Big Guy on a Bicycle: Tech

Yet Another Tech Article

Wed, 21 Jun 2006 22:51:35 GMT

We still have a guest ("Juniper" - not her real name) from out-of-town that we are entertaining tonight, so we'll be leaving soon for dinner (I think the ladies have decided on sushi). Thus, it's back to the personal archives once again to recycle another technical article first written for my car club's monthly newsletter. Just let me brush a little dust off of it and .... there, good as new. One warning: I actually wrote this article before the one I published yesterday (you may notice that somewhere in the text).

Driven to Exhaustion

Or, "Blow It Out Your Tailpipe"

by [Big Guy Science Press]

In previous articles, I’ve talked about intake systems and octane ratings (as related to combustion), so it’s probably time to discuss the last step in the process of producing power in an engine, exhaust.

Quite simply, what goes in, must come out. Air and fuel, in a controlled mixture, have been fed into the combustion chamber. Combustion was simply a chemical change in an enclosed environment (albeit under pressure – boom!). For those of you who remember high school or college chemistry, every chemical reaction is balanced. Gasoline and oxygen plus a little bit of energy (from the spark plug) on one side of the equation become carbon monoxide and steam plus heat on the other side. Or, written in chemical shorthand, O₂ + gasoline + e (for “energy”) = CO + H₂O + (heat). That’s an oversimplification, but good enough for our illustration purposes. (Remember that gasoline is mostly carbon and hydrogen atoms). Now combustion is complete and we have to make room for fresh fuel and oxygen, so out with the old and in with the new.

Now we have carbon monoxide and steam venting from the combustion chamber, neither of which are something we want coming out from under the hood since they could easily go from there to the passenger compartment. So some brilliant person way-back-when decided to hook up a pipe to the exit of the combustion chamber to carry these undesirable by-products away from the driver and passenger either up overhead or behind the vehicle. Smart, eh? The beginnings of the exhaust pipe.

One other by-product of combustion I haven’t mentioned yet: acoustic energy, or noise. Combustion is loud (and louder on some cars than others). Someone else came along and hit on the idea of attaching a canister with baffles in it to the exhaust pipe to “muffle” the noise coming out of the engine (hence the “muffler” moniker). Let’s discuss how this works. Acoustic energy travels in waves through the pipe. The waves aren’t really disturbed much by the bends in the pipe, but when the wave fronts hit the baffles, they tend to break apart and bounce around a lot inside the canister, interfering with each other and dissipating a lot of the acoustic energy before exiting through the open end. How much noise is muffled depends on the shape of the cylinder and baffles, and of course the condition of the muffler (if the baffles are degraded and the canister has rusted through, it’s not going to function properly, is it?)

So there’s the pipe and muffler…what else is there to the exhaust system? Two major components; the oxygen sensor and the catalytic converter. The oxygen sensor monitors the amount of, you guessed it, oxygen in the exhaust gas and reports it to the ECU, the computer that controls how the engine runs. If there’s too much oxygen (indicating too little fuel in the air/fuel mix), the ECU will cause additional fuel to be injected into the combustion chambers and vice versa. The catalytic converter is there to control some of the more noxious gases in the exhaust. I don’t have space to really get into the chemistry behind it, so I’ll just say that it contains a catalyst, which is a substance that promotes a chemical reaction, to convert the harmful exhaust components into something less harmful. The catalytic converter has an interesting history, but that’s for another time.

The stock exhaust systems in most cars are (typically) good ones and were well designed, but that doesn’t mean you can’t do better. The system is in three parts: the header, the catalytic converter (or “cat”), and the “cat-back” (everything from the “cat” to the back of the car, including the muffler). You can replace any of them or all of them. There are some systems available that are less restrictive and can help increase your engine’s output horsepower to some degree (the engine’s energy isn’t wasted on having to push the exhaust gases out quite as hard). These are often louder, but are usually “tuned” to produce a deeper sound, which some people find pleasing for various reasons.

The stock setup is usually a trade-off between restriction and noise control, but if you can live with (or desire) a louder exhaust to get that extra bit of power, there are lots of aftermarket options open to you. Caveats to this are that you should consider not only the local noise ordinances in your area, but your own potential hearing damage and also damage to your car caused by disgruntled neighbors with blunt and/or sharp objects (it has happened to the son of someone I know).

I should be back to my quasi-normal blogging schedule tomorrow (especially since it's trivia day).

Another Tech Article

Tue, 20 Jun 2006 22:50:38 GMT

OK, here's the scoop. I had a physical therapy session after work. When I got home I had to walk and feed the dogs (yeah, and feed the cats). Also, Mrs. Guy is bringing in a co-worker with her when she returns tonight ("Juniper", who has been mentioned once before), so I had to vacuum Mrs. Guy's office and the guest room. I also vacuumed up the dog hair downstairs for good measure. Now I have a meeting I need to leave for that I just found out about yesterday. What does this mean? I don't have time to think up something and write about it, that's what it means. Therefore, I am again going back to my personal archives and dusting off another one of the technical articles I wrote for my old car club's newsletter. Enjoy.

