A diesel engine is a compression ignition engine which is a type of internal combustion engine. The fuel that is inside a diesel engine is ignited by sudden exposure to a high temperature, pressing factor of a compressed gas containing oxygen that is generally climatic air, instead of a different wellspring of start energy which is mainly a spark plug.
Like the gasoline engine, it is a reciprocating engine consisting of a crankshaft connected to the piston by a connecting rod. This system transforms the rectilinear round trips of the piston in its cylinder into rotation of the crankshaft, then of the driving wheels.
Belonging to the large family of internal combustion engines, it has the particularity of using the phenomenon of self-ignition. Thus, the air compressed by the head of the piston in the small volume of the cylinder head reaches a range of 500 to 700 ° C. At such temperatures, it will suffice to spray the fuel in the cylinder head to get it ignited without the need for a spark plug.
One of the most important criteria to consider when buying is price. Diesel cars are more expensive and have a longer payback period, not to mention the cost of maintenance, which is also higher. If they are less polluting than gasoline engines, it is in particular thanks to the addition of pollution control systems (FAP, EGR, etc.) which significantly increase the score.
What are the different types of diesel engines?
- Indirect Injection Engines
Even if it is declining, the diesel market remains at the top of sales in the world. Reputed to be more robust, diesel engines are less greedy than gasoline units and often have a longer lifespan
The application of the industrial diesel engine to light motor vehicles sometimes favors the silence of operation to the detriment of slight overconsumption. To meet these conditions, diesel engines fitted to cars were, until recently, of the divided chamber type (indirect injection). Two principles are used: pre-chambers and swirl chambers.
In these two cases, the combustion takes place in two separate volumes: a chamber, representing 30 to 60% of the total volume, which receives the fuel injection and where combustion begins, and the main chamber in which it s’ complete. Injecting fuel into this small, relatively hot volume allows the fuel ignition delay to be reduced.
Only the minimum quantity of fuel necessary for the ignition of the combustion ignites, the rest is expelled from the chamber by the increase in pressure and combustion continues in the main chamber. Indirect injection engines meet the conditions required for their application to the automobile, namely relative silence in operation and a low rate of NOx emissions.
The second oil shock in 1973 and ever more severe pollution control standards led manufacturers to rethink the diesel engine in terms of economy and low pollution.
- Direct Injection Engines
The direct-injection engine stands out for its higher efficiency than indirect injection engines.
Indeed, the ratio between the surface area and the volume of the combustion chamber is markedly lower for an engine with a single dead space chamber (direct injection) than for a prechamber engine (indirect injection); moreover, the combustion time is shorter in a direct injection engine.
These two parameters reduce the heat exchanges between the combustion chamber and the cooling system, factors of loss of efficiency. The problems associated with direct injection are twofold: combustion noise and nitrogen oxide (NOx) emissions. The appearance of electronic regulation in injection systems has made it possible to stabilize and refine the basic settings, both in terms of injection moment and fuel flow.
These various mechanical injection systems using a distributor pump, whether or not electronically regulated, have as a common characteristic the variation of the injection pressure as a function of the speed of rotation of the engine. The common rail injection system overcomes this constraint.
Common Rail High-Pressure Injection Engines
Unlike distributor pump systems, the injection pressure is independent of the engine speed and remains constant during the injection phase. The control of the injection by an electronic computer leaves a great latitude of programming to the engine manufacturers.
The quantity of fuel injected can be fractionated in order to carry out a pre-injection, which makes it possible to reduce combustion noise and the formation of NOx.
This small quantity of fuel (1 to 4 mm3) makes it possible to prepare, by increasing the temperature and the pressure in the combustion chamber, the ignition of the fuel during the main injection.
Vehicles fitted with particulate filters have a cleaning phase that requires post-injection, made possible by controlling the injectors by an electronic computer. The control by digital electronics of all the injection parameters makes it possible to optimize the operation of the engine.