The principle of operation of a diesel engine.
Design features and operational characteristics predetermined the passion or rejection of motorists in relation to units on “heavy fuel”. So how does a diesel engine work, what is its design, operating principle and advantages?
The times when a car with diesel engines were associated with smelling and quiet, have long been left behind the corner. Every motorist knows that a vehicle with a unit on “heavy fuel” emits characteristic rumbling sounds, its exhaust smells strange. Modern engines reward their owners with moderate fuel consumption, impressive elasticity (torque available in a relatively wide range of revolutions) and sometimes stunning dynamics to the envy of some petrol cars. But they are also demanding on the quality of diesel fuel, and repairs to fuel system components can be quite expensive.
Design features
Diesel engines, of course, do not have such colossal differences as the Wankel rotary piston engine, the structure of which is absolutely different from the “anatomy” of a traditional internal combustion engine, but it has a number of features that draw a line between it and petrol engines.
The diesel also has a crank mechanism, but its compression ratio is significantly higher – 19-24 units versus 9-11 units, respectively. The fundamental difference between a diesel engine and a petrol engine is how the fuel-air mixture is formed, ignited and burned.
The diesel engine has no spark plugs and, consequently, the fuel-air mixture is ignited by compression. At the same time, air and fuel are supplied separately. It should also be noted that virtually no modern diesel engine is without a supercharging system, which is used to improve the performance of the unit. Variable geometry turbochargers are used to optimise supercharging over the widest possible rpm range. The diesel unit has a higher efficiency, but it is heavier and produces more torque at low speeds than a petrol engine.
Principle of operation of a diesel engine
How does a diesel engine work and, most importantly, how does the fuel ignite in the combustion chamber if this type of unit has no spark plugs? First, air enters the cylinders. At the end of the compression stroke, when the piston has almost reached the top dead centre, the air temperature in the combustion chamber reaches high values (about 700-800 degrees) and then diesel fuel is injected into the cylinders, which is ignited independently, without spark ignition. However, there are still spark plugs in the diesel unit, but they are glow plugs, not ignition plugs, which heat the combustion chamber to facilitate starting the engine in cold weather.
They are spirals (metal and ceramic), can be installed in a vortex chamber or in a pre-chamber (in the case of units with a separate combustion chamber) or directly in the combustion chamber (if it is not separate). When the ignition is switched on, glow plugs glow almost instantly, in a matter of seconds, they glow to temperatures in the region of a thousand degrees and heat the air in the combustion chamber, facilitating the process of self-ignition of the fuel-air mixture.
Types of diesel engines
Separate combustion chamber engines are widespread – fuel is injected into a special chamber in the head of the block above the cylinder and connected to it by a channel, and the combustion process is not exactly the same as in petrol combustion engines. In this vortex chamber, the air flow is more intensively swirled, which contributes to more efficient mixing and self-ignition, which continues in the main combustion chamber. Incidentally, diesel engines with a split combustion chamber are less noisy due to the fact that the use of a swirl chamber reduces the intensity of pressure build-up during auto-ignition.
In non-separated combustion chamber diesels, the auto-ignition process takes place directly in the super piston space. This type of unit is somewhat noisier.
What is Common Rail
Common Rail is a modern fuel injection system developed by Bosch that uses the principle of supplying fuel to the injectors from the fuel ramp, which is a high-pressure accumulator. Common Rail allows the unit to be quieter, while being more economical and environmentally friendly. Another advantage of using a common fuel ramp is the wide possibilities of adjusting fuel pressure and injection timing, as these processes are separated.
The system includes a high-pressure fuel pump, piezoelectric injectors, a fuel ramp, a fuel pressure regulator and a fuel dosing valve. Interestingly, at the dawn of their evolution, diesel units had a simpler fuel equipment with mechanical injectors and incomparably lower pressure of diesel fuel against the background of modern systems.
Child of progress
Not so long ago diesel engines were environmentally “dirty” and rather weak, but since some time units of this type have changed dramatically, and some representatives of the tribe are worthy of sports cars. This includes BMW’s 3.0-litre inline six-cylinder motor with four turbochargers.
By the way, the design of this motor clearly demonstrates the progress of “heavy fuel” units. The techno-masterpiece is equipped with two low-inertia low-pressure turbochargers and two high-pressure turbochargers, one of which comes into action beyond 2500 rpm. The piezo injectors inject fuel at a whopping 2500 bar. The output is 400bhp and 760Nm. Interestingly, 450Nm is available at 1,000rpm! That’s what modern diesel engines are all about.