Bourke engine

The Bourke Engine was designed by Russell Bourke in the 1920s, as an improved two-stroke engine, based around harnessing the power of hydrogen-oxygen detonation instead of using the more progressive, slower carbon-oxygen burn normally found in Otto Cycle engines. Despite finishing his design and building several working engines, the onset of World War II, lack of test results,^[1] the poor health of his wife and investors moth-balling further developments in 1958^[2] compounded to prevent his engine from ever coming successfully to market. The main claimed virtues of the design are that it has only two moving parts, is light weight, powerful, has two power pulses per revolution, and does not need oil mixed into the fuel.

The only other type of Engine known to operate on similar hydrocarbon fuel chemistry as the Bourke Engine (high temperature shock wave fuel detonation instead of lower temperature progressive Carbon-Oxygen burn) is the Wave Disk Generator.

Contents 1 Overview 2 Operation 2.1 Mono-stroke Cycle 3 Design features 3.1 Mechanical features 3.2 Gas flow and thermodynamic features 3.3 Lubrication 4 Claimed and measured performance 5 Engineering critique of the Bourke engine 6 Design Controversy 7 Replication of Bourke Engines 8 References 9 External links

Overview

The Bourke engine is basically a two-stroke design, with one horizontally opposed piston assembly using two pistons that move in the same direction at the same time, so that their operations are 180 degrees out of phase. The pistons are connected to a Scotch Yoke mechanism in place of the more usual crankshaft mechanism, thus the piston motion is perfectly sinusoidal. This causes the pistons to spend a sufficiently^[3] longer time near top dead center than conventional engines so as to allow hydrogen-oxygen detonation - complete combustion - of the fuel to occur (as opposed to slower, progressive and incomplete burning whilst undergoing expansion, as is the case with a standard combustion engine).^[4] The incoming charge is compressed in a chamber under the pistons, as in a conventional crankcase-charged two-stroke engine. The connecting-rod seal prevents the fuel from contaminating the bottom-end lubricating oil.

Operation

The operating cycle is very similar to that of a current production spark ignition two-stroke with crankcase compression, with two modifications:

1. The fuel is injected directly into the air as it moves through the transfer port.
2. The engine is designed to run without using spark ignition once it is warmed up. This is known as auto-ignition or dieseling, and the air/fuel mixture starts to burn due to the high temperature of the compressed gas, and/or the presence of hot metal in the combustion chamber.

Mono-stroke Cycle

Bourke's work was within living memory of the design of the Otto Cycle engine: his documentation therefore uses the term "Mono-stroke", which is equivalent to the modern two-stroke. One crank revolution therefore has the following stages^[5][6] :

1. The crank bearing rolls across the yoke for a significant period (at the top of the sine wave), holding the piston for a prolonged period at or close to TDC, such that the gases burn very quickly at very high pressure and temperature until completely consumed and there is no longer a flame. Maximum pressure is developed. During this time at TDC, the piston has aligned with the intake ports in the crankcase and, due to the vacuum which was created, a fuel-air mixture is sucked quickly into the area underneath the piston.
2. As the crank turns, the yoke begins to move the piston: the intake port is no longer aligned and the fuel-air mixture on the other side of the piston is compressed. At the same time, on the other side of the piston, the gases (which were fully combusted due to high-explosive detonation) expand, pushing the piston and thus turning the crank.
3. When the piston reaches the other end of the chamber, the window in the piston lines up with the transfer ports, and at the same time the exhaust ports line up with the exhaust, and the compressed fuel escapes under pressure into the cylinder head, forcing the exhaust gases out the open exhaust ports. On its way through intake port, the air-fuel mixture passes a fin which causes it to become turbulent, thus fully mixing the air and fuel and causing cyclonic vortices.
4. At the same time, the opposing cylinder is beginning its power impulse cycle: expansion is occurring in the opposing cylinder, and compression in the original.
5. The original piston is now compressing the air-fuel mixture as it is no longer lined up with the transfer ports or the exhaust ports.
6. As the original piston compresses the air-fuel mixture, there is a vacuum created on the other side.
7. At around 90 degrees into crank travel before TDC, ignition occurs (of a low-grade fuel, taking considerable time to burn due to being a Carbon-Oxygen reaction) and compression continues
8. The air intake port lines up, either allowing in an air-fuel mix or air to mix with the injected fuel, beneath the piston.
9. On the other side (in the chamber), as compression continues the temperature increases, and fuel burns more rapidly (1800F). Sufficient force is created to stop the combined mass of the pistons, rods and the yoke. As TDC is reached, the fuel is completely burned and the pressure causes the piston to move.

