Homebrew Hvid

Conversion to Naphtha


Spark Ignition

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Click on the picture for the high definition version.


22 December 2009:


Drawing of spark ignition head

I arbitrarily decided on a compression ratio of 4:1.  That means that, with a stroke of 4.000" and a cylinder deadspace (the space between the piston head and the top of the cylinder) of 0.200" with all the compression shims out, I need 0.800" of combustion chamber depth.


The parts prior to starting on the head.

While trying to make up my mind whether to hunt scrapyards or purchase a piece of 4.500" diameter steel bar, I made up the rest of the parts needed for the conversion.  I also made the command decision to not modify the Hvid head, fuel block, etc. in case I wanted to convert it back to Hvid in the future.  The large bar at the top of the photo is the foot long (minimum length) of 4.500" diameter ductile iron bar I got from McMaster-Carr.  Kinda pricey but at least I know what I have to work with.  The steel shafting I used for the Hvid head was a bit too "gummy" to suit me.


Valves, guides and springs.

The valves, springs and keepers are from an unknown and deceased lawn mower engine.  The length of the exhaust valve (the smaller of the two) had to be cut down to be the same as the intake.  Carbide tooling in the lathe did this easily.  The guides were turned out of leadloy steel.


Exhaust fitting.

Line on the Hvid head, the gas exhaust adapter is pressed into the exhaust port in the head.  I turned the I.D. of a piece of 1/2" iron pipe to clean it up then turned the O.D. to 0.825 to match the exhaust port.  The fitting that it screws into is the adapter to the flex exhaust hose.


Non compression release rocker shaft.

I removed the Hvid rocker shaft and compression release eccentric and made a new shaft.



The mixer was turned from a length of 1" diameter leadloy steel.  It was bored on the engine side of the venturi to 0.750" with a venturi of 0.375".  The bell mouth is a pressed on extension in case I need to make a flutter choke for it.


The brass parts are the fuel delivery jet and the float chamber.  The float chamber is made from an old steam radiator condensate valve, or whatever it is called.  These things were used in single-pipe steam heating systems.  They had a cup shaped float that closed a bleed valve if condensate filled the radiator.  When the valve closed, the pressure in the radiator stabilized and, since steam was no longer flowing up into the radiator, the condensate could flow back down, usually accompanied by a clanking of pipes.


I modified the condensate valve by making a valve seat and sweating it into a 1/8" tubing fitting.  The tubing fitting connects to the fuel line from the tank and fuel flows by gravity.  The needle on the float closes the valve when the chamber fills.  We'll see how well this works.  You will also note that it's possible to change the alignment of the float chamber.  That's because the intake port is angled like the exhaust and I needed a way to get the float chamber level.


Ignition timer.

I had to come up with a timing device for the ignition.  A logical place to get timing is from the end of the camshaft.  I cut a circle out of a piece of P.C. board material then drilled a small hole into which I glued a small rare-earth magnet which triggers a Hall-Effect transistor.


The rest of it consists of what is needed to make the timing variable.  I can pull the handle to rotate the sensor in relation to the magnet, thus changing the timing.  Here it is shown in the full retarded position, just past TDC.  Fully advanced is about 20 degrees BTDC.


Cutting the head off of the ductile iron bar.

Cutting the head blank was a little more than the cheap bandsaw could handle in one "chomp".  When it had cut about 3/4 through, the frame bumped into the bar so I had to rotate the bar about a quarter turn so it could finish the cut.


Machining the head.

The head was mounted in the 4-jaw chuck and centered as well as the rough surface of the blank allowed and the face cleaned-up.  Then the piece was reversed in the chuck and again centered and faced to the finish thickness.  While chucked-up, the combustion chamber was bored.


After removing the head from the lathe, it was put in the mill and the flat for mounting the rocker stand was made.


Then, the head was clamped to the mill table and the 1/2" bolt and valve guide holes were drilled.


Tomorrow, time permitting, I will plunge mill the valve ports.  I will have to make a special tool to machine the valve seats.  I wish I could use the same tool I used for the Hvid head but, since this head has a recessed combustion chamber, the tool is too big in diameter.  I'll probably end-up with something that will go on the boring bar.


23 December 2009:

The head's just about done.  To finish-up, I need to drill and tap the mounting pad for the rocker stand, lap the valves and drill and tap the mixer mounting set screw.


I figured out how to cut the valve seats in the head.

Angle of seat cutter tool.

My boring head uses 1/2" boring bars and since I had a worn 1/2" drill bit, I simply ground one face to 45 degrees.  Voila!  Valve seat cutter!

Valve seat cutter in action.

Worked like a champ!  Shown here is the intake seat.  I have yet to bore the exhaust port and do the seat and drill and tap for the 10mm spark plug.


It should be running soon.


24 December 2009:

I made it a short day today.  


