Diesel vs gasoline

Most people agree it is environmentally important to focus on non-fossil fuel car engines NOW. 
This aside, we will continue to buy fossil fuel burning automobiles as long as economically feasible.
How long can we (the planet) afford to continue to refine gasoline and diesel engines? 
I had always thought gasoline was less harmful to the environment than diesel.

I have always thought generation of biofuels was energy expensive. 

Here are two very interesting charts showing lifecycle GHG reduction from biofuels displacing fossil fuels and producers of CO2 in the USA. Biofuels (ethanol and biodiesel) are the way to go in the near future, as they produce much less CO2 and will become the fuel of choice (as fossil fuels are depleted).

However, HEVs (hybrid electric vehicle) and EVs (electric vehicle)  engines will produce significantly less emissions and are the longer short-term solution. Hydrogen fuel cell engines will be an even better solution as they produce NO emissions (but do cause emissions in their current production methods).CO2 is, by far, the greatest contributor to GHGs (approx 85%) in the USA.

While we continue to buy gas and diesel fuel, we should understand which is less harmful to the environment (generates less lifecycle greenhouse gas emission (GHG)).

Here is a list of the most environmentally friendly automobiles for 2009. Hybrids and FFVs (flex fuel vehicles using 85% ethanol) lead the chart at this time. Check here for your vehicle or next purchase.

A diesel engine doesn't appear until around number 42. Biodiesel automobiles don't appear in the list as they are not yet available in Canada. 

Here is the list of diesel cars available in Canada. These are clean diesel (see below) engines and their CO2 output is greater than FFVs, gas hytbrids and a few small gasoline engines ... but comparable to the larger gas engine vehicles. The diesel engines, prior to clean diesel technology, were much worse environmentally than gas engines. Previous diesel engines produced more CO2 and NOx than gas engines. Previous diesel engines produce more NOx than gasoline engines. 
Diesel engines have advanced in the last few years and now use clean diesel technology (the ones in the chart above). Mercedes-Benz BlueTec (car engine) and AdBlue (truck engine) are examples.

Clean diesel technology utilizes a catalytic converter to convert all NOx into nitrogen and water only. The clean diesel engine is now comparable to the larger non-hybrid and non-biofuel gasoline engine in lifecycle GHG reduction. 

What will happen in the future?

In my opinion, from a GHG perspective, fossil fuel HEVs with biofuels will overtake FFVs, if the price of fossil fuels doesn't increase too quickly. EVs will take over, if fossil fuel prices increase dramatically, more efficient batteries are developed, and the hydrogen fuel cell isn't ready. Eventually, hydrogen fuel cell vehicles will become the clear winner.