What we know as diesel is actually only one form called petrodiesel, which is a derivative of petroleum. Petrodiesel is made by fractional distillation of petroleum at temperatures between 200°C and 350°C. Thus it is an unrefined fuel, which is used in a variety of fields.
The engine was in fact so flexible, that Rudolf Diesel tried out various vegetable oils to fuel the engine and they actually worked. The new form - biodiesel, is a continuation of the same idea that did not quite take off the last time. Other substances used to power a diesel engine include BTL (biomass to liquid) and GTL (gas to liquid) fuels.
A diesel engine enjoys a very high compression ratio and low fuel consumption per horsepower as compared to the gasoline powered engines. It also has better torque ratios. They don't have spark plugs for ignition and can ignite themselves. Moreover, they are quite accommodating and allow the use of bio-diesel.
However, there are some problems associated with these engines which are discussed here.
The engine is much higher priced as compared to its gasoline counterpart. This is because the design of such an engine is inherently more complicated. It also involves a higher cost of maintenance.
Diesel releases a lot of pollutants as it is a crude, fractionally-distilled form of petroleum. Although research is being done on ultra-low sulfur diesel, emissions are still quite high.
These engines need to be warmed up by using oxygen for oxidation. Hence, such an engine needs to have an air tank attached to it, to constantly supply oxygen to the engine. It also needs an oil tank. An oil tank is very crucial to the working of a diesel engine, as diesel is not a refined fuel. Hence, these will need to be changed regularly.
Moreover, it is also going to need an intercooler that will reduce the temperature when the heat becomes too much for the engine. Also, the air filters and fuel filters need to be changed regularly as they are extensively used to curb the emissions from this engine. Glow plugs are also to be changed after 2 years.
While a diesel engine is not troubled by a spark plug failure, it is going to be troublesome in the winter. In cold temperatures, diesel changes into a gel, which is immobile. This immobile diesel will not enter the combustion area and causes the heat that gets the engine going. Thus, in winter most people face problems starting such engines.
As hard to believe as this may be, diesel also fuels growth of microbes. Certain bacteria, fungus, yeast, and mold are known to thrive in such an engine. This bio-degrade gets stuck in the fuel filters, thereby plugging them. Hence, they need to be changed often.
Thus diesel engines have their own share of problems. Ignition, maintenance, price, etc., are some of the problems you need to consider while making a decision regarding whether to buy a diesel-powered engine or a petrol-powered one.