STOICHIOMETRY

The Stoichiometric air/gasoline ratio, with respect to the most efficient burning of the mixture, is approximately 97:3 - And I will take bets on that - John Allen

Air gas was used to light wealthy people's houses and public buildings, including hospitals, in the early 1900s. The most well known producer was the Mercury Gas Company, located in the small town of Newmilns, Ayrshire, UK - which exported a number of its air gas generating systems all over the world.

Air gas, a stoichiometric mixture of air and gasoline, had a considerable economic advantage over coal gas. (Gasoline was at that time not subject to special taxation). Delivered to the lamps at a pressure of around 20 lbs, it also eliminated the risks of carbon monoxide poisoning and explosion, which were associated with coal gas.

That circa 97.5% air, 2.5% vaporised gasoline (at a pressure of 20 lbs per square inch) is a stoichiometric mixture, is proven by the fact that the flame burns without taking oxygen from, and without releasing carbon monoxide into, the surrounding atmosphere. In other words, it is a perfectly 'balanced' reaction, which is what is really meant by the term 'stoichiometric'. It is also claimed to give the whitest light, although we never thought to confirm this.

Air gas is not best suited to internal combustion engines, but it is ideal for producing a good steady flame. A good steady flame is ideal for carrying out spectral analysis. Spectral analysis is the easiest and cheapest way to detect 'if' a process is actually able to effect changes to the molecular composition of gasoline. We used the development cracker on liquid fuel being fed into a small air gas generator, feeding a simple flame torch - to test the fundamental theory, and to determine which signals would have what effects.

You may be wondering: If there are at least two stoichiometric air/fuel ratios, might there be more, and might not the rather glib assertions of the majority of IC engine combustion experts be just a little bit suspect.

The US legislature [once] ruled (at the behest of the IC engine experts) that all 'modern' engines should use the 'established' stoichiometric air / fuel ratio. We have to assume that the poor simple souls believed this was best for the world, for to assume anything else, would be scandalous...

Before I talk about engines, I would like to point out that there are thousands of highly efficient industrial chemical processes, which do not employ stoichiometric ratios. Indeed, there are many textbooks which advise that, "The reaction is best carried out in an excess of….." This is particularly true where an ingredient is valuable, potentially harmful, or even dangerous. The idea is generally that the excess of an innocuous component will ensure that none of the 'other' components are likely to survive the reaction. This tends to be especially critical where the reaction takes place during a sequence of processes, and even more so if the reaction parameters are not well defined. The reaction chamber of an IC vehicle engine comes pretty close to being the worst reaction environment with respect to the above considerations.

Stoichiometry was devised as a method of determining and communicating the constituent parts essential to any chemical reaction or composition. That is not to say that it is artificial, any more than is, say, mathematics. But it is not the gospel, which many would have us believe. The stoichiometric ratio with respect to IC engines is almost always expressed as so much fuel, for so much air (oxygen). That is to say, as just 2 items.

A description of the burning of complex hydrocarbon fuels would be a little too involved for this article, so I shall use hydrogen as my example, but I promise, it does obey the same set of physical laws.

We all know that water is two parts hydrogen and one part oxygen. That is the 'stoichiometric' analysis of water, which is fine, if we leave it at that. 2 Hydrogen + 1 Oxygen = 1 molecule of Water. (3 items)

Let us say that we wish to extract the gasses from water. Let's go for extracting hydrogen (for our vehicle fuel, of course) as an example of stoichiometry application.

First we get a flask and fill it with x molecules of water. The chemical reaction is endothermic, that is to say, we have to supply energy (DC current) in order to 'drive' the process. So, we insert two electrodes into the water, and pass a DC current through it (4 items) and out bubble the gasses. But how much current do we need? In terms of energy, it should be the same as that which would be produced if we subjected the hydrogen to the exothermic reaction of recombination with oxygen, to get water. And how much hydrogen will we get? Well, should be 2x, of course.

