If you’ve made it this far you’re no doubt thinking “This man’s fallen off his rocker!”  “A locomotive engine design for a motorcycle!?!”  That’s not really the case, although it was the EMD engine and their prolific use on local short line railroads that introduced me to the concept of the uniflow engine.

To understand why the EMD engine is inspiring I should backtrack a little and explain a bit about the two-cycle engine, the history of motorcycle racing and what makes the uniflow engine the right choice for the Paulding Racer.

The Two Cycle Engine

It is the two-cycle engine design, not the four-cycle, which has produced the largest array of displacements, service types and power levels.  One of the smallest two-cycle engines produced is the loop scavenged Cox .049 cubic inch “Babe Bee”, a control line model airplane engine fueled by nitromethane.  On the other end of the scale is Wartsila-Sulzer’s uniflow RTA-96, a direct-drive uniflow marine diesel engine displacing over 111,000 cubic inches per cylinder.


Cover art for the Wartsila RT-FLEX96C and RTA96C Technology Review brochure showing bedding of the crankshaft. The man on the left should provide sufficient scale.

According to Cyril Lovesey, Sir Stanley Hooker once poked fun at the four-stroke engine, stating that it uses “one stroke to produce power and three to wear it out.”  While the four-stroke engine can devote one full stroke of the piston (half a crankshaft revolution) to each of the four portions of a combustion cycle, the two-stroke engine must do everything in only one full revolution.  The four portions of a complete combustion cycle in the four-stroke engine are:

1)  Intake:  The camshaft operated intake valve opens and the piston moves towards bottom of the cylinder, sucking in a fresh air and fuel charge.

2)  Compression:  All valves are closed and the mixture is compressed prior to ignition.

3)  Power:  With all the valves remaining close, the fuel mixture is ignited and expanded.  Power derived from the expansion is transmitted to the crankshaft.

4)  Exhaust:  The exhaust valve opens and the spent combustion gasses are forced from the cylinder in preparation for a fresh charge.


The four strokes of a four-stroke engine. Animation Credit: Wikipedia user Zephyris

On the other hand, the two-stroke engine has only one crankshaft revolution to complete everything the four-stroke engine does in two revolutions.  This is accomplished by combining the intake of a fresh fuel and air charge with expelling the exhaust gasses from the cylinder.  This operation is called scavenging.

The animation below is of a loop-scavenged two-stroke reed valve engine using the crankcase as an induction air compressor and is typical of chainsaw, weed trimmer and dirt bike engines.  The lubrication system is the total loss type, depending on oil inducted into the engine through the carburetor to lubricate the engine internals.

The cycle begins when the piston rises in the compression stroke.  Atmospheric air (blue) is drawn though the carburetor where it picks up a combustible mixture of fuel and lubricating oil (green).  The mixture passes though the reed valves into the crankcase.  As the piston reaches Top Dead Center (TDC) the reeds close and the piston begins moving down, reducing the volume of the crankcase and compressing the mixture slightly.

As the piston continues downward, it uncovers the exhaust port (known as the release point of the cycle), then the intake port.  The mixture compressed in the crankcase is forced though the transfer ports into the cylinder.  Some of it escapes out the open exhaust port.

As the piston rises in the next compression stroke the intake port is blocked off by the advancing piston.  At the same time, an exhaust pulse traveling down the tuned expansion chamber pushes the majority of the fresh mixture back into the cylinder, but only if the engine is operating within the range of the tuned exhaust.  If the engine is operating outside the range of the tuned exhust that portion of the mixture is lost, contributing to poor volumetric efficiency.

This piston continues advancing to the trapping point (the point at which the exhaust port is blocked off by the piston.)  Mixture compression occurs followed by ignition and expansion.  As the piston is forced downwards past the release point again, an exhaust pulse begins traveling down the tuned exhaust pipe and the cycle starts again


Animation of a two-cycle loo-scavenged reed valve engine. Credit: Wikipedia /

The Uniflow Engine

At this point, the more motorcycle-oriented of you are saying “The two-cycle bike is a peaky wheelie monster and unsuitable for modern road racing unless you want to get killed.  Besides, today’s 4-cycle bikes are more powerful than the old 2-smokes ever were and are better at helping the rider get that power to the road where it counts for something!”

That is true of port scavenged engines, yes.  However, it is not the case with the uniflow engine as evidenced by the fact that it’s an excellent choice for moving the heaviest loads ever moved by an internal combustion engine.  The same things that make an engine suitable for pulling a freight train or driving a ship make the perfect engine for a cutting edge road racing motorcycle.  Namely, a broad and flat torque curve and copious, predictable power.

By negating the need for a organ pipe tuned exhaust and through the use of forced induction, the uniflow engine can have the throttle characteristics of a 4-cycle engine.  In addition, it can produce more power than a four-cycle engine from the same displacement as a two-cycle engine fires twice as often.  This can place less stress on many of the rotating components of the engine and produces a smoother power response.

Operating Principle of the Uniflow Engine

Unlike a reed valve engine, the uniflow engines require a scavenge blower to “breathe” as it does not use the crankcase to compress the mixture before transferring it to the cylinder.  The scavenge blower can be an engine-driven supercharger such as a Roots Blower, a centrifugal supercharger, or even a turbocharger which is powered by the engine at low throttle settings to provide starting air.  The uniflow engine is so named because gas passes in only one direction though the engine, as seen below.


Illustration from the EMD 567 Engine Maintenance Manual illustrating the scavenging and compression events.

The start of the cycle begins at position A, with the piston at bottom dead center.  Scavenging air provided under pressure by the roots blower enters the cylinder ports uncovered by the piston and forces the spent fuel gasses up the cylinder and through the exhaust valve s.

