Flex Fuel Caravan Question

[tomt2]

Does anyone know how the 2000 3.3L Flex Fuel Caravan detects the amount of Ethanol in the gas tank? I heard some cars just use a Lambda sensor to detect the amount of Ethanol. I'm not sure if that is done with a narrow band oxygen sensor or if it necessitates a wide band oxygen sensor. The other method I heard of was to use a fuel sensor that can detect the dielectric strength (if I remember correctly) of the fuel mixture to determine how much ethanol is present.
Thanks!

 

[dgallup]

I believe it has a fuel sensor but I have no proof.

 

[tomt2]

Anyone know for sure on this question? I want to use a wideband oxygen sensor on this fan but don't know enough about how the engine detects alcohol content.

 

[Lou Scannon]

Might be best just to weld in a new bung for the wideband sensor.

 

[tomt2]

I was planning on doing that anyways. But I would like to use my wideband O2 sensor to simulate the Narrow Band sensor ouput so I can run the engine a little leaner than stoich. The problem is that I don't know how that will effect things if I add E85 to the gas tank. Maybe I could use the Lambda readings instead of the Air to Fuel readings. When looking at Lambda, the stoichimetric value is 1 for gas and also 1 for E85. The Air to Fuel Ratio would be 14.7 for gas or 9.8 E85. Does that sound plausible?

 

[Lou Scannon]

You have just discovered the secret formula for relating A/F to Lambda. The oxygen sensor does not sense air/fuel ratio or even lambda; it does sense the presence of excess oxygen or excess fuel in the exhaust. The controller uses this signal to adjust the fuel so the lambda dithers with a small amplitude and high frequency around lambda=1.
This will occur regardless of the fuel's stoichiometric air/fuel ratio, as long as adjustment is within the control's calibrated range.
If you have a way of biasing the apparent switching point of the signal sent to the control, the end result will be the same regardless of the fuel. If the control is very rudimentary (unlikely), you may succeed in shifting the calibration lean. However, I suspect there are catalyst diagnostics that will be triggered, resulting in illumination of the CEL, and possibly disabling closed loop control. At that point you're probably worse off than when you started.
Running "a little leaner than stoich" might provide a measurable increase in fuel economy. A catastrophic increase in emissions is guaranteed.

 

[tomt2]

Good to hear that this might work. I do have a wideband oxygen sensor controller that uses software that allows me to shift the voltage input to the ECU by a certain amount. That way the ECU still "sees" 14.7 air fuel ratio but it is actually getting 15 or 16 air fuel ratio (depends on how much I offset the voltage by). Of course this removes the narrowband oxygen sensor from the loop.
I argee that the NOx emissions will increase with a leaner AFR but, wouldn't the rest of the emissions (HC, CO) go down because less fuel is being consumed?

 

[Lou Scannon]

Engine out CO will go down as you lean out. Engine out HC will go down as you lean out, to a point, then they will increase sharply due to slow combustion & misfire.
Feedgas temperature into the catalyst will go down, which may reduce conversion efficiency of HC.

 

[tomt2]

As far as slow combustion goes, could I advance the timing a little to compensate for the slower combustion speed of the leaner mixture to help keep the engine efficiency optimum? Any ideas on how to do that on the fly, not just base timing adjustments? Or would base timing adjustment be good enough if I keep the same AFR throughout the rpm range?

 

[patprimmer]

Having to advance the ignition might more than lose the gains from leaner mixture.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tomt2]

If the flame front is slower from a lean mixture than the spark should happen sooner in the cycle to allow the mixture to burn more completely. So I would assume advancing the ignition timing would get you higher peak combustion pressures and consequently more power which would give you even better fuel efficiency because you could let off on the throttle a little. The mixture would have more time to burn completely and therefore give you less emmissions as well.
That is my assumption. Why do you think advancing the ignition will loose any gains from running a lean mixture?

 

[patprimmer]

How does having to start pressure buildup earlier in the compression stroke to end up with the same peak pressure a 14 deg ATDC help economy. You have the same pressure on the way down, but more on the way up. I cant se how that is not more parasytic losses on the compression stroke.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tomt2]

I agree that you want your peak cyl pressure between 10 and 20 deg ATDC for the best mechanical advantage on the connecting rod. If the stock setup at 14.7 AFR gets you peak cyl pressure at 15 deg ATDC, then slowing down the burn rate with a leaner AFR will give you peak cyl pressure at 25 deg ATDC (hypothetical example). That is too late in the cycle. Power will be lost because the maximum cyl pressure did not happen at 15 deg ATDC.
I'm not talking about advancing the timing on a stock setup. I'm talking about advancing the timing on a modified lean burn setup.

 

[patprimmer]

You totally missed my point.

If about stoich needs say 25 deg advance to give peak cylinder pressure at 14 deg ATDC, and you lean it off say 10% and get a slower burn, two things will happen.

1) You will need wider throttle to get the same power, say 7% more airflow, so 7% of the 10% saving is already lost.

2) as it burns slower, you need say another 5 deg advance to get peak cylinder pressure t 14 deg ATDC. That means the piston is moving up against a burning charge for 5 extra degrees, all happening when the rod angle is effective at using that pessure to work against the piston moving up. ie it takes more power from the crank to work against the expanding charge on the compression stroke.

Net result is most likely a negative.

I expect there will be a sweet spot where combustion pretty well completely consumes the fuel, but burn rate is not greatly reduced. That will be very close to where conventional wisdom gained by years of experience already has it.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tomt2]

Ok, now I see your point. I would like to add to your first point. You said,
1) You will need wider throttle to get the same power, say 7% more airflow, so 7% of the 10% saving is already lost.

The wider throttle opening does result in less pumping losses. This will add a little bit to the savings. I don't know how much but I do know Diesels are a lot more efficient for that exact reason.

On your second point you said,
2) as it burns slower, you need say another 5 deg advance to get peak cylinder pressure t 14 deg ATDC. That means the piston is moving up against a burning charge for 5 extra degrees, all happening when the rod angle is effective at using that pessure to work against the piston moving up. ie it takes more power from the crank to work against the expanding charge on the compression stroke.

If the charge is burning slower you will not have more pressure at TDC. You will have the same pressure as before. It just took longer to get there. The crank angle is a good point, though. The 5 extra degrees of crank angle rotation is happening when the air/fuel charge first sees the spark. The burn rate at this point of time is much slower than when the cylinder is at TDC and after TDC. The added pressure due to spark advance against the connecting rod during those first 5 degrees is probably very low.

Great points. I could be wrong, but that is why I am asking the question.

Just a side note, I don't mind losing power due to a lean burning setup. I'm not looking to make the same horsepower as my stock setup. I don't care about accelerating hard. I'm just concerned about fuel economy. What I don't want to do is loose out on fuel economy because the engine wasn't running efficiently due to the spark advance being off.