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Diesel Tuning Fundamentals: Boost Control

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Boost Control


00:00 - Turbocharging and common rail diesel engines go hand in hand in order to provide the advantages of diesel along with a reasonable power density from the engine.
00:10 With this in mind, boost control is going to be a significant part of the tuning considerations on any turbocharged diesel engine.
00:17 In general the principles of turbocharging a diesel engine are identical to that of a gasoline engine but again the diesel engine's operation does offer some significant differences too.
00:29 Before we move into these though, it's important to have a basic understanding of turbocharger operation.
00:35 The turbocharger consists of a compressor wheel that is fitted on a common shaft along with a turbine wheel that's exposed to the exhaust flow.
00:44 The exhaust wheel harnesses energy from the exhaust flow in order to spin the compressor wheel at sufficient speed to compress the inlet air flow and create positive boost pressure.
00:55 Now that's a highly simplified view of turbo operation, but for now this is all we really need to understand.
01:02 One of the keys to turbo performance is the energy that is available to drive the turbine wheel and a subtle but important aspect is that the energy is provided in two forms.
01:13 Heat and exhaust flow which creates a pressure drop across the turbine wheel.
01:19 The more heat and flow that's provided, the faster the turbine wheel spins and in turn the more air the compressor wheel can force into the engine, creating more boost pressure.
01:30 In a gasoline engine, since we're always operating across a relatively narrow air fuel ratio, the exhaust temperature in particular is always relatively constant, and for example, if we're operating at 3000 RPM and 15 psi of boost, we might expect to have an exhaust temperature in the region of perhaps 700 degrees centigrade plus or minus perhaps 10%.
01:53 This means that the amount of energy available to drive the turbocharger is relatively consistent and predictable.
02:01 In a diesel engine on the other hand, we can see a huge variation in the air fuel ratio and as we increase the fuel supply and richen the mixture, this in turn creates more exhaust gas temperature, providing more energy to drive the turbocharger.
02:16 For the same operating point of 3000 RPM and 15 psi of boost for example, the exhaust temperature in a diesel engine may vary from 300 degrees centigrade to perhaps greater than 900 degrees.
02:30 What this means is that as we increase the fuel volume, more energy is available to drive the turbocharger and the effect is that as we richen the air fuel ratio we also tend to see the boost pressure increase.
02:44 In this way, boost control in a diesel engine becomes a little more complex to tune because the final boost pressure will change when we make adjustments to the fuel volume being delivered.
02:55 In turn though as the boost changes, this has the effect of varying the air flow into the cylinder and this then changes the air fuel ratio.
03:04 In this way there is a complex relationship between boost and fuel that we need to consider when making tuning adjustments, and adjustments tend to become an iterative process of going back and forth between fuel and boost control settings until the desired targets are achieved.
03:21 The other area that turbocharged diesel engines differ from mainstream gasoline engines is that often diesel engines will be fitted with a VNT or variable nozzle turbine turbocharger.
03:33 These may also be referred to as VGT which stands for variable geometry turbine.
03:40 Either way these turbochargers include a set of veins in the turbine housing that can be opened or closed via the ECU in order to control the exhaust flow onto the turbine wheel.
03:51 This style of turbocharger has the advantage of being able to act like a smaller turbo at low RPM, providing better boost response, while acting like a larger turbo at higher RPM and reducing exhaust gas back pressure.
04:06 In a conventional turbocharger that we may see more commonly fitted to a gasoline engine on the other hand, we're likely to control boost pressure via a wastegate.
04:16 The wastegate is a flap or valve that diverts exhaust flow around the turbine wheel and reintroduces it to the exhaust downstream of the turbine.
04:25 By varying the amount of exhaust gas flowing through the turbine wheel, the speed of the turbine wheel and hence the boost pressure can be accurately controlled.
04:36 We'll now look at a practical demonstration on the dyno to help reinforce the relationship between the air fuel ratio and the turbocharger performance.

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