Tuesday, February 27, 2007
AV - Manually starting the VW Engine
> > Please explain to me the adjustments/mods to be made to my 2.0l Type 4
> stock distributor to facilitate easy hand propping. The engine will
> drive from the flywheel end.
There are four key factors involved here. (Although I may add more later :-)
1. Engine Assembly
2. Ignition Timing
3. Propeller Orientation
4. Type of Ignition System
1. Engine Assembly
There are two areas of critical concern, the first is cam selection, the second is accurate assembly of the engine, especially with regard to your valve train geometry.
With regard to the cam, most VW engine converted for flight are hot-rod engines -- something you'd expect to find in a dune-buggy rather than an airplane. And as a dune-buggy engine, they are fitted with dune-buggy cams, meaning the torque band has been shifted toward the upper end of the rpm range. This is accomplished by grinding a lot of overlap into the cam and extending the duration, two features never found in a direct-drive aircraft engine. What you want is the stock cam, which happens to have a torque band suitable for slinging a propeller of useful diameter. (There are a few other cams that are suitable. They were used in VW's industrial engines or in VW's converted to serve as orchard blowers.)
With regard to engine assmebly, there's no way I can cover the subject via email. I'm working on a manual that does the job fairly well but it's already over 200 pages in length and I've only just gotten to the heads :-) But as a general rule, you need to ensure that your cam is correctly indexed to the crankshaft. Simply lining up the dots is only an assembly-line approximation, usually accurate to withing +/- 2 degrees or so. You not only want it dead-on, for a low rpm, high-torque engine you generally want to retard the stock setting by 4 degrees. This task is typically referred to as 'dialing in the cam' and requires the ability to rotate the camshaft very slightly relative to the cam gear when the cam gear is locked in place and the engine is precisely at TDC. (How do you manage that? You reach in through the opening provided for the oil pump.)
I covered this procedure in some detail for the Type I engine in an illustrated article that appeared in VW Trends magazine a couple of years ago but the basic procedure will be found in any tome covering professionally built high-output engines.
Once the lower-end is dialed in you focus on accurate valve-train geometry, which is a fancy way to say you ensure minimum lost motion in transferring the movement of the cam to the depression of the valve. This matter is worthy of your attention because it's not uncommon to see loses as great as 25% in this area. (That is, an anticipated 10mm of movement at the valve reduced to as little as 7.5mm due to improper assembly of the valve train components.) The point most fail to appreciate here is that faulty valve train geometry effectively alters the engine's mechanical timing. It isn't uncommon to see a casually assembled stock engine give away as much as 15% of its normal output. (Indeed, it is more the rule than the exception.)
The key point here is that an improperly assembled engine can effect your valve timing, and improper valve timing can make an engine very difficult to hand-prop.
2. Ignition Timing
Don't waste your time here unless you've already confirmed that the engine is properly assembled, because for every degree you're off relative to the crankshaft/camshaft combination, you'll be off by 2 degrees in your ignition timing.
But assuming a properly assembled engine, to ensure easy starting you want to set your static timing somewhere between three and five degrees BEFORE top-dead-center. The closer to TDC, the easier the thing will start... but the farther the timing will need to advance when you open the throttle.
This assumes you are using an ignition system which allows the firing point to advance as the rpm increases (ie, a centrifugal- advance mechanism). You'll not find this in a magneto and only half of it in a stock, late-model Bosch distributor (ie, the 'composite type,' having both vacuum- and centrifugal-advance). The Bosch -009 distributor will serve but you should know that it was never installed on any VW vehicle. It is in fact a generic after-market replacement for the dozen or so centrifugal distributors used on the early Transporter, and for at least that number that were found on VW industrial engines. But it is beloved of dune-buggy types because it is inexpensive and can be adjusted to give as much as 30 degrees of advance. (Typical is 17 to 19 degrees.)
You will want at least 28 degrees total ignition advance, assuming you want to spin your prop at least 2700 rpm. To increase the advance range you simply fiddle with the stops on the advance plate and bob-weights -- a good mechanic can show you how. But proceed with caution. If you file away a tad too much metal you'll find it hard to replace :-) (To increase the advance rate you reduce the mass of the bob-weights and use springs of lighter tension. But that's only needed on dune-buggies, dragsters and the like.)
I hope you can see the quandry here: To make the engine easy to start the static firing point must be near (or even slightly AFTER) top-dead-center. But for the engine to run fast enough to be useful, the firing point must be advanced to about 28 crankshaft degrees BEFORE top-dead-center. If your distributor can only provide twenty degrees of advance (fairly common for a -009 straight out of the box) then your static firing point will have to be at 8 degrees BTDC, which will make the engine slightly hard to hand-prop. Move the static firing point nearer TDC and you automatically limit your peak rpm. (Need I mention proper engine assembly again? I hope not. But just in case... understanding that the relationship between ignition timing and rpm should offer a hint as to why many sloppily assembled engines never live up to their potential.)
Luckily for me I won't have to figure it out since you are the Mechanic-in-Charge :-)
3. Propeller Orientation
You've probably never owned a Model-T Ford but if you had, you would know that there was a certain feel -- a kind of 'springiness' -- in the crank as the piston approached TDC. You would feel for that (with the ignition off) as you charged the cylinders. Then with the ignition ON (and SPARK set to fully retarded), you'd flip the crank past the springy point and the engine would clatter into life. (The Model T's cam didn't have any overlap at all, limiting its maximum rpm to about 1800... and making it superbly easy to start.)
The T4 engine isn't a Model-T but when it comes to manually starting, your prop is still a crank.
