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Practical Diesel Tuning: Boost Control Fundamentals

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Boost Control Fundamentals

04.03

00:00 - One of the most common areas we're going to make changes on a diesel engine is in the boost control.
00:04 In particular, we'll want to increase the boost pressure when the driver's commanding maximum power.
00:09 This has the effect of increasing airflow to the engine which means we can add more fuel while maintaining a safe air/fuel ratio that isn't excessively rich.
00:16 In this way our use of boost pressure when tuning a diesel engine is quite different to the petrol engine.
00:20 And within reason, boost is our friend.
00:22 But of course there are a few caveats we want to be aware of.
00:25 The first and most important is that we need to operate within the capabilities of the turbocharger.
00:30 This means understanding what the turbocharger's capable of in terms of airflow and boost pressure as well as the mechanical strength limits of the turbo.
00:38 Secondly we also need to consider that if we increase the boost pressure and create more power from the engine, we may exceed the mechanical strength of the engine components or struggle to adequately seal the cylinder head to the block under very high cylinder pressures.
00:51 Lastly, increasing boost pressure also entails creating more heat in the airflow into the engine.
00:58 Hot air of course is less dense which means it contains less oxygen for a given volume.
01:01 So hot air is the enemy of power.
01:04 This is why the modern turbocharged diesel engine is equipped with an intercooler which cools the air charge back down after it's been compressed by the turbo.
01:12 The stock intercooler is almost certainly adequate for the factory designed boost level but when we start increasing the boost the intercooler needs to remove more heat from the air and it may not be able to cope, resulting in air temps climbing.
01:24 If we get to a point where the charge air entering the engine becomes too hot, we won't see the benefit we may expect from the increase in boost.
01:31 This is also made worse as we start pushing the stock compressor wheel harder and harder to make that boost.
01:38 Physics dictates that when we compress air it must increase in temperature.
01:41 However another factor that will affect the final compressor outlet temperature is the efficiency of the compressor wheel.
01:47 To understand this, we really need to have a look at the compressor map which you can see here.
01:52 On the vertical axis we have the pressure ratio which is simply the pressure at the outlet of the compressor divided by the pressure at the inlet which for simplicity we can assume to be atmospheric pressure.
02:02 On the horizontal axis we have corrected airflow out of the compressor.
02:06 In other words how much air the turbo is pumping into the engine.
02:09 On the graph itself we have several islands plotted which are called efficiency islands and you can see that each island covers a range of both pressure ratio and airflow.
02:18 Right in the middle of this particular compressor map we have an island where the compressor efficiency is operating at 79% which is about as good as you can expect from a modern turbocharger.
02:27 We can see that as we increase the pressure ratio and airflow, we move further away from the peak efficiency island because the efficiency of the turbo drops we end up adding more heat into the compressed air.
02:38 You don't need to be a turbocharger engineer to tune a diesel engine and really, all we're pointing out here is that when we start increasing boost on a small factory turbocharger, we're likely to see an increase in airflow which we want, however as we push the turbo harder and harder its efficiency drops off and we start putting more heat into the air.
02:55 So between the compressor efficiency and the mechanical strength of the turbocharger, there's a limit to how much we can raise the boost pressure.
03:02 The next obvious question is how much boost is safe for my application? Unfortunately there isn't a blanket rule we can apply here.
03:10 Every turbo is different and some will take a considerable increase in boost safely while others are close to their limit in stock form.
03:17 A good example of the latter is the stock turbo fitted to the Toyota 1KD engine found in the Hilux and Hiace.
03:23 From our own experience we've found that the tiny shaft in these turbochargers causes them to fail anything over stock boost so obviously care is required.
03:31 My advice here it to start by researching what others are achieving with your particular engine and to see where failures are occurring.
03:38 Particularly with popular diesel engines, there's an active community of enthusiasts modifying them and we can learn from others rather than risking our own failures.
03:45 As a general rule of thumb however, in most instances it should be relatively safe to raise the stock boost targets in the 10% range without many concerns.