Electronic Diesel Control
Just like it is the case with gasoline engines and carburetors, a mechanical device—even one as complicated as a diesel injection pump—cannot match the versatility and sensibility of a microcomputer coupled with various sensors, applying sophisticated rules to regulate the whole process of fuel injection.
The only input a mechanical pump can measure is the engine speed. The amount of air entering into the engine, unfortunately, is far from being proportional to engine speed, and the turbo or the intercooler disturbs this relationship even further. As the injection always has to inject less fuel than the amount which would already generate smoke, the mechanical pump—capable only of a crude approximation of what is actually going on in the engine—wastes a significant amount of air, just to be of the safe side.
The satisfactory combustion in diesel engines relies on the exhaust as well—if this is plugged up, more of the exhaust gases stay in the cylinder, allowing less fresh air to enter. A mechanically controlled injection pump has no feedback from the engine (except for the engine speed)—it will simply pump too much fuel into the engine, resulting in black smoke. An electronically controlled injection pump, on the other hand, can tell how much air has actually entered by using a sensor (although only the latest systems use such a sensor).
There are also other factors never considered by a mechanical system. The details of the combustion process depend heavily on the chemical characteristics of the fuel. The ignition delay, as we have already seen, depends on the cetane number of the diesel oil. In spite of the fact that correct timing has a paramount influence on the performance and the low pollutant level of a diesel engine, the mechanical system can have no information about this very important input factor. Less essential but still important is the temperature of the incoming air. With measuring all the circumstances and conditions in and around the engine (air, engine and fuel temperatures), the injection system can achieve better characteristics, lower fuel consumption and less pollution.
All in all, the electronically controlled injection pump not only adds precision to the injection process as its gasoline counterpart does but introduces completely new methods of regulation; therefore it represents a much larger leap forwards than fuel injection in gasoline engines. In spite of this, it is quite similar to its mechanical predecessor. From the five subparts, four remain practically the same, only the regulator is replaced with a simple electromagnetic actuator that changes the position of the same regulator collar 5 as in the mechanical pump, in order to regulate the amount of fuel to be injected.
The real advantage over the former, mechanical pumps is that an electronic device, a small microcomputer can handle any complex relationship between the input values and the required output. With mechanical systems, only simple correction rules are possible, and as the rules get more complicated, the mechanics quickly becomes unfeasible. In contrast to this, the ECU just have to store a set of characteristic curves digitized into lookup tables, describing the amount of fuel to be injected using three parameters: engine speed (measured by a flywheel inductive magnet), coolant temperature (measured by a sensor protruding into the coolant liquid), air temperature (measured by a sensor in the air inlet).
The newer HDi engines use an air mass sensor using a heated platinum wire. Having the exact amount of air to enter the engine, these latest EDC systems can deliver true closed loop regulation.
A potentiometer attached to the accelerator pedal sends information about the pedal position to the computer. This signal is used as the main input, conveying the intentions of the driver. The ECU uses this sensor to learn about special conditions like idle speed or full load as well.
Air temperature is measured by a sensor in the inlet manifold (but if the air mass is measured by a heated platinum wire sensor, this already provides the necessary air temperature correction, thus there is no need for an additional sensor).
The ECU stores the basic engine characteristics, the intrinsic relationship between the air intake and the engine speed (plus the manifold pressure if a turbo is fitted). The values obtained from this table are corrected according to the inputs of the various sensors, in order to arrive at a basic timing and smoke limit value. The actual amount of fuel injected and the accurate timing are a function of these results and the position of the accelerator pedal.
The final amount of fuel calculated will be used to control the electric actuator 8 which—by moving a lever 10—changes the position of the regulating collar 5. To ensure the necessary precision, the factual position is reported back to the computer using a potentiometer.
As we have already mentioned, the exact timing of the injection is of utmost importance in a diesel engine. The electronic system uses a needle movement sensor built into one of the injectors (the other are assumed to work completely simultaneously) notifying the computer about the precise time of the beginning of the injection. Should there be any time difference between the factual and designated opening times, the electro-valve 14 of the injection adjuster 15 will receive a correction signal until the difference disappears. If the electro-valve is completely open, the injection start will be delayed, if it is closed, the start time will be advanced. To achieve the timing required, the valve is driven with a modulated pulse signal, with the duty cycle (on-off ratio) determined by the ECU.