The Truth About Cats and Smogs (Not a Veterinary Discourse)

By: [Big Guy]

A long time ago, I promised to one day write a technical article about catalytic converters. That day has come. What are they? Why does my car have one? What's it all about? What's the meaning of life? I’ll try to explain (except that last part).

A catalytic converter, or “cat” is a small device installed along the exhaust system between the engine’s exhaust header and the muffler. The cat converts some of the more harmful components of the engine emissions into something hopefully a little less harmful. The typical components of engine exhaust are:

Nitrogen (N₂) – Air is 78% nitrogen, and most of it passes straight through the engine
Carbon Dioxide (CO₂) – Relatively harmless, from the bonding of carbon to oxygen in the air
Water vapor (H₂O as steam) – Also harmless, from the bonding of hydrogen to oxygen
Carbon Monoxide (CO) – A poisonous gas generated by an inefficient burning of the fuel
Hydrocarbons or Volatile Organic Compounds (VOCs) – Mostly from unburned fuel that evaporates. These form oxidants in the atmosphere, leading to smog
Nitrogen Oxides (NO) – Another contributor to smog, and also acid rain

These last three are the main regulated emissions, and are the ones the cat is designed to deal with.

Most modern cars are equipped with “three-way converters”, meaning that they work in, you guessed it, three ways to reduce the three worst emissions. The cat has two different types of catalysts, a reduction catalyst and an oxidation catalyst, and a control system to monitor the exhaust stream. First, we’ll discuss the catalysts.

There are two different types of structures used in cats, but most cars use a honeycomb structure, so that’s the one I’ll mention. The two catalysts coat the surface of the structure. The idea is to maximize the surface area of contact with the exhaust stream while minimizing the amount of catalyst needed. Why minimize the amount of catalyst? They’re expensive!!! Very expensive!!!

The reduction catalyst is the first stage of the cat. It uses platinum and/or rhodium as the catalyst to reduce the nitrogen oxide emissions. When the nitrogen oxide molecules hit the catalyst, the catalyst grabs hold of the nitrogen atom, allowing the oxygen to go free as O₂ (natural oxygen) The nitrogen atoms then bond with other nitrogen atoms stuck to the catalyst and are released as N₂ (natural nitrogen). I could go into the detailed chemistry behind it, but I don’t have the space here. You’ll just have to trust me.

The oxidation catalyst is the second stage. Unburned hydrocarbons, VOCs, and carbon monoxide are burned, or oxidized, on a heated platinum and/or palladium catalyst. Burning these components allows them to react with the remaining oxygen in the exhaust stream to form CO₂ and water vapor. To burn the components, though, the cat has to get hot. It is heated by the exhaust gases themselves. In other words, the oxidation catalyst doesn’t work until the engine has been running for a little while. As you can see, it isn’t a perfect setup, but it’s better than nothing.

Catalytic converters have been standard on automobiles since the mid-70’s and helped drive the push toward the use of unleaded fuels. Leaded fuel can quickly ruin, or “poison”, a catalytic converter, rendering it useless and probably clogging it (which really cuts engine performance in a hurry). In fact, it is illegal to put leaded fuel in a car with a catalytic converter. It is also illegal to remove the cat from your car in order to be able to use leaded fuel, or for any other reason. In fact, you shouldn’t have to worry about your cat at all since it is intended to last for the life of your car, unless… did I mention that leaded fuel can poison a cat? Having an engine badly out of tune can also poison a cat. Too much unburned fuel reaching the cat can coat the catalysts, which will cause irreparable damage. That’s what the control system is for (you thought I’d forgotten about that, didn’t you?).

The control system consists of a heated oxygen sensor mounted just in front of the cat. It monitors the amount of oxygen in the exhaust and lets the engine control unit (the ECU, or computer) know if the engine is running with too much fuel (rich) or too little (lean). The ECU then regulates the fuel injectors to keep the air-fuel mixture at the optimal, or stoichiometric*, ratio. Theoretically, this ratio is 14.7 to 1, meaning that for each pound of fuel consumed, 14.7 pounds of air will be burned as well.

So just a few parting comments on cats. Remember not to park over tall, dry grass or piles of leaves. Several fires have been started when the grass or leaves get too close to the hot cat. For the same reason, please do not touch your cat after you have been driving around, unless you really enjoy the sensation and burning smell. Also, don’t overuse fuel additives, as they don’t always burn completely and can shorten the life of your cat. Remember, unlike our furry friends, this kind of cat only has one life.

*This word you’ll just have to look up, unless you took Chemistry and/or Thermodynamics in college like I did.