REFLECTOR:Engine Combustion Theory ... was: Exhaust systems
Brian Michalk
reflector@tvbf.org
Tue, 15 Jul 2003 21:49:34 -0500
Oh, this is going to be fun.
Regarding EGT measurements, I quote John B. Heywood, "Internal Combustion
Engine Fundamentals", Copyright 1988, ISBN 0-07-028637-X.
Page 703-704:
Increasing the compression ratio in an SI [spark ignition] engine decreases
the total heat flux to the coolant until Rc [compression ratio] is around
10; thereafter heat flux increases slightly as Rc increases. The magnitude
of the change is modest; e.g., a 10 percent decrease in the maximum heat
flux (at the valve bridge) occurs for an increase in Rc from 7.1 to 9.4.
Several gas properties change with increasing compression ratio (at fixed
throttle setting): cylinder gas pressures and peak burned gas temperatures
increase; gas motion increases; combustion is faster; the surface/volume
ratio to TC increases; the gas temperature late in the expansion stroke and
during the exhaust stroke is reduced. Measured mean exhaust temperatures
confirm the last point, which probably dominates the trend at lower
compression ratios. As the compression ratio increases further, the other
factors (which all increase heat transfer) become important.
The effect of changes in compression ratio on component temperatures
depends on location. Generally, head and exhaust valve temperatures
decrease with increasing compression ratio, due to lower expansion and
exhaust stroke temperatures. The piston and spark plug electrode
temperatures increase, at constant throttle setting due to the higher
peakcombustion temperatures at higher compression ratios. If knock occurs
(see Sec. 9.6), increases in heat flux and component temperatures result;
see below.
--------
All typos are mine.
Unless you have a reference that indicates otherwise, I take these last to
paragraphs to mean that increasing the compression ratio lowers the EGT.
Also, since "combustion is faster", I conclude that more of the endgas is
consumed before the exhaust stroke begins, therefore increasing efficiency.
Also, since "cylinder gas pressures ... increase", I conclude that torque is
increased, therefore increasing horsepower.
The section 9.6 (page 450) referenced above is titled, "Abnormal Combustion:
Knock And Surface Ignition".
Regarding your remark below, "I can also easily get 220 HP out of my
Lycomiong by installing high compression pistons."
Maybe you can, but it might be a little difficult.
Heywood does not wrap up cylinder geometry/burn rates/detonation factors as
nicely as he does factors related to compression ratio. We have what he
calls "slow burning engines", defined by the "rapid burning angle". The
longer the rapid burning angle, the slower the flame burn rate. The Rapid
Burning Angle is defined on p.389:
"The crank angle interval required to burn the bulk of the charge. It is
defined as interval between the end of the flame-development stage and the
end of the flame-propogation process."
On page 394, "Mixture burning rate is strongly influenced by engine speed.
It is well established that the duration of combustion in crank angle
degress [NOT TIME] only increases slowly with increasing engine speed."
So burning rate is determined by 1) cylinder geometry, 2) RPMs, and 3) other
environmental factors. As you increase the cylinder size, you lengthen the
burn time. As you decrease the RPMS, you lengthen the burn time.
Page 423, "One of the major advantages of fast burning engines is now
apparent. The magnitude of the variations in the flame development process
and subsequent flame propogation rate are decreased as the burning rate is
increased ... Thus, these smaller combustion variations in fast burn
engines, ... have little effect on torque. In contrast, the larger
combustion variations of slow burning engines result in significant cyclic
torque variations."
Since cycle to cycle cylinder pressures are greatest for slow burning
engines, we have to set our aviation engines up with a very conservative
spark timing. We have the worst of all worlds. High cylinder pressures,
large cylinders, and slow RPMS. These three are detonations playground.
Remember how your old distributor ignition cars/trucks would clatter in low
gear with the accelerator pedal mashed to the floor? (high pressure low
RPM)
As a contrast to a fast burning engine:
Honda CBR929RR
Bore and Stroke: 74mm x 54 mm
Compression ratio: 11.3:1
Fuel: "regular pump gas" --- I couldn't find an octane requirement.
Compare the Franklin to the Lycoming. The Franklin is 10 cubic inches
smaller, and it has two more cylinders. This makes its cylinder geometry
such that it has a higher burn rate, and therefore less cycle-to-cycle
pressure variation.
If there is a company making 10.5:1 compression ratio Lycoming IO360
engines, I would like to know. I think I've heard people going 10.1:1, but
never as high as 10.5. I don't think high compression engines are in our
best interest. This is why I'm turboing my Franklin. I gotta be nuts to
turbo a 10.5 CR engine, right? Well, maybe. For now, it turbo normalized.
I strongly feel that 100LL is going to get too expensive to burn. When that
day comes, I'm going to remove the pistons and have them machined down so
that I get about 8:1 compression ratio. Now the turbo really comes in
handy, because it's essentially a variable compression ratio valve. If I
ever fill up with 100LL, I'll be able to turn the boost up, and get my 205
(or 220) horsepower like I always did. If I fill up with 87 octane, then I
won't be able to boost as much.
How do I keep track of all of this? Monitor the Peak Pressure Position
(PPP). There are a few people on the internet cobbling up circuits such
that you can monitor on a per-cylinder basis, the pressure in that cylinder,
and also detect detonation by a characteristic signature. It's called ionic
cylinder pressure sensing, and it uses the spark plug for the sensor. Most
of the work is taken from SAAB patents, and SAE papers. It's a surprisingly
simple circuit. Total parts cost for six cylinder monitoring would be well
under $200. One guy had a single cylinder system on his car with a laptop
for about $15, but a complete embedded processor solution is perhaps a year
away.
Brian Michalk <http://www.michalk.com>
Life is what you make of it ... never wish you had done something.
Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
musician, home-brewer, entrepreneur and mostly single
> -----Original Message-----
> From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
> Behalf Of Scott Derrick
> Sent: Tuesday, July 15, 2003 5:02 PM
> To: reflector@tvbf.org
> Subject: Re: REFLECTOR: Exhaust systems
>
>
> I can also easily get 220 HP out of my Lycomiong by installing high
> compression pistons. No magic there. It requires a high octane fuel and
> it does wear the engine out out a tad faster, probably not measurable
> unless you look at 100's of engines.
>
> I think the EGT's your seeing are lower because of probe placement
> alone. I have two holes in one of my exhaust pipes. Because the first
> placement was not in the main flow and showed 75 degrees cooler than the
> other probes. about 90 degrees around at the same distance and the temps
> were aligned.
>
> Scott
>
>
> Brian Michalk wrote:
> >>observed that peak EGT is about 1400 degrees on engines as
> diverse as A/C
> >>and two-cycle motorcycles, so I will be surprised if the Franklin
> >>engine is
> >>really 500 degrees cooler.
> >
> >
> > I would be surprised if it were this much cooler as well. 50,
> okay. 100
> > degrees barely maybe.
> >
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