Van's Air Force
Western Canada Wing
Judging by the RV List, one of the most consistently hot topics among RV builders is alternative engines--especially auto engines. I'm in the strange position of being more comfortable with auto engines than aircraft piston engines. I only have about 30 hours in the air with pistons, and I've scarcely even seen the insides of a Lycoming. But I've rebuilt quite a few auto engines, for both road-going cars and racing cars. And I've watched with interest as the durability of auto engines has shot up over the years. I can remember--and I'm a youngster compared to many RV builders--when it was a real milestone to get 100,000 miles out of your car, especially if it was a four-cylinder. Now, the manufacturers are talking quite seriously about having the first scheduled service at 100,000 miles! So I'm inclined to be skeptical when people confidently claim--without any analysis--that an auto engine couldn't have reasonable durability in an RV.
But it's quite correct to claim that the duty cycle of an aircraft engine is very different from the duty cycle of an auto engine. While it's true that auto engines typically last a lot longer, in hours, than aircraft engines, they also operate well below their peak power output for most of that time. So it's quite appropriate to ask, "How long will that auto engine last if it's putting out 120 horsepower in sustained cruise, and 160 horsepower in TO and climb?"
On the other hand, one very inappropriate discussion that often takes place is about the percentage of rated power that the auto engine runs at. Auto engines aren't rated the way airplane engines are. An aircraft engine's rating is, by definition, a measure of how much power it can put out for some defined period of time. So, naturally, it can be expected to last that long. An auto engine's rated power is simply the peak power that it can produce--far more than an aircraft engine of the same size would be expected to produce. For example, the Chevy 4.3 V6, with a displacement about the same as an O-235, is rated at 200 HP. If the auto engine was rated the way airplane engines are, it's rated power would be much less, and you would find that it's cruising at a much higher "percent of rated power" than the 20 or 30 percent that's often quoted.
Given all this, is
there some way that we can fairly compare an auto engine to a Lycoming
when used in an airplane? Here's my suggestion: compare the two on the
basis of mean piston pressure (MPP), mean piston speed (MPS), and peak
piston acceleration (PPA). These factors are used by engine designers to
provide a feel for the amount of stress the engine is experiencing. You
can find the equations to calculate them in most mechanical engineering
handbooks. I used the Bosch Automotive Handbook, 2nd Edition, to compare
the Lycoming O-360 and O-320, and the Chevy 4.3 V6. I have assumed that
the Chevy would use a reduction drive, allowing it to rev to 3,600 RPM.
|MPP (peak), psi||146||147||134|
|MPP (cruise), psi||110||110||101|
I was quite surprised
to find that the MPP for the Chevy was actually less than for the Lycomings,
despite producing the same power as the O-320 with only 80% of its displacement.
This lower pressure is a direct result of higher RPM. That is, less piston
pressure is required to produce the same horsepower at a higher RPM. The
penalty of the 900 extra RPM, of course, is piston speed and acceleration.
Nevertheless, because of having more cylinders (allowing a shorter stroke),
the Chevy's piston speed is only slightly higher than the Lycoming's.
Piston acceleration is another story, though. The Lycoming's is quite a bit less than the Chevy's, again due mostly to lower RPM. How significant is this difference? It's important to bear in mind that piston acceleration is a function of RPM only, not power output. In the days before overdrives became common, a V6-equipped car with 15-inch wheels and a 3.73 rear end would have turned about 3,500 RPM at 120 kph (75 mph). So, presumably, GM would have anticipated sustained speeds in that neighbourhood. And, because piston acceleration is proportional to the square of engine speed, the piston acceleration that the Chevy experiences at 3,600 RPM is not even half of what it experiences at peak power in an automotive application. So there's every reason to believe that the main bearings, wrist pins, and so on in the Chevy are designed to withstand the 3,600 RPM accelerations with a decent fatigue life.
All things considered, it appears that the Chevy wouldn't be experiencing a lot more distress in an RV than a Lycoming is. You could even make a case that it's operating further below it's operating limit than the Lycoming. There are a lot of other considerations to an auto engine conversion, of course: auxiliaries, reduction drives, cooling system, and weight, to name just a few. But durability doesn't look like a problem, to me.
I'd be interested
to know what other builders think of this analysis. Are there other factors
you think should be analyzed? Other engines? If you have any thoughts about
this, please write and I'll include them in a future issue of the WCRVator.
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