Mark75H
04-02-2004, 07:17 PM
If you have the books written by Peter Hunn you have come across several mentions of a motor built back in the 1950's by a man named Russell Bourke.
There has always been a great deal of mystery regarding these engines.
After a bit of research I've come to a few conclusions about them.
There seem to have been only a half dozen or fewer prototypes, mostly bigger than would have been suitable for conversion for outboard use ... more in the size for light aircraft use.
Mr. Bourke's testing was not very scientific. For his horsepower claims his dyno was simply a motor set up on a test stand with an aircraft propellor. This means was probably not too bad, but after looking at the available photographs and descriptions I am guessing that his estimates may have been off because he might have ran the test motor indoors rather than out where the prop would have been more heavily (and correctly) loaded. Addtionally Bourke ran the prop faster than the prop manufacturer had plotted the horsepower curves.
His tests of his engines as an outboard were even less scientific: he bought the biggest and strongest racing lower unit available at that time a 1930's OMC 4-60 racing lower unit.
The wet trials ended abruptly when the lower unit scattered after just a few seconds of super high speed operation. This should have been easily predictable since the standard motors the lower unit was being used on barely turned 7,000 where his motors easily turned 10,000 or 12,000 rpm. With loads on bearings increasing by a squared factor with rpm any trained engineer would have predicted the lower unit's failure knowing that it was near its limit under normal torque and rpm loads.
Let me go over the rpm load thing a little: Let's suppose the 4-60 made 60 foot pounds of torque at 5,250 rpm. By the definition of horsepower it would have made 60 hp as well. If a racer bumped up the power of his 4-60 with some nitromethane and got 66 foot pounds of torque the load on his bearings would have increased by 10%. If by some magic he doubled his torque the horsepower would be doubled to 120 hp and the load on the bearings would be doubled as well. On the other hand if Mr. Bourke's motor made the same 120 hp by producing 60 foot pounds of torque at 10,500 rpm the loads on the bearings would increased 60 times as if the torque was increased to 3600 foot pounds.
How was the Bourke engine capable of such high rpm?
It had very unusual construction. Some of the features were unique and followed ideas two stroke manufacturers and hot rodders had been trying to work out for years before and since.
The crankshaft was not an ordinary crankshaft as most engines use. Instead it used a kind of slotted link or "Scotch Yoke" mechanism.
http://www.bourke-engine.com/images/altbrk.GIF
From the image you can see that the Bourke does not use crankcase scavenging, but instead uses a cavity below the piston that the piston nearly fully scavenges (minimal crankcase volume, very high pumping efficeincy).
Another thing that pushed the Bourke engine rpm up was that it was basically a very large mechanical ocillator. One piston fired and pushed the other directly linked piston from BDC to TDC.
Drawbacks I see are the wear at the shaft & seal going into the crankcase and oil consumption from the crankcase. For any kind of range a Bourke would have to have a fuel tank and an oil resupply tank.
There has always been a great deal of mystery regarding these engines.
After a bit of research I've come to a few conclusions about them.
There seem to have been only a half dozen or fewer prototypes, mostly bigger than would have been suitable for conversion for outboard use ... more in the size for light aircraft use.
Mr. Bourke's testing was not very scientific. For his horsepower claims his dyno was simply a motor set up on a test stand with an aircraft propellor. This means was probably not too bad, but after looking at the available photographs and descriptions I am guessing that his estimates may have been off because he might have ran the test motor indoors rather than out where the prop would have been more heavily (and correctly) loaded. Addtionally Bourke ran the prop faster than the prop manufacturer had plotted the horsepower curves.
His tests of his engines as an outboard were even less scientific: he bought the biggest and strongest racing lower unit available at that time a 1930's OMC 4-60 racing lower unit.
The wet trials ended abruptly when the lower unit scattered after just a few seconds of super high speed operation. This should have been easily predictable since the standard motors the lower unit was being used on barely turned 7,000 where his motors easily turned 10,000 or 12,000 rpm. With loads on bearings increasing by a squared factor with rpm any trained engineer would have predicted the lower unit's failure knowing that it was near its limit under normal torque and rpm loads.
Let me go over the rpm load thing a little: Let's suppose the 4-60 made 60 foot pounds of torque at 5,250 rpm. By the definition of horsepower it would have made 60 hp as well. If a racer bumped up the power of his 4-60 with some nitromethane and got 66 foot pounds of torque the load on his bearings would have increased by 10%. If by some magic he doubled his torque the horsepower would be doubled to 120 hp and the load on the bearings would be doubled as well. On the other hand if Mr. Bourke's motor made the same 120 hp by producing 60 foot pounds of torque at 10,500 rpm the loads on the bearings would increased 60 times as if the torque was increased to 3600 foot pounds.
How was the Bourke engine capable of such high rpm?
It had very unusual construction. Some of the features were unique and followed ideas two stroke manufacturers and hot rodders had been trying to work out for years before and since.
The crankshaft was not an ordinary crankshaft as most engines use. Instead it used a kind of slotted link or "Scotch Yoke" mechanism.
http://www.bourke-engine.com/images/altbrk.GIF
From the image you can see that the Bourke does not use crankcase scavenging, but instead uses a cavity below the piston that the piston nearly fully scavenges (minimal crankcase volume, very high pumping efficeincy).
Another thing that pushed the Bourke engine rpm up was that it was basically a very large mechanical ocillator. One piston fired and pushed the other directly linked piston from BDC to TDC.
Drawbacks I see are the wear at the shaft & seal going into the crankcase and oil consumption from the crankcase. For any kind of range a Bourke would have to have a fuel tank and an oil resupply tank.