With the prop mounted on the clutch end of the crankshaft it's going to rotate clockwise relative to the pilot (Volksplane assumed). That means you want TDC to occur at about 9 o'clock when you're standing in front of the plane facing the prop. The firing point is going to be a few degrees up from the 9 o'clock position, allowing you a 'swing' of nearly 90 degrees. That is, with the cylinders charged you'll bring your 'signature' blade to about the 12 o'clock position then turn on the ignition, then put your hand flat on the blade out near the tip (do not allow your fingers to curl around the edge of the blade) and flip it down toward the 9 o'clock position -- using a motion that carrys your hand and arm out and away from the arc of the propeller.
(All of which assumes you've got the tail-wheel chained to a fence- post and the slack pulled out... 'cause if you don't, it'll chase you :-)
So why 'Prop Orientation'? Because you need to know when a cylinder is coming up on TDC. And you need to know that relative to the blades of the prop.
VW engine fires once every 180 degrees. If you have a two-bladed prop you'd think it doesn't much matter how the thing is bolted on but it turns out, you do, and aligning one blade to #1 cyl is generally best. (Of course, that means the same blade is also aligned to #3 but I'll get to that in a minute.)
With a four-cylinder, horizontally opposed engine what you don't want is to have your signature cylinder in the same sequence on the same bank.
Clear as mud, right?
Go look at the VW's firing sequence: 1 - 4 - 3 - 2.
(Ed.Note: Cylinders 1 & 2 share the right-hand bank when facing the pulley-hub; cylinders 3 & 4 on on the left-hand bank.)
That means you don't want to have it fire on #2. Nor on #4. Because the next cylinder will be on the same bank... and the odds are, that cylinder won't have a full charge... because you just fired it's paired cylinder. What you do want is for the thing to initially fire on #1. Or #3. Because the next cylinder will be on the opposite bank. And -- trust me here -- the odds of the engine starting and continuing to run are about 100% better when the second cylinder to fire is on the opposite bank.
So mark one of your blades; the one that is going to fire on #1. Or on #3.
Now, you may have a problem with the 'clock'-related alignment if your prop-hub is drilled so a pair of holes aligns with the throws of the crankshaft. Because if you look at your prop, the usual arrangement is to have one of the prop's bolting holes aligned with the center of the blade. In theory, this should work okay -- and a lot of props are installed that way. (If your holes are so aligned, go aheady and try it.) But with a wooden prop you'll generally find the VW runs smoothest when the prop is not aligned with the crankshaft throws. And that presents something of a problem because you've only got six holes -- only three orientations -- to play with and only one of them is good for hand-propping... and it seems to have nodal points. (This is for the Type I engine. I don't think the T4 engine is any different.)
One solution is to use a prop extension in which the bolting holes are offset by thirty degrees. This gives you a bit more latitude. Or your prop, pitch, engine mount and crankshaft may present an entirely different torsional system than the T1 engine, which is what most of my experience is based upon.
4. Type of Ignition System (Finally!)
Hand-propped, even with an impulse coupling, a magneto puts out a weak spark. That means you'll need to use a narrow spark plug gap and a modest compression ratio, typically 7.5:1 or less, and those things can combine to make the engine notoriously hard to hand-prop. Not when everything is new and fresh but after it accumulates a few hours. Your plug-gap widens in use, as does the distributor air-gap, and the compression ratio falls as the engine accumulates wear.
At starting-speeds the stock Kittering-type ignition system (ie, as found in most vehicles up to about the 1995 model year) is vastly superior to a magneto. That's because the Kettering system delivers its maximum spark energy at the lowest engine speed. Makes things easy to start. But once its running, the spark energy drops steadily as the rpm increases, thanks to the declining amount of time the coil has to build up its 'charge.' (It isn't really a charge, it's just a magnetic field. But it's output is proportional to its strength and its strength is proportional to the amount of time so while it isn't electrically correct to say 'charge' it generally gets the idea across.)
A lot of folks -- especially the ones trying to sell you stuff -- will tell you that replacing the points with a transistor will give you a hotter spark. It won't. You've still got a Kettering ignition system and the output is still a function of the coil's current over time. But such systems do have a better wear factor and tend to give you a better spark because of it. That is, in the stock system your spark energy will decline as the points accumulate wear. Eliminate the points, you eliminate the wear, allowing you to enjoy maximum spark-energy the system can produce for a longer period of time.
Most modern-day ignition systems are some form of the Kettering System. The only ones that are truly different are Capacitance Discharge Systems, in which the points (or other trigger) discharge a capacitor through the coil. The advantage here is that the capacitor is charged with an invertor that may operate at voltages as high as 400, allowing it to re-charge the capacitor rapidly enough to allow the system to provide as much as 40,000 volts of spark energy up to speeds as great as 12,000 rpm.
Most of which is as useless as tits on a boar when it comes to flying Volkswagens :-)
But the bottom line is that that higher spark energy ensures more reliable starting. So if you're running points, replace them more often than you would in a car. And consider replacing them entirely, substituting some form of solid-state switch. Just be sure to not use any form of optical sensor. The VW distributor is not sealed and so long as you retain the distributor function (ie, the wires, rotor and cap) the central graphite button guarantees the optical sensor will eventually become obscured by oil vapor and carbon particles.
There's a few thousand things I haven't mentioned but the above should be enough to get you started. (Little play on words there, I suppose :-)
Ed.Note: In general, the factors discussed above must be taken into account when configuring any Otto Cycle engine for manual starting. In many cases Continental A-series engines with a reputation for being difficult to start need only to have the propeller re-oriented to make them start on the first flip. This is especially important with a 'Cub' on floats, where the engine must be started with one hand whilst balancing atop the starboard-side float.