Diesel automobile engines

The main difficulty encountered when developing a diesel engine lies in the very short time available to vaporize the injected fuel and mix it with the air. Combustion in a heterogeneous environment with fuel-rich zones inevitably produces smoke and a slowdown in combustion that is detrimental to efficiency.

Diesel engineers have always endeavored to accelerate the mixing speed between fuel and air. The most effective way to do this is to increase injection pressure, creating very fine droplets of fuel (diameters of around 10 to 15 μm) projected at high speed (over 400 m/s) into the chamber. Mixing is further improved by creating a swirling movement of air in the cylinder, thanks to the appropriate shape of the intake pipes.

In direct injection, the fuel is injected radially into the "bowl" hollowed out in the piston from an injector nose positioned towards the center of this bowl, by several jets, the number of which is usually between 5 and 8.

This arrangement was developed on large industrial engines, but ran into difficulties on small car engines. On these, due to the small size of the chamber, the fuel jets hit the chamber walls and overlap each other, creating rich zones that burn poorly.

Engine designers then turned to another principle of mixing fuel and air: indirect injection, in which the fuel is injected into a pre-chamber connected to the cylinder by a transfer channel (Ricardo swirl chamber) or several communication ports (Mercedes pre-combustion chamber). This creates powerful aerodynamics that activate the air-fuel mixture over a wide rpm range, with a single-hole injector (pintle injector), without requiring very high injection pressures (400 to 500 bar).

But this powerful aerodynamic design also had its shortcomings, many of which had to do with the increased heat exchange coefficient at the walls:

• cold start difficulties ;

• poorer performance ;

• need for a larger cooling radiator ;

• limitation of specific performance, due to increased thermomechanical stresses in piston and cylinder head materials.

In the 1990s, advances in injection systems enabled the development of direct-injection diesel engines for the automotive market, with pressures in excess of 1,300 bar, and up to 1,600 bar in 2007. Specific performance, which was limited to around 45 kW/L with pre-chamber engines, exceeds 65 kW/L with direct injection. High specific power enables what is known as "downsizing": a given level of performance can be achieved with a smaller-displacement engine that has less internal friction, resulting...