Efi Related Thoughts

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
Consider this a general brain dump of EFI concepts for us Average Joe's.

by
Mark Bradley

Maybe I need to get a life but for now I enjoy this too much.

IM NOT CLAIMING TO BE AN EXPERT. Just someone willing to share and be called out if I'm wrong!
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Modern fuel injection works off of the idea that we should be continuously correcting the fuel delivery volume based on atmospheric conditions and feedback from various sensors including an oxygen (O2) sensor in the exhaust.
All of the general principles of an internal combustion engine still apply. As we all know a good running motor needs air, fuel, compression, spark and they must happen in the correct sequence under the correct timing.

Now the differences:
With our carbureted motor if we crack an exhaust manifold it has no "performance" bearing on the motor. On a vehicle with an O2 sensor this crack is a serious problem.

Why? I need to get a few concepts down then it will become clear why this is a problem.

The O2 sensor (may see Lambda sensor) is used to quantify the amount of Oxygen in the post burn combustion chamber by sampling the exhaust. This is accomplishes in a couple ways but for now.. it just does (see the URL link at the end).
We should know that the engine needs air and fuel in the correct ratio to run correctly and that ratio for gasoline is 14.7 pound of air to 1 pound of fuel (expressed as an AFR of 14.7:1). You can run different ratios for different reasons but let's stick with this for now.

Fuel delivery:
In fuel injection we use calibrated injectors so that we understand how much fuel is delivered over a given period of time the injector is fully energized. They are commonly rated in pounds per hour for fuel delivery. To do this two things MUST be controlled and that is the fuel pressure to the back side of the injector and the voltage source to energize the injector. I'm not going to try and cover this in detail here although the power cycle (duty cycle) driving the injector is altered (% on time vs % off time) to open it more or less time and that controls fuel "volume" delivery.

Air Delivery: (Boost and N2o is not covered here).
Air delivery typically happens with a mechanical valve like the butterflies in your carburetor body. The further you open them the more outside atmosphere you can pull into the engine and mix with the fuel delivery just like we do with a carburetor.

Measuring Air Volume:
For an accurate AFR we need to know how much volume of air we are pulling past the butterflies.
If we know this we can adjust the duty cycle driving the injectors to give the appropriate fuel delivery for the 14.7:1 AFR we want.
The air volume can be measured multiple ways depending on the quality of the system although it is common to use a Mass Air Flow sensor.
For our discussion know it is accurately measured and you change the flow with the butterflies.

O2 feedback:
For this discussion let's just accept the industry has done its research and knows how to calibrate the O2/Lambda sensor.
When the engine runs we know how much unconsumed oxygen should exist in the exhaust from our 14.7:1 AFR.
If there is excessive remaining O2 then the engine must need more fuel to use up the O2 (a lean state) and if the O2 is too low then it is perceived as being rich and less fuel is desired.

Keeping the mechanical air valves (butterflies) in the same position controls the air flow so the signal from the O2/Lambda sensor indicates if a longer or shorter duty cycle to the injectors is required (for us carb guys... larger or smaller jets). Longer "on" cycle makes it rich and a shorter "on" cycle makes it leaner.

Timing and other factors:
Since we are comparing a carbureted motor to a an electronic fuel injected (EFI) motor I'm going to say the timing an all other variables are perfect (except for that exhaust leak) with this motor.

NOW THAT PESKY CRACKED EXHAUST:

Remember that the O2/Lambda sensor is authoritative here because it knows the oxygen concentration in the exhaust in real time.
By allowing extra atmosphere (containing oxygen) into the exhaust the sensor reports it. It neither knows or cares how it is happening... it just reports a value.
Now the electronics in the EFI sees a lean state (excessive O2) reported and drives the injectors open longer to correct for the exhaust lean reading.

Hopefully you see the problem here.

Once the exhaust gas oxygen concentration has been adjusted with additional fuel the "AFR IN THE CYLINDERS" are actually excessively rich.
This will result in poor fuel mileage, fouling of plugs and possibly excess fuel making it into the crankcase thinning the oil.

So the moral of all this is that EFI is great but just because your car ran well with a carb does not blindly imply it is healthy for EFI.

I found this article on O2/Lambda sensors to be of value.
http://www.austincc.edu/wkibbe/lambda.htm

Cheers.Mark Bradley
 
OK Mark, I will play devil's advocate here -
With a running engine, there is exhaust gas pressure in the manifold THAT LEAKS OUT.
So then, how does air enter the manifold?
 
Cracked manifold, blown gasket between the head and then manifold will pull in air.
If the exhaust is truly under pressure all the time (restrictive) then it is unlikely much air will pull in but then you have another issue.
Otherwise keep this in mind that the exhaust has pulses resulting in moments of high and low pressure. An opening to the atmosphere will pull in outside air... guarantee that.

I hope that helps.
 
I know an oil furnace with a small leak will suck in air and never show a leak and its under more pressure than an EFI setup
 
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