Diesel oil has been a contender to gasoline for many decades. Earlier diesel engines were not refined enough to win the hearts of many drivers but recent advances in technology made these engines not only worthy competitors in all areas but in some features—fuel economy or low end torque, to name just two—even exceeding the characteristics of their gasoline counterparts. And in addition to the general technological advantages, Citroën’s diesel engines have a widely accepted reputation—even among people blaming the quirkiness of its suspension or other features—of being excellent and robust.
As it is widely known, diesel engines have no ignition to initiate their internal combustion, they rely on the self-combustion of the diesel oil entering into a cylinder filled with hot air. Due to this principle of operation, the supply of the fuel has to comply with much more demanding requirements than it is necessary in the case of gasoline engines.
Unlike in the gasoline engine, not a mixture but air enters into the cylinders via the inlet valves. During the adiabatic compression all the energy absorbed is used to increase the temperature of the gas. The small droplets of fuel will be injected at high velocity near the end of the compression stroke into this heated gas still in motion. As they start to evaporate, they form a combustible mixture with the air present which self-ignites at around 800 °C.
This self-ignition, however, is not instantaneous. The longer the delay between the start of the injection and the actual ignition (which depends on the chemical quality of the diesel oil, indicated by the cetane number), the more fuel will enter the cylinder, leading to harsher combustion, with the characteristic knocking sound. Only with the careful harmonization of all aspects—beginning of injection, the distribution of the amount injected in time, the mixing of the fuel and air—can the combustion be kept at optimal level.
Small diesel engines suitable for cars were made possible by a modification to the basic principle, that allowed these stringent parameters to be considerably relaxed. It includes a separate swirl chamber connected to the cylinder via a restrictor orifice. The air compressed by the piston in the cylinder enters this chamber through the orifice, starting to swirl intensively. The fuel will then be injected into this swirl, and the starting ignition propels the fuel-air mixture still incompletely burned into the cylinder where it will mix with the air, continue and finish the combustion process. Using a prechamber results in smaller ignition delay, softer combustion, with less noise and physical strain on the engine parts, but introduces some loss of energy because of the current of air having to pass between the chambers. Citroën engines of this type use a tangentially connected spherical prechamber.
As diesel engine evolution continued, better simulation and modeling techniques became available, which, together with the improvements in fuel injection technology, lessened or removed the problems initially solved by the introduction of the prechamber. The direct injection engines of today have no prechamber, instead, the piston has a specially formed swirl area embedded in its face.
Although the basic principles of fuel injection are similar to what we have already discussed for gasoline engines, there are some notable differences. First of all, diesel engines operate without restricting the amount of air entering the engine: there is no throttle, the only means of regulating the engine is to vary the amount of fuel injected.
The fuel is injected into the engine, creating a combustible mixture in the same place it is going to be burned. Because the forming of this mixture results in its self-combustion, the diesel injection system is, in essence, an ignition control system. Unlike on the gasoline engine, fuel injection and ignition cannot be separated in a diesel engine.
The complete mechanical injection system is built into a single unit which can be divided into five individual—although interconnected—subsystems:
- a low pressure fuel pump to deliver the fuel for the rest of the injection system;
- a high pressure pump and distributor that routes the fuel to the appropriate cylinders in firing order (similar in purpose to the distributor on gasoline engines) and generates the high pressure needed for the injection as well;
- a regulator that determines the amount of fuel to be injected in relation to the engine speed, modified by additional factors like idle speed, cold starting, full load, etc.;
- an injection adjuster to compensate for the higher engine speed by advancing the start time of the injection;
- a fuel stop valve to cut off the fuel supply when the ignition has been switched off.
The diesel fuel is drawn—through a filter—from the tank by the low pressure pump 1 operated by the engine. A pressure regulating valve 2 ensures that the fuel pressure will not exceed a preset limit; when the pressure reaches this value, the valve opens and lets the fuel flow back to the primary side of the pump.
