Operating Difficulties
The foregoing abbreviated and paraphrased process gives the reader a good indication of what pilots/flight engineers from a bygone age had to contend with in order to safely transport passengers. With such a complex and idiosyncratic mechanical marvel, it was inevitable that operating difficulties would arise-and they did in spades. Like the skilled organ player alluded to above, the life expectancy of the R-2800 hinged on good operating practices. As with all large, high performance aircraft piston engines, it had its sensitivities. Even so, some problems could not be overcome even in the most skilled hands. Perhaps the most common difficulties experienced by commercial operators were spark plug fouling and top piston ring land failure. As we shall see they were both connected.
By the time the CB series of engines was being commercially operated it was what is known in industry as a "mature" product. Furthermore, it had been developed into a high performance engine and consequently shrinking margins of error. And yet commercial operators demanded ever increasing times between overhaul.
Starting out with a simple spark problem, a chain of events could lead to catastrophic engine damage. At wet takeoff power, a CB engine is producing 2400 horsepower at 2800 rpm. This means 23 power events per second. Although the flight deck or cockpit is well instrumented, these instruments cannot instantly react to the aforementioned 23 power strokes per second. For instance, the CHT has a relatively slow response to temperature change. At the beginning of a takeoff roll and full takeoff power is applied, it takes approximately 15 to 20 seconds for peak temperatures to be reached. Clearly, at maximum power and minimum speed cooling is inadequate. This results in hot spots occurring, particularly at the rear of the cylinder heads. This situation is aggravated if hot heat-range spark plugs are installed in the rear position. These hot spots now induce pre-ignition which quickly degenerates to the point of top piston land overheating, broken spark plug nose ceramics, and loss of BMEP. This chain of events typically originates in the upper cylinders and yet ironically, the only cylinders instrumented for CHT are #8 and #9, both of which are located in the cooler running lower section of the engine. By the time over-temps are observed for CHT serious damage has already wreaked havoc inside the engine and the hot running upper cylinders are already well into pre-ignition. One preventive measure that some operators found to be successful was the practice of installing a colder heat range plug in the rear positions. With hot plugs installed in the rear position, some degree of pre-ignition would occur at each takeoff. With this kind of thermal and mechanical distress, the top ring land is softened. After many takeoffs, the top ring land opens up and allows an excessive clearance to develop. After hundreds of takeoffs, the top ring land is opened up to the point where it no longer gives adequate support for the ring. Under normal operating conditions, rings rotate in their groove. With a damaged top ring land, the ring gap eventually coincides with the widest part of the ring groove-typically during a takeoff when temperatures are at the maximum and probably aided by pre-ignition. The now-softened ring land now offers minimal support and V2 inch to 1/8 inch of the top ring breaks off due to fatigue failure. As takeoff power is reduced to cruise power, the piston land cools off and the piece of broken piston ring is trapped in the groove. As the flight continues or on subsequent flights, the piece of broken piston ring flutters until it finally hammers its way out and escapes into the combustion chamber. As this piece of piston ring bounces around inside the combustion chamber, it eventually peens over the spark plugs. If the operator is in luck, the offending piece of piston ring will pass harmlessly through the exhaust. The damaged piston, particularly around the area of the broken land, acts as a glow plug that quickly deteriorates into complete piston failure by burning down through the lands. This classic piston failure invariably occurs at the rear of the piston adjacent to the exhaust port area.
The foregoing describes the most common manifestation of piston failure and pre-ignition along with spark plug failure, i.e., nose ceramics. By far the most common form of piston failure is by excessive temperatures. This can lead to other types of mechanical failure. When operated in pre-ignition mode. pressures inside the cylinder are two to three times that of normal operation. This puts additional load and stress on the valve train. Remember, the R-2800 opens its exhaust valve 70 degrees before bottom dead center. With an engine that is operating normally, a significant amount of residual pressure still resides inside the cylinder at the point of exhaust valve opening. Initial opening of the exhaust valve imposes a heavy load on the cam ring, pushrods, rocker arm, and valve. When this residual pressure is increased two or threefold, the resulting excessive loads on the valve gear lead to accelerated wear and eventually failure, usually a scuffed cam ring lobe. By "tricking" the engine with colder heat range plugs in the rear positions, many of these failures can be headed off by postponing the onset of pre-ignition. However, a ham fisted pilot/flight engineer can still cause distress to the engine if takeoff power is applied too suddenly. For several reasons, power applications on the R-2800 need to be added slowly: (1) temperatures have more time to stabilize without developing hot spots and (11) many gear trains inside the engine are heavily loaded, especially the blower drive gears which need to spin up a supercharger impeller 7 to 8 times crank speed. With a sudden acceleration of the engine these gear loads are magnified.
