Rudolf Diesel was an inventor and a visionary. He wanted to produce an engine that was simple, economical and did not need complex electrical accessories to achieve combustion. That’s why he invented a self-ignition engine which would ignite the fuel-air mixture by producing its own heat. At first many rejected his ideas and said that his engine would be impossible to construct. It took many efforts and failures in the process before the engine finally ran.
The idea was simple: compress air quickly and forcefully so the temperature becomes high enough to ignite fuel when it is injected. However, the pressure needs to be contained and injection of fuel has to occur quickly at a precise point in order to make the process work. How does one get the necessary air in and out, and how can the fuel be injected at the right point with very high pressure to achieve rapid combustion?
It starts with a piston traveling up and down in a cylinder that generates much higher compression than a piston in a comparable gasoline engine.
Intake and exhaust valves control the flow of gases like in other combustion engines. When the intake air is at its maximum compression through rapid piston movement a stream of fuel mist is injected rapidly through an injector by a pump producing very high pressure. The fuel pressure generated in modern Diesel engines can be a couple of thousand pounds. Rudolf Diesel first used charged air pressure to inject fuel. Today small mechanical pumps deliver the fuel.
Ignition is generated by the heat from the compressed air in the cylinder. However, if the engine is cold then pistons and cylinders will absorb some of the heat and prevent the engine from reaching combustion. That’s why Diesel engines have glow plugs. These are small metal sticks which reach into the combustion chamber and their tip literally glows when activated with electricity. When the fuel is injected it will ignite by passing the hot glow plug. Once ignition has occurred it generates enough heat to continue through the mechanical action of the engine.
Timing is critical. The space between cylinder head and pistons is so small at the highest position (top dead center) that it is measured in fractions of millimeters. If the timing between pistons and valves is not accurate then they will make contact and something will need to give. There will be severe damage. The valves are most often controlled by timing chains or rubber belts with a gear like surface on one side. They will need to be checked and replaced in regular intervals.
It’s the right mixture between very small fuel droplets and air that creates the explosion.
The small space that’s left when the piston is at its top dead centre (changing direction from up to down) creates a problem for the fuel injection. There’s not enough space for the fuel to expand to form a fine mist that can explode. In indirect-injection engines a small cavity in the cylinder head allows the fuel to swirl and create an explosive mixture through injectors that pray fuel at a predetermined angle.
In newer direct-injection engines the piston has a cavity that allows fuel and air to mix with the injector spraying fuel sideways from a central position in the combustion chamber.
Injection timing is also critical for the development of power. If it is too early it will slow down the travel of the piston and if it is too late then less energy is generated. Most Diesel engines have springs in the injectors which release at a certain pressure. That peak pressure requires the movement of the piston in the injection pump, which generates the pressure, is accurately correlated with the movement of pistons and valves through the timing belt. Today more and more engine management is controlled electronically by measuring air flow, engine temperature, engine rotations, power demand and other parameters.
Injection systems are changing too. Fuel pumps are now loaded with electronic systems. There are unit injector systems where every injector creates its own pressure which eliminates one central injection pump. Then there are common rail systems where a central pump produces high fuel pressure and the injectors release fuel based on electronic control mechanisms.
There is certainly more complexity to modern Diesel engines and the drive for simplicity is history. Certainly Diesel engines have become much more efficient and they are already 30-40% better than gasoline engines in energy consumption. Maybe Rudolf Diesel would be proud of the development of his engine as it powers most of our goods in ships and trucks, helps to feed us in agricultural applications and generates electricity for some. At his time he resisted the use of his engine in military applications. Navies were very interested in his compact engine to replace the cumbersome steam engines. The inventor disappeared mysteriously from a ship while crossing the English channel just before the first world war.
As Rudolf Diesel ran his engine on peanut oil at the world exhibition in Paris early in the last century we can now go back to his idea by running modern Diesel engines with renewable waste oils. There is no war required to access that fuel…