As the crankshaft advances at position B, the piston ports are closed by the piston (trapping point) and compression begins.


Illustration from the EMD 567 Engine Maintenance Manual illustrating the injection and expansion events.

As the piston reaches top dead center, as in position C, fuel injection occurs and combustion takes place.  The piston is forced downwards to position D.  As the cylinder pressure falls and before the intake ports are uncovered again, the exhaust valves open and begin venting the remaining exhaust gasses to prevent blowback into the induction air belt.

A Locomotive Engine for a Racing Motorcycle?

The uniflow engine offers several distinct advantages with only a few disadvantages if developed for motorcycle use:

Advantage:  The uniflow engine has the potential for the highest volumetric efficiency of any 2-stroke engine across a broad range of speeds.  While the scavenging and charging events are substantially shorter than the equivalent 4-cycle engine, the uniflow engine has around four times the exhaust and intake port area of any 4-cycle engine which means faster scavenge and charge times.

Advantage:  Unlike loop-scavenged two-cycle engines, which are found in the vast majority of two-cycle road racing motorcycles, dirt bikes, weed eaters and chainsaws, the direct injection uniflow engine does not require a tuned expansion chamber exhaust to effect charge trapping.  As a consequence it should be possible to have a smooth and steadily building power curve with a fairly flat torque curve.  The result is predictable power as opposed to the “light switch” power hit of a loop-scavenged engine.

Advantage:  The uniflow two-cycle engine can provide all the predictable power characteristics which make the 4-stroke a dominant performer in motorcycle racing while providing more power per cubic inch of displacement.

Advantage:  While the uniflow two-cycle engine is more complicated and heavier than the 4-cycle engine or the loop-scavenged 2-cycle engine, it is my estimation that the weight penalty is more than made up for by the increase in engine performance.  The main increase in packaged weight will be the weight of the scavenge blower and drive arrangement.

Advantage:  The 2-cycle engine will provide twice as many power events per revolution of the crankshaft.  A substantially smoother engine should result.

Disadvantage:  The uniflow engine is one of the most complicated types of internal combustion engine due to the need for direct injection and a scavenge blower.

Disadvantage:  The piston skirt must keep the intake air ports closed as the piston approaches TDC.  This requires a trunk-type piston which will weigh more than modern slipper piston.  The heavier pistons will require more crankshaft counterbalance and may reduce the top end speed of the engine.

Disadvantage:  A uniflow engine achieves highest volumetric efficiency at a scavenge ratio of about 1.2 to 1.4   By directly injecting fuel into the cylinder after trapping, over scavenging the engine does not substantially alter fuel economy, but direct injection increases the engine complexity.

Why Hasn’t it Been Done Before?

To answer that question let’s go back in time to before the second world war.  Before WWII, British motorcycles dominated the Isle of Man Tourist Trophy race, one of the oldest, most prestigious and certainly the toughest road race in the world.

On June 16th, 1939 however, a BMW factory bike ridden by Georg Meier ended the British domination when he won the Senior TT race after setting a new lap record on his supercharged BMW.  Jock West, another BMW factory rider placed second on his supercharged bike, a full 30 seconds ahead of the competition.  The BMW factory team was on a roll.  Meier had already won the 1938 German, Belgian and Italian GP races along with many domestic races.


A much older Georg Meier riding the Lap of Honor at the 1989 Isle of Man TT on a 1939 BMW Type 255 Kompressor, the same model he rode to victory in the 1939 Senior TT.  Photo Credit:  Wikipedia Commons

On September 1, 1939 the world changed forever as Germany invaded Poland, marking the start of WWII.  As the world scrambled to choose sides, motorcycle racing was forgotten and BMW went back to building airplanes.  It’s said that many of the race-winning Kompressor BMW’s were hidden in pig pens and barns during the war.  There is probably some truth to that, as some BMW race bikes survived the war and it’s insatiable appetite for scrap metal.

When the war ended in 1945, German racers were forbidden from competing in international competition by the FIM (Federation Internationale De Motocyclisme) and BMW was forbidden from building motorcycles.  As a final blow to the BMW Kompressor bikes, FIM banned supercharging entirely.  It’s a rule that still stands in the FIM MotoGP rules today:

2013 FIM Road Racing World Championship Grand Prix Regulations
2.4.3  Engines  Engine Description
1.  Engines may operate on the reciprocating piston  four stroke
principle only.
The normal section of each engine cylinder and piston in plan view
must be circular. Circular section cylinders & pistons are defined as
having less than 5% difference in the diameter measured at any two
2.  Engines must be normally aspirated

The BMW Kompressor motorycles, stripped of their mechanical superchargers, were no longer competitive.  To win a GP race meant BMW would have to completely re-design their engine from scratch.  This time, a naturally aspirated one.

Successful campaigning of a racing motorcycle is perhaps the most expensive and effective method of advertising a sport bike manufacturer has at his disposal.  Going fast and proving it against the competition has always caught the attention of those of us who live to go fast.

Because of the expense required to develop their technologies, manufacturers tend to design their production bikes around the technology they develop for their racing motorcycles.  As superchargers are forbidden from most, if not all national and international competition (FIM, AMA) very little has been done since the 1930’s on supercharged motorcycle development.  This was true even when two-cycle engines were still allowed in competiton.  As the uniflow engine requires a scavenge blower to operate it has been excluded from powering bikes entering the major racing classes since the end of WWII.


1 Comment

Filed under Inspiration, Moto History


  1. Riley

    I can not describe the admiration I hold for your efforts, truly inspiring. Having spent a good deal of time operating and maintaining heavy equipment in a marine environment, I am fascinated in your vision of the adaptation of proven technology’s using modern resources. This is a link I recently viewed that may prove useful to you as you proceed through your development efforts. Keep charging!


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