Improving fuel economy for the 1991 200q 20v
cord4530 at uidaho.edu
Fri Mar 4 18:31:01 EST 2005
> I am curious about
> possible causes for lower than expected mileage for the 1991 200q 20v cars.
> Conversely, I am thinking about what changes could be made to improve fuel
> economy. It seems many things are controlled by the ecu, but there are
> still many things that could affect fuel economy.
There *are* many factors that go in to fuel economy. You wrote a pretty
good list. But there are some factors that are a *lot* more important
On the engine side...the best you can realistically hope for (ECU/sensor
constraints) is to run a stoichiometric air/fuel ratio. Your ECU uses
the O2 sensor as the feedback for this. If that's not functioning
correctly, fuel economy will suffer. On another note, (not sure on the
specifics of our ECU), the ECU typically disregards the O2 sensor at
higher throttle conditions. For some ECU's, anything over 70% will go to
open-loop. The O2 sensor is the main item, but many other things can
cause your air/fuel ratio to be incorrect. MAF sensor, air leaks....this
list alone could get exhaustive.
Other engine items....anything that reduces friction (thinner oil,
synthetic...etc.) will help. Also, running *higher* compression ratio
pistons will help. Of course, you couldn't run a turbo...but, but the
fuel economy would improve. Likewise, anything that will improve
combustion efficiency would help. If everything is in working order
(plugs, wires,...) there won't be much to gain. But any parts *not*
working properly can really hurt you.
Transients (not homeless people) in the engine load/speed require extra
fueling. In theory, if you could maintain a single load and speed, that
On the chassis side, (already mentioned) alignment makes a big
difference. Technically, no toe would offer the least rolling
resistance. Camber...hard to say. Running zero camber would make your
tires flat on the road (good for braking/accelerating), but puts the
largest contact patch down too. Excessive camber would have a smaller
contact patch, but would lead to higher tire deflection (and horrible
tire wear too). I believe Bernie prefers to run at zero camber, but my
memory isn't great... Also, tires make a big difference. With our HEV
and EV vehicle projects, we found large improvements when running
special 'low rolling resistance' tires. The tires last a long time, and
reduced road load....but they handle horrible. As previously notes, a
skinny tire at higher pressures will offer less resistance than a big,
fat, sticky one. Honestly, after the cool winter, your tires could be
slightly under pressurized...
You didn't mention anything about aerodynamics. At highway speeds, the
drag force is quite significant. Hard to say what would improve the
aerodynamics of our cars....but it could be improved upon. Since drag is
proportional to the cube of velocity (the power to overcome drag
anyway), cruising at 60 mph will likely get you better fuel economy than
cruising at 80 mph.
So, are you looking for hardware changes, or driving style changes?
Unless you have some defective hardware (O2 sensor, plugs, wires, etc.)
you probably can't justify any cost savings by replacing parts. $1000 of
aerodynamic enhancements will probably take 400,000 miles of driving to
pay for itself in fuel savings. But, driving style changes can make a
HUGE immediate difference. If you're serious about this, I highly
recommend getting yourself a wide-band O2 kit with a display. Mount this
in your field of view while driving. When the meter reads around
14.5...you're doing okay. When it dips to 12:1....not so good. This
serves as a reminder for the driver, but can also alert you to when the
engine isn't being fueled properly.
I'm only touching the surface here, but since this is what I do a lot
of, I thought I should chime in with some input.
University of Idaho - Engine Research Facility
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