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What is a purpose built engine

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ekimneirbo

I was thinking about the discussions I've participated in as to an automotive engine not being built for aircraft use and it dawned on me that if you set down and look at the attributes and detriments all engines have, automotive engines rate rather highly in most catagories. I'll list in what I consider order of importance (I'm sure sequence will be debatable). I'll probably miss some things too.

1. Reliability
2. Sufficient Power
3. Weight (Light enough to work)
4. Cost


Where I think the most discussion should initially occur is #3 Weight. The typical aircraft engine is considered purpose built. To me that means that certain factors in the design were chosen with an eye on reducing weight while insuring sufficient power at lower rpms, and establishing reasonable reliability. In other words, even though more reliability might be available with a different design, it would be at the detrimental cost of adding weight. So, although its considered as a purpose built engine, what compromises had to be made to get there.

Aircraft Engine (opposed) ...........................................................................................................................................................................................................Automotive Engine

1. It had to be air cooled because that was a major factor in reducing weight. ........................................................................................................................Mostly water cooled and heavier

2. Few cylinders as possible were used which then required large pistons to get displacement..................................................................................................... Nope

3. Using large pistons meant cylinder wall to piston clearance had to be very large to prevent seizure........................................................................................... Nope

4. Using large clearances meant higher oil loss past the rings and pistons..................................................................................................................................Nope

5. Using directly opposed cylinders meant that the crankshaft had to have a single plane so the forces could offset each other......................................................... Nope

6. Using a single plane crankshaft means that the engine will not run as smoothly and will generate more resonance and vibration....................................................... Nope

7. Compression ratios must be kept low in order to prevent detonation and higher cylinder head temperatures..................................................................................Nope

8. Exhaust valves will require special construction (sodium filled or inconel) in order to dissipate heat..............................................................................................Nope (may have high quality valve but surrounded by water jacket)

9. Exhaust guide wear will be critical because too much clearance will prevent the exhaust valve from transferring its heat and result in catatropic valve failure................Nope

10. If using a carburetor you will have to have heat assist to prevent the carburetor from icing up................................................................................................ Nope (unless you are running a carb version)

11. You will have to prime the engine mechanically by hand and its often prone to flooding which will wash oil from the cyl walls and may cause a no start situation.......... Nope

12. Ignition system is pretty reliabile and redundant.................but somewhat antiquated with moving parts that wear...................................................................... Nope (easily made redundant)

13. Camshafts are flat tappet (except in some new engines) which allow less cylinder filling, more subject to wear, and some may develop rust if allowed to sit for a short time period.... Nope

14. Rocker arms are non adjustable so as they wear, tolerances increase and power is lost.............................................................................................................Maybe (but new rocker arms are cheap)

15. Spark Plugs are of a special design which makes them extremely expensive and often cause reuse rather than replacement..............................................................Nope (even the expensive ones are cheaper)

16. Motor mounts are of varying types which are expensive to replace and do a marginal job of limiting vibration. I have to wonder about the effectiveness of their mounting design.... Nope (take a look at new Corvette gel filled mounts)

17. Exhaust systems do not appear to be overly efficient but are reasonably light............................................................................................................................Probably slightly heavier but more efficient

18. Sensors are mostly nonexistant for monitoring engine parameters and conditions................................................................. Lots of ways to see everything going on with your engine and log the data for later review

19. Fuel efficiency and fuel type is more limited and expensive..................................................................................................................................Readily available anywhere and don't have to worry about seals

20. Maintenance is required in order to insure that the engine does not have deficiencies prior to reaching TBO .................................................................Minimal if any maintenance

21. It is designed to have a propeller bolted directly to it with no adapter required.........................................................................................................Requires Adapter


So, off the top of my head I would say that purpose built is not all its cracked up to be. Its basically a series of compromises in order to get light weight. If someone can use a heavier combination, they don't have to compromise on many other important features.
 
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