The piston 6 of the high pressure part is driven through a coupling 4 consisting of a cam disc and four cam rollers. The piston rotates together with the shaft coming from the engine but the coupling adds a horizontal, alternating movement as well: for each turn, the shaft and the piston 6 performs four push-pull cycles.
It is the pushing movement of this piston 6 that creates the high pressure and sends the fuel to the injectors. The fuel, provided by the pump 1 arrives through the fuel stop electro-valve 17, which is constantly open while the ignition switch is on but cuts the fuel path when it is turned off.
First, the piston 6 is pulled back by the coupling 4, letting the fuel enter the chamber and the longitudinal bore inside the piston. As the side outlets are blocked by the regulator collar 5, the fuel stays inside the chamber (phase 1).
In the next phase, the piston rotates and closes the ingress of fuel from the stop valve 17. On the other side of the piston, the high pressure outlet opens but as the fuel is not yet under pressure, it will stay in the chamber.
In phase 3 the piston is energically pushed by the cam disc and rollers of the coupling 4, injecting the fuel stored in the chamber into the output line with a significant force.
As the piston 6 moves to the right, at some point the side outlets will emerge from under the regulator collar 5—the fuel injection into the real output will stop immediately, and the rest of the fuel stored in the chamber will leave through this path of lesser resistance. This is phase 4, the end of the injection cycle.
Actually, this operation is repeated four times for each revolution of the incoming shaft. There are four high pressure outlets radially around the piston, each serving a given cylinder. As the outlet slot 19 of the piston turns around, it allows only one of the outlets to receive the fuel.
The pressure valves 7 serve to drop the pressure in the injector lines once the injection cycle is over. To reduce the cavitation caused by the pressure waves generated by the rapid closing of the injector valves, a ball valve minimizing the back flow is also used.
The length of phase 3, thus the amount of fuel injected depends on the position of the collar 5. If it is pushed to the right, it will cover the side outlets for a longer time, resulting in a longer injection phase, and vice versa. If it stays in the leftmost position, no fuel will be injected at all.
And this is exactly what the regulator part does: it moves this collar 5 to the left and to the right, as the actual requirements dictate. The lever 9 attached to the collar is rotated around its pivot by several contributing forces. The two main inputs are the position of the accelerator pedal as communicated through a regulator spring 12 and the actual engine speed, driving a centrifugal device 8 via a pair of gears 3. The higher the engine speed, the more the shaft 20 protrudes to the right, pushing on the lever 10.
When the engine is being started, the centrifugal device 8 and the shaft 20 are in their neutral positions. The starting lever 10—pushed into its starting position by a spring 11—sets the position of the collar 5 to supply the amount of fuel needed for the starting.
As the engine starts to rotate, a relatively low speed will already generate a large enough force in the centrifugal device 8 to push the shaft 20 and overcome the force of the rather weak spring 11. This will rotate the lever 10, moving the collar 5 to the left, setting the amount of fuel required for idling. The accelerator pedal is in the idle position as well, dictated by the adjustment screw 14. The idle spring 13 keeps the regulator in equilibrium.
Normally, the amount of fuel will be regulated by the position of the pedal as both springs 11 and 13 are fully compressed and do not take an active part in the process. When the driver pushes on the pedal, the regulating spring 12 stretches, both levers 9 and 10 rotate and move the collar 5 to the right, to allow the maximum amount of fuel to be injected. As the actual engine speed catches up, the centrifugal device 8 opens up, pushing the shaft 20 to the right, countering the previous force, gradually returning the collar 5 towards the no fuel position, until the point is reached where the amount of fuel injected maintains the equilibrium. When the driver releases the pedal, the inverse of this process takes place. During deceleration—pedal at idle, engine rotated by the momentum of the car—the fuel is cut off completely.
Without such regulation, if enough fuel is provided to overcome the engine load, it would continue accelerating until self-destruction (this is called engine runaway). Speed regulation is a feedback mechanism comparing the actual speed of the engine to the one dictated by the gas pedal and modifies the amount of fuel as necessary. If either the engine speed changes (because of varying load, going over a hill, for instance) or the driver modifies the position of the accelerator pedal, the regulation kicks in, adding more or less fuel, until a new equilibrium is reached. If the engine is powerful enough to cope with the load, keeping the pedal in a constant position means constant cruising speed in a diesel car; gasoline vehicles need speed regulated fly-by-wire systems or cruise controls to achieve the same.