Design features

The following design features have been identified:

1. The high compression ratio means that the temperature of the exhaust gases is considerably lower.
2. Use of lower-grade fuels and ignition pre-TDC results in an initial slow burn that becomes a highly explosive combustion (detonation) at top dead center. The fuel chemistry is therefore entirely different from a standard Otto Cycle Engine.
3. An arrangement of 4 cylinders (two on each crank) with each pair firing in opposite directions results in a completely mechanically-balanced engine.
4. The scotch yoke results in a pure sine wave, end-result being that compression at TDC is longer than on a normal piston with a camshaft, as can be seen from graphs comparing Scotch yoke with standard piston design.
5. There are no valves, only ports, reducing complexity and maintenance.
6. Piston blow-by does not go into the crankcase, because the chamber under the piston is also ring-sealed and is used to store (lower-pressure) incoming charges. Piston blow-by therefore gets recirculated and mixed with incoming charges.^[7]
7. The triple sleeve bearing on the crankshaft is counterbalanced with a counterweight, resulting in no vibration of the crankshaft.^[8]
8. Exhaust ports are deliberately similar sized as the inlet ports; the intake air-fuel charge deliberately smaller than the piston chamber's size at BDC; a small amount of completely burned exhaust (comprising water vapour and carbon dioxide) is deliberately left in the piston chamber; these factors combine so that there is a residual amount of water mixed with the air-fuel mixture to catalyse the detonation at TDC.^[9]

Mechanical features

• Scotch yoke instead of connecting rods to translate linear motion to rotary motion: Piston motion equations for standard crank and slider mechanisms show reduced dwell time at top dead center.
• Fewer moving parts (only 2 moving assemblies per opposed cylinder pair) and the opposed cylinders are combinable to make 2, 4, 6, 8, 10, 12 or any even number of cylinders
• Smoother operation due to elimination of crank and slider mechanism: Scotch yoke motion is sinusoidal.
• The piston is connected to the Scotch yoke through a triple-sleeved slipper bearing (a type of hydrodynamic tilting-pad fluid bearing)
• Mechanical fuel injection, using vacuum in the chamber under the piston to suck air-fuel mixtures in.
• Ports rather than valves.
• There are effectively three separate main chambers: main piston chamber, underside of the piston, and crank-case chamber (shared between cylinder pairs).
• Use of transfer channels that line up with the ports, to arrange for the air-fuel mixture to enter the piston chamber under pressure.
• Easy maintenance (top overhauling) with simple tools.
• The Scotch yoke does not create lateral forces on the piston, reducing friction, vibration and piston wear.
• O-rings are used to seal joints rather than gaskets.
• The use of the Scotch Yoke reduces vibration from the motions of the connecting rod—for example, the peak acceleration in a Scotch yoke is less than the acceleration in a conventional crank and slider arrangement. The piston movement and therefore vibration is sinusoidal so the engine could theoretically be perfectly counterbalanced, unlike a conventional engine which has harmonics in the piston movement courtesy of the lateral movement of the crankpin.
• The Scotch Yoke makes the pistons dwell very slightly longer at top dead center, so the fuel burns more completely, much faster, at higher temperatures, in a smaller volume. In combination with increased air intake, this results in completely different chemical reaction: a hydrogen-oxygen combustion (which has a flame speed of 10 ft/sec ^[10]as opposed to 25 to 75 ft/sec for the carbon-oxygen combustion that is normally seen in Otto Cycle Engines).

