Optimizing a small engine for propane?

[BioHazard]

I'm going to convert a simple "Coleman Powermate" 5hp Briggs and Stratton 13ci flathead generator to run on propane. Part of the reason is to have a generator with fuel that never goes bad, but I also want to experiment with gaseous fuels. Propane is becoming very comparible in price to low octane gasoline when you adjust for BTU content.

I know I can buy a "kit" to convert the carb to propane and that's that, but I'm not satisfied with 6.5:1 compression on 100+ octane fuel. The 5hp flathead is a favorite for go kart racers and they reach upwards of 20-30hp out of one!

I don't intend on creating a bunch more HP, I just want to make the engine as efficient as possible. Starting with the head, I'll have it milled down to increase CR as much as I can. Kind of hard to do with a flathead without killing flow though. To compensate some material between the valves and cylinder can be removed from the block. Possibly a higher popup piston too. Is there an "ideal" compression ratio for propane, or does it vary with the engine?

Now, the motorhead part of me wants to open up intake and exhaust, use a more agressive cam, etc, but I worry if some of that stuff might actually kill the torque in the 3600 RPM range, go karts run double that. On the other hand I also think propane gas takes up more room in the intake runner than gasoline vapor thus allowing less space for air. Do you guys think it would be a good idea to enlarge the carb, intake openings, or the valves, or remove restriction from the exhaust? Should I use a cam with higher lift or keep the stock type?

Timing - should it be adjusted from factory spec for propane? I'm thinking about converting to a CDI type ignition, Honda is using this on some of their new small engines and are claiming better fuel efficiency over standard magnetos. I'm also considering adding a second spark plug, the go kart racers have mixed opinions on this but they do claim a smoother idle. More efficient burn maybe?

Anybody have any other thoughts on this? I know there are better engines to start with than an old flathead but it's kind of a sentimental thing for me. Sorta like listers for some. First engine I ever owned, first engine I ever learned to fix. I will be making modifications one by one and testing specific fuel consumption in between, so I know what works and what doesn't.

[Henry W]

Going with a more agressive cam will make the engine loose torque. I would stay with the stock cam. See what the increased compression will do. you might be able to advance the timing a bit to help with 100 octane.

Another thing to look into is to see if there are aftermarket adjustable cam gears available to advance or retard timing.

Leave the Carb alone putting a larger carb on will hurt torque.

These engines have already been tuned to run at 3600 and lower so it will be tough to squeese more out of it.

Flat head air cooled engines are not the most fuel effecient engines because of thier design. And they run hotter.


I think it might be best to look into a 5 Hp OHV Honda. That is an easier engine to increase compression out of it. Lots of aftermarket racing parts for them since the started using them in carting.


Henry

[BioHazard]

Flat head air cooled engines are not the most fuel effecient engines because of thier design. And they run hotter.

One could say the same about a listeroid, no?   I already have the generator with this engine and a shitload of flathead parts to play with. I may see how much heat I can extract from the cylinder head and exhaust.

Sometimes you just gotta go with "cool" instead of "useful".  2 strokes aren't efficient either, but I love my little harbor freight mosquito fogger.

[vdubnut62]

Find an old 4hp tiller motor head. It's gotta be a 4hp, they share a common block, but the 4 has a smaller bore. They are the same bolt pattern and the area around the valves is the same, but the area over the piston is visually much shallower and will give higher compression without a sacrificing airflow. Take the flywheel key out and throw it away, use valve lapping compound on the crank and flywheel area and it will fit tight enough to not need a key to hold timing. Then you can custom set your advance.
Yes I raced a methanol-burning Briggs a time or two.
Ron

[TimSR2]

I'd favor an ohv design with a higher CR. It's pretty hard to get any CR over 7 to 1 or decent flame pattern in a sidevalve engine.

With propane you have a lower btu content than gasoline, and the gas displaces 7 to 8 percent of the air in the throttle body. These can lead to a 15 pct drop in horsepower.  By increasing the compression ratio  to take advantage of the octane rating you can end up with equal or more horsepower than the same engine on gasoline.  I think the ignition advance will be OK the way it is. This is a constant speed engine so  only the total advance is important.  For a really hard worker on constant speed duty about 25 to 27 degrees should be good.  On OHV design modified to to give a 9.5 to 1  CR running 25 degrees BTDC would be my starting point.

Design to the full 2.0 hp/kilowatt , or even 2.5.  This thing will get really hot if run hard. 

