Diesel cars justifiably have a great reputation for excellent fuel economy and low impact on the environment.
This essentially is a result of the diesel operating cycle being thermodynamically more efficient than the petrol engine.
A diesel engine in a similar sized vehicle is much more fuel efficient in terms of kilometres per litre than a petrol engine.
One of the reasons why a diesel engine is more fuel-efficient is because it operates at higher pressures than a petrol engine.
Fuel savings are significant with fuel efficiency typically being 25-35% better than a petrol engine vehicle.
Historically the diesel engine was also considered to be lacking power and driveability in a passenger car, however the modern passenger diesel engine is almost indistinguishable from the performance of a petrol engine.
This is due to advances in electronics and computer management systems that have enabled more precise control of the combustion process and the adoption of turbochargers to increase the power output and responsiveness of the diesel engine while retaining its advantages of fuel efficiency and environmental performance.
As stated in the introduction, a diesel engine is also generally considered to be more environmentally friendly than a petrol engine. This is very true in terms of hydrocarbon (HC), and Carbon monoxide (CO).
The only drawback of the diesel engine is that it produces far more soot as a result of combustion. They have been known to be a "dirtier" alternative to petrol vehicles.
The PSA Peugeot-Citroen group however, had pioneered the Diesel Particulate Filter (DPF) technology for this purpose. The DPF filters almost all the soot released and the end result is emissions that are significantly lower than before.