Stoichiometric Ratio
The relationship of the O2 Sensor, ECU, and the catalytic converter.
What it all represents
When I was a kid I use to watch my dad, (Who started out as a diesel mechanic), tune up cars and trucks from time to time. He would turn the adjustment screws
on the carburetor until the engine smoothed out and started to purr. Then after
listening closely to the rumbling of the motor the adjustment screws would get
one final tweak. He would always bend down and cup his hand around the
belching tail pipe exhaust and take a big sniff of the fumes. Then go back to
his adjustments. That was back in the 60’s and 70’s before PCM’s, O2 sensors, and catalytic converters.
Many years later, I attended some classes where I finally realized what he was doing with a hand full of exhaust. He was actually taking an “old school” measurement of the “Stoichiometric ratio” (stoi-kE-‘a-me-trek). Stoichiometry: is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions.
We need 4 things to operate a gasoline engine, fuel, air, compression, and spark. If the fuel or air isn’t at the right values you’re not going to get much of a bang out of that engine, no matter how much compression or spark you have. The ratio to fuel and air is the key to achieve complete combustion, in simpler terms: “the perfect air/fuel mixture”.
The ideal mixture ratio is 14.7:1 that’s; 14.7 parts air to 1 part fuel. Even though the perfect mixture is hard to achieve throughout every condition we put an engine under, it can be extremely close to perfect. This is made possible with the computer driven systems we use in today’s cars. The heart of this fuel/ratio control is actually in the exhaust. Just as my father would “sniff” the exhaust so does the O2 sensor. It takes a sample of the exhaust and relays that information back to the main computer in the car. The O2 sensors values (the presence of oxygen) are sometimes referred to with the Greek letter – “L” or “Lambda”. On some older vehicles I’ve even seen the O2 sensor actually referred to as the “Lambda sensor”. A Lambda reading of 1 is considered a perfect setting. Less than 1 indicates a rich condition, and over 1 indicates a lean condition.
Why do I need to know this, you’re asking? Well, it has more to do with the health of the catalytic converter. In the past you could get away with the mixture being off because we didn’t have to worry about the catalytic converter melting or clogging up. (Can you remember driving down a highway and the car in front is belching out huge black clouds of smoke, you don’t see that as much anymore.) But these days that “perfect” mixture is crucial to the life of the converter.
The design of the converter relies on that perfect mixture to maintain its ability to remove the nitrogen oxides (NOx) and other harmful hydrocarbons. If an O2 sensor fails, the computer is going to start taking guesses at the mixture, or default to a preset mixture/injector pulse setting. This is fine for a while, but the longer this is left alone, the more likely permanent damage will be done to the converter. Maybe a better way of putting it is: “The catalytic converter is built on the assumption that the Stoichiometric ratio is maintained for the life of the vehicle.”
Some time ago I had an old customer bring in an after-market ECU system he wanted me to install on his hot rod. This system was a complete “do-it” yourself ECU. (I think it was a Holley setup) It had a few basic preset programs you could load into the ECM that would allow the engine to run, but there was a lot more “tweaking” that could be done with it. Every parameter on the engine could be adjusted and viewed on a scope or graph on the laptop. Everything from the Map sensor, TPS, spark advance, coolant temp, injector pulse width, etc…. At first I figured I could put some basic values in from the manual that came with the ECM and get it pretty close. That didn’t work to well. If you took the car out for a run it would have flat spots through the power range, or at times, even cough up a huge flame from the intake. None of it was due to a mechanical problem; it was with the mapping of the ECU, spark, timing, and the air/fuel mixture. What made the difference in this case? The understanding of the Stoichiometric ratio. Mapping the engine thru the power band I could see where the ratio was by way of the “Lambda” readings from the O2 sensors. (Plus or minus 1) It took some time, but I eventually got it running like the hot rod he was looking for.
So why do I need to know this?
Better performance, longer lasting components, and longer lasting customer cars. If we know we want to maintain a perfect Stoichiometric ratio (14.7 to 1) then we know that the “Actual Fuel Ratio” should be as close to the Stoichiometric value as possible. This is where looking at the Mass Air Flow values, the O2 values, injector pulse, etc… can be so important.
The idea is to achieve that perfect mixture. You’ll notice that on the newer CAN systems there are a couple of new parameters added too. Like Catalytic temperature for instance (A good thing to know) and something else that should be a real boost to diagnosing is the “Command Equivalence Ratio” which is nothing more than the “Lambda” reading, usually indicated by the number “1”. Knowing the actual value “Lambda” reading can offer a lot of information to the conditions inside the combustion chamber. These days with fuel economy a huge concern, keeping that perfect mixture is crucial, but, it would never have been possible without the computer systems we have today. Engineers have managed to clean up the exhaust, raise the fuel economy, and improve performance, and all this is because they followed that one little formula, the Stoichiometric ratio.
As my dad would turn those idle screws on the carburetor and adjust away all the vibrations from the engine. Then reach back to get a hand full of exhaust and take a whiff of it, he could tell if it was rich or lean. He too, was trying to reach that optimum Stoichiometric ratio too. I do the same thing today, however not with a screw driver, but with a laptop. To think, it takes me a laptop, some complicated computer software and data stream information to keep a precise control on the Stoichiometric ratio. While in good old dad’s era, close was pretty good, and a whiff of the exhaust told all that needed to be known. I guess you could say; He could accomplish the same thing, with his own brand of software.
I suppose, that’s what ya call… progress.
