Saturday, July 19, 2008

The Orphaned Engine

Most Experts Aren’t. That's something the late Smokey Yunick said back when I was a seaman deuce. Every month my mail delivers one or two messages saying it's still true. The messages usually come from some superbly experienced fellow who has literally spent his life working on cars or trucks. He is the Local Guru when it comes to engines for homebuilts and he's taking the time to let me know that the automotive engineers I like to cite in my articles aren't quite as bright as I seem to think they are, offering an experience-based example to prove his point.

Unfortunately, the offered example invariably deals with cars or trucks, things in which the fellow has a life-time of experience, whereas automotive in the sense used here, does not, although it could include them. To an engineer, automotive means something that can move under its own power. Like an oil tanker, the Space Shuttle, or a gold dredger.

My usual reaction is to hit the delete key. I get more mail than I want, most from people with real problem, some of whom I can help. But it's always sad to hear smart people say dumb things. And on the whole, these are smart people, even though a life-time of experience hasn't tipped him off that we're taking about two different meanings for automotive.

We all start out pretty dumb. As we age we gather information and gain experience and, assuming a fair share of native wit, we end up a bit smarter than when we began. Mebbe all this guy needs is a nudge in the right direction. So you say hello and the odds are the fellow is having the same problems as everyone else except he was a bit too proud to say so.

With this type of Expert you'll often discover his life-time of experience has been with just one type of engine or perhaps one type of car and he has been trying to transfer that experience to a Corvair or a Volkswagen and isn't having much luck. I mean, who ever heard of a head torqued to only eighteen foot-pounds! That has to be wrong... right?

If the fellow hasn't figured out the meaning of automotive there's a good chance he won't have any idea in the blue-eyed world about Class of Service but a good understanding here is the real key to a successful conversion so you give it a shot.

A car or light truck uses a variable speed, high-rpm, low-torque engine whose nominal output approximates 25% of its peak output. Nominal output is defined as the amount of power the engine was designed to deliver for approximately 98% of its service life. The only time it’s expected to produce more… that wayward 2%… is when accelerating or climbing a hill. Once on the flats -- once you've reached a Stable State of cruise -- the figures are a good match. For hilly regions vehicle manufacturers offer different ratios for the rear-ends. Economy takes a hit but over-all, the figures match up. Respect an engine's Class of Service and you'll be rewarded with 2,000 to 5,000 hours between overhauls.

You can always demand more output from either type of engine but doing so will reduce it's service life. With a converted VW, for example, your Mean Time Before Failure will typically fall from about 2,000 hours in vehicular service to about 200 hours when powering a plane.

By comparison, an aircraft engine is a single-speed, low-rpm, high-torque engine whose nominal output approximates 75% of its peak output. Peak output may be defined further as maximum sustainable output, and as Peak-sub I, meaning an instantaneous value or dyno blip, something you might use to impress the newbies.

Since our goal is to produce thrust throuigh the rotation of a propeller, our primary interest is in the amount of torque that appears in the crank, and in the propeller's efficiency at a given rpm. The measurement of thrust is quite simple and articles describing different types of homebuilt thrust stands have appeared in the literature and on the internet. You will note that horsepower, which serves no useful function at this stage, has not been mentioned.

It usually takes an exchange of half a dozen messages or thereabouts to arrive at this point, if in fact we arrive at all. In the overwhelming majority of cases the Local Expert simply vanishes. Which is doubly unfortunate because the best is yet to come.

When we convert an auto engine for use in an airplane we are trying to convert it from one Class of Service to another to make it more suitable, usually in the area of mechanical reliability. By comparison, the typical flying Volkswagen starts out as a marginally suitable auto engine that is then made even less suitable for aircraft use by turning it into a hot-rod enigine. Why? Usually because the person doing the conversion has little understanding of an aircraft powerplant. Indeed, most such experts are merely the local Guru grown old, selling dune buggy engines to the kiddies. And after all, it does fly the plane, right?

So why even bother.

Well.... because we should. A properly built engine is more efficient. It produces the required torque at a lower rpm and wear increases exponentially with rpm. That means a properly built engine uses less fuel to deliver the same thrust and last longer, too.

But a properly built engine is also a lot less expensive to build and nowdays that's becoming a critical factor.

