Turbos, boost and horsepower go hand in hand – but only if you’re in control of the turbocharger.
In order to understand how boost control works we first need to understand how a turbocharger works.
A turbocharger uses engine exhaust gases to spin up a turbine wheel (30), which is mounted on a common shaft (36) with a compressor wheel (32). The faster you spin the exhaust wheel the faster the compressor wheel will spin.
The Compressor wheel (32) pulls fresh air through the air filter (46) and jams it into the engine (42), resulting in what we know as Boost Pressure.
The more compressed the intake air is – the more oxygen-dense or heavy it is, therefore the more oxygen you can get into an engine – the more power it’s going to make.
Of course there are limits. You can’t just put unlimited boost into an engine, you need to regulate the speed of the exhaust turbine wheel and in turn regulate the speed of the compressor turbine wheel.
This is achieved by bypassing exhaust gasses around the exhaust turbine wheel with a component known as a Wastegate.
The wastegate uses a boost referenced, spring loaded diaphragm in order to determine just how much exhaust flow should be directed around the exhaust turbine wheel and how much should go through it.
It seems odd to waste all this energy but there are good reasons for that. The turbocharger assembly spins at over 100,000 RPM and it’s important to regulate the exhaust gasses to make sure it doesn’t over-spin resulting in a catastrophic failure.
It’s also important to regulate the exhaust side to control the intake side. When we have control over the speed of the compressor we effectively have control over the boost pressure.
That’s just mechanical boost control. With electronic boost control we can have different boost pressures at different Engine RPM or depending on the Ethanol content of the fuel, or via a simple boost switch on the dash. There are endless combinations of boost control strategies you can employ to make the most effective use of the boost generated by your turbocharger.
In order for the ECU to control the boost pressure it needs to measure the boost pressure – for that we use a Manifold Pressure Sensor.
This sensor’s mounted after the throttle body but before the intake valves and measures the pressure in the intake manifold.
We also need a boost control solenoid. We use this solenoid to manipulate the amount of boost pressure the wastegate sees. The more pressure we bleed off, the more exhaust gasses the wastegate will direct through the turbine wheel resulting in more boost pressure.
In the ECU we can set our Boost Target against numerous parameters like Engine RPM and Ethanol Content. Let’s say when we’re at 85% Ethanol and 6000RPM we want 20 PSI of boost pressure – the ECU will pulse the boost control solenoid directing the pressure between the wastegate and out to atmosphere in a delicate balancing act to achieve our target boost into the engine.
Because exhaust flow isn’t consistent across all RPM and engine loads the boost control solenoid has to do plenty of work in order to manipulate the wastegate and achieve the target boost pressure.
So next time you put your foot on the throttle and feel the boost come on – spare a thought for your wastegate, the environment it’s working in and how hard it’s working to control your boost!