The input from this sensor is also used for compensating calculations on the amount of fuel injected, and to provide the on-board computer with the exact amount of fuel used up so that it can calculate the momentary and average consumption.
The computer has extensive self-diagnostic functionality. Many sensors can be substituted with standard input values in case of a failure (serious errors will light up the diagnostic warning light on the dashboard). Some sensors can even be simulated using other sensors—for instance, the role of a failing engine speed sensor might be filled in by the signal generated from the needle movement sensor.
As there is no standalone ignition in a diesel engine, the only way to stop it is to cut off the fuel supply. The mechanical default position of the actuator 8 is the position where no fuel enters the injectors at all; this is where it returns when the computer receives no more voltage from the battery, the ignition switch having turned off.
As it has already been mentioned, the inlet pressure is one of the principal EDC parameters for a turbocharged engine. Later Citroën turbocharged diesels—starting with the 2.5 TD engine of the XM—pioneered variable turbo pressure technology. The wastegate on these turbines has several actuators, fed with the turbo pressure through electric valves. The ECU, based on the relevant engine operation parameters obtained from the sensors, controls these actuators in various combinations, providing a selection of two or three different wastegate limit pressures. This lets the system ease the compromise between the turbo pressure and turbine speed: the pressure is kept at the usual value for higher engine speeds (limited by the maximum turbine speed) but is allowed to go higher than that in the middle rpm ranges, adding a significant amount of torque in the range where it is most needed.
Green versus Black
Diesel oil, just like gasoline, is a mixture of various hydrocarbons (CnHm), and burned together with the oxygen (O2) of the air, transforms to carbon-dioxide (CO2) and water vapor (H2O). However, as the combustion is never ideal, the exhaust gas also contains various byproduct gases: carbon-monoxide (CO), various unburned hydrocarbons (CnHm), nitrogen-oxides (NOx). The relatively high lambda value a diesel engine is operating with reduces the hydrocarbon and carbon-monoxide content to 10–15%, and the amount of nitrogen-oxides to 30–35% of the corresponding figures measured in gasoline engines without a catalytic converter. The sulphur content of the fuel—drastically reduced during the recent decades—is responsible for the emission of sulphur-dioxide (SO2) and sulphuric acid (H2SO4).
Conversely, these engines emit 10–20 times more particulates—or black soot—than gasoline engines. These are unburned or incompletely burned hydrocarbons attached to large particles of carbon. These substances are mainly aldehydes and aromatic hydrocarbons; while the first only smells bad, the second is highly carcinogenic.
The much higher amount of particulates is due to the different combustion process. The various aspects of mixture formation, ignition and burning occur simultaneously, they are not independent but influence each other. The distribution of fuel is not homogenous inside the cylinder, in zones where the fuel is richer the combustion only takes place near the outer perimeter of the tiny fuel droplets, producing elemental carbon. If this carbon will not be burned later because of insufficient mixing, local oxygen shortage (large fuel droplets due to insufficient fuel atomization, caused by worn injectors) or the combustion stopping in cooler zones inside the cylinder, it will appear as soot in the exhaust. The diameter of these small particles is between 0.01 and 10 μm, the majority being under 1 μm. Keeping the amount of fuel injected below the smoke limit—the lambda value where the particulate generation starts to rise extremely—is essential.
Similarly to gasoline engines, the exhaust gas can be post-processed to reduce the amount of pollutants even further. There are two different devices that can be used:
- Soot burning filter: as the diesel engine always operates with excess air (its lambda is above 1), there is enough oxygen in the exhaust gas to simply burn the carbon soot present. The burning filter is manufactured from ceramic materials that can withstand the resulting high temperatures (up to 1200 °C). As the diesel engine is very sensitive to excessive back pressure, the filter has to be able to self-regenerate. This is solved by the addition of organic metal substances.
- Catalytic converter, identical to the simpler ones used on gasoline engines before the proliferation of three-way, controlled converters. It reduces the carbon-monoxide and hydrocarbon content of the exhaust gas.