Abstract: The Como Comet was a “diesel” 2-stroke loop scavenged chainsaw that was commercially sold in the early 1950’s and later produced by Jonsereds through the late 1950’s and early 60’s . It utilized “pre-mixed” diesel combustion. Could modern technology application to the concept make it work better today? This article explores the possibility and experiments conducted or planned.

Background and History:

In the past decades there has been increasing interest in premixed diesel combustion. It is an attractive means of simultaneously reducing NOx and particulate emissions while maintaining the fuel efficiency of the diesel cycle. There have been many versions of this concept and that can be identified or associated with names like:

PCI – Premxed Compression Ignition

PPCI – Partially Premixed Diesel Combustion

HCCI – Homogeneous Charge Compression Ignition

GDCI – Gasoline Direct Injection Compression Ignition

RCCI – Reactivity Controlled Compression Ignition (highly associated with University of Wisconsin Research Center work)

Research and development work on the concept(s) has been or is ongoing at multiple companies and organizations such as Hyundai, Delphi, University of Wisconsin, Aramco and many more. People like Mark Sellnau have been champions of the concept for a number of years. Recently developed or emerging technologies like VVA (Variable Valve Actuation) and VCR (Variable Compression Ratio) can provide new degrees of freedom for the successful engineering of such concepts.

It is not the intention of this article to provide a review of these concepts as hundreds of references to the recent work are available through internet search. Rather, it is to discuss some interesting engine history that predates recent work by decades and resulted in a small commercial engine. I will also discuss some tests conducted to experiment with the concept.

Historic Engines

Prior to and in parallel to early diesel engines there were a wide variety of hot bulb or hot tube engines. These engines relied on a heated surface within the combustion system to assist with the vaporization and auto ignition of the fuel. Thus these engines did not rely solely on the heat of compression for ignition as diesel engines did. A 2002 paper published by SAE (Society of Automotive Engineers), 2002-01-0115 -Early Swedish Hot-Bulb Engines – Efficiency and Performance Compared to Contemporary Gasoline and Diesel Engines by Olof Erlandsson, of Lund Institute of Technology, represents a good summary of the technology. It compared hot bulb and diesel engines of the same time period. These engines were primarily 2 stroke cycle engines, however 4 stroke cycle engines also existed. The Hornsby- Akroyd engine being a very successful example.

The 2 stroke engine had the advantage of a relatively hotter cycle with less time for surface cooling between firings. It also had more residual heat from unscavenged hot gases that aided the auto ignition process. The overall simplicity of the early 2-stroke engine construction also favored the 2-stroke design. These engines generally utilized low pressure fuel injection/metering systems to deliver the fuel to the combustion chamber and relied more on the heat of chamber surfaces for vaporization. The combustion system or part of the chamber was heated externally, normally by a burner or torch, to create hot chamber walls to assist in vaporization and auto ignition. Once running the chamber was kept hot by the combustion process and external heat could be removed. Low compression ratios, relative to true diesels, reduce structural loads helping to reduce the weight of the engine and the internal friction. Compression ratios were generally between 6:1 and 12:1. These engines were slow speed stationary engines and or engines used in agricultural tractors. Most were single cylinder. These engines generally had superior efficiency to spark ignited engines of their day and were competitive with early diesels. The hot bulb engines are often thought of a predecessors to prechamber diesels.

Fuel injection in the hot bulb engines was generally timed much earlier than diesels. This allowed time for the fuel to evaporate and mix with air forced into the hot chamber near TDC (top dead center). As such the combustion was primarily pre-mixed with combustion ignited through auto-ignition of mixture near the hot surfaces of the “Hot Bulb”. The balance of combustion was through a progressing flame front and or auto-ignition from the rising temperatures.

Mietz and Weiss Hot-Surface Ignition Engine, circa 1897

Another form of the engine utilized a “hot tube”. A long tube extending from the combustion chamber was heated externally for starting. The tube was heated a distance from the combustion chamber such that the compressed premixed fuel charge would not reach the heated area to support auto ignition until late in the compression stroke

Schematic of a “Hot Tube” Ignition System (Daimler)

The ignition timing of hot tube engines could be controlled by adjusting factors as the unheated length of the tube, the length and volume of the tube and the position of the heated portion. A mechanical valve could optionally be placed at the entrance of the tube. Ignition timing would then be controlled by the timing of the mechanical opening of the valve.

Hot Tube Ignition System with Mechanical Valve

The Como Comet

A Norwegian inventor, R.K. A. Wiig, conceived a concept for a 2-stroke diesel chainsaw. The US patent was filed for in 1950 and granted in 1955 ( US Patent # 2,722,928). The saw was originally produced in Oslo, Norway by the company Norsk Sagbladd Fabrikk. Later it was produced as the “Comet” by a Swedish company Como M&T Bjerke. In final years it was produced for them by Jonsered as the Raket (Rocket). Models XA and XC were produced.

