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The Bourke Engine was an attempt to improve the two-stroke engine by Russell Bourke in the 1920s. Despite finishing his design and building several working engines, the onset of World War II, lack of test results, and the poor health of his wife 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 lightweight, powerful, has two power pulses per revolution, and does not need oil mixed into the fuel.

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 acceleration is perfectly sinusoidal. This causes the pistons to spend more time at top dead center than conventional engines. 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.

Design features

The following design features have been identified:

Mechanical features

• Scotch yoke instead of connecting rods to translate linear motion to rotary motion
• 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
• The piston is connected to the Scotch yoke through a slipper bearing (a type of hydrodynamic tilting-pad fluid bearing)
• Mechanical fuel injection.
• Ports rather than valves.
• 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 25% 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 in a smaller volume.

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
• 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.

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 dropfull 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, 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 witnessed by a third party, the actual fuel consumption was 1.1 hp/(lb/h), or 0.9 (lb/h)/hp, equivalent to a thermodynamic efficiency of about 12.5%, which is typical of a 1920s steam engine.
• Power to weight 0.9 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, or 0.1 hp/lb better than a Graupner G58 two-stroke. The lower claim is unremarkable, easily exceeded by production four-stroke engines, never mind two strokes.
• 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 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.

Engineering critique of the Bourke engine

This article's "criticism" or "controversy" section may compromise the article's neutrality. Please help rewrite or integrate negative information to other sections through discussion on the talk page. (May 2014)

The Bourke Engine has some interesting features, but the extravagant claims for its performance are unlikely to be borne out by real tests. Many of the claims are contradictory.

1. Seal friction from the seal between the air compressor chamber and the crankcase, against the connecting rod, will reduce the efficiency.
2. 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.
3. 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.
4. Each piston pair is highly imbalanced as the two pistons move in the same direction at the same time, unlike in a boxer engine. This will 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.
5. High speed two-stroke engines tend to be inefficient compared with four-strokes because some of the intake charge escapes unburnt with the exhaust.
6. Use of excess air will reduce the torque available for a given engine size.
7. Forcing the exhaust out rapidly through small ports will incur a further efficiency loss.
8. 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.
9. Emissions - although some tests have shown low emissions in some circumstances, these were not necessarily at full power. As the scavenge ratio (i.e. engine torque) is increased more HC and CO will be emitted.
10. Increased dwell time at TDC will allow more heat to be transferred to the cylinder walls, reducing the efficiency.
11. 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.
12. 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 cause nitrogen dioxide to be formed.

——————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————— During the 1980’s, this writer conducted a test of a 30 cubic inch Vaux engine, (a replica of the 30 cubic inch Bourke design), actually constructed using much of the original Bourke tooling. The goal of the test was to attempt to reproduce Bourke’s rather remarkable claims of 76 BHP at 10,000 rpm with stated brake specific fuel consumption values around .25 lbm/ bhp-hr. The project was undertaken with an optimistic approach to verifying the performance figures and to understand the thermodynamic processes that would allow these high specific power outputs to be obtained with high thermal efficiency.

The engine was tested on a water brake dynamometer after careful preparation and thorough break in. The test results were very disappointing, with power outputs and fuel consumption measurements being nowhere near what Bourke originally claimed. After many hours of careful adjustments and repeated tests, the best power recorded was 8.9 BHP at 4000 rpm where specific fuel consumption was a horrific 1.48 lbm/bhp-hr! The unloaded (free) speed was 5000 rpm maximum, a far cry from the stated figures. An induction airflow measurement revealed that the engine’s delivery ratio (volumetric efficiency) was only slightly better than 40% at 4000 rpm, dropping off sharply at higher engine speeds. It was obvious that this engine’s air handling characteristics were very deficient. My findings were as follows:

1) There was no indication of any unusual combustion phenomena occurring. Various low octane fuels were run and spark timing was advanced and mixture leaned in order to induce the “Bourke Cycle”. The engine responded like any other two-stroke – there was audible detonation with no increase in power output. 2) Best spark timing was around 35 degrees BTDC…any further advancing reduced power output. Leaning the mixture (via an adjustable main jet carburetor) past MBT (mean best torque) resulted in reduced power output. Using a 50:50 mixture of 87 octane unleaded gasoline and diesel fuel did increase the detonation intensity with no increase in power output or improved fuel consumption. The engine actually performed best on high octane gasoline. 3) The cool exhaust gasses were determined to be caused by fuel mixture short-circuiting out the exhaust ports during the scavenging process. The scotch yoke configuration allows the scavenge pump compression ratio to be very high – Bourke made use of this feature to obtain a ratio of approximately 3:1 (conventional two-strokes are around 1.3 to 1.5:1). When the transfer ports open, the pressure ratio was above the critical (Po/P = .528) thus choked flow occurred during the initial portion of the scavenge phase. The effect was that transferring fuel mixture short circuited right over the top of the piston deflector and out the open exhaust ports, without properly scavenging the cylinder of exhaust residual. Such a process could never result in the low fuel specifics and low exhaust emissions that Bourke claimed. 4) Vibration of this engine was indeed severe. Bourke 100% rotationally balanced the crankshaft without incorporating a reciprocating balance factor, thus all reciprocating forces were unresolved. Later, my engine’s counterweighting was altered to incorporate a 50% reciprocating balance factor. This made for somewhat tolerable vibration, but still not acceptable for a production machine. 5) Shearing of driveline components, which Bourke touted as high power potential, was actually high cyclic torsional inputs into the drive system. Bourke claimed that running a flywheel on his engine was detrimental to performance. I found that a flywheel was an absolute necessity to keep the dynamometer coupling in one piece. 6) Bourke states that the scotch yoke allows for a longer piston dwell time at TDC, thereby inducing the hydrogen/oxygen combustion process. He also states that the inherent straight-line motion allows for better resistance to failure when operating under detonation. As combustion was found not to differ from a more conventional engine, these claims are without merit. 7) The poor air delivery ratio was the result of both restrictive cylinder porting and lack of any induction or exhaust tuning. Cylinders were of the cross-scavenged type with drilled inlet, transfer and exhaust ports. The design was very similar to outboard motor technology of 1930’s vintage.

