Diesel vehicles operate with an internal combustion engine and a compression-ignited injection system. Diesel fuel is injected into the combustion chamber of the engine and ignited by the high temperatures achieved when the gas is compressed by the engine piston. 

Diesel vehicles are available in either manual and automatic transmission. 

Diesel vehicles are more fuel efficient than petrol equivalents and are the preferred option for high mileage drivers and motorway drivers. They emit lower CO2 than the equivalent petrol car, but higher NOx. They benefit from both lower fuel duty than petrol and the entitlement to a VAT input credit for companies, while as the current VRT system places a greater emphasis on CO2 than NOx, they attract lower VRT.  

Lower mileage urban drivers are more suited to other engine types, as NOx emissions are higher and the start/stop and low speed nature of urban driving does not allow diesel engines perform effectively and can lead to increased servicing costs. New diesel cars (Euro 6 Standard) are now much cleaner and have significantly reduced both their CO2 and NOx emissions. Despite a reduction in its share of the new car market in recent years, diesel remains the second most popular choice of new car for Irish motorists.