Gas flow and thermodynamic features

• Low exhaust temperature (below that of boiling water) so metal exhaust components are not required, plastic ones can be used if strength is not required from exhaust system
• Combustion is initiated at 90 degrees in advance of top dead centre, taking advantage of the low flame speed of lower-temperature carbon-oxygen progressive burn times (25 to 75 ft per second) to allow the crank and scotch yoke to pressurise the burning gases and bring them up to detonation temperatures (1800 F).
• Extremely fast hydrogen detonation burn time (5000 ft per second) of the lean mixture so the engine can be considered to be a hydrogen detonation (i.e., explosion not deflagration) engine. If the mixture is not lean enough, detonation will not occur and the Bourke Engine will not operate correctly or efficiency.
• 15:1 to 24:1 compression ratio for high efficiency and it can be easily changed as required by different fuels and operation requirements.
• Fuel is vaporised when it is injected into the transfer ports, and the turbulence in the intake manifolds and the piston shape above the rings stratifies the fuel air mixture into the combustion chamber.
• Lean burn for increased efficiency and reduced emissions: the high oxygen content results in no Carbon Monoxide emissions.

Lubrication

• This design uses oil seals to prevent the pollution from the combustion chamber (created by piston ring blow-by in four-strokes and just combustion in two-strokes) from polluting the crankcase oil, extending the life of the oil as it is used slowly for keeping the rings full of oil to hold and use to lubricate. Oil was shown to be used slowly by the dropful as needed, but checking the quantity and cleanness of it was still recommended by Russell Bourke, its creator.
• The lubricating oil in the base is protected from combustion chamber pollution by an oil seal over the connecting rod.
• The piston rings are supplied with oil from a small supply hole in the cylinder wall at bottom dead center.

Claimed and measured performance

• Efficiency 0.25 (lb/h)/hp is claimed - about the same as the best diesel engine,^[11] or roughly twice as efficient as the best two strokes. This is equivalent to a thermodynamic efficiency of 55.4%, which is an exceedingly high figure for a small internal combustion engine. In a test (carried out after Bourke's death, by enthusiasts insufficiently familiar with Bourke Engine Chemistry) witnessed by a third party, the actual fuel consumption was 1.1 hp/(lb/h),^[12] or 0.9 (lb/h)/hp, equivalent to a thermodynamic efficiency of about 12.5%, which is typical of a 1920s steam engine.^[13] (Note: Bourke points out in the Documentary that if such poor efficiency is achieved, the engine setup has been mis-configured)

• Power to weight 0.9^[14] to 2.5 hp/lb is claimed, although no independently witnessed test to support this has been documented. The upper range of this is roughly twice as good as the best four-stroke production engine shown here,^[15] or 0.1 hp/lb better than a Graupner G58 two-stroke.^[16] The lower claim is unremarkable, easily exceeded by production four-stroke engines, never mind two strokes.^[17]

• Emissions Achieved virtually no hydrocarbons (80 ppm) or carbon monoxide (less than 10 ppm) in published test results, however no power output was given for these results, and NOx was not measured.

• Low Emissions As its design is based around Hydrogen detonation, the engine is claimed to be able to operate on hydrogen or any hydro-carbon fuel without any modifications, producing only water vapor and carbon dioxide as emissions. However, Bourke points out that the spark plugs required for ignition would need to be made of Platinum in order to generate temperatures exceeding 2,000F, and that atmospheric air instead of pure oxygen would be recommended, to avoid severe damage or explosion of the engine. He also points out that, due to the extreme temperatures and flame speeds, a Bourke Engine should be operated at very high RPM in order to reduce the amount of time that the chamber is exposed to the intense heat of the combustion.^[18]

Engineering critique of the Bourke engine

The Bourke Engine has some interesting features, but many of the claims are contradictory to those familiar with Otto Cycle Engines, making it difficult for people familiar only with Otto Cycle Engines to accept the efficiency, emissions and power claims.^[19] When reading the critiques below it has to be pointed out that Bourke himself observed many people attempting not to duplicate his work, failing to first fully understand the chemistry behind the design decisions made, but to tamper detrimentally with the design, usually by applying lessons learned from Otto Cycle Engine development which is based around a completely different chemical combustion process.^[20]

