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Diesel Tuning Fundamentals: Boost Control Practical

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Boost Control Practical


00:00 - In this practical example we're going to have a look at the relationship between the mass of fuel being delivered into the engine, or in other words the lambda or air fuel ratio we're running the engine at, and the boost pressure produced by our turbocharger.
00:13 Remember as we add more fuel and richen the air fuel ratio, we're producing more heat inside the combustion chamber, which results in a higher exhaust gas temperature and this in turn provides more exhaust gas energy to help drive the turbocharger.
00:28 So what we can expect here is that as we richen the air fuel ratio, we're going to end up with an increase in our boost pressure.
00:36 Now it's important to mention that in stock form, our Toyota 1KD engine is equipped with a VNT or variable nozzle turbine type turbocharger.
00:47 We found in our testing that these proved to be unreliable so it has been replaced with a conventional turbocharger that uses a wastegate to control turbo speed and in turn, boost pressure.
00:58 Regardless whether you're running a VNT or a conventional wastegate style turbocharger though, the exact same results can be expected as we adjust our air fuel ratio.
01:09 So let's get into our test and what we're going to do is run the engine in steady state at wide open throttle and 2500 RPM.
01:17 And this is above the point where we would expect to reach full boost from our turbocharger.
01:23 So this gives us a really good indication of how the air fuel ratio affects the turbo boost pressure.
01:29 Now in our table here, in our fuel delivery table we can see that currently I've massively reduced the fuel delivery at wide open throttle and 2500 RPM.
01:39 So we're gonna be starting with a very lean air fuel ratio, we'll then richen that and we'll see the effect on boost pressure.
01:46 So let's get our engine up and running.
01:47 With our engine up and running in steady state we can see that our 40% value in our fuel table has resulted in a fuel mass of 47 milligrams being delivered to the engine.
01:59 We can see we've got a very lean lambda 1.9 lambda, as a result of that smaller mass of fuel that's being delivered into the engine.
02:06 And we can see that this is resulting in 151 kPa of manifold pressure.
02:12 So that's 51 kPa of positive boost pressure.
02:15 OK so let's increase our fuel delivery now and we'll go from 40% up to 60%.
02:22 So we can see that that results in the mass of fuel being delivered, increasing up to 78, 79 milligrams.
02:29 Our exhaust lambda is still quite lean, we're sitting at about 1.3, 1.4 lambda.
02:34 But we can see the increase in our boost pressure as a result.
02:38 We're now sitting at 200 kPa, one bar of positive boost pressure, or approximately 15 psi.
02:44 I'll just pause that time graph for a moment though, and we can see that the massive jump there as a result of adding that additional fuel.
02:53 Let's go one step further now, let's increase our fuel delivery up to 70%, let's actually increase that even further, let's take it up to 75%.
03:03 We can see that that's resulted in our lambda now sitting at 1.2 Still a little bit leaner than we probably need to be, fuel mass delivered now is at 92, 93 milligrams, and we can see that our boost pressure has increased again up to 220 kPa.
03:20 Let's just bring the engine back to idle and we can discuss those results.
03:24 We can see at the start of our test, with 47 milligrams of fuel being delivered we ended up with a lambda of 1.9 and that's given us enough exhaust gas energy to spool the turbo and produce 150 kPa of boost pressure.
03:40 Or 51 kPa of positive boost pressure.
03:43 Our next change netted us a fuel mass delivered of 78 to 79 milligrams.
03:49 That's brought our lambda down from 1.9 to 1.3, 1.33, and that's given us approximately 200 kPa of boost pressure or inlet manifold pressure, 100 kPa of positive boost pressure.
04:01 Then our final change there, we can see that with 92 miligrams of fuel being delivered, our exhaust lambda's now dropped to 1.16, and that's given us 220 kPa of boost pressure.
04:14 Now the important takeaway from this is that if we were chasing a fixed boost pressure, every time we made that change to the fuel delivery, this would in turn require us to go and make adjustments to our boost pressure, to our wastegate control, or our VNT control.
04:31 For example with that final change to our fuelling, if we were wanting to maintain 200 kPa, we'd need to reduce the boost pressure.
04:40 Now that also has a knock on effect, because as we reduce the boost pressure, we're reducing air flow into the engine and the effect of this of course is that then with less air flow and the same mass of fuel being delivered, our air fuel ratio actually becomes richer.
04:56 So this is where the iterative process of adjusting our fuel, adjusting our boost, and then going back and adjusting our fuel comes into play.

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