Thursday, February 28, 2008

Another 100hp VW Conversion

In January, 2008, a fellow popped up on Usenet touting the glories of a another of those 100 horsepower VW conversions that have filled the skies with airplanes, which lead to the following exchange:

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> "...the great little VW > conversion with a 2.0:1
reduction producing 103 HP."
> ---------------------------------------------------------

To All:

In the context of an engine converted for flight the figures above are wildly fallacious. At best, they represent a 'dyno blip,' at worst they may be an out-right lie. Here's why:

The maximum sustainable power available from any air-cooled engine is determined by the engine's ability to cool itself -- to couple its waste-heat to the atmosphere. And with a carburetted, spark-ignited, gasoline-fueled engine there is a lot of waste heat to be managed since such engines are no more than 25% efficient when it comes to converting the heat of combustion into torque at the crankshaft. That means that for every horsepower measured at the crank you must generate at least four horsepower's-worth of heat in combustion. These basic rules of thermodynamics are made even worse by two additional factors, the first being 'Economy of Scale' in that smaller engines are less thermally efficient than larger engines, and the basic definition of Standard Day conditions -- 59.9 degrees on the Fahrenheit scale and an atmospheric pressure of 29.92 inches of mercury -- a fairly cool day.

With those laws of physics as preamble the next factor worthy of note is the physical dimensions of the Volkswagen cylinder head and the fact that all VW heads have the same exterior dimensions. This is because they must fit under the stock VW engine shrouding. No manufacturer of VW heads, either stock or after-market, offers a head having more fin area. Indeed, most after-market heads have less, due either to thickening of the combustion chamber wall or even eliminating one of the eight fins -- and in a few cases they have done both.

All -- ALL -- Volkswagen heads in common use today are derived from the heads developed for the 1300cc engine; their external physical dimensions remained exactly the same for the later 1500 and 1600 engines. The output of the 1300 engine was approximately 40hp and could sustain that level of output indefinitely under Standard Day conditions. This engine was bored-out to 83mm to produce the 1500 engine, then over-bored to 85.5mm to create the 1600 (actual displacement 1584cc), the maximum output of which was 57bhp for carburetted models, achieved in the 1971 model year. But that level of output could only be sustained for a bit less than four minutes, until the cylinder head temperature exceeded safe levels, again under Standard Day conditions.

So what's this 'safe level' of CHT? About 450 degrees on the Fahrenheit scale. This reflects the fact that VW heads are made of cast aluminum (as opposed to a forging) and the fact aluminum is a 'white short' metal, meaning it becomes frangible when its temperature enters the 'plastic' range. A characteristic of white-short metals is that when heated they fracture like a cube of sugar when subjected to stress. The floor of the frangible range is a bit higher for a forging -- about 550F according to Pratt-Whitney -- but can be as low as 400F in a casting, depending upon the alloy.

A common thread used to impress technologically naive buyers is tales of driving a Volkswagen bug or bus for hours on end with the throttle wide open. The fact the engine was was probably producing less than twenty horsepower goes unsaid. This involves the Horsepower Myth and generally leaves a large black question mark hanging over the heads of those without an engineering background but it needs to be touched upon since ignorance can be as deadly as a machine gun when it comes to aviation.

The Horsepower Myth was create by James Watt in order to sell his modified Newcommen steam engine to mine owners. To do so he added the element of time to the equation and from that day to this the general public has been comfortable with the idea that 'horsepower' represents a given quanta of energy... which it does... but only within a defined unit of time. From that day to this, that arithmetical loophole has been used by those eager to prey upon technologically naive consumers.

Indeed, in the early days of aviation those predations cost so many lives that government agencies had to step in, requiring the manufacturers of aircraft engines to justify their claims of power and durability.

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All of which tends to leave the average homebuilder with more questions than answers. Fortunately, the engines themselves are incapable of lying, especially when it comes to fuel consumption.

The Specific Fuel Consumption (SFC) of all -- ALL -- air-cooled, gasoline-fueled, normally aspirated Otto-cycle engines is clumped near the 0.5 mark, meaning it takes about 0.5 pounds of 'gasoline' (*) per hour to produce one horsepower's-worth of torque at the crankshaft. For aviation gasoline that works out to about 12bhp per gallon per hour. For a 103hp engine that works out to 8.58 gph.

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(*) -- Thanks to additives and dilutants (such as alcohol) gasoline intended for automobiles has less potential energy.

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So when someone tries to sell you their Whiz-Bang 103hp VW engine, simply ask about its fuel consumption. If they give you an honest answer, such as 'nine gallons per hour' your next question should be 'For what TBO?' (And if they try to feed you the usual '3gph' bullshit, simply walk away.)

The truth is, by simply spinning an engine faster you can claim an impressive amount of 'horsepower' -- up to 1500bhp for some 'VW' powered dragsters (but with a TBO measured in minutes). Some years ago turbosupercharged VW engines were all the rage... until people learned they needed a valve job about every ten hours, no big deal if you're only selling such engines -- but of some importance to the folks who actually fly them :-)

-R.S.Hoover
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