RPM signal converted to Variable resister question

[Mag00]

Hi Ya'll, I was referred here by #12 at all about circuits.

I have a basic understanding of electronics.

The challenge is to take a tachometer signal and tie it to a variable resistance to alter a lambda sensor output going to a computer (ECU).

I am in the discovery phase, (just starting LOL) and do not know what semiconductor or IC chip or arrangement of electronic components can do the job.

The RPM needs to be read without bleeding much power off the wires running to the coil. I have taps on the wires for a dyno rpm reading.

The engine runs lean at low rpm, and rich at high rpm. The lambda (02 sensor) sends a voltage signal to the ecu to correct to predetermined specs residing in the ecu. If I can change the resistance, thus changing the voltage in proportion to RPM, I think I can have a good solution. I may be able to use an on/off type setup, that cuts on or off at a selected rpm.

Unfortunately nobody has been able to crack into the ecu and upload new fuel and ignition maps, so a piggy back fix is the next best thing to a real programming solution.


What semi conductor should I be looking at, or do I need to design something to do the job using transistors and other hardware? Been 30 years since I took electronics, maybe I need to crack a book again.

Any suggestions will be appreciated.

Dave

 

[crutschow]

Generally it's not easy to generate an accurate variable resistance. What exactly does the lambda sensor output look like? What are the resistance and voltage levels? What variation in resistance do you need?

 

[Mag00]

The signal from the lambda (O2) changes as the oxygen content changes, and it cycles anywhere between .2 and .7 with the median point about .45 volts.

What I understand is more oxygen will produce a lower voltage. I think I need a lower voltage to create the illusion of being lean at low rpm so the computer will compensate and feed more gas.

We have been using a variation of this on the Air Intake sensor to trick the computer into thinking it is colder out than actual temp and that in turn enriches the mixture.

The ECU is smart and figures this out and compensates to a degree. The fuel map tells the computer what target values should be and uses all the sensors to figure this especially the lambda. The problem I have if I just add a resistor to lower the voltage, it will do this at the higher rpm where it is running rich. (according to the dyno tests). That means I need to remove the resistance after 4k rpm.

Thus something to detect rpm and either on off at the target rpm or a variable resister with rpm, and I will have to figure what curve I need etc, then buy the semiconductor that can meet the requirements. Or make some small controller circuit board to do the job.

I just don't know how to read the rpm and translate that to talk to a transistor, or if there is a chip that may already be configured to do similar. Alot has changed since I went to school in the semiconductor area. The basics seem to be all good.

My understanding is that the lambda has about 1,000,000 ohms of resistance, but I will measure when the time comes to purchase parts.

I'd like to enrich the low rpm range by about 6%. If range is .2-.7 volts that is a .5 variance. Reaching an effective .4 voltage shoud do it.

Don't pay attention to my ramblings on these calculations, just dusting the cobwebs out of my brain LOL. Am I guessing a 20megohm resister to be about right, hooked in parallel to ground? (for a non variable solution, not using the RPM cuttoff)

It's getting late, thanks for your interest and help in this project. I will check back in the morning.

Dave

 

[debe]

A bit of information on Lambda sensors from when I did an EFI course some years ago. One thing we were told not to do was measure the resistance of the sensor as it can be damaged. Only measure its out put voltage

 

[Mag00]

Always good to keep in mind about damaging things.

The one meg ohm resistance came from another site. I need to know the resistance so as to create the proper ratio for the added resistor. I suppose I can do a trial and error, as the resistors are cheap enough. But the basic calc should give me a starting point.

Let's assume that the signal voltage is 12 and the rpm range is 0-8k rpm.

If I want to create an open circuit in the added resistor circuit, at 4500 rpm, what type of semiconductor do I need? There are two separate circuits, one with the resistor to ground and onetapping the output voltage of the lambda. The resistor circuit needs switching to be open at the 4500 rpm. The second side of the circuit has to identify the rpm and throw the switch. The two circuits cannot use each others voltages.

In a perfect world, I would like a variable resistance based on RPM.

What would I use? Something like a rev limiter?

