Pneumatic turbocharger controls

[digidocs]

A couple of times in the past, Ross F. has made reference to the use of automotive style pneumatic wastegates as an alternative to aircraft style hydraulically actuated ones.

I was thinking about this, and I don't understand how it works. For example, consider a turbonormalized engine where the turbo control system trys to maintain 30" of MP. At SL we need essentially zero boost, but at altitude we might need 7-8psi over ambient. My understanding of a pneumatic wastegate controller is that it consists of a diaphragm with a spring on one side and a chamber connected to the "upper deck pressure" on the other side. At boost levels below the desired set point, the spring overcomes the low pressure on the diaphragm and keeps the wastegate shut. However, as the boost pressure gets higher, it begins compressing the spring and thus causes the wastegate to open. The problem I see in an aircraft application is that the pneumatic system opens the wastegate at a fixed gauge pressure (relative to atmospheric) instead of a fixed absolute pressure (30" Hg). The aircraft systems use an aneroid reference to solve this problem, but I don't think this is standard practice on cars. Is there a way to make this work?

Thanks,
David Carr

 

[Andy_RR]

Automotive systems have an electronic control over the mechanical/pneumatic system. Solenoid valve bleeds off the boost pressure from the wastegate capsule to close the wastegate and raise the boost pressure. Being electronically controlled allows the use of cheap absolute pressure sensors to provide the reference. No reason you couldn't adopt a similar system on an (experimental) aero engine.

 

[rv6ejguy]

Andy is right with respect to most post 1995 cars. While the wastegate valve is moved pneumatically, control pressure to the diaphragm is modulated electronically by the ECU.

In the slightly older days, boost pressure acted directly on the diaphragm and this is what I use in my RV6A with cockpit controlled adjustment feeding air to the other side via a miniature air regulator (twin port integral Garrett actuator). Above 9500 feet, I need to tweak the regulator slightly to maintain 35 inches in the climb. Below this altitude, you can have 35 inches or anything less, just through throttle adjustment. Below or at 4000 feet (my field elevation) full throttle gives me a max of 38 inches so you can't overboost. Since 90% of my flying is below 10,000 feet, I almost never touch the regulator and you fly it just like an atmo Lycoming using throttle.

While you could build an absolute referenced mechanical controller, it is far easier today to add a commercially available electronic one to existing pneumatic wastegates.

Most of the Reno guys still use a system similar to mine since it is dirt cheap and dead reliable.

 

[digidocs]

Ross and Andy,

Thanks for the description of the bleed control system. That makes sense. However, I don't see how one could use this system to create say 31" Hg on SL takeoff (<0.5psi boost). It seems that the bleed solenoid can only cause the wastegate to open at a higher pressure than it normally would. Are there wastegate actuators with very light springs (1 psi)? It seems that most are in the 3+ psi range...

Thanks again,
-DC

 

[rv6ejguy]

It's true that on pneumatic systems, minimum boost pressure is determined by the spring pressure and we can only add boost above this, not take it away.

None of the integral actuators have such light springs and usually have clinched diaphragm chambers (non-serviceable). External types can be fitted with very light springs but you tend to get lazy control as exhaust pressure upstream of the turbine can be several psi and this pushes open the wastegate valve. A stepper motor type actuator driven by an electronic controller might be your best bet for low boost/ turbo normalizing.