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Q: The concept of a tailpipe filter is controversial. The idea is actively debated online, for example, on Reddit Science. The climate of opinion if one were to capture this tailpipe CO2 seems to be:

Well, if you did that your car would weigh so much at the time of each fill-up, it would be totally impractical.

The reasoning behind these opinions are based on straight-forward hydocarbon combustion calculations that assume all the fuel burns; and all the CO2 is captured.

How do you answer that?

A: First, If you could burn it all - the heat would exceed the melting point of the engine.

Second, the quantity of chemicals entering a reaction, and those being produced by it, need to be understood. Also, what is added to the exhaust stream before it leaves the tailpipe, and how much of it is accountable.

For example, a large portion of energy entering a modern engine is totally wasted - roughly 38% goes out of the tailpipe. Chemical waste including carbon monoxide (CO), oxygen (O2), hydrocarbons (HC), hydrogen (HO), nitrates (NOx), sulfur oxides (SOx), carbon soot (C), gaseous water with carbon dioxide (H2O vapor, with CO2), and CO2 neat are all waste products. Air is about 79% nitrogen and 21% oxygen by volume.

Among vaporous species many hundreds are produced. The difficulty of realistic numerical calculations involves the amount of reactions over a range of time-scales,10E-9s to 1s (see: Curran, H.J. et. al., 2002, Combustion and Flame, 129, p253, 280, and Ren, Z., Pope, S. B., 2006, "Geometry of reaction Trajectories and Attracting Manifolds in Composition Spaces", Combustion Theory and Modeling, 10, 361-388).

Buffer molecules of HCO3(-) are common - bicarbonate ions which are polyatomic (hydrogen ion + carbonate ion). Further, high temperature gives rise to orbital bonding and dissociated transfer resulting in one configuration to the possibility of another.

Exhaust products include H2CO3, a carbonic acid molecule. Involved too - common radicals of hydrocarbons with their various derivatives containing oxygen and nitrogen atoms, among others.

Bonds broken by force (combustion) re-bond; the job of a CO2 filter is to persuade left-over species in the waste stream to re-bond again by offering a non-kinetic alternative. Carefully chosen hydroxides of Group I and II metals in fluid pathways provide persuasion toward other orbital gains and losses.

One method is to introduce K+ and OH- to evolve KHCO3 first, then K2CO3 with additional KOH + H2O; this goes to K2CO3 as an alternative to some of the thermochemical species going out of the tailpipe.

Most vehicles today run with an Air/Fuel combustion ratio that produces large amounts of CO2 and water vapor. This is considered good by industry standards. The air/fuel ratio is "ideal" (Toyota Motor Corporation Sales, USA), at 14.7/1. This is the A/F ratio "where the CO2 production is highest" and where "CO2 is an excellent indicator of efficient combustion."

To complicate stoichiometry further, manufacturers often introduce oxygen to the exhaust after combustion as a secondary stream in order to oxidize outgassing species.

It's important to measure quantities of chemicals entering a reaction, and those actually being produced by it.

In our case, a car gets 20 miles to the gallon, travels 100 miles, and burns 5 gallons of gas. Five gallons is about 20 liters. Twenty liters weighs about 20 kilograms (Kg). This will make over 60 Kg (over 100 pounds) of CO2.

However we assume all the fuel burns, and all the water vapor, and all the CO2 absorbed by the water vapor are ignored. Also, by design, the filter acts as a flow-by structure, so it experiences boundary layers, vortices, shear forces, and other features of fluid flow which all apply here to discourage covalent bonding.

We capture some of the CO2. A fact we think is better than capturing none at all. We know how to capture more because we have learned what not to do.

If the filter surface reaction stops after 100 miles, we can measure that. A simple water rinse dissolves collected CO2 and the carbonate indicates success. The resulting carbonate also tells us about the differences between idle and cruising speed in regard to capture efficiency - data which agree with laser measurements.

Both non-dispersive infrared (NDIR), and carbonate mass values support the claim - capture efficiency is about 7% of the 14% total volume flow of CO2 at idle speed (like sitting at a red light).

Another misunderstanding is the capture efficiency at cruising speed (a range of rpms between 2500 and 3500). This turns out to be 3.5% of the 14% total volume flow of CO2 as measured by NDIR and carbonate paste.

We welcome all discussion about our efforts. We also agree to a demonstration anywhere on any light duty vehicle.