The engine used the concept of the earlier Hot Bulb or Hot tube engines to provide a source for heat for auto ignition. The original engines used a device referred to as a tändrör (translated as detonator). This was a tube connected to the combustion chamber that was initially preheated externally and eventually kept hot by engine combustion. It was basically the “hot tube” or “hot bulb” from the earlier engine history. In the earlier Comet engines, propane storage was built into the handle to provide fuel for a burner to preheat the hot tube for starting. Once running combustion heat would sustain the tubes temperature for continued operation without external heat.

Illustration of the combustion system’s “hot tube” ignition system in US Patent 2,722,928

There are several references to the engine specification. One source for the early Comet “B” lists the bore and stroke as 41.2 and 42.6 mm or 1.625 and 1.687 inches. This reflects a displacement of 3.50 cubic inches or 57.35cc. The same site indicates a displacement of 49cc or 3.0 cubic inches – which does not agree with the previously mentioned calculations. The power is listed a 3.0 hp at 3000rpm. Assuming 3.5 cubic inch displacement, this reflects a bmep (brake mean effective pressure) of 113 psi. This value seems to be an exceptionally high for a for loop scavenged engine without apparent exhaust tuning and makes me question the power value.

The later model XA engine produced by Jonsered, was reported to have bore 39.7 mm and a stroke of 40 mm giving a displacement of 49.5 cc or 3.02 cubic inches. The reported reference lists the power as 1.3 kW @ 4500 rpm (1.74 hp) and a maximum rpm of 6000 rpm. These numbers reflect a bmep of 50.7 psi, which would appear to be a more reasonable for a small loop scavenged engine.

The reference on the XA engine mentioned a fuel to oil ratio of 10:1, with 50 or 60 weight oil. This seems to be a very oil rich mixture. The the reference also indicates it could run on diesel fuel, kerosene or gasoline. Assuming the engine is operating at a higher compression ratio with higher cylinder pressures it makes sense to provide better lubrication. However, the 10:1 ratio still seems extremely high. One possibility is the 10:1 ratio is meant for use with gasoline – not for lubrication, but to decrease the auto ignition temperature.

There are several videos available on the internet (Video Link 1, Video Link 2) from collectors showing the engine starting and running with a very high degree of blue gray exhaust smoke. It is difficult to believe that the level of smoke demonstrated would be acceptable with a commercial product. Are these “worn out” old engines or does it accurately reflect the engine’s exhaust character? The blue -gray smoke color is what one would expect from unburned fuel or oil and not the normal black diesel smoke associated with rich diesel combustion. If one were to run a spark ignited gasoline 2-stroke with 10:1 fuel/oil mixtures I would also expect a similar level and type of smoke.

In 2 stroke engines with premixed fuel and crankcase scavenging, through scavenging is possible for part of the charge. Gasoline, being more volatile than diesel, is easier to evaporate. With diesel fuel, through scavenging of part of the charge is more likely to be un-evaporated liquid droplets. If fuel is not fully evaporated, fine droplets are more likely to exist causing a visible cloud. White smoke of an idling diesel, upon start in cold weather, is an example of that. This is another possible explanation of the smoke.

If one were to build and market the concept today any significant level of smoke would not be acceptable. It would need to be kept low enough to be addressed by an appropriate exhaust catalyst.

Later models of the Comet abandoned use of the burner type hot tube heater and replaced it with an electrically heated glow plug. The configuration of this design is unknown to the writer beyond that a reference indicates initial heating was provided by 2 D cell batteries. It is assumed the glow plug was recessed in a chamber connected to the main combustion chamber, similar to the earlier hot tube. The glow plug was likely of the heated wire loop kind that was prevalent during the time period.

Modern Considerations

There are several reasons one might consider this technology or variations of it. For military applications there is a strong desire to have a single battlefield fuel to simplify logistics. Operating on diesel or jet fuel with small engine support equipment would be desirable. The use of small 2 stroke engines that have superior fuel consumption to spark ignition engines has been a goal for applications such as UAVs (unmanned Arial vehicles).. The Cosworth AG2 engine being an example. https://www.cosworth.com/case_studies/case-study-ag2-engine/. A picture I took of the engine in 2017 at a show in Novi, MI is shown below.

Cosworth 2-Stroke Diesel UAV Engine (Novi, Mi 2017)

Use of a hot surface ignition in such engines would potentially offer benefits of being able to operate with a lower compression ratio for reduced weight and friction, perhaps simplifying the fuel system and reducing costs. The ability to vary the surface temperature could also provide a means of operating with a wide range of fuels and ignition properties.

