Davezj
16-11-2011, 09:32 PM
i had a bit of an Epiphany when i was at work today whilst sat on the bog. well i thought it was an Epiphany.
i will let you decide,
why do we use actual boost pressure to activate the wastegates, in a mechanical way, i know it has always been done like that, but the standard wastegates fitted to the vr4 do seem to have there limitation.
they are prone to creeping, and leaking.
this is what i mean, when one of our turbos is off the car you can easily see the wastegate valve next to the exhaust turbine wheel and all the turbos i have seen weather it is supposed to be like this or not they have been all the same, the valve flap does not seem to be slammed shut. the flap disk seems to be able to rotate quite freely, which implies it leaks all the time. not be much but it does leak and that is with no exhaust pressure behind it.
let me explain my dilemma, the exhaust gas come from the cylinder and into the manifold and these gases get forced against the exhaust turbine wheel and the wastegate valve, now i believe that the wastegate valve closes onto the flow of exhaust vales and the only thing that holds this valve shut is the spring in the wastegate actuator.
hope i have everything correct up to this point.
49577
this is the exhaust mounting side of the turbine
49578
picture of the wastegate actuator if you are not familier
now to the nitty gritty of the rambling, how can you tell if the wastegate value is completely closed, up to the point where you want the valve to open to regulate the boost pressure.
with out watching the wastegate actuator rod whist the car is boosting i don't think you can.
i believe the wastegate spring force is about 4-5lb (this is a guess as it has to be less than the stock boost pressure and that is about 8psi) which equates to 4-5psi pressure on the diaphragm in the wastegate actuator. for this scenario lets remove the boost control valve from the equation and assume the wastegate is connected directly to the manifold pressure.
so with that 4-5psi being applied to the actuator the valve is fully open, so as the pressure builds the valve starts to open a bit then open a bit more and more until fully open. this i believe is called wastegate creep.
am i right in thinking that the ideal wastegate valve operation would be slammed sult until you reach the desired pressure then fully open to vent the pressure, then closed again to build pressure again. or would this lead to boost spiking. there could be a bit of a soft open and close on the valve to avoid boost spiking.
anyway my thought is to remove the wastegate actuator (spring, diaphragm, boost pipes) and replace it with a servo motor that has a 90 deg travel to open the valve and a driver linked to a pressure sensor in the manifold, the pressure converted to a voltage would be compared to a reference signal that would be set in the car via a switch. so the boost control would be via the rotary switch in the car. no need for any other manual or electronic boost control, which is a bonus.
i hear you say this is all a bit much to effectively get to the point we are already at. but there is a big plus point with the servo activation is that the servo will keep the wastegate valve slammed shut all the time you want it to be shut.
i understand that most people that already have some kind of external boost control making and changes like this would not be cost effective. but i am looking at this from first principles.
do i have some assumption i have made wrong or do you think there are major flaws in my thought.
don't be shy in posting your comments even if it is just to ask me to explain a bit more about it.
i will let you decide,
why do we use actual boost pressure to activate the wastegates, in a mechanical way, i know it has always been done like that, but the standard wastegates fitted to the vr4 do seem to have there limitation.
they are prone to creeping, and leaking.
this is what i mean, when one of our turbos is off the car you can easily see the wastegate valve next to the exhaust turbine wheel and all the turbos i have seen weather it is supposed to be like this or not they have been all the same, the valve flap does not seem to be slammed shut. the flap disk seems to be able to rotate quite freely, which implies it leaks all the time. not be much but it does leak and that is with no exhaust pressure behind it.
let me explain my dilemma, the exhaust gas come from the cylinder and into the manifold and these gases get forced against the exhaust turbine wheel and the wastegate valve, now i believe that the wastegate valve closes onto the flow of exhaust vales and the only thing that holds this valve shut is the spring in the wastegate actuator.
hope i have everything correct up to this point.
49577
this is the exhaust mounting side of the turbine
49578
picture of the wastegate actuator if you are not familier
now to the nitty gritty of the rambling, how can you tell if the wastegate value is completely closed, up to the point where you want the valve to open to regulate the boost pressure.
with out watching the wastegate actuator rod whist the car is boosting i don't think you can.
i believe the wastegate spring force is about 4-5lb (this is a guess as it has to be less than the stock boost pressure and that is about 8psi) which equates to 4-5psi pressure on the diaphragm in the wastegate actuator. for this scenario lets remove the boost control valve from the equation and assume the wastegate is connected directly to the manifold pressure.
so with that 4-5psi being applied to the actuator the valve is fully open, so as the pressure builds the valve starts to open a bit then open a bit more and more until fully open. this i believe is called wastegate creep.
am i right in thinking that the ideal wastegate valve operation would be slammed sult until you reach the desired pressure then fully open to vent the pressure, then closed again to build pressure again. or would this lead to boost spiking. there could be a bit of a soft open and close on the valve to avoid boost spiking.
anyway my thought is to remove the wastegate actuator (spring, diaphragm, boost pipes) and replace it with a servo motor that has a 90 deg travel to open the valve and a driver linked to a pressure sensor in the manifold, the pressure converted to a voltage would be compared to a reference signal that would be set in the car via a switch. so the boost control would be via the rotary switch in the car. no need for any other manual or electronic boost control, which is a bonus.
i hear you say this is all a bit much to effectively get to the point we are already at. but there is a big plus point with the servo activation is that the servo will keep the wastegate valve slammed shut all the time you want it to be shut.
i understand that most people that already have some kind of external boost control making and changes like this would not be cost effective. but i am looking at this from first principles.
do i have some assumption i have made wrong or do you think there are major flaws in my thought.
don't be shy in posting your comments even if it is just to ask me to explain a bit more about it.