The excess fuel will finally leave the pump unit through an overflow valve 18, flowing back to the fuel tank.
Something needs to be corrected…
The chemistry involved in the combustion dictates some parameters of fuel injection, the most important being the smoke limit, the maximum amount of fuel injected into a given amount of air, that results in combustion without resulting in soot particles. Although gasoline engines also have this limit, they normally operate with a constant fuel to air mixture that automatically places the amount of fuel below this critical limit. Diesel engines, in contrast, operate with a variable fuel to air mixture, using this very variation for power regulation. With diesel fuel observing the smoke limit is a much stricter task because once soot starts to develop, this changes the character of the combustion itself, resulting in a sudden and huge increase in the amount of particulates—a bit like a chain reaction.
Because the maximum amount of fuel injected depends on how far the lever 10 is allowed to rotate counter-clockwise, the inability of the pump to inject too much fuel, thereby crossing the smoke limit, is insured by an end stop 21 for this lever. This very basic means of smoke limit correction, adjusted for worst case conditions, was developed further on turbocharged engines, and still further on electronically controlled injection systems.
Timing is of enormous importance in a diesel engine. During the stroke of combustion, several events take place in close succession: the fuel injection system starts its delivery, then the fuel is actually injected (the time elapsed between these two is the injection delay), slightly later the fuel will self-ignite (this delay is the ignition delay), then the injection will stop but the combustion is still raging, first reaching its maximum, then dying away slowly (on the scale of milliseconds, that is).
Just like in a gasoline engine, the ignition delay remains constant while the engine speed changes. The fuel has to ignite before the piston passes its TDC position, but with the increasing engine speed, the distance the piston travels during a given period of time becomes longer. Therefore, the injection has to be advanced in time to catch the piston still in time. The injection adjuster 15 feeds on the fuel pressure provided by the pump 1, proportional to the engine speed.
This will move the piston, which in turn, through the levers, modifies the relative position of the cam rollers to the cam disc inside the coupling 4, increasing or decreasing the phase difference between the revolutions of the engine and the rotating-alternating movement of the distributor piston 6.
Some engines also have additional minor correction mechanisms 16 that modify the idle speed and timing depending on engine temperature, to provide better cold start performance. The engine temperature is measured indirectly, through the coolant acting on cylinder and piston-like elements filled with paraffin. As the paraffin expands or contracts as the coolant temperature dictates, the transformed mechanical movement, coupled through cables to two movable end stops for both the lever 9 and the injection adjuster 15, modifies the idle speed and the injection timing of the engine. Because correct timing depends on temperature, the corrections, although relatively slight, insure that the amount of fuel injected as well as the timing provide better combustion and lower pollution when the engine is started and operated at low temperatures. They do not have any effect once the engine reaches the normal operating temperature.
Now that the correct amount of fuel is carefully determined and the necessary high pressure generated by the pump, it has to be injected into the swirl chamber. The pressurized fuel entering the injector through a filter 1 tries to press the piston 2 upwards but a spring 3 counters this force. As soon as the pressure exceeds the force of the spring (which can be adjusted by placing appropriately sized shims behind it), the piston jumps up and the fuel rushes into the swirl chamber through the small orifice now opened. After the injection pump closes its pressure valve at the end of the injection period, the spring 3 pushes the piston 2 back, closing the orifice until the next injection cycle.
Each swirl chamber has its own glow plug whose only purpose is to heat up the chamber in cold weather. They start to glow when the ignition key is turned into the first position and stay glowing for some time afterwards unless the starting was unsuccessful.
