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Measuring liquids and solids in vehicle exhaust

Hanna Bergman
Machine Design
KTH, Sweden
June 2003

Particles from car exhaust make up many of the liquids and solids that pollute the environment. These particles may cause problems to human health, and it is important to limit how much is emitted into the environment. Today, a passenger car is not allowed to emit more than a certain mass of particles per kilometre. However, researchers have recently found that a small particle with little weight can be as dangerous as a big particle with much larger weight. Therefore, the limits for how much a car is allowed to emit should possibly be based on something else than mass. You cannot decide what the limitation should be based upon before knowing what the particles look like and how they change as they are transported from the engine to the environment. In my research project we will first try to find the right tools to study the particles, and later use these tools to describe how particles behave when coming from different engines.

Exhaust from a typical truck or bus engine consists of both gases (such as carbon oxides, nitrogen oxides and hydrocarbons) and liquid or solid parts called particles. These particles have a diameter about one thousand the width of a human hair. Many particles, but not all, are made from carbon structures. These carbon particles are the ones that make the visible black smoke behind the truck, and they are commonly referred to as smoke. As the exhaust flows from the engine through the exhaust pipe and out into the air, the size of the particles change. The particles then grow, since the gases cool and condense outside of the particles. Also, the total number of particles changes as new liquids and solids are formed from gases. In my research, I will study the conditions that determine whether particles grow, form or merge together.

To study these particles, I first need to find the right tools. Unfortunately, vehicle exhaust cannot be brought directly to a measuring instrument. Firstly, the exhaust is hot and needs to be cooled down. Secondly, the exhaust has high concentrations of particles, and it would be impossible to count how many they are because too many would pass your "eye" at the same time. Therefore, it is necessary to dilute the exhaust before measuring what is in it. I have recently chosen a diluter and I am currently testing whether it works properly. I have also chosen instruments that count how many particles there are of each size, and in the near future I will do experimental work to find out how the diluter and the measuring instrument work together.

Once the proper tools are found and understood, I will use them to study particles under different conditions. These conditions include using different types of engines, different qualities of fuels and different working patterns of the engine. The temperature of the exhaust depends on whether the engine has heavy load and whether it is maintaining speed or accelerating. The temperature influences how big the particles will get and how much they change as they move down the exhaust pipe.

Understanding what particles from typical engines, fuels and engine working conditions look like and how they behave is essential in deciding what specific characteristic, or parameter, of the exhaust particles should be limited. This parameter might be the total number of particles, the number of particles of a specific size or something else. The parameter that is limited should preferably have relevance to human health, so that the car manufacturers at the same time can both meet the demands of this limitation and produce engines that are more friendly to humans and to the environment.