Cutting screwdriver grooves in the valves for lapping.

I chucked-up a Dremel cut-off disk in the drill press and, running it at full speed, ground the lapping slots in the valve heads.  It worked out fine using the drill press vise and slowly working the valves against the disk.


The valves are lapped, the rocker stand is in place and the head is mounted on the engine.


Exhaust side.                                                                     Intake side.

Even though the spark plug head is 3/8" taller than the Hvid head, I can use the pushrods and, leaving the washers out, the head studs work out all right, too.  All I had to do was to move the locknuts to the other side of the rockers and I have adequate thread left for adjustment.  I'll have to lengthen the governor shaft and make the linkage to the mixer.


Now, if everything goes well, it should be running by the end of the next day's work.




26 December 2009:

It runs!  I finished up the governor linkage (it took a bit of fiddling), put on the fuel line, filled the fuel tank with naphtha and turned on the fuel.


Right off the bat, I thought I was going to have trouble with the float chamber because it was dripping a little.  I gave it a gentle whack with a screwdriver handle not expecting anything but it stopped dripping.  Must have been a little dirt making trouble.


I hooked up one of my solid-state ignitions for temporary spark and gave the crank a spin.  Danged if, after the third time over compression it fired a couple of times.  Then nothing.  Crank, crank, crank.  Nothing!  I came to find out that my ignition module had cooked an I.C. so I got out the other one and hooked it up.  Engine fired right off and, after a bit of twiddling with the fuel mixture, it was running steadily, although faster than I'd have liked.


After fiddling with governor springs, I finally found one that is almost right and the engine settled down (under load) to around 800 RPM, still too fast.  Investigation revealed that the throttle linkage, which is a bit on the Rube Goldberg side, is binding a little.




Note exhaust has been at blue heat.


Oddly, the exhaust port stub turned blue after a few minutes of running under load.  I'm not sure if that's good or bad.  The fuel mixture's good and I've got the timing set to where it knocks slightly at load but the exhaust is still hot.  I suppose I could make up a 0.200" shim to put in the rod to move the piston flush to the deck at TDC.  This would raise the compression ratio from 4:1 to 5:1 and may improve the power some.  I'm not sure what effect that would have on the exhaust temperature, though.


I ran the tractor down the street a few times and the engine can just keep up with the drive motor load in high/high.  The alternator is making about 25 amps at this point.  It's a LOT better than the Hvid setup and it has the advantage of NO CLAG!


27 December 2009:

I made a 0.200" shim for the connecting rod to raise the compression and the engine seems to run a little better.  Now, it is having some fuel starvation issues and leans-out under heavy load with the needle valve out all the way (and a screw in it's place!).  I'll make a flutter choke and then if it doesn't do enough I can always drill out the main jet a few thousandths.  It would be neat to just have a fixed jet, though.  None of those pesky adjustments.


When the engine is running with timing advanced, I'm not sure whether I'm hearing detonation or a loose wrist pin.  It doesn't seem to be bothering anything at this time so I'll leave it be 'til I get other issues sorted out.


31 December 2009:

I figured out why the engine tried to quit under heavy loads.  I didn't have a ground on the ignition circuit and the spark was diverting through the sensor to ground.  This caused the sensor to latch up occasionally.  Ground now in place and the problem went away.


Now to the mixer issues.  First thing, I made a flutter choke.


Flutter made from scrap bin stuff.

The flutter choke is just a spring loaded plate that fits against another plate that is bolted to the mixer air inlet.  It allows fast and slow running of the engine.  The way it is adjusted is to set the needle valve for good acceleration and then set the flutter choke for best idle.  It takes some fiddling but works reasonably well.


Not shown in the above photo is the enlargement of the venturi.  I originally bored it to 0.375", knowing it may be a bit small.  While I had the mixer off the engine, I stuck it back in the lathe and bored the venturi out to 0.475".  This gives a little more power.


Now, I might start fiddling with the cam timing.  For the Hvid, there can be no reasonable amount of overlap (where the intake valve opens before TDC and the exhaust valve closes after TDC).  With the Hvid, there's just not enough headspace to keep the valves from hitting the piston.  I'm thinking it might run a bit better if I give it a little overlap, opening the intake about 8 degrees BTDC and closing the exhaust about 8 degrees ATDC).  It'll be interesting to see if that messes-up the idling which right now is pretty good.



After fiddling around with the gas version of the engine, I decided that it was easy to get a good-running 4-cycle spark ignition engine to run well but not that easy to make a Homebrew Hvid into a good, reliable engine.  Because of this, I converted it back to a Hvid and refined it until it's now the best running of my home-made engines.  Go to the end of the Homebrew Hvid series to see how I accomplished this with the advice of Denis Basson of Australia.


BOY!  This is fun!

If you have any questions or comments, please email me at:

[email protected]