jarheadk5 said:
The reason today's cars don't run stoich all the time is because the materials automobile engines are made of won't withstand the combustion temperatures that running at stoich can generate.
My understanding of aluminum is that (generally, since I'm sure different AL alloys melt at different temps) under normal combustion, the melting point of the piston material is exceeded every single combustion event. Right around 800*, at least for some aluminum.
As you mentioned with 5 million $ jet engines, companies can afford this, individual consumers can't. In a vehicular engine, to make it easy, say a TBI truck like one of ours, you are measuring *8* combustion events with ONE sensor, and governing the air fuel mix based on that one sensor. (crude explanation) The system can do nothing but average the results from all cylinders and hope that all cylinders are within a certain range of each other, to be accurate. However, one "failure" on one cylinder offsets the balance so much that it upsets that average. In any case, 8 cylinders being exactly the same in regards to fueling needs and efficiency isn't possible. The A/F ratio jumps around because the events in an internal combustion engine are "broken up" due to firing order, combustion efficiency being different on each ecylinder being different, slightly different injector flow rates, etc.
I would doubt (again, me theorizing) that temperature has anything to do with not running 14.7:1 ALL the time (besides it's not the best mix for power, etc.) in an automotive application. First and foremost is economy. I'm sure that if you had a system capable of monitoring each cylinders performance independently, the steadiness of the A/F ratio would be exponentially greater. However, O2 sensors alone are expensive if talking about 8x the cost over millions of vehicles, and there is no requirement at this point (at least federally) for that kind of efficiency. If there was, the consumer would pay that cost, and the manufacturers would do that, or something less costly, but as effective.
I'm pretty unfamiliar with how fuel is "governed" in a jet engine, (I'm assuming no O2 sensors for instance) but the functional differences between an 8 cylinder 4 stroke and a turbine engine are probably so great that there is no way that both could be monitored and run the exact same way.
In the most crude possible explanation, as compared to a multi cylinder 4 stroke engine, isn't a turbine essentially one "cylinder"? As in, you don't dump fuel into 8 seperate chambers and hope they combust in a turbine engine, isn't it more of multiple injectors into one chamber? Therefore monitoring exhaust temp (for instance) would be a very easy way of measuring a turbine engines operation, while measuring 8 cylinders with exhaust gas temps might give you an idea something is wrong, but not WHERE the problem is, just like an O2 sensor.
At least those are how I see it.
