This is a subject seen often in antique engine and tractor forums, and quite a few seem to be interested (especially in the small engine crowd). I am an antique small engine enthusiast who became interested in this subject a number of years ago. When I asked questions, I found few answers. I kept at it and did my homework, so now I’m writing the article I wish I’d found 5 years ago. Hopefully I can clear the air a little on this issue...
In order to burn "distillate fuels" like kerosene or diesel, first, it must be atomized/vaporized. Either mechanically (like a diesel injection pump) or with heat. Any gasoline engine can be set up to run on kerosene as long as two issues are addressed, how to prevent preignition, and how to vaporize the fuel.
First, lets deal with fuel vaporization. I’ll begin with a brief discussion on the characteristics of gasoline vs. kerosene vs. diesel. All of these are petroleum distillates, which means they are refined from crude oil. As with any distillation process, the “lighter” elements are the first to evaporate. Gasoline is one of these elements. Diesel is considered a “heavier” element, which means it does not evaporate easily. Kerosene is roughly between the two. So here is a very simplified comparison.
Gasoline: 125,000BTU/gal. Flashpoint is -40F
Kerosene: 135,000BTU/gal. Flashpoint is 100-162F
Diesel: 138,000BTU/gal. Flashpoint is 126-204F
The key factor here is the flashpoint, which is the minimum temperature at which the fuel will vaporize with air to produce a combustible mixture. Note the jump from gasoline to kerosene is at least 140 degrees! Diesel’s flashpoint is as much as 240 degrees higher! The reason for this is the hydrocarbon “chain” of molecules is much longer in kerosene and diesel than in gasoline. This chain must be broken apart before the fuel will vaporize. The easiest way to accomplish this is HEAT.
Another important thing to remember is the difference between “atomization” and “vaporization”. To keep this a simple explanation, atomization is the conversion of a liquid to a fine mist, and vaporization is the conversion of liquid to a gaseous form. Why am I boring you with this? Because in order to effectively burn “middle distillates” like kero or diesel, they must be VAPORIZED. Fuel that is too cool will be atomized by a carburetor. The result is the poor, very smoky performance that some of us experimenters have seen. When the microscopic droplets of fuel are burned, only that fuel which is actually vaporized will burn. The rest either sticks to the cylinder head as carbon, or goes out the exhaust as smoke. This is great for ‘skeeter control, but it is a waste of fuel and not the best for your engine. Heat must be used to vaporize these heavier fuels.
Since the goal of running kerosene was to reduce operating expenses (years ago anyway), mechanical atomization (as in a diesel engine) is out of the question. Also, whatever method used needs to be simple and low maintenance. So for simplicity and minimizing cost, heat is the method of choice. On the tractor engines of old, this was easy since most integral intake/exhaust manifolds had a "hotspot" where the exhaust manifold heated the intake manifold immediately downstream of the carburetor. The fuel in the intake stream that was not completely vaporized would not be able to make the sharp turn into the intake manifold log and would be thrown into this very hot surface and immediately vaporize. Even automotive engines used this in the form of an exhaust crossover in V8 intake manifolds, and the same "hotspot" style was used on inline sixes. The principle is used on gasoline engines to improve efficiency and cold weather operation. In some cases, the fuel line from the kerosene tank was run in close proximity to the exhaust manifold to preheat the fuel so it would vaporize more readily in the carburetor.
The second problem with kerosene and diesel in a gasoline engine is preignition. Kerosene and diesel have a very low octane rating (somewhere around 25) and thus makes it very prone to compression ignition. This is why, of course, they run great in diesel engines. In a diesel, the fuel is not introduced until it is time to burn it, so the low octane is a non issue. When the intake air is carrying the fuel, low octane is a problem. There are two ways to resolve this. The simplest, and most common, is to use a low compression ratio so that the intake air will not be compressed enough to ignite the fuel. Any engine with a ratio of around 6.5:1 or less should be able to run kerosene without problems. This would include almost every flathead engine made, as well as a good number of antique tractor I-head engines. Another slightly more complicated option is to use an anti detonant. The best example of this would be the Rumely Oil Pull engine. It utilized a three bowl carburetor. One for starting gasoline, one for kerosene, and one for water. The water jet was located ahead of the venturi (where the main jet is located) so that water was only drawn when the engine was under load (half throttle or above). The water prevented compression ignition thus allowing higher compression ratios to give performance equal to (some will argue better than) a gasoline engine of equal size. Another advantage to the water/kero mix is it produces a much smoother (less violent) burn in the cylinder, making it much easier on the engine. Therefore, it lasts longer. For more information on the Secor-Higgins carburetor, follow this link to a fantastic article on the “Rusty Iron” webpage.
There were a surprising number of OEM small engines built for gasoline/kerosene operation by Kohler, Briggs & Stratton, Clinton, and Tecumseh. All were off the shelf gasoline engines modified for kerosene operation. These modifications included:
1. Either a low compression cylinder head, or a stock head installed with two head gaskets.
2. A divided fuel tank with the large side for kerosene, and a small (usually a pint or less) for starting/warm-up gasoline. In some cases, the bowl capacity or the fuel bowl in the carburetor had sufficient capacity for an adequate amount of gasoline to start the engine, so a small flip top “fill cup” was T’d into the fuel line so that the carburetor bowl could be filled with gasoline prior to starting.
3. A carburetor bowl drain of some fashion for draining kerosene from an improperly shut down carburetor. On Briggs “Flo-Jet” (updraft) carburetors, a special main jet screw was installed with a drain cap that allowed the bowl to be drained. Most other engines with stamped metal bowls used a small button installed in a drilled hole in the bottom of the bowl. These are still used on some Tecumseh “Sno-King” and other engines for seasonal use.
4. A hotter spark plug was utilized to prevent fouling, and in some cases (on the Kohlers) timing was retarded to aid in the prevention of detonation.
The fuel heating on small engines is inherent in the design. The close proximity of the intake to the exhaust on most L-head single cylinders heats the intake elbow sufficiently to somewhat vaporize kerosene. Diesel fuel would be slightly more difficult since the hydrocarbon chain is longer than that of kerosene. Pretty much any single cylinder L-head engine with a float bowl carburetor should not be difficult to convert for kerosene use.
Opposed twins like Onan’s CCK and Kohler’s K-582 (or the monstrous K_-662) will pose some problems due to the long intake runners sitting atop the engine right in the cooling air. The expense of designing and building a reliable kerosene system would probably negate the fuel savings advantage of kerosene.
The old Briggs manuals state that engine's horsepower must be derated by 15% when running on kerosene, and engines with “Vacu-Jet” and “Pulsa-Jet” carburetors (those mated directly to the fuel tank) cannot be converted for kerosene use.
If you are willing to do the research and design/build a kerosene carburetion system, a lot of the parts are still out there sitting on distributors shelves as “New Old Stock”. For example, Briggs & Stratton, and Kohler dual fuel tanks, brackets, and carburetor parts are waiting for someone to claim them. I know, because I have a few brand new Briggs gas/kero tanks sitting in my shop. Other items the OEMs used were simple brass and copper parts (three-way valves, fill cups, and other hardware) that are still available at most hardware stores.
The bottom line is that in order to burn “middle distillates”, the fuel needs to be heated to the point where it will vaporize. To keep the fuel from knocking and destroying your engine, you need to lower the compression ratio and/or use an anti-detonant.
Hope my little treatise helps in some way. I did a lot of research on the subject a few years back and spent many hours researching on the web, reading books on petroleum chemistry, and poring over pages and pages of old parts/service manuals on a microfiche viewer. I hope it sparks interest and saves folks who are interested in this some time and headache.