The Ethyl Corporation, the world's largest supplier of tetraethyl lead when it was commonly used, issued a report in the 1950s. They found that lead deposits on spark plug nose cores reacted with each other. As temperatures increased, new compounds were formed of higher melting points and higher insulating value. This in turn caused the core nose insulating value to stay above the critical value at which spark plug misfire occurred. If the temperature rise was rapid, i.e., our ham fisted pilot/flight engineer slammed the throttles forward, the deposit would lower in insulation value. The critical point, or worse, could occur allowing the classic copper flow scenario whereby the plug is shorted by bridging the gap. Of course, at this point the plug is rendered useless. Again, good operating procedures could have alleviated some of these plug maladies.
Ground Fouling
Ground fouling of the spark plugs is a function of idle mixture strength and the temperature of the plug electrodes and nose ceramics. Since the plug temperatures at idle are essentially the same as the CHT, it follows that ground fouling tendencies are the same for all plug heat ranges. In other words it make no difference whether the plug heat range rating is hot or cold. If the idle mixture is too rich, ground fouling will occur. The plug heat range rating is only valid at maximum or takeoff power. When ground fouling occurs, it is due to excessive carbon deposits on the plugs which then cause misfiring. This, however, can be rectified by burn-out procedures.
Problems of Oil Viscosity at Start-Up
At low temperatures aircraft oil congeals and takes on the consistency of molasses. Highly viscous oil can wreak havoc with an aircraft engine lubrication system by causing excessively high oil pres-sure and little or no oil flow. 0il flow may be reduced to the point where metal to metal contact can occur resulting in local seizure. Abnormally high oil pressures are generated, both on the pressure side and the scavenge side. This leads to blown seals, burst oil coolers, and burst oil lines. Two methods of controlling this damage are at the disposal of the designer. One is the controversial "oil dilution" system. As its name suggests, the oil is diluted via the oil scavenge line. The pilot/flight engineer has the option to inject raw gasoline into the oil system, while the engine is still hot, prior to shutting the engine down. Even on a frosty morning, the oil will be considerably less viscous and consequently cause less harm to the engine on start-up. As the engine warms up, the gasoline evaporates off. However, there is a serious downside to oil dilution. Gasoline acts as an excellent solvent, especially inside the bowels of an engine. Sludge is that nasty looking black stuff resulting from the by products of combustion. Its make-up includes carbon, acids, and water. After a number of hours have accumulated on an engine, sludge builds up, particularly in those areas exposed to centrifugal force such as inside crank pins and propeller domes. After oil dilution, special maintenance procedures are usually called for such as cleaning oil screens and filters to remove the sludge broken loose due to the dilution process.
Another method of increasing the rate of oil temperature warm-up is the stand pipe. A typical oil system for an R-2800 may contain up to 30 or more gallons. Rather than circulate all 30 gallons, it is advantageous to only circulate the amount of oil necessary to accomplish lubrication requirements. This may be as little as a couple of gallons. Built into the de-aerator is a stand pipe that only allows its contents to be circulated. As the engine warms up, a valve inside the oil tank opens and allows the full contents of the tank to be circulated. Another key role of the stand pipe is that of ensuring an oil supply in the event the propeller needs to be feathered. This is especially important if the engine suffers a catastrophic failure that result in the contents of the oil tank being dumped overboard.
Last edited by Brian Abraham; 24th Mar 2010 at 08:47.