From: John De Armond
X-Source: The Hotrod Mailing list
Date: Mar 1993
Subject: El Camino Mileage mod
Here's my r.a.t posting. Never heard anything back from this guy.
From: email@example.com (John De Armond)
Subject: rec.autos.tech, EGR/Fuel Economy
Date: Thu, 28 Jan 93 21:23:46 GMT
firstname.lastname@example.org (Tom Leone) writes:
I missed your first post, Tom, so pardon me if I duplicate some comments.
>: >YES, EGR does improve fuel economy. There are several reasons:
>: >(1) Reduced "pumping work" needed to move gases from a low-pressure
>: >intake to a high-pressure exhaust. When you dilute the air/fuel
>: >mixture with EGR, you must increase intake pressure (open the throttle
>: >more) to get the same mass of air/fuel for the same torque. Note
>: >EGR is automatically turned off at wide-open-throttle, so you get
>: >the same maximum torque/power.
I would think the reduced pumping work would come from the lower
differential pressure across the piston between the near atmospheric
pressure in the crankcase and the fairly high vacuum in the intake.
This DP lasts longer and should be greater than that between the crankcase
and the exhaust. Most exhaust system I've measured run <1psi at cruise.
Side question: Why does no production engine (at least that I'm aware of)
apply partial vacuum to the crankcase? This seems to be a mostly
no-brainer to reduce drag, pumping losses and oil leaks. I modified a
Datsun Z engine to suject the crankcase to manifold vacuum via checkvalves
(to retain it during WOT) and gained a couple MPG at cruise.
>: >(2) Reduced heat transfer losses, due to lower combustion temperatures.
>: >(3) Less dissociation in the burned gases, due to lower combustion
>: >temperatures (allows fuller use of fuel's energy for useful work).
>: Explain a little further on 2 and 3.....
>Well, diluting the mixture lowers combustion temperatures because you
>have to heat up some unreactive gases. That's the main idea of EGR,
>because lower temperatures decrease emissions of NOX.
>It also reduces heat transfer, because convective heat transfer
>is proportional to the difference in temperature (between gas
>and walls), and radiative heat transfer is proportional to the
>difference of the FOURTH POWER of the temperatures. Less heat
>lost to the walls means a greater percentage of the heat is
>going into increased gas pressure, therefore increased work.
I looked up your reference (Internal Combustion Engine Fundamentals,
Heywood, page 837,838). Very interesting. Appears from his chart
that BSFC minimizes at about 25% EGR. he also notes that fuel disassociation
is a minor issue and notes that induced air has the same effect (more on
that later.) I'm really wondering about the magnitude of the radiative loss
issue vis a vie reduced Carnot efficiency resulting from lower combustion
temperature. Seems to me like radiative loss would be a rather small
component for a lambda=1 mix because of the low luminousity of the
combustion flame. Wonder if you would have any other information
on this aspect?
>Even the obsolete carbureted cars have "flapper" valves to take
>warm air from around the exhaust during part throttle. They are
>vacuum-operated, so they take cold air at wide-open throttle
>(less vacuum). And again, the EGR is automatically off at
>wide-open throttle, so it does not heat up the air. And, I
>don't like your "tone of voice", especially when you are wrong.
Well sorta. None of my (older) vehicles use ported vacuum or anything else
other than intake temperature to operate the heat stove flap. Has
this changed in later model cars?
>: >Reference: Heywood, "Internal Combustion Engine Fundamentals", p. 837-8,
>: >including graph showing approximately 10% improvement in fuel economy
>: >for 20% EGR (for a certain engine and speed/load point).
>Bob Valentine wrote:
>: Exactly. At some point, there will be a gain. But at what
>: percentage of the time does the engine run at that point, and will the
>: losses to either side of that point compensate for the gain at that
>: point. Given, that point is not a tiny little peak, but still....
It should be pointed out that the chart in question shows large gains
for fast burn chamber designs. On the same chart is a line for slow
burn chambers that shows much less improvements and then only at
low (5-10%) EGR injection. This graph shows why EGR has such a bad
rap. Older designs were CRAP. The EGR valve slammed open at part
throttle and dumped a large but fixed amount of EG in. One could
typically FEEL the EGR kick in on a 70s and early 80s car. There is
now a large education task at hand to reverse the perceptions of the
last couple of decades.