More power requires more fuel. An efficient way to boost the performance is to provide both more air and fuel to the engine. The exhaust gases rushing out from the engine waste a great deal of energy; a turbocharger 4 spun by the exhaust flow taps into this source of energy to provide added pressure in the air inlet. Diesel engines are particularly well suited for turbocharging. Gasoline engines may not have the inlet pressure raised too much because the air and fuel mixture may subsequently self-ignite when it is not supposed to, and instead of burning controllably, detonate. In a diesel such a situation is not possible because the fuel is injected only when combustion should actually happen in the first place. As a result, relatively high inlet pressures can be used, considerably improving the power output of a diesel engine, and with proper attention to the subtleties of the design, engine efficiency and fuel consumption.
On its own, once the amount and pressure in the exhaust manifold reaches a level high enough to power it, with the engine fully loaded, the turbine would spin proportionally to engine speed squared, because both the pressure and the volume of the air pumped into the engine are increasing.
Because the engine is required to deliver as much torque as possible at the widest possible range of engine revolution, the requirements on the turbine are somewhat contradictory. If the turbo is made very small and light, it will spin up very quickly due to its low mass and inertia, ensuring its full benefit already at low rpms. However, with a moderate increase in engine speed, the rotational speed of the turbine (note the quadratic relationship) would become excessively high. When the turbine blade speed approaches the speed of sound, a supersonic wave effect occurs that can abruptly leave it without any load, at which point runaway would occur, resulting in severe damage to the turbine.
On the other hand, if the turbine was dimensioned so that even at the highest engine speed it is still operating within safe limits, it would not be useful at all in the middle range where the engine is most often used. A compromise can be achieved using an overpressure valve, the wastegate valve 5. The turbo pressure is constantly monitored by this valve opening above a set pressure limit, letting the exhaust escape through a bypass. This avoids turbo runaway by making the turbo rotational speed proportional to that of the engine, once the limit pressure is reached. This way the quick spin-up resulting from the quadratic relationship can be preserved while the turbocharging effect is extended over a significant percentage of the usable engine speed range—typically the higher 70-80%. But it comes at a price: because of the simplicity of such a regulation, the limit pressure is dictated by the maximum turbine speed, which is usually calculated for maximum engine speed plus a safety margin. The maximum pressure is already reached at lower engine and turbine speeds, where the turbine could conceivably still provide more pressure because of a lesser demand for air volume. Although with a simple wastegate a certain amount of the turbocharging potential is lost, the increase in power output is still substantial.
Citroën is a pioneer in implementing variable wastegate limit pressure using a controllable wastegate valve, to tap into this previously unused turbo potential.
Essentially, a turbocharged diesel engine runs in two different modes: atmospheric pressure or turbo-charged. The atmospheric pressure mode prevails while the exhaust gas produced is not yet sufficient to power the turbine (below a given engine speed and load). Once this limit is crossed and the turbine starts generating higher than atmospheric pressure, the engine is running in turbocharged mode.
The injection pump regulator needs to know about the changes in the inlet pressure, because those changes mean differences in the amount of air entering the engine. And this also means that the upper limit of fuel injected needs to be changed correspondingly. These injection systems are tuned for the turbo producing the rated waste pressure (also known as full boost). However, the amount of fuel injected during the atmospheric mode of the engine—before the turbo kicks in—has to be reduced in order to avoid crossing the smoke limit. The turbo pressure drives a limiter in the injection pump: with the increasing pressure the piston 21 moves down. Its varying diameter forces the lever 22 rotate around its pivot, which then acts as a stop to limit the allowed range of operation of the regulator lever 9, limiting the amount of fuel to be injected.
Towards a cleaner world
Exhaust Gas Recycling (EGR) systems were used—depending on the market—as add-on units. An electronic unit measuring the coolant temperature and the position of the gas pedal control on the pump (with a potentiometer fitted to the top of the control lever) controls a valve which lets part of the exhaust gas get back into the inlet.
Post-glowing is also used as a pollution reducing mechanism. A definite post-glow phase, lasting for up to minutes is usually controlled by a combination of a timer and the engine coolant temperature: either the timeout of 4 minutes runs out or the engine reaches 50 °C. An additional mechanism prevents post-glowing if the engine was not actually started.