[Christ]

Originally Posted by jamesqf

It's obvious that you're not thinking about what "parts per million" really mean. It's the same as percent, except per million rather than per hundred. So you've got burned air:fuel mixture (14.7:1, roughly) coming out the exhaust, and so many parts per million of that are unburned fuel. So if your engine is moderately well tuned and emitting 20 ppm HC, then for every liter of fuel burned, you have (20 * 15.7 / 1000000) liters of unburned fuel, or about 0.3 milliliters. So burning that completely would increase your mpg by 0.03%. (Plus a bit for burning the CO to CO2, but still not enough to even be measurable outside the lab.)

I understand this, but without a reference point, you have no idea how many millions of particles you're actually talking about.

How can you be sure that engine is running at or approximately at stoich? How do you know it's not running lean or rich, or burning a different fuel, or any of a few other variables?

You still need a reference point to quantify a PPM count, no matter what it's in reference to.


If I said 7% of your breath is deadly gas, it means nothing until you know what volume your breath actually occupies, and then how many times you're taking a breath in a determinate time to calculate how much of that deadly gas you're spewing per breath/per time.