Common-rail diesel technology
A guide to troubleshooting and fixes for high-pressure systems.
Diesel engines has been under fire recently in a game of political ping-pong, with the Government reversing its stance of encouraging diesel usage. Accusations of ‘dirtiness’ have resurfaced, long after it was believed the charges were no longer relevant. Nevertheless, the Society of Motor Manufacturers and Traders continues to defend modern diesel vehicles, citing that they are cleaner than ever before. Even taking into account the ‘Dieselgate’ scandal, the statement is correct.
In the 1980s, Citroën’s BX was Great Britain’s best-selling diesel car. Praised at the time for its refinement, economy and power, its Peugeot XUD engine was responsible for a great number of European motorists considering diesel as a serious alternative to petrol for the first time. Yet, things have moved on. Viewed in today’s context, the venerable XUD is inefficient, noisy, smelly and smoky.
More for less
Even as motorists have demanded more power and greater efficiency from diesel engines, legislators have been tightening the regulations on emissions through the EURO standards, and these requirements have had to made without sacrificing fuel economy. While emissions control hardware, such as diesel particulate filters (DPFS) and exhaust gas recirculation (EGR) valves, play beneficial roles, the advent of high-pressure common-rail direct fuel injection has allowed modern vehicles to meet the conflicting demands of engineers and environmentalists.
Direct vs indirect injection
Many diesel passenger cars, including the aforementioned BX, used indirect fuel injection that was developed by the British pioneer Harry Ricardo. For this, the fuel injector nozzle is not positioned directly within the cylinder but inside a special pre-chamber, cast into the cylinderhead. This increases the burn duration and reduces diesel ‘knock’, but prioritises refinement over efficiency. To correct this, direct injection (DI) places the fuel injector within the cylinder and the piston crown is reshaped to form a bowl. Fuel is injected towards the crown’s centre, prior to dispersing outwards, as the mixture burns. This
dictates greater control over the fuel delivery, because DI systems require higher fuel pressures, due to the resulting shorter injection period.
As diesel pumps are driven by the engine, the fuel pressure generated is proportionate to engine speed. On many indirect injection units, mechanical fuel injectors are connected directly to the pump and inject fuel only when pulses of a pre-determined pressure are delivered. With common-rail systems on most DI engines, the fuel pressure is stored at much higher pressures, up to approximately 2500 Bar, within a hollow metal tube (the common-rail – which acts as an accumulator) to which the injectors are connected. Instead of the high-pressure fuel activating the injectors, they are controlled electrically via the Electronic Control Unit (ECU). Therefore, both higher pressures and more precise electronic regulation over the injectors has allowed manufacturers to control engine timing, fuel atomisation and injection quality far more accurately.
Modern systems can also perform several injections within each short cycle, which can comprise several pre-injections for refinement, a main injection period that produces the power, and several post-combustion injections to allow for DPF regeneration, if required.
More recent high-pressure pump developments have made them smaller and less power-hungry.
Prolonging life
Given their precision, common-rail pumps and fuel injectors are both complex and expensive. This has made them more sensitive to fuel quality, which is why using more viscous fuels (such as Straight Vegetable Oil) is likely to damage the high-pressure parts. As the diesel fuel lubricates both the high-pressure pump and the fuel injector nozzles, accidentally filling up with petrol, which has virtually no lubricating qualities and acts as a solvent, can seriously damage the high-pressure pump before the fuel even reaches the engine.
According to Carwood of Birmingham, which is a specialist diesel system remanufacturer, fuel contamination is the main cause of failures. Not changing diesel filters regularly, or using lowquality cartridges that collapse within the fuel filter housing, will reduce the life of the high-pressure components, especially on pumps fitted with gears that lift fuel from the tank. Failure to drain water from the fuel filter housing also promotes corrosion. Mechanical failure, such as an in-tank lift pump disintegrating internally, can also introduce metal fragments into the workings of the high-pressure pump. Should this swarf find its way into the injectors, metal shards cause wear as they denigrate into a fine grinding paste.
Further problems can be caused by well-intentioned owners. Not only can aftermarket tuning raise pump pressures to such an extent that thermal overheating occurs, but fuel cetaneboosting additives that are overdosed can affect the vital pump and injector lubricating properties of the diesel fuel.