What are some possible reasons why cleaner exhaust has no significantly reduced total emissions from cars?

Answer 1

Well total emissions although related to how "clean" each individuals cars emissions are there are a few variables that are overlooked. First the number of cars on the road. As technology has improved with time cleaning emissions so has the demand for cars. Because there are many more cars on the road today the total emissions may increase regardless of how clean each car may be. Motor displacement is another emission levels may not have changed. Emissions are measured as a ratio (parts per million) this does not reflect how many particles are actually being exhausted. Meaning a 500 cubic inch motor and a 100 cubic inch motor may both exhaust .1 parts per million of carbon dioxide but the 500 cubic inch motor is exhausting more particles overall therefore also more carbon dioxide. Expanding on this idea further in the 70s due to the lack of technology the trend to meet federal emissions requirements was to decrease the motors size and reduced performance to meet emissions requirements. However in the mid late 80's as technology improved (including fuel injection) to help reduce emissions motor displacements have started to increase in size again, again increasing the overall emissions of a motor. This is also a big issue when you look at what people are driving today. Back during the gas crisis small efficient economy cars were big however since up until lately fuel has not been much of an issue and the economy was good which led fuel economy taking a back seat in terms of priorities when it came to buying a vehicle. Trucks, SUVs, vans made up a larger portion of what people are driving and although cleaner then their predecessors there not as clean as economy cars and there's simply more of them being driven. Then theres driving habits. With the economy in good shape and gas prices relatively low people were driving a lot more than during the gas crisis or when every family only had one car. Granted this has changed recently but this recent change would not likely show up in the numbers yet.