 

[picbits]

The Lamba sensor outputs a voltage - not a resistance. You can just buffer it with an opamp and use electronics to add or subtract voltage from your circuit.

The LM2917 will convert your RPM frequency to a voltage too. You can then use something like a comparator to turn things on and off at a certain voltage.

Reading through your apparent lack of electronics knowledge, I'd suggest skipping this project. You aren't just dealing with simple signals, you also have to deal with the harsh electronic environment car electrics present. I designed something for automotive use many years ago and it took me over a year and many of thousands of £££ to get it perfected. I also have a 30+ year background in electronics.

 

[dr pepper]

I thought stoichometric fuel/air gave 0.6v from from the lambda, or is a that a lambda of 0.6 I'm a little rusty.
This project is begging for a pic microcontroller, but I guess thats probably a little too deep for you.
Lambda response times are not quick so maybe you could use a motorised pot to control the level from the lambda, or maybe the fresh air/recirc air servo from a car heater, these just have 3 wires and 2 rest positions, you could link this to a pot, moving the arm out from the centre of the pot would change the rate, and the servo could be controlled from as you say a programmble rev limiter.
This seems like a more doable way.
In my experience ecu's get all upset when the i/p isnt from a ego sensor, maybe they try and sense the condition of the sensor by measuring its impedance or something mad like that.

Are you into racing?

 

[Mag00]

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The LM2917 will convert your RPM frequency to a voltage too. You can then use something like a comparator to turn things on and off at a certain voltage.

I took a look at the LM2917 (https://www.ti.com/product/lm2917-n) and have a couple questions. Looks like more than I would need for the circuit, but then again, I am now seeing some advantages to this.

"The LM2907, LM2917 series are monolithic frequency to voltage converters with a high gain op amp/comparator designed to operate a relay, lamp, or other load when the input frequency reaches or exceeds a selected rate. "

I am going to assume that the extra terminals are so I can design and incorporate circuits to determine the frequency and other variables at which to operate a switch. Is this a fair assumption?

Anyway, added to a board and enclosed in a case, the module would still only need four wires to operate. Tach signal (2 wires) and the bleed off (for lack of proper term). The high ohm resistor to drop voltage slightly. After I do another comprehensive dyno run and get more accurate O2 readings accross all RPMS, I can then narrow down the specifics.

I do not wish to produce voltage, or add voltage to the lambda output at this time. I have a lean condition, which the computer will identify as a higher voltage from the O2 sensor, so dropping the voltage using a resister in parallel will do the job.

So my next question is, what electronic gadgets, microwaves, vcr tv or other might I find the LM2917 in? Also as long as I am looking, are there any chips anybody else might be interested in while I am scrapping?

Dave

 

[Mag00]

I thought stoichometric fuel/air gave 0.6v from from the lambda, or is a that a lambda of 0.6 I'm a little rusty.
This project is begging for a pic microcontroller, but I guess thats probably a little too deep for you.
Lambda response times are not quick so maybe you could use a motorised pot to control the level from the lambda, or maybe the fresh air/recirc air servo from a car heater, these just have 3 wires and 2 rest positions, you could link this to a pot, moving the arm out from the centre of the pot would change the rate, and the servo could be controlled from as you say a programmble rev limiter.
This seems like a more doable way.
In my experience ecu's get all upset when the i/p isnt from a ego sensor, maybe they try and sense the condition of the sensor by measuring its impedance or something mad like that.

Are you into racing?

I like racing, I do not compete. I love performance and I like things to work right. This is for my New Husqvarna and they come too lean from the factory to run proper. Nobody has cracked this ecu as of yet, to change fuel maps. There are add on gadgets to help out, like the PC5, but I do not need anything that elaborate for my driving style

Also, I have a 4x4 that I am doing an engine upgrade, and going to install the Wolf 3d engine management system to.

I may in the future look to the micro squirt for an ecu replacement. If you know of anybody who specializes in ecu programming etc, I am all ears.

BTW, the jargon should come back eventually.

I hadn't considered the self test on the lambda. That is a concern. I remember reading about how ecu goes into open loop if voltages did not check out as when the O2 is not up to operation temp. That is the beauty of the on off resister bleed off. I think now, it may need to be an off on off type setup. Off in a resting state, on at low rpm, and off again at target rpm of about 4k.