First Experiments

Having no first hand experience with hot surface ignition beyond that of a gasoline engine “dieseling” or “run-on” that can occur after ignition turnoff due to a hot valve, or spark plug or glowing carbon, I decided to experiment with an old 4-Stroke Briggs and Stratton single cylinder air cooled engine available to me. The engine was a flathead type, probably about 3 hp. The stock head gasket was replaced with a thin paper one to increase the compression ratio slightly. A fabricated a hot tube was added to a hollowed out spark plug. The tube as about 2 inches long, 0.2 inch diameter about an 0.080 inside diameter. The engine was to run unloaded with diesel fuel supplied to the standard carburetor. The glow tube was heated with an acetylene welding torch until glowing red. The engine started easily with the pull rope starter after the preheating of the tube. The engine would run for a few seconds and then die. After several failed attempts to achieve sustained operation a different approach was tried. In an attempt to keep the engine running, sustained heat from the torch was applied to the hot tube. This was a bit more successful. However, the attempt ended abruptly when the tip of the torch flame locally over heated the tube resulting in a jet of flame shooting through a melted hole. This put an abrupt end to the experiment.

The next attempt utilized a 12 volt diesel glow plug built into an adapter installed in the spark plug hole using a battery charger to power the glowplug.

Glow Plug Adaptor with Glow Plug
End View of Glow Plug in the Adaptor

This attempt achieved success. However, the engine would only run on gasoline and not diesel fuel. This was despite the lower ignition temperature of diesel fuel. I tried having the engine running on gasoline and filling the tank with diesel when it approached empty only to have the engine stall. My conclusion was that the carbureted diesel fuel was not vaporized well enough to ignite and or to reach the glow plug cavity. When running on gasoline the engine easily started and would run throughout the speed range with a occasional backfire through the intake during rapid accelerations. Some missing occurred at high speed. A video of this operation is attached below.

Briggs and Stratton 4-Stroke Running with Glow Plug Ignition on Gasoline

With the limited success it was decided to move on with a plan to use a 2-stroke loop scavenged engine to more closely emulate the Como Comet. The two stroke engine offers potential to have more fuel mixing and vaporization time in the crankcase as well as a hotter running combustion chamber with better surface to volume ratio.

Two Stroke Considerations

In assessing small 2 stroke engines, a multitude of inexpensive Chinese built engines of approximately 49 cc displacement were found. These engines were generally designed to modify bicycles to provider engine assisted power. A “performance” model marketed as a generic brand by Banggood was chosen. The selection was primarily due to it’s low cost ($89.14 with shipping from China) but more importantly due to it having a removable cylinder head with a combustion chamber having a central vertically mounted spark plug. The exact specifications for the engine were not published by the supplier beyond a 49 cc advertised displacement and a claim that this performance version had better porting. They inferred a 44 mm bore and claimed that it could rev. to 12,000 rpm and produce approximately 4 hp. I found a virtually identical looking engines (even with the same cast number on the jug) from another supplier claiming their model 44-6 had a bore of 43.5 mm, a stroke of 35 mm with 2.6 hp at 7000 rpm and 2.8 Nm of torque at 5500 rpm. Without disassembling the engine, my rough measurements of the bore through the exhaust port and the stroke via the spark plug hole determined that the bore is 44 mm and the stroke is 33 mm for 50.17 cc displacement. More precise measurements of the bore and stroke will be made once the cylinder head is removed.

2 Stoke Gasoline Engine Purchased for Next Phase of Testing

The bore and stroke of the purchased engine, are not ideal for a diesel engine. Normally a smaller bore and longer stroke would be chosen for reduced surface to volume ratio at TDC and lower the force on the crank. However, the removable head and combustion chamber insert of the purchased engine are ideal for enabling the experimentation with combustion chamber shapes and materials. Many of the other small engines considered had the head and cylinder jug cast a single piece. They also had angled spark plugs and non symmetric combustion chambers. These factors would have made modifications difficult.

The next planned step is mounting of the engine for testing. A test setup is being designed and built to drive a small hydraulic pump with the engine through a 3:1 chain drive to allow the engine to be tested under load. By restricting flow output of the pump pressure can be increased to load the engine. With knowledge of the pump displacement, pressure and speed the power and torque of the engine can be approximated with the assumption of the pump and chain drive efficiency. An exhaust pipe, with an expansion chamber and silencer designed for small two stroke engines has been purchased to minimize noise and enhance the engine’s performance.

The first priority in testing will be to break in the engine under mild loading. Then establish a baseline for the spark ignited performance with gasoline. Hot operation on diesel fuel will also be attempted with spark ignition. Following this the cylinder head will be removed and the combustion chamber modified for glow plug ignition. A variable power supply means will be sought to vary and control glow plug temperature.

Stay tuned for Part II once the work is completed.

Side Note:

Another interesting 2 stroke premixed diesel engine is the historic Lohmann Engine. This engine used manually adjusted compression ratio to provide the required temperature for auto ignition. Search on Lohmann Diesel and there is a wealth of interesting information. Here is a link to one reference to get you started. https://onlinebicyclemuseum.co.uk/1951-lohmann-18cc-diesel-engine-new-old-stock-unused/

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