Emissions Testing

by Bill Abbott

What is it about the infernal internal combustion engine (ICE) that causes it to emit:

  1. CO ?
  2. NOx ?
  3. HC ?


Complete combustion would burn Gasoline, which is modeled as:

Octane (C8H18) a.k.a.:

  H H H H H H H H
  H H H H H H H H

and produce C02 and H20. There are two problems: Incomplete combustion gives CO (vice CO2- needs one more Oxygen atom) and 'HC' which is unburned hydrocarbons- complete octane molecules, pieces of octane molecules, etc. High temperature combustion gives Nox.

Really worn rings or valves, leaking oil into the combustion chamber or exhaust stream, also cause HC but this is worn-out, end-of-life stuff, not misadjustment or a maintenance problem. An engine can wear-out without showing such symptoms. (Ie the BEARINGS can wear out, eh??)

You'll note that a heathy engine gives much lower CO and HC values at high RPM than at idle: The reason is higher velocity of the fuel and air mixture, giving better mixing, and more uniform combustion. Distributor advance starts the burning earlier but RPM mean less time to burn- not sure which dominates. If your generator is dying you might get a slightly stronger spark at high RPM but that's marginal. A super-rich mixture, too much fuel, gives CO and HC at high RPM because there just isn't enough O2 to burn it all.

A super-lean mixture burns very hot and produces a lot of NOx. The richer the mix, the cooler. So for best results a perfectly adjusted mixture is used. This used to be accomplished by leaning out for maximum power and then backing off a little. Modern fuel injection equipment monitors O2 in the exhaust and adjusts so there's just a little, but not a lot. It may adjust the spark advance by monitoring for knock too.

Which pollutant above do each of the following devices address?:

  1. Catalytic Converter
  2. PCV
  3. A.I.R. pump
  4. Capacitive Discharge Ignition (CDI)
  5. other

Catalytic Converter completes combustion of CO and HC into CO2 and H2O. Note that the O2 to do this must be coming along in the exhaust gas, so it can't cure super-rich conditions, but it can clean-up sloppy combustion at idle.

PCV is Positive Crankcase Ventilation. It takes care of another HC source, engine oil! Yes, oil fumes from the engine crankcase and valves used to just be dumped overboard. PCV valve connects these spaces to the intake system and allows the fume-laden air from the engine to be passed through the combustion chambers, where the oil vapors are largely burned.

Air pump is just a pump that puts fresh air into the exhaust stream. Just before Catalytic Converters, Air Pumps were popular with car makers to add extra oxygen to slightly-rich-but-cool-burning mixtures AFTER they came out of the cylinder. Add a little more air, the CO and HC finish combustion in the exhaust pipes.

CDI Makes nice hit sparks. Means you get better combustion at low RPM particularly. Lowers HC and CO.

EGR Exhaust Gas Recirculation Taps some of the exhaust and adds it to the normal mix at high rpm. I'm not sure if this cleans up idle combustion (second pass) at low rpm or what. I replaced an EGR valve once, it was broke, new one worked. But I didn't figure out when it opened and when it was closed. ;)

[Ken Hooper's note: What shooting exhaust into the mix accomplishes is that it enables a leaner mix without the (otherwise) corresponding high heat, making for better mileage and lower emissions with no downside. Well, except that it's another part to break eventually.

EGR can drop the internal cylinder temp by over 2000 degrees. Symptoms of a faulty EGR valve include...severe ping. Lack of an EGR system is the primary reason us upright engine folks will always be gross polluters. Sort of interesting trivia: the PCV valve when it was developed had nothing to do with emissions concerns. It was developed for military Jeeps around 1941 in order to seal the crankcase, to make the vehicles better able to drive with their engines submerged in water, for the sake of stream crossings and such. If not for the PCV valve the water would come into the crankcase through the road draft tube, which was how street engines were vented until the early 60s.]


O2 monitoring in the exhaust stream for electronic control. Can be used in FI or carbs, but on VWs found only stock in FI.

Which pollutants above increase due to:

1) worn rings & valve guides?


HC: You're getting oil past the rings and dropping down the valve shaft. Maybe a little CO from damped combustion if the amount of oil is large.

2) worn valves?


HC: unburned mixture is pumped out the exhaust valve's bad seal before the spark fires. Combustion at lower compression ratio isn't, perhaps, as good too.

3) valves out of adjustment?


No pollution effect unless they're HORRIBLY out of adustment.

4) incorrect ignition timing?


HC/CO: You can't make the mix burn HOTTER with bad timing but you CAN make it burn badly. Distributor advance starts the spark early enough to get power out of the mix at the RPM the engine is at- start the spark too late and you get bad combustion. Well, I guess if you've got killer knock it might make more NOx.

5) incorrect Air/Fuel ratio?


CO/HC from too rich, NOx from too lean.

6) other wear/adjustment problems?


NOx isn't the only problem with too-lean- you can melt a VW engine with a lean mixture, some knock from cheap gas/late spark. I've seen it: molten aluminum sprays out of the hole in the head and covers the cooling tin. #3 cylinder, my brother's wife's bug. 87 octane Shell, damn hot day, climbing a big hill.

Super-rich mix will cause soot to build-up inside the engine- makes for bad ring and valve seal, causes wear on moving parts, increases compression ration making for pinging/detonation more likely.

Generally, pollution is waste- gas or parts of the gas going out the tailpipe only partially used. For maximum horsepower you might pump WAY too much fuel/air mix through the engine to keep it cool, and not burn it all but this isn't the same as just richening the mix- go ahead- turn up the richness and you'll LOOSE power in a normal engine.

A well maintained engine with a reasonable life expectancy, making the power its designed for, is a stunningly efficient machine for converting air and gasoline into power, water and CO2. If your engine is running clean, its running good, and vice-versa.

Back to Library Back to Exhaust