
MB

 

[MaxRaceSoftware]

Maybe due to the greater expansion of burnt gasses, or from the greater work done by burnt mixture ??
What would be the relative difference in the EGT. Enough to require a change in the length/dia of a set of tuned length headers??
Regards,
MB
----------------------------------------

due to the greater expansion of burnt gasses sooner in
power stroke

if both 8.5 and 10.5 CR were optimized in dyno tuning
then 10.5 CR would be very much lower in EGTs and BSFC
BSAC

The 10.5 CR would have lower EGT and header pipe length
would have to be SHORTER because speed of sound is now slower....but differences in length changes would be very small.

Also related to EGTs ;

the leaner A/F Ratio the higher EGTs
the richer A/F Ratio the lower EGTs






Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[SBBlue]

Let me see if I can muddy up the waters a little here. . .

Actually, its not increasing the compression ratio that increases power output and decreases exhaust temperature. Its increasing the expansion ratio that does the trick. Now for most engines the expansion ratio is the inverse of the compression ratio -- an engine with a 10:1 compression ratio will have a 1:10 expansion ratio.

However -- with an engine with variable valve timing its possible to have, for instance, a 5:1 compression stroke and a 1:10 expansion (combustion) stroke.

How much of a difference do the different expansion ratios make?

Assuming a combustion temperature of 3500 deg F, here are some brief simplified calculations from the standard gas tables that are not completely accurate (neglects efficiencies, friction, doesn't model combustion right, etc) but should give you a rough idea of what's going on:

For a 10:1 compression/expansion ratio: Compression work: 137.9 BTU/lb gas: Output 441.09 BTU/lb, Net output: 303.18 BTU/lb, Fuel energy: 756.05 BTU/lb, Efficiency 40.1%, Exhaust gas temp 1520 deg F

for a 12:1 compression/expansion ratio: Compression work 155.4 BTU/lb gas, Output 465.1 BTU/lb, Net output 309.7 BTU/lb, Fuel energy 732.8 BTU/lb, Efficiency 42.26%, exhaust gas temp 1403 deg F

Hope this helps!!!

 

[Tmoose]

>snip< also related to EGTs ;
>the leaner A/F Ratio the higher EGTs
>the richer A/F Ratio the lower EGTs

If the time is spent to optimize ignition timing, OK.
If the A/F unexpectedly changed, or time is NOT spent to optimize ignition timing, then I think a point will come when when the increase in EGT may be more a result of effectively retarded ignition timing, which in the extreme case would have most of the combustion energy going right out the pipe. I sometimes wonder how much of the "burnt up my motor running lean" actually started from roasted exhaust valves when power riming is applied to slow burning lean mixtures.

 

[MaxRaceSoftware]

For a 10:1 compression/expansion ratio: Compression work: 137.9 BTU/lb gas: Output 441.09 BTU/lb, Net output: 303.18 BTU/lb, Fuel energy: 756.05 BTU/lb, Efficiency 40.1%, Exhaust gas temp 1520 deg F

for a 12:1 compression/expansion ratio: Compression work 155.4 BTU/lb gas, Output 465.1 BTU/lb, Net output 309.7 BTU/lb, Fuel energy 732.8 BTU/lb, Efficiency 42.26%, exhaust gas temp 1403 deg F

( doesn't model combustion right, etc) but should give you a rough idea of what's going on
------------------------------------------------------

i come up with ;
10:1 CR = 1421 to 1449 EGT (average thru 600 RPM/SEC Dyno test)

12:1 CR = 1298 to 1324 deg F EGT
(average thru 600 RPM/SEC Dyno test)



Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[RobHughes]

Thanks guys for the help with this one, but it's real hard to dispell my customers ideas that higher compression equals more heat (as well as more power) things like the following aren't helping things...you'd think NGK would be more careful about such "blanket statements.

From this web site:

http://www.ngksparkplugs.com/techinfo/spark_plugs/techtips.asp?nav=31000&country=

"Page 4 of 5

Below is a list of some of the possible external influences on a spark plug's operating temperatures. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur.

Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.

Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability
Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

Compression can be increased by performing any one of the following modifications:

a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)

b) adding forced induction (Nitrous, Turbocharging or Supercharging)

c) camshaft change
As compression increases, a colder heat range plug, higher fuel octane, and careful attention to igni-tion timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage
Advancing Ignition Timing

Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C
Engine Speed and Load

Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed
Ambient Air Temperature

As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
As temperature increases, air density decreases, as does intake volume, and fuel delivery should be decreased
Humidity

As humidity increases, air intake volume decreases
Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's tem-perature and a reduction in available power.
Air/fuel mixture should be leaner, depending upon ambient temperature.
Barometric Pressure/Altitude

Also affects the spark plug's tip temperature
The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature de-creases, so does the plug tip temperature
Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
The real answer is to adjust jetting or air/fuel mixtures in an effort to put more air back into the en-gine
Page 4 of 5"


Also someone pointed out this websites quotes (I'd bet these guys are pretty bogus after looking at the products they sell):

http://www.castle-comply.com/Pages/articles/RSGDAN.html

"There are two ways to control NOx production from today’s vehicles: Pre-combustion and post-combustion. Pre-combustion NOx control is the primary method of reducing NOx emissions. Simply stated, it is necessary to keep combustion temperatures low. Methods of doing this include lowering compression, retarding timing, enriching the fuel mixture and EGR flow. Post-combustion NOx control occurs in the catalytic converter."


Larry (or anyone else), would you have any analogy I could show my customers that puts it in semi plain english, lots of them are still confused when I tell them that higher octane fuel on it's own will not make more power..lol

Thanks again, I'm so glad I found this site.

 

[SBBlue]

Try this approach:

It is true that increasing compression ratio will increase the temperature of the compressed gas. Here's some examples, assuming an ambient intake air temperature of 80 deg F:

8:1 compression -- 751 deg F
10:1 compression -- 854 deg F
12:1 compression -- 943 deg F
18:1 compression -- 1157 deg F

So it is true that increasing the compression ratio will increase the temperature of the compressed gas.

But -- when fuel is added to the mixture, the temperature goes way beyond that -- to a flame temperature of maybe 4000 deg F and a mixed gas mass temperature of 2500 deg F.

So the increase in temperature just from the compression is not all that great, and the exhaust temperature winds up being lower.

 

[Turbinator]

This is my first post, I've browsed here for a while and it is a great site!

Fuel has a certain amount of energy in it. From that burned fuel, a percentage of the energy is absorbed into the coolant as heat, some heat leaves through the exhaust, and the rest of the energy is used to produce power. Increasing the compression ratio increased the percentage of fuel energy converted into power, while lowering the percentage turned into wasted heat, meaning lower EGTs.

 

[MaxRaceSoftware]

Larry (or anyone else), would you have any analogy I could show my customers that puts it in semi plain english, lots of them are still confused when I tell them that higher octane fuel on it's own will not make more power..lol
=======================================================
RobHughes,
i can't think of analogy at moment, but have real world dyno example => i had a customer years back that ran Buick V6 w/ 13+ Comp Ratio in IMSA road racing
with VP C-16 race gas . He then decided to try a Chevy V8 Low-Compression Ratio (9.4:1 CR) that was allowed in same IMSA class to race against Hi-CR Buick V6

he had about 11 or so drums of VP C-16 left over , so tried out C-16 with 9.4 CR Chevy V8 on my dyno
we kept increasing timing 2 degrees at time till we reached 48 deg BTDC was best average Torque/HP Curve

i talked him into trying Exxon 93 prem for back-to-back tests ...we started around 32 deg BTDC and kept increasing timing till at 36 BTDC engine made best average Torque/HP Curve

9.4:1 CR with Exxon 93 Prem and 36 BTDC was 20+ HP better
than with VP C-16 with 48 deg BTDC in that combo

that particular combo test was the most HP/TQ difference i've seen from Octane being overwhelmingly too much for the Comp Ratio ...usually its no where near as much of a difference as it was in those dyno tests !




Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[MaxRaceSoftware]

http://www.maxracesoftware.com/egt_probes.htm

a little more EGT info and a few EGT probe pictures


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[ivymike]

the max for combustion temp is actually a little to the lean side of stoich, but not very far (5% excess air or so). I'm not certain that exhaust temps track in-cyl temps directly, but I expect they're close. Usually when people say that a "lean" afr results in higher temps, they're talking about lean as compared to "very rich," not lean as compared to stoich.

 

[MaxRaceSoftware]

leaner mixture burns slower and continues to burn relatively later in exhaust stroke
thereby increasing EGT temps, exh valve temps, coolant temps, etc.


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3