See that chart down there? The one title BORE VS STROKE? (It's embedded in the article in the blog; you guys on r.a.h. will have to go dig it out and print yourself a copy.) The chart shows the bore & stroke combinations for most common conversions and for everything using 88mm jugs or larger, or a 78 mm or longer crank, is going to have to machine the crankcase & heads to match. What they'll end up with is a dune-buggy combination -- a high-rpm engine that produces most of its torque up high. Itty-bitty toothpick of a prop. Not very efficient at all. Lots of machining to do. Lots of tricky bits to go wrong during assembly... which is why some folks don't even offer the thing assembled.

But it's all a bit of a joke because no matter HOW BIG the engine, it's MAXIMUM SUSTAINABLE OUTPUT is going to be between 35hp and 45hp.

Yeah, I know -- everybody is selling 80hp and up. Which is a dyno blip, not a steady output. Lotsa cubes is going to get you out of the weeds quicker but once you get the puppy cleaned up you're flying behind your basic 40hp engine, depending on the local atmosphere.

The limitation has to do with the heads, not the displacement. The cylinder heads only provide enough fin-area to manage the waste-heat from about 40hp. Unless its nice and cold or you are nice & high. But the dune crowd only knows how to build big-bore strokers.

Now go take anohter look at that chart. Limit your jugs to the stock 85.5mm.s and your cranshaft to a 78mm. At those sizes there's NO MACHINING REQUIRED. Your displacement is 1791cc, your maximum SUSTAINABLE output is about 45hp and your peak torque is going to come in at about 2800rpm.

Did I mention that no machining is required?

You've altered your cam timing but you're running a stock cam or a Schneider 'chugger,' the one used in the orchard-blower engine. You're running SINGLE PORT HEADS... because you're now an airplane engine, not a hot-rod. Your Volumetric Efficiency is pushing 70% and you're about a $1000 dollars ahead of the game because you haven't had to buy all that machining and you're using a higher percentage of stock, off the shelf parts. You're also running a longer, more efficient prop -- hopefully one you've carved yourself.

The thing starts on the first flip because it has an efficient ignition system, one that automatically adjusts itself to the load and a 20A. electrical system. But no starter, please. As it is, it weighs about twenty pounds less than any engine offered by anyone else.

But of course, it's not a dune-buggy engine. And the Instant Experts will stand in line to damn it with faint praise for that fact alone even while it flys circles around them and is still going strong when they're doing their second valve job of the year.

In my opinion, this is the perfect engine for an aerodynamically clean single-seater, like Bruce Kings little beauty. Had fate dealt me a different hand, that's what it would be going into. It would also be a good match for a KR-1, the early Jodel, Druine, the Teenie Two and similar designs.

Kill the parent, you got orphans. And that applies to engines, too.



phlsti said...

I really enjoyed your comments, Bob. I have been wondering how to estimate TBO for an auto engine conversion (VW, Ford, Chevy), and how to compare torsional vibration encountered in autos versus rotating a prop. Does it make sense to include the standard flywheel to help reduce torsional vibration? What kind of TBO can you expect from an auto engine, say an LS1 crate engine running 4200rpm behind a geared reducer (assuming you have good cooling setup)? Can a 'standard' VW with broad torque curve cam be expected to make 1,000hrs (assuming again good cooling ducting to heads)? Not much information out there about this, so thanks for any comments along this line.

mark smith said...

Thanks for all your sensible comments. I am currently building a 40hp HVX engine. You have seperated the wheat from the chaf so many times with your explinations, pictures and drawings. I wish the folks selling products had as clear diagrams.
God Bless You,
Mark Smith, builder of the VP1 Rose Minnette

Anonymous said...

Back in the 80's, Smokey Yunick was one of the best parts of Popular Science magazine. I learned quite a bit reading his columns month after month. Then I discovered the VW beetle, and found your sermons...and the rest is history.

Keep up the great work, Bob. You're in my prayers.


Shaun Dustin said...

Cancer sucks, Bob. Guess I don't have to tell you that. I've been reading your internet stuff for nigh on 12 years now, I think. As an engineer, it's a pleasure to read your articulate and ordered approach to things. Good luck, and fight the good fight.


"May you live as long as you wish and love as long as you live."

R. A. Heinlein

Curt the Dead Sled Wrencher said...

Back in the day I read the trip reports you'd written, like the story of Grendel. I wonder if that stuff exists someplace. I'd love to read it again.