>What losses on either side? The EGR valve is designed to give the
>optimum amount of EGR at all times. The modern systems are
>computer-controlled, and even the older systems use a vacuum signal
>to get approximately the right amount. Even a little EGR is better
>than none, and the only possible loss is if you have way too much
>EGR and hit misfire. And during typical driving, a huge percentage
>of the time is spent at part throttle, where EGR is a great fuel
>saver. When you are accelerating hard, the EGR goes off automatically
>and you have good power. What's the problem?
I realized over 10 years ago that a lean-of-stoich cruise mixture, and
the resultant higher manifold pressure, would improve mileage dramatically and
built a device to prove it. These devices were fitted to many of my cars but
the best result was on my 79 El Camino. The device consists of a fresh air
metering valve and some controls. The metering valve is a modified
Datsun A.I.R injection anti-backfire valve. The seat orfice is about 10 mm,
a relatively high flow device. This device feeds air from the air
cleaner to a plate fitted where the EGR valve once went. (This is one of
those garbage EGR implementations that could be felt) The controls
consists of ported vacuum switched by a thermal valve to defeat the
actuation when the engine is cold and an adjustable orfice and volume
arrangement whereby the slope of the applied vacuum to the metering valve
could be adjusted and time-delayed.
The anti-backfire valve was modified so that instead of pulsing open
it opened and remained open when vacuum was applied. The original had
a diaphram across a sealed chamber with a small orfice to bleed pressure
across the diaphram. When vacuum is applied, the diaphram opens the
valve until the pressure in the sealed side is equalized by bleed through
the orfice. My modification was simply to drill a hole through the housing
into the sealed chamber so it always had atmospheric pressure on it. The
orfice bleed is low enough not to matter.
The thermal valve was already fitted to the engine for some other
purpose. It is closed until the engine is almost fully warmed.
This mechanism, fitted to the 305 small block, improved the long
trip mileage from 18-20 to 25-27 mpg. Town mileage improved some but
not dramatically because the delay, used to improve throttle response,
kept it un-actuated most of the time. The down side of the device is
that throttle response at cruise was somewhat soggy, since additional
throttle had to be dialed in to respond to the opened air valve.
My homemade cruise control system handled that quite nicely :-) I'm sure
electronic controls would have greatly improved the performance of this
Tom, I have a couple of questions for you:
* Does the EEC-IV EGR algorithm use the IAC stepper to increase the
effective throttle angle during EGR introduction? This would
seem logical and would mitigate the sogginess EGR often causes.
I have studied GM's algorithm, which basically involves using
manifold pressure and RPM to point into an EGR flow lookup table.
There is no connection to idle air control. If Ford (or anyone
else) does not do it, why not?
* Assuming a fly-by-wire system, has there been any consideration
to using massive EGR as a means of throttling the engine? It
seems to me that with the fast burn chambers and effectively
stratified charge designs now popular, that massive EGR could
be used to throttle the engine with fresh air being admitted
only as needed to burn the fuel required for the power setpoint
(accelerator pedal position.) It seems that the manifold
pressure could be essentially atmospheric at all times.
If this has been experimented with, could you point me to
Thanks. Interesting discussion.
From: emory!merlin.gatech.edu!ucsd!btree!hale (hale)
X-Source: The Hotrod Mailing list
Date: Mar 1993
Subject: Re: El Camino Mileage mod
John described his manifold air bleed which he used instead of EGR
to get mileage improvements. Since exhaust gas has virtually no
oxidizer left it wouldn't be expected to cause an oxidizing
environment in the combustion chamber, but allowing plain air into
the manifold will add extra oxygen. Does this pose a problem for
things like valves and piston tops? If you run out of gas at WOT,
many engines will burn parts. It seems as though the same problem
would exist here but to a lesser extent.
Bob Hale ...!ucsd!btree!hale
[Not a problem because it activates only at low throttle settings where
there is ported vacuum. The engine's maybe making 20-30 hp at that
As a practical matter, I've put over 100k miles
on this engine with no problems until I er, kinda forgot to keep it
full of oil. it now rattles its rods in protest :-( It still doesn't
use any oil, though it does pour it out the rear main seal. JGD]