Very interesting indeed.

 

[dr pepper]

The lm2917 is not all that common in domestic stuff, you might find one in a 70s or 80s car electronic tacho, your better going to somewhere like digikey or mouser.

To shunt the lambda I spose it depends what you mean by high ohm resistor, I'd start with a 1k, you might even need one a few hundred ohms or less to drop the voltage, you could switch in the resistor with a relay or transistor.

If you do a google you'll find tacho's using the lm2917, and probably some led ones that use a lm3914 chip which has 10 led outputs, if you had 2 of these as a 20 led bargraph you could tap off one or more of the led outputs to control a relay or transistor switch to bring in the lambda shunt resistor.

 

[Mag00]

Interesting, very interesting. I haven't fully wrapped my head around this.

My initial thoughts were on adapting to use a transistor as in this vid,

 

[Mag00]

The lm2917 is not all that common in domestic stuff, you might find one in a 70s or 80s car electronic tacho, your better going to somewhere like digikey or mouser.

To shunt the lambda I spose it depends what you mean by high ohm resistor, I'd start with a 1k, you might even need one a few hundred ohms or less to drop the voltage, you could switch in the resistor with a relay or transistor.

If you do a google you'll find tacho's using the lm2917, and probably some led ones that use a lm3914 chip which has 10 led outputs, if you had 2 of these as a 20 led bargraph you could tap off one or more of the led outputs to control a relay or transistor switch to bring in the lambda shunt resistor.

I guess I am thinking more along the lines of a parasitic draw. Variable with regard to RPM. Since the O2 is not producing voltage when cold or the engine off, wouldn't matter to the battery, but if the drop happens before the ecu, the ecu would feed more fuel to compensate for the low input.

I really only need a 6 or 7% enrichment, and really do not know what drop will cause that. It is about trial and error. Before I get to the dyno, I need to figure out where and how I am going to tap the lambda output, then can do so on the dyno and see the results in action.

Thanks for the suggestions, I am looking into all of them best I can.

 

[shortbus=]

Or you could do this another way. By injecting air into the exhaust ahead of the O2 sensor. It would take some experimenting to figure out the amount of air to introduce in the exhaust. The amount could be linked to RPM also.
A bung to hold a fuel injector, and the exhaust gas moving past it would be used to introduce the air.

Before some one says this won't work, an exhaust leak at a manifold often times is the cause of an ECU giving a rich condition.

 

[Mag00]

Wow, I have another thought that may eliminate the need for the RPM trigger.

The lambda is going to output voltage based on the A/F ratio. So why can't I use a feedback from the true output compared to the target output and compensate. Probably didn't say that right.

If the ECU is programed using a baseline lambda output of .45 volts, and I spoof the ecu to put more fuel in, the lambda will be putting out 5 volts in reality in the corrected state, to enrich the fuel mixture. If that voltage were to go to .51 it could trigger or run a transistor or some equivalent circuit to bring the corrected voltage down, thus cutting back the enrichment, bringing the A/F mixture back to the stoichiometric value.

So would I be correct in saying a proper rated transistor could do this job?

Thanks,
Dave

 

[Externet]

Any signal you decide to use, buffered-amplified to not put a load on it, integrating tachometer (FtoV) or lambda signal can vary the brightness of a light bulb or led accordingly. A light dependent resistor attached to the light will provide a variable resistance.

 

[dr pepper]

Air injection is a good idea, however this will only work on a 4 stroke engine.

Heres a page that shows you an interface to a ego sensor, last time I saw something like this there was 2 gas sensors, one for the original ecu and another for the display system, might be a good idea and saves you messing up and blowing your ecu, if you get one thats heated you'll need to wire up the heater coil to a relay off the ignition.

http://www.the944.com/lambda.htm

Heres a tacho circuit that uses the lm2917 and the lm3914, 20 or 30 led is also possible:

http://www.edutek.ltd.uk/Circuit_Pages/Rev_Counter.html

 

[KeepItSimpleStupid]

Check out www.jaycar.com for a frequency switch kit as well as other car stuff.