"Silicon nitride (ceramic) turbines are very rare, even in racing applications."
I remember many years ago working on a twin turbo V6 race engine. We built up some low inertia turbos using a SiN ceramic turbine and SiN/M50 hybrid ball bearings we got through NISMO (from a '90 Nissan GTR engine I believe). We mated them to a modified Garrett water-cooled center section, aluminum compressor and scroll housing. Garrett also supplied some thin wall, investment cast stainless steel (347 alloy I think) turbine housing castings that we machined to match the ceramic turbine wheel. Apparently, these stainless castings were left over from the Alfa Romeo 4 cylinder F1 engine program.
It all seemed to work pretty well, even though none of the compressor and turbine geometries were optimized. The only issue I recall was a bit of difficulty getting the axial preload on the ball bearings correct. The SiN turbine wheel was brazed to a steel shaft and was very reliable.
Regards,
riff_raff
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
Edis wrote:
Silicon nitride (ceramic) turbines are very rare, even in racing applications.
For the turbine housing there are several options. Up to roughly 700 degC grey iron can be used. Up to 900 degC nodular iron, Ni-resist, Si-Mo alloyed iron are possebilities. Above 900 degC heat resistant casting steel and stainless casting steel such as HK30 are options. Garrett racing turbochargers usually use thinwall HK30 castings.
This is not my impression, both Toyota and Nissan are using ceramic turbines on their top-models. Another popular application is ceramic coating of the turbine housing, in order to contain the thermal energy.
"I spent most of my money on wine and women...I wasted the rest"
It's not the pressure ratio that causes problems with ceramic turbine wheels. In fact, the SiN material typically used has low density, high stiffness and excellent hot strength properties. What usually causes failures with ceramic turbines is impact damage from debris in the exhaust gas flow. Ceramics are strong but brittle (ie. low elongation rates), so they tend to fracture upon impact as opposed to bending like high temp nickel alloys do (ie. high elongation rates).
The reason for using SiN turbine wheels is to reduce the polar inertia of the turbo spool, and thus reducing turbo lag. But SiN is a very expensive raw material (>$500/lb) and is difficult to fabricate. It must be sintered at extremely high temperatures and pressures using carbon tooling, and can only be finish machined by grinding with diamond coated tools. For production cars with turbos, it is usually less expensive and almost as effective to use variable geometry (vanes or nozzles) on the turbine inlet.
Also, due to the extremely high kinetic energy present in a turbine wheel spinning at 150,000 rpm or more, any disintegration failure presents a serious fragmentation hazard to anything in the rotation plane. That is why production car turbocharger turbine housings have very thick cast walls.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
When I was doing a little turbine project, I went to junk yards to pick up turbos, and almost all the Ceramic blade turbos I found had broken blades. I thought it was interesting. I think Nissan and Toyota stopped using them?
I would rather use the supermetals than the ceramic because of those reasons. But just out of interest, can Ceramics be reinforced with metal or carbon fibres? Forming temperatures too hot?
n smikle wrote:When I was doing a little turbine project, I went to junk yards to pick up turbos, and almost all the Ceramic blade turbos I found had broken blades. I thought it was interesting. I think Nissan and Toyota stopped using them?
I would rather use the supermetals than the ceramic because of those reasons. But just out of interest, can Ceramics be reinforced with metal or carbon fibres? Forming temperatures too hot?
Probably, but they would really need to be bonded almost at a molecular level to stop them from just falling apart at high speeds.
The sensor is an oxygen sensor. There's probably an optimum or desired level of oxygen requested in the exhaust gas with the sensors job determining the amount and reporting back to the ECU so changes can be made to keep it at such a level.
