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An engine outputs the same weight of fuel and air as exhaust that flows into the engine. The exhaust mass is simply the sum of fuel and air masses entering the engine.

Exhaust is much hotter than intake air, which means exhaust gas occupies more volume at any given pressure. As exhaust cools the mass occupies less volume, more mass fitting in a smaller space. The relationship between volume and heat, where heat increases volme, makes our systems work. As racers, we do all we can to increase the volume of air and fuel running though our systems, and the thermal difference between unburned mixture and burned mixture in the cylinder. We want to get as much cold mixture as we can packed into the cylinders, ignite it at the correct time, and get the hot exhaust out of the cylinder.

Turbo systems increase power by packing more air and fuel into the cylinder. Wastegates are a critical part of controlling power created by a turbo. The wastegate controls turbine pressure, which controls compressor speed. This controls boost, and boost directly controls power.

Power added by boost directly relates to the pressure ratio increase over atmosphere, less system power loss to run the turbine and compressor and any change in air temperature from compressing the air. By controlling intake pressure, horsepower can be controlled. 

How A Wastegate Works

The turbine spins from exhaust mass moving through the turbine. The faster  mass flows through the turbine, the faster the turbine spins. The more flow mass pushing against the blades, the more torque the shaft has. Of course blade size, shape, and pitch comes into play but that is set by the particular turbo.

The exhaust mass flow is exactly equal to the mass of air and fuel pushing into the engine. For a given thermal efficiency, a certain horsepower on a given fuel will always flow the same mass into the engine. The system at a given horsepower will flow exactly the same exhaust mass out for a given fuel and thermal efficiency, it does not matter what size the engine is!

If we are going to control turbo boost to some stable level below maximum, the wastegate must divert enough exhaust mass to slow the turbine and compressor to the proper speed. Obviously this is a mass flow ratio concern, not a power limit. As a matter of fact the more power we want to make the less flow the gate needs. The gate flow is opposite the power needed, with the highest gate flow needed at the lowest desired high RPM power level. Conversely, increasing power requires more exhaust through the turbine, and that means the system needs less gate flow. This is especially true at lower RPM or below the engine horsepower peak, where the engine flow volume from pumping is reduced.

The system needs the highest gate flow when commanding lowest target boost at or around the engine's peak horsepower point. This is why so many systems have "boost creep", where boost climbs higher and higher with increased engine RPM and load. Boost "creeps up" because the wastegate cannot flow enough exhaust mass to pull back exhaust mass going through the turbine. This is a flow ratio problem, the turbine cannot be slowed enough so the compressor spins too fast. We are dumping too much drive power into the turbine.

Gate Stability and Control

The waste gate system is based on a closed feedback loop. A typical wastegate is shown in figure 1 below.

A boost pressure sample signal is applied to b. Any boost pressure creates a force on the diaphragm area Ad. The force is boost*Ad. Let's assume boost is 10 pounds (psig) and with a 3" diameter diaphragm Ad is 7 square inches. The net opening force Fo is 10*7 = 70 lbs.

If the pressure Fc of spring S is less than 70 lbs, Fo will push the gate open until Fc and Fo become equal.

 The gate will move until Fo and Fc  with force Fo, reducing turbine drive and ultimately reducing boost. The spring or other pressure source on the dome tries to hold the wastegate closed. These two forces fight each other.

As in any feedback system, control error heavily depends on  feedback loop gain. The wastegate has an internal pneumatic pressure amplifier created by the ratio of dome diaphragm area to the exhaust control valve area. The dome diaphragm area to gate valve area ratio is often overlooked, but it is as critical to gate operation as valve size.     


Wastegate Function Fig 1