• Seal friction from the seal between the air compressor chamber and the crankcase, against the connecting rod, will reduce the efficiency.^[21]
• Efficiency will be reduced due to pumping losses, as the air charge is compressed and expanded twice but energy is only extracted for power in one of the expansions per piston stroke.^[22][23]
  • Note: this claim does not take into account the higher amount of energy extracted from complete combustion (detonation) of the fuel.
• Engine weight is likely to be high because it will have to be very strongly built to cope with the high peak pressures seen as a result of the rapid high temperature combustion.^[24]
  • Note: what is actually done is that far less fuel (and a leaner mixture) is used than in an Otto Cycle Engine, thus remaining within engineering tolerances yet extracting more power. If however the same amount of fuel was used that would normally be inserted into an Otto Cycle Engine of the exact same cylinder size, the resultant Hydrogen-Oxygen-based detonation in a Bourke Engine would clearly (as pointed out) require much stronger materials.
• Each piston pair is highly imbalanced as the two pistons move in the same direction at the same time, unlike in a boxer engine.^[25] It is claimed that this would limit the speed range and hence the power of the engine, and increase its weight due to the strong construction necessary to react the high forces in the components.^[26]
  • Note: in 1953, Bourke demonstrated a 30 cubic inch Engine running at 15,000 RPM to Boeing Engineers (5 years later in 1958, Boeing announced the invention and use in its Jet Engines of the same triple-sleeved bearing invented by Bourke),^[27] Bourke also occasionally tested the engine up to 20,000 rpm (a limit imposed by the spark plugs!)^[28]
• High speed two-stroke engines tend to be inefficient compared with four-strokes because some of the intake charge escapes unburnt with the exhaust.^[29]
  • Note: Bourke points out that Otto Cycle engines are based around lower temperature (Carbon-Oxygen) progressive burn of fuel, which occurs during the expansion phase (pressure drop and temperature drop): thus of course the intake charge will escape unburnt from the exhaust of an Otto Cycle engine. The Bourke Engine is based around much higher temperatures (above 1800F), much higher pressures and much higher air-fuel mixture ratios, resulting in Hydrogen-Oxygen "detonation". In a correctly-operating Bourke Engine, there is no unburnt fuel escaping with the exhaust.^[30]
• When the charge is transferred from the compressor chamber to the combustion chamber it will cool down, reducing the efficiency of the engine.^[31]
  • Note: this claim has not been properly substantiated or evaluated. The fact that the charge was drawn into the compressor chamber under vacuum (thus reversing adiabatic expansion effects) has probably not been taken into consideration by the original evaluator.
• Use of excess air will reduce the torque available for a given engine size.^[32]
  • the Bourke Engine uses detonation (not progressive burning as in an Otto Cycle Engine) so the chemistry is completely different, being based on Hydrogen-Oxygen reaction, not a Carbon-Oxygen reaction.^[33]
• Forcing the exhaust out rapidly through small ports will incur a further (significant) efficiency loss.^[34]
  • Note: Bourke points out that this loss would only significant if the gases were still burning (as would be the case with an Otto Cycle Engine, due to the lower flame speeds of Carbon-Oxygen combustion: 25 to 75 ft per second). The Bourke Engine's chemistry is completely different: combustion is entirely completed at TDC, with flame speeds of 5000 ft per second.^[35] Additionally, the design relies on catalytic effects of small amounts of water vapour being left in the piston chamber from the previous cycle, to catalyse the burning of the Hydrogen and Oxygen from the air-fuel mixture at lower temperatures: if the exhaust gases were completely expelled, this would not occur and also there would be a risk that the air-fuel mixture would exit the exhaust port.^[36]
• Operating an internal combustion engine in detonation reduces efficiency due to heat lost from the combustion gases being scrubbed against the combustion chamber walls by the shock waves.^[37]
  • Note: Bourke points out that the combustion gases are fully burned even before the piston begins the majority of its exhaust stroke, and that the amount of energy obtained from Hydrogen-Oxygen detonation produces approximately four times more energy per unit volume than that of the slower, progressive Carbon-Oxygen burn of Otto Cycle Engines.^[38][39]
• Emissions: although some tests have shown low emissions in some circumstances, these were not necessarily at full power. It is claimed that as the scavenge ratio (i.e. engine torque) is increased more HC and CO will be emitted.^[40]
  • Note: again, these claims again do not take into account the lean mixture (much more oxygen) which is so in excess in a Bourke Engine that it results in zero CO emissions, nor do these claims take into account the much higher temperatures (over 1800F) that result in Hydrogen-Oxygen "detonation" in a Bourke Engine. These claims would however be correct in the case of an Otto Cycle Engine because the temperatures in Otto Cycle Engines are far lower.
• Increased dwell time at TDC will allow more heat to be transferred to the cylinder walls, reducing the efficiency.^[41]
  • Note: this claim does not take into account the ultra-high flame speed (5000 ft per second) of Hydrogen-Oxygen combustion: burning of fuel is entirely completed at TDC, leaving only Carbon Dioxide and superheated water as the exhaust gases, which rapidly expand and drop in temperature as the piston moves away from TDC. The cylinder head has no contact with any moving part, and can therefore be air-cooled for greater thermal efficiency.^[42]
• When running in auto-ignition mode the timing of the start of the burn is controlled by the operating state of the engine, rather than directly as in a spark ignition or diesel engine. As such it may be possible to optimize it for one operating condition, but not for the wide range of torques and speeds that an engine typically sees. The result will be reduced efficiency and higher emissions.^[43]
  • Note: the power stroke of a 2-stroke engine is only approximately 45 degrees, whereas a Bourke Engine's power stroke is 135 degrees.^[44] Yet despite this, Bourke himself was fully aware that engine designs can - and should - be adapted and set up to run under optimal conditions: an engine that is configured for high torque at low RPM is not then suitable for use over a wide RPM range, and vice-versa: the Bourke Engine is no different in this regard, and Bourke recommended adjusting the length of the piston rods to suit the usage required.^[45]
• If the efficiency is high, then combustion temperatures must be high, as required by the Carnot cycle, and the air fuel mixture must be lean. High combustion temperatures and lean mixtures in Otto Cycle engines cause nitrogen dioxide to be formed. It is thus claimed, without proper testing being carried out, that the Bourke engine (which operates on the basis of extremely rapid Hydrogen-Oxygen Detonation in excess of 1800F) must suffer from the same flaw.