[BioHazard]

Find an old 4hp tiller motor head. It's gotta be a 4hp, they share a common block, but the 4 has a smaller bore. They are the same bolt pattern and the area around the valves is the same, but the area over the piston is visually much shallower and will give higher compression without a sacrificing airflow.

I think I'm going to have this guy make me a head like this, without the extra valve pockets. He said he can do the machine work for $15 if I send him a head.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=220739424719&viewitem=&sspagename=STRK%3AMEWAX%3AIT

Design to the full 2.0 hp/kilowatt , or even 2.5.  This thing will get really hot if run hard.  

Yeah, it's only got a 2250 watt head on it.

As inefficient as flathead engines can be, I would like to point out B&S still make flathead lawnmower engines based on the same thing they've been using forever. From what I understand all they have had to do to make the EPA happy is increase the CR and lean out the carb. There's just something about the old flathead that I love, kinda like the guys that are into the Whitte engines. Outdated but we still want 'em.

[BioHazard]

With propane you have a lower btu content than gasoline, and the gas displaces 7 to 8 percent of the air in the throttle body. These can lead to a 15 pct drop in horsepower.

So doesn't that mean the engine would benefit from a slightly larger carb bore (they sell cheap reamers for this) and/or a larger intake valve? If I stuck to gasoline that would be bad for torque, but if the propane is displacing air...shouldn't it need a bigger opening to bring in a bit more?

Briggs also makes four different carb designs that work with the governor linkage on this engine, so I'll probabaly try a few to see what's best.

[TimSR2]

Even if you don't up the cr a bit she'll probably pull just fine. 5 hp on a 2.2 kw head is lots of power. The standard propane derate will still give you better than 2.0 kw/hp .  But if the cr is higher it will use less fuel, and have more power, and will be a much more interesting and informative project. :-)

[BioHazard]

Yes, my goal here is to find the absolute maximum efficiency I can for the flathead. Don't care if I end up with more than 5hp...although it might be helpful for surge loads if it has a little more kick.

I'll also try to compare my results vs gasoline as much as I can, so we can see the difference in consumption. I wish I had the equipment to analyze the exhaust gasses...

What do you guys think about the spark? Would a "hotter" spark, or dual plugs make for a more efficient burn? Or do you think the stock magneto is good enough? Briggs has a bolt on coil for twin cylinder engines that will fire two plugs at the same time. I can have the machinist put the plug where ever I want...

Tonight I noticed gas was $3.49 for alcoholic regular and only $2.79 for propane. Propane would of course be cheaper if I had it deliverd in bulk to my house and that price hasn't changed in months. Diesel is up to $3.79, I think that works out pretty close to the same $/BTU as propane.  

Believe it or not, the first word I ever spoke was "lawnmower", pointing at a Briggs flathead.  I guess my parents weren't very concerned if I kept all my fingers or not...

[BioHazard]

Hmmm...what about a turbocharger?!
http://www.mbe-motorsports.com/catalog/product_info.php/products_id/265?osCsid=oeapamr0ou3b6hdpm06hbmccm4

That seems like the best way to increase the effective compression in a flathead engine. How might that effect efficiency if you're still only asking 5hp from the same engine?

[Henry W]

That is a very nice small Turbo Charger.
One problem is the 5 hp briggs has splash lubrication. Another thing is I feel it still might be to large for your application. I think it would work fine for racing cart engines that spin over 3600 rpm.

Now that will be cool to put that small turbo on one of those 10 hp air cooled diesel engines. or a Changfa water cooled engine. It might make it pass emmisions.

Henry

[BioHazard]

One problem is the 5 hp briggs has splash lubrication.

I was thinking about that, but given how small and simple the engine is, does it really make much difference? I think if you took a new engine and ran it on gas until it would run no more it would eventually die from lack of compression. I don't think pressure lube really helps that though? Usually the crankshaft lasts through several rebuilds. Using propane of course would increase the life significantly.

The major drawback of course is oil filtration. I'm working on that + a larger sump though....

[Henry W]

My concern is getting oil pressure to the turbo bearings. Oil also helps cool the turbo.

Henry

[wiebe]

Where is the trust in splash lube .
our gokarts getting the hell riding out of them on lpg.
Somtimes the fan has al the fins broken of end stil it runs .

If the engine is running in spec rpm there is no problem .
Try to get the biggest carb ore make the ventury bigger .

i wil see if i can make some pics tomorrow at work .
Of our systems .