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Diesel Tuning Fundamentals: Fuel System Fundamentals

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Fuel System Fundamentals


00:00 - The fuel system in a typical common rail diesel engine consists of a high pressure fuel pump that is driven directly by the engine, the common rail, which distributes the fuel to the injectors, and then the injectors themselves which are mounted directly into the combustion chamber.
00:16 The fuel initially is drawn from the tank into the high pressure pump either via an integral lift pump located inside the high pressure pump, or in some instances there may be an external lift pump that pumps fuel from the tank to the high pressure pump as found on some GM systems.
00:33 With systems that rely on an external lift pump, it is worth noting that if the lift pump fails for any reason, the engine's going to stop.
00:41 Once the fuel has made its way into the high pressure pump, it's compressed via a multi lobe and plunger arrangement.
00:48 The actual fuel pressure is controlled directly via the ECU which will be aiming for a specific target fuel pressure and the fuel pressure is monitored via a fuel pressure sensor fitted directly to the common rail.
01:01 In order to achieve this target fuel pressure the ECU will operate a suction control valve.
01:07 This can be accomplished in one of two ways.
01:10 The first method is via a simple pulse width modulated output that uses a PID control algorithm to control the fuel pressure.
01:19 The valve opens to allow fuel into the plunger chamber for delivery to the common rail and the ECU monitors fuel pressure and operates the suction control valve accordingly.
01:30 In the second method the suction control valve is operated sequentially in time with the engine and the lobe position in order to deliver a pre calculated volume of fuel into the rail.
01:42 This method allows for better control over the fuel pressure as the pump attempts to deliver exactly the amount of fuel to the rail that the ECU has calculated to be delivered from the injectors.
01:54 This second technique is also identical to the way a direct injection high pressure gasoline fuel pump operates.
02:01 Unless you're dealing with a standalone aftermarket ECU which is still rare in the diesel tuning world, it's not likely you'll need to worry too much about this since the factory ECU will be configured to correctly control the pump and achieve the target fuel pressure.
02:17 Once the fuel is delivered from the pump to the common rail, it's typically only released via an open injector into the engine and hence the fuel system can be considered as returnless.
02:27 It is common however for a safety release or over pressured to be included in the system to release fuel back to the fuel tank if required.
02:36 These safety release valves do still allow the fuel system to retain some fuel pressure even though the valve may be completely open, although this pressure will be well below the normal operating pressure and may be somewhere in the region of perhaps 40 megapascals or less.
02:52 As we'll see further into this course, increasing the fuel pressure is a common technique employed when tuning diesel engines, and it is important to understand that if we get to a point where the safety release valve opens, often the valve tends to stay open due to the volume of fuel flowing and under normal circumstances they only fully close when the fuel flow has been reduced dramatically.
03:16 If the valve is forced open then due to an over pressure situation and the driver was to remain at full throttle, this would result in sustained operation at much lower than normal fuel pressures.
03:28 And while this isn't necessarily going to be dangerous for the engine, performance will be severely hindered.
03:34 For this reason it's important to maintain control over the fuel pressure, and ensure that the fuel pressure target is kept below the relief valve opening point to ensure the valve doesn't open during normal operation.
03:47 Typically the relief valve is a simple mechanical device, however some engines such as the Toyota four cylinder diesel engines, use an electronically controlled relief valve.
03:58 And these can be used to help smooth out pressure overshoots that can occur in over run situations during deceleration.

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