The other automobile generated pollutant, nitric oxide (NO) contributes to smog/ozone when mixed with hydrocarbons and subjected to sunlight. NO is formed mostly under heavy load, when the engine is running a little lean and when the catalyst and other NO control measures are not operating.

Smoke (usually white) behind a gasoline powered vehicle is not normally caused by air/fuel mixture chemistry, but rather by crank case oil leaking past valve stems or piston rings and being blown out of the exhaust.

Diesel powered vehicles, when properly tuned, emit very low CO and HC because they operate at high temperatures with a lot of excess air. Nitrogen oxide emissions are therefore often elevated. When more than the normal amount of fuel is injected into a diesel engine, power output increases, but black soot smoke (probably carcinogenic) is emitted.

Automobile manufacturers have known all these things for a long time. We have had to learn them relatively recently. Thankfully, books such as John Heywood's Internal Combustion Engine Fundamentals are a great source of information. We have had to learn this material because we invented a device (Stedman and Bishop, "Apparatus for Remote Analysis of Vehicle Emissions," U.S. Patent No. 5,210,702, May 11,1993) that measures the exhaust emissions from each car as it passes by using remote sensing. Figure 1 shows a diagram of the system. The video camera takes a picture of the rear of each passing vehicle, from which the license plate can be identified.

Technologically, the remote sensor is not different from a conventional tailpipe emission test. In a conventional test, the exhaust is pumped from the tailpipe into an instrument. A source of Infra-red (IR) light shines through an optical cell in the instrument. At the other end of the cell, detectors measure the