The electronically controlled short-nozzle injectors can each deliver two precisely metered shots of fuel on each power stroke – a pilot injection and a main injection. The very small volume pilot injection comes earlier in the stroke, followed by the bigger volume of the main injection, which produces the power. The ultra-fine fuel vaporisation, two-phase injection and precise metering and timing, together have the effect of making the combustion process more progressive and more efficient. This means the engine runs more smoothly, is quieter, and delivers more power for less fuel. This efficiency is achieved at all engine speeds: at idle, noise levels are reduced by the ECM adjusting the pilot injection; at higher speeds, torque and power output are increased by extending the main injection to give a longer fuel-burn duration.
The ‘brain’ of the system, the ECM, controls both the diesel pump and the injectors. It takes information from sensors that monitor crankshaft, camshaft and throttle positions, and uses this data to assess engine speed, load and other factors. From there, for every single power stroke of each individual cylinder, the ECM computes the unique injection profile that will deliver optimum engine performance, and varies the rail pressure, the fuel volume and the injection timing to deliver it.
In practice the injection pressure ranges from 230bar (3,380psi) at low engine speeds to 1,500bar (22,000psi) when engine speed exceeds 2,000rev/min.
Beyond this technology, the Jaguar diesel engine has several additional features that optimise efficiency and maximise refinement. The first is a new system for detecting and reducing combustion noise - the CNS Combustion Noise Sensor. This ‘listens’ to each stroke in each cylinder, and by fine adjustments to the pilot and main injectors every 0.3 milliseconds, the CNS ensures the Jaguar X404 diesel engine is exceptionally quiet. It is also a ‘learning’ system, whose effectiveness increases as it operates.
Fundamentally, the CNS calculates the Minimum Drive Pulse (MDP) needed to provide the required output with the lowest combustion noise, especially at idle speeds. During assembly, the ECM system is programmed with an approximate value for the MDP. Every time the engine runs thereafter, the ECM continuously updates the MDP settings according to road- and other conditions.
So, unlike some engines, the Jaguar diesel isn’t at its quietest potential when it first comes off the production line. In effect, it learns to become quieter, especially at idle speeds, the more it is driven. If the injectors are replaced in service, it repeats the process just as effectively. Combustion noise is optimised after approximately 500 miles of typical road driving.
A further refinement in the X404 engine involves the way the individual fuel injectors are constructed to produce optimum performance as an assembly. Although they are manufactured with a very high degree of precision, the manufacturing tolerances involved are so minute that each injector has its own, unique ‘fingerprint’. These fingerprints are crucial to the way the ECM controls each injection parameter, in order to maintain the engine’s smooth running, low noise and optimum performance.
During manufacture, every injector is calibrated and its characteristics are written into a barcode label that is attached to the individual injector. As each engine is assembled, four injectors are selected at random. Their barcodes are scanned, and a new ‘composite’ barcode is generated, incorporating the collective data from the separate components of the injector set. That composite barcode (which by definition is itself unique) is attached to the engine. When the engine and ECM come together in the vehicle, the data from the composite barcode is transferred to the ECM – so that every ECM is perfectly tailored, individually, to its own injector set.