As a Canadian expecting to have to buy a newer car this year, I've been doing some research as to what kind of car to look for. Of course both gas/electric hybrids and vehicles using alternative fuels (compressed natural gas) are very clean, but for the amount we use the car (very little), I decided it's not worth it for us to spend that much money on a car.
Instead, I've been trying to find out whether or not I should favour a diesel vehicle. Sure diesel engines use less fuel, but do they produce less emissions? Are the emissions overall more or less harmful than those from gasoline engines?
First, I found out that both the Canadian and US governments are aiming to clean up diesel emissions, which would be done by installing particulate filters on diesel vehicles. However, these filters will only be introduced once ultra-low sulphur diesel fuel becomes widespread, because this is a prerequisite for the filters to work effectively. Government legislation comes into effect (in Canada) in 2006 which requires refiners and importers to reduce sulphur content to ultra-low levels, and the first vehicles with "advanced emission control technologies" (particulate filters) will not be available until at least 2007.
Second, I was curious to know how current diesel emissions compare to gasoline emissions. The EPA produces a listing of vehicles rated by emissions on a scale of 1-10 (10 being the "cleanest"). I noted that there are only gasoline vehicles in the 7-10 range; the cleanest diesel (the New Volkswagen Beetle) is rated only 6 out of 10.
My conclusion is that at least in the short term (the next 3-5 years), gasoline is favourable over diesel in terms of emissions. Even after 2006, when ultra-low sulphur becomes the norm, an existing diesel vehicle would need to be retrofitted with a particulate filter in order to reduce emissions. And as this will be very new technology, it will likely be expensive to have this done.