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. Put more simply, more energy is converted into useful work and less energy in the form of heat which is transmitted to the cooling system and subsequently to the atmosphere. This means that a diesel engine in a similar sized vehicle is much more fuel efficient in terms of kilometres per litre than a petrol engine.
This characteristic of higher thermodynamic efficiency with less heat going into the cooling system means that a diesel engine takes longer to reach the most efficient operating temperature and requires a higher engine load to maintain that temperature. This means that a diesel engine is more suited to longer distance driving at higher vehicle speeds or loads. If a diesel engine is not driven at its optimum operating temperature then its fuel and environmental efficiency significantly deteriorates. In a nutshell, the diesel engine in a passenger car is not suitable for short trips at low loads in a city environment i.e. one or two passengers travelling to the shops and back.
One of the reasons why a diesel engine is more fuel-efficient is because it operates at higher pressures than a petrol engine. This means that structurally the engine needs to be significantly stronger than a petrol engine. This is why a Boxer engine is particularly suitable for use as a diesel engine, because the engine crankshaft, that carries the majority of engine load and pressure, is provided with a high level of support by being sandwiched between the two halves of the crankcase. Never the less, even a Boxer diesel engine structure and its fuel delivery system that injects fuel directly into the high pressure combustion chamber needs to be manufactured with higher levels of strength and durability. This means that a diesel engine is typically approximately $3000 more expensive than a petrol engine and therefore to recover this increased capital cost longer driving with higher kilometres per year are required.