Heat energy liberated in the combustion of hydrocarbon fuels is determined by classic bomb calorimeter experiments. Here a known mass of fuel is burned in the presence of pure oxygen in excess. Virtually all of the fuel is consumed, the reaction goes to full completion, and results are therefore highly consistent and repeatable. Flame rates in the combustion bomb are the same as those encountered in engine cylinders when detonation occurs, this being due to combustion occurring in a pure oxygen environment. The conclusion is that there is no scientific possibility that more energy can be extracted from a given hydrocarbon fuel than that determined in the aforementioned experiment. As heat and work are mutually convertible quantities (Joule’s law), and knowing the fuel consumed during a given test, the engine performance can be predicted and checked against the dynamometer results. For this test, agreement was verified and indicated that approximately 9 BHP was all that was attainable.

Knowing the work out at the crankshaft, assuming realistic operational efficiencies and combustion being complete at TDC, and knowing the physical geometry of the engine, one can quite accurately calculate the temperature of the exhaust gasses when the exhaust ports open using a polytropic expansion relation. Had Bourke’s engine produced 76 horsepower at 10,000 rpm, exhaust gasses would be more like 1100 deg F, much higher than the claimed 200 deg. But as stated previously, the cool exhaust gasses were the result of poor scavenging, not from any magical combustion phenomena, and thus wrongly interpreted by Bourke.

As the engine’s delivery ratio was falling off sharply after 4000 rpm (and only 40% maximum), there is no possible way that it could ever turn up to 10,000 rpm. This effect, combined with poor charge trapping (due to the highly deficient scavenging), would never allow the high rpm, power outputs, or thermal efficiency to be obtained as Bourke stated. Air is the working fluid for any internal combustion engine – it is the properties of air expanding when heated that produces work on the pistons. If deficient air moving and trapping is occurring, as was evident in the tests, it would be impossible for the engine to ever approach the performance claims that the inventor made.

It is this writer’s opinion that the Bourke engine was a product of an overenthusiastic inventor who made unsubstantiated claims of high performance and fuel efficiency, mainly to draw attention to his invention in the hopes of acquiring developmental funding. Until validated test data can prove otherwise, this engine offers no revolutionary benefits and is not a valid contribution to internal combustion engine technology.

Patents

Russell Bourke obtained British and Canadian patents for the engine in 1939: GB514842 and CA381959

He also obtained an US Patent in 1939.

References

1. http://bourke-enginefiles.i8.com/146.htm
2. The Most Powerful Diesel Engine in the World Archived July 16, 2010, at the Wayback Machine
3. best two strokes
4. Paul Niquette. "The Bourke Engine". Niquette.com. Retrieved 2011-12-06.
5. GS Baker "Ship Form, Resistance, and Screw Propulsion" p215
6. "Bourke Engine Com". Bourke-engine.com. Retrieved 2011-12-06.
7. http://www.sportscardesigner.com/hp_per_lb.jpg
8. "Unbenannt-1" (PDF). Retrieved 2011-12-06.
9. "aircraft engine development". Pilotfriend.com. Retrieved 2011-12-06.
10. The Bourke Engine Project L.L.C. - Confirmed Test Results Archived September 28, 2007, at the Wayback Machine
11. Bourke Engine#Claimed and measured performance
12. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 pp240-245|Trade-off between efficiency, emissions and power
13. "Archived copy" (PDF). Archived from the original (PDF) on 2010-06-29. Retrieved 2007-12-16. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)CS1 maint: archived copy as title (link) |Friction of seals
14. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 p723|Pumping losses
15. C Feyette Taylor "The Internal Combustion Engine" 4th edition, p194 para 2-3, p205 fig 124b, p258|Pumping losses in two strokes
16. C Feyette Taylor "The Internal Combustion Engine" 4th edition, p119|stresses due to detonation
17. Engine balance#Single-cylinder engines Balance of single-cylinder engines
18. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 p20|Importance of primary balance
19. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 pp240-245, p881|Scavenging ratio and low efficiency
20. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 pp240-245|Scavenging ratio effect on torque output
21. C Feyette Taylor "The Internal Combustion Engine" 4th edition p194 para5|Pumping losses in two strokes
22. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 p452-3|Increased thermal losses due to detonation
23. JB Heywood "Internal Combustion Engine Fundamentals" ISBN 0-07-100499-8 pp240-245, p881|Scavenging ratio and high emissions
24. "Science Links Japan | Effect of Piston Speed around Top Dead Center on Thermal Efficiency". Sciencelinks.jp. 2009-03-18. Archived from the original on 2012-01-27. Retrieved 2011-12-06. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
25. Hot bulb engine
26. "Espacenet - Bibliographic data". Worldwide.espacenet.com. Retrieved 2013-01-21.
27. "Espacenet - Bibliographic data". Worldwide.espacenet.com. Retrieved 2013-01-21.
28. https://www.google.com/patents/US2172670

External links

• Running engine and Cad modeling

• Proposed engines
• Piston engines
• Engine technology
• Two-stroke engine technology