We collect the hydrogen and it is less than 2x – we find that we've been short-changed. So who, or what, stole our hydrogen? And we further find that the process has used too much electricity! So where did that go?

Unfortunately, passing a current through the water also generates heat (5 items) but that, at least, explains the missing current. The raised temperature caused some of the water to evaporate (6 items) so there's the missing hydrogen. And so this is where that simple stoichiometry 'model' can be seen to fall a wee bit short of the mark.

The same sort of thing applies to the internal combustion engine. Let's put the hydrogen from the water into an engine and use it to drive an electrical generator. Do we get back the electrical energy that we used to get the hydrogen? No! Nothing like it.

The stoichiometric "problem" with the conventional IC engine chamber, is that we cannot arrange for every fuel molecule to find enough oxygen. It wouldn't matter if we used hydrogen, of course, because it's harmless, but with most other fuels, pollution is a real problem. Stick an engine on a dynamometer, get it running at a nice even pace, and you could get some pretty acceptable results. But, put it in a vehicle and make it work right across its power band, with widely varying loads, and the amount of partially converted hydrocarbon molecules will soar. That is, if the air-fuel has the stoichiometric ratio. On the other hand, as we increase the amount of oxygen available, the percentage of hydrocarbons and carbon monoxide will fall. Of course it will, anyone can see that. (Except some of the experts, that is).

But we haven't finished yet. Ask the guys at ICI and they will tell you that we have been ignoring another very important aspect of stoichiometry – time (7 items). Try to convert the water into hydrogen in 'zero' time? Of course not. Every process requires time, but reaction time is rarely seen expressed as a part of the IC world's notion of stoichiometry. Watch a heavy lorry struggling up a steep hill and what do you see? Smoke! (Of course it's a diesel lorry). You may not 'see' the evidence of the same thing happening to petrol (gasoline) vehicles, but stand along side and your nose will certainly 'smell' it. Heavy load + high engine revs = not enough time to burn all the fuel.

In order to design a more efficient IC engine, one might very well choose to pursue a spontaneous reaction. (Certainly, all of the chemistry books would support such a direction). Clearly, with such a design, the duration of the reaction could be of extreme importance, and the advice regarding the employment of "an excess" of the most benign component (air) might become important to the new arrangement's efficiency. So to legislate against following this advice can only be seen as legislation against such improvements. Which, of course, was the intention.

Never mind that there is a way by which means we can better rely upon the science of stoichiometry. By employing "Spontaneous Combustion" to cut the reaction time down dramatically. Never mind that we may then be advised to shove in a goodly excess of oxygen and then, and only then, can we expect the reaction (which is determined by the thermodynamic laws which apply to stoichiometry) to occur. The legislators know best, and those who teach chemistry know nothing?

So how much extra oxygen do we need? Actually, there is a way of working it out, but it's very complex, and not the least bit helpful here. It's a bit like working out how much you're going to spend at the store, then taking the exact amount. No, of course we don't do that, we just take plenty. And that's what we can do with our engine. But we can only do that if we employ spontaneous combustion. Just burning it as we do in any conventional engine – forget it.

To sum up then: The proper application of stoichiometry requires that all pertinent factors be taken into account, not just those that happen to suit a particular argument. For an IC engine, we need to take account of the fuel and the reactant (air), the type of reaction (spontaneous or progressive), the time (to conclude the reaction) and the distribution (to ensure reactant pairings). And if it's overall engine efficiency that we are after, we need also to match the reaction time to the engine cycle time (as best we can) but definitely not make the cycle time too short. Get it right and all of the fuel will react exactly as the real stoichiometric experts say it will.

Now if the US government had legislated for mandatory stoichiometric conversion – then it would have made sense. Then it would have cleaned up our air. But they couldn't do that, of course, because no regular engine could be made to comply. So they legislated against stoichiometric conversion. Well, they had to do something about global warming!!

This stoichiometric thing - It really is a bit more complicated than those "experts" pretended it was, wouldn't you say?