Design Controversy

Bourke spent 14 years of study - 1918 to 1932 ^[46] - prior to creating the first prototype.^[47] In 1933 he showed a set of blueprints to the Professor of Engineering at Berkeley University, California, and, on pointing out that a working engine was in the back of his truck, received a curt response, "There is no use wasting your time and mine, Mr Bourke - this engine cannot possibly run. Good day.".^[48] That first experience of an adverse reaction to the design became an inexplicable trend throughout Bourke's life.

As the section in this article shows on the Engineering Critique, the design is sufficiently different from an Otto Cycle, and its design parts critically inter-dependent as well as taking advantage of hydrocarbon chemistry (detonation) which the modern designs of Otto Cycle Engines go to extreme lengths to avoid, as well as Bourke's understanding of chemistry being based on scientific texts that are in a completely different context from modern chemistry, makes it very challenging for people to accept what Bourke discovered.

Replication of Bourke Engines

Bourke trusted some investors with the future of his engines: in April 1958 they took control of the Corporation, moved the workshop and commercially-developed engines being assembled to a secret location and terminated all development.^[49] Decades later, these mothballed engines found their way into the hands of Roger Richards.^[50] Between 1958 and 1968, many people contacted Russ Bourke, attempting to replicate the engine: almost all of them failed, by virtue of attempting to make modifications to the design before fully understanding it.^[51] From having access to a number of original engines that were still operational after 40 years, Roger Richards is one of the very few people to have not only succeeded in replicating the Bourke Engine but also in having made some incremental improvements of the earlier designs, based on improvements learned by Bourke in his later work.^[52]

From bitter experience, Richards patiently explains why it is so critical that, prior to replication of a Bourke Engine,^[53] it is so absolutely essential to understand that the chemistry involved is very different,^[54] and also that every part of the engine's design is critically inter-related.^[55]

There is however evidence of other people making successful replication and incremental improvements on the Bourke Engine design. Daniel M. Reitz is the registered holder of an expired patent, granted in 1975,^[56] that allows the piston rod some lateral play, to overcome issues associated with the scotch yoke when the engine is run under design conditions different from those envisaged by Bourke himself.^[57]

References

Note: The Bourke Engine Documentary, referenced above, was not published with an ISBN number.

Patents (expired) include:

• http://www.freepatentsonline.com/2172670.html
• http://www.freepatentsonline.com/2172677.html
• http://www.freepatentsonline.com/2172676.html

External links

• Running engine and Cad modeling

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