Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (March 2001, week 1)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Tue, 6 Mar 2001 13:57:58 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: O2 sensors effect on power
Comments: To: Mark Keller <kelphoto@ISLANDNET.COM>
In-Reply-To:  <3AA51CBD.B8FDD14E@islandnet.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 12:22 PM 3/6/2001, Mark Keller wrote: >The O2 sensor is only used on engines with catalytic convertors. The >pre cat Bosch fuel injection system is quite capable of giving accurate >fuel mixture in operational service. The O2 sensor's main function, IMO >is to safeguard a too rich mixture and thus a cat meltdown.

Actually its major function is to allow precise enough control of mixture so that a three-way catalytic converter will operate properly and produce minimum emissions. The three-way cat takes oxygen from oxides of nitrogen, reducing them to nitrogen; and gives that oxygen to the hydrocarbons and carbon monoxide, oxidizing them to carbon dioxide and water. In order to have the available oxygen from the NOx match the needed oxygen for the HC and CO, the mixture must be almost exactly at the theoretically correct amount (the "stoichiometric mixture," the mixture where the oxygen in the air mass is exactly enough to oxidize the fuel charge). This amount, designated as lambda = 1, is a ratio of almost exactly 14.7 air to one fuel. This is the *required* mixture for the cat to operate correctly. If the mixture is the slightest bit lean the NOx rises dramatically; if it is rich the CO and HC output rise on a steady slope.

The mixture for maximum power is 12.6:1, or lambda = 0.86; the mixture for maximum economy is 15.4:1, or lambda = 1.05. Both of these values are outside of the practical measuring range of the standard O2 sensor in an automotive environment -- in fact in those conditions the sensor really only provides crossover data for the FI system. The grounding and shielding requirements to get numerical mixture info would be extreme -- and are unnecessary, since the FI system is perfectly satisfied to know only where the crossover is.

>IMO the reason a the vanagon horsepower rating drops from 95 to 90 is >that the O2 sensor loop or rich lean cycling means that the engine is >running lean half of the time.

No, it's running lean all the time if you want max power, and rich all the time if you want max economy. The control loop of the FI never gets even close to either of those points.

>The rich side of the loop is at .75 >volts, while max power is at .84 volts of O2 sensor output.

This is unmeasurable under the practical conditions of the system. The output of the sensor for these purposes may be considered to be a step function; the useful information consists of the change from high to low voltage, not the actual value of the voltage. O2 sensors exist which provide practical measuring ability, but they are very expensive and completely unnecessary to the operation of the system (I understand that Honda has an engine now that needs and uses a sensor that gives numerical info for some range on the lean side of stoichiometric).

>It is impossible for a Digifant O2 sensored fuel injection system to >provided more power than a non-senosred digijet, because the digifant >will never set optimum power fuel mixture, while the digijet system can >be set to deliver optimum fuel mixture all the time.

True except for one little detail: at full throttle both systems ignore the lambda sensor and provide a programmed enrichment. The amount of the enrichment could be reprogrammed in either one...

david

David Beierl - Providence, RI http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85 GL "Poor Relation"


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.