CHT & mixture control

[shumway76]

I have flown certified aircraft (C172, C150) using Lycoming engines and also kit planes using Jabiru & Rotax engines.

I am just wondering why is it:

1. Certified engines (eg. Lycoming) does not have a CHT, but the Jabiru & Rotax (non-certified) do have CHT.

2. Certified engines have manual mixture control but non certified engines have "auto mixture" in the carb. Simplicity of design?



p/s I am just making reference to these engines only, as I'm sure someone out there will say "there are non-certified engines with mixture!"

[thing]

Certified engines (eg. Lycoming) does not have a CHT

It's nothing to do with the engine, it's whether that aircraft has one fitted or not.

[Whopity]

I suspect the answer is the cost of certification. Non certified enginge, no cost, so you have all the latest add-ons, but a certified engine will cost a fortrune to add the items.

[Johnm]

but a certified engine will cost a fortrune to add the items

Fitting our JPI engine monitor didn't cost a fortune, compared to the general running costs of the aircraft.

[Genghis the Engineer]

A lot of it is innate conservatism I think.

The certified GA world tend to be very reluctant to adopt innovations - automatic mixture control, additional engine instrumentation, electronic engine monitoring, coolant based carb-wrap are all pretty common in permit / experimental aeroplanes, but the certified fleet are all still sadly flying with what are basically 1950s American car engines.

Part of that is nervousness of stuff they don't understand, a lot of it is fear of the cost and complexity of getting approval on any changes.

G

[phiggsbroadband]

Any Economist would tell you that 'Restrictive Practices' drive up prices.


Whereas 'Competition would ensure the sale of goods at prices corresponding to the lowest practicable costs of production'.


It seems as if nobody at the CAA has studied Economics.

[Silvaire1]

The certified GA world tend to be very reluctant to adopt innovations - automatic mixture control, additional engine instrumentation, electronic engine monitoring, coolant based carb-wrap are all pretty common in permit / experimental aeroplanes, but the certified fleet are all still sadly flying with what are basically 1950s American car engines.

Not to take anything away from powerful and reliable automotive engines like the Chevrolet V8, I think the only 1950's American car engine with anything in common with aircraft engines past or present is that of the Tucker - which was propelled by a Franklin aircraft-derived engine. There is also some common car/plane ancestry in the 30's, during which time central European auto engine designers and US aircraft engine designers were both developing air-cooled boxer engines with short crankshafts, aluminum crankcases and heads. That architecture remains a very good one for aircraft engines in which weight and complexity cannot and should not grow in the 21st century automotive style.

Answering the (entirely legitimate) question about automatic mixture control, older larger aircraft engines did have automatic mixture control but it was not considered worth the trouble for smaller engines. The Rotax 912 ended up with a degree of automatic mixture control through using constant vacuum (CV) Bing motorcycle carbs, which were what was readily available in mid-1980s Austria. The downside of those CV carbs is that they have a thin rubber rolling diaphragm to seal the vacuum actuated moveable slide, and when it cracks the throttle closes until its replaced. Not an ideal situation for an aircraft engine, and neither is the close fitting sliding seal used by SU - which originally developed the CV concept in the 1920s. Simpler fixed jet aircraft carbs have basically one moving part, the throttle plate, which is directly cable controlled... plus a mixture control. Nothing comes for free, and (BTW) that also applies to fuel injection.

[Johnm]

Certificated aircraft engines have little in common car engines from US or anywhere else. They were based on stationary engines designed for long term continuous use at high power to drive pumps and the like.

[smarthawke]

It's basically down to the different types of carbs used.

Some other points to add:

The Rotax 912 is available in certified and non-certified variants - the same parts are used. There are a number of certified aircraft using 912 engines (including the IFR Tecnam P2006T twin).

As for wear life (or if you like, reliability), the 912S (or ULS) has a 2000hr/15 year TBO. The average Lycoming also has a 2000hr but 12 years by calendar life.

The cylinder heads in Rotax 91x series engines are liquid cooled so the CHT sensor is effectively measuring coolant temperature.