Duty Cycle
Duty cycle
TDC
Kevin Cameron
As WE HUMANS DO, ENGINES EITHER LIVE intensely for a short time, or plod along dully for a much longer period. Until new F-1 rules required engines to be capable of more than one hour of operation, such engines were 18,000-rpm one-use throw-aways. Top Fuel dragster engines make a 2-second burnout at 500-1000 hp, idle briskly through staging, and then make their joyous noise (reputedly up to 8000 hp!) for ¼ seconds. After that they get an intensive 90-minute overhaul that typically includes new pistons and rings, con-rods and rod-bearing inserts.
The other side of the duty-cycle coin is the old Continental flathead-Six once used in Checker Cabs, and first manufactured during Hammurabi’s reign. At least one is surely still idling at a cabstand on some lost backstreet in NYC or Philly, with 10 million miles on the h clock. Or those trusty roller-rod BMW 500 Twins of the kind that were always being ridden around the world by Rolexwearing adventurers. I’m sure a few are still out there, pausing on their global laps high in the Hindu Kush or on the banks of the great, gray, green, greasy Limpopo River to straighten crashed parts using only a forked tree and a stone. Their modest power makes it impossible to wear them out, so if the rider has the patience, they reach at least the same mileage as those Checkers.
Somewhere between such extremes were the great aircraft piston engines. When Strategic Air Command ran the 28-cylinder 4360 in B-36 bombers, they ran 350 hours before needing complete overhaul. When essentially the same engines powered Air Force Reserve transports, they lasted 600 hours, and when the profit-and-loss types in the airline biz got them, they ran 1200-2000 hours. Why the difference? It has to do with how much time the engines spent at high power and temperature. Hot metal is weak, and is more vulnerable to wear and chemical attack. Bad things happen faster when everything is hot. SAC was always on a mission, so their engines grunted and strained on full take-off power for long hot minutes. The Reservists got points for economy and reliability so they were more careful, using only the power actually required by specific circumstances. The airlines had to pay cash for all the parts they used, making them even more conservative.
Sometimes you can see an engine live out its life, reach its MTBO (Mean Time Before Overhaul) right before your eyes. I did at Loudon, New Hampshire, in the late 1960s. A hot 50-hp Triumph 500 Twin led the national roadrace, and then at about three-quarters race distance it began to smoke a little. Then it smoked a lot more, faded out of first, and presently clattered to a stop. The same has happened to many an aircraft engine, many an Isle of Man TT racer, many a generator or pump engine left by itself to carry too much load with too little oil.
The heat balance of a hard-working piston is a delicate thing. Combustion heat enters the dome of the piston and is conducted outward through the aluminum to the rings and skirt. Much of the piston’s heat passes through the rings because they are in the most intimate contact with the cylinder wall. The harder it is for heat to be conducted out of the piston and into the cylinder wall, the hotter the piston gets. There are two basic goals: 1) to keep the piston cool enough to lubricate and to retain the mechanical strength it needs; and 2) to keep the piston dome cool enough that it doesn’t overheat the incoming fresh mixture, pushing it toward detonation. This is easy on a low-duty cycle, and becomes harder as full throttle is used more often for longer periods.
This heat balance is especially hard to achieve in an air-cooled engine, but remains a serious problem for any hardworked engine. Just in the past five years more than one advanced, computer-controlled, liquid-cooled 600cc sportbike engine has fried its pistons because Daytona Supersport speeds put heat in faster than the cooling system could take it out. Cadillac recently reckoned that piston oil-cooling jets permit its supercharged Northstar V8 to tolerate extra spark advance sufficient to boost torque by 18 foot-pounds.
If the load does outrun the cooling system, piston temperature rises, perhaps enough to cause light detonation to begin. This can be a vicious circle, for although deto causes a fall in exhaustgas temperature, it causes a rise in engineparts temperature. Now the piston gets hotter yet, intensifying the detonation. The succession of sharper shocks hitting the heatsoftened piston may now be enough to forge the top piston ring land down against the compression ring, trapping it in its groove. Combustion gas, with detonation shouting behind it, now attacks the second ring land. With the loss of each ring’s seal, more and more oil reaches the combustion chamber, causing the light smoking I saw at Loudon. Another part of the vicious circle is that as piston rings stick, the piston loses its heat-transmitting, cooling effect, so its temperature rises further. Jets of leaking combustion gas boil oil off the cylinder wall, then blow it down into the crankcase and out the engine breather. This creates unlubricated regions that now scuff and score. Bad stuff is happening fast. It’s a race between two disagreeable outcomesoutright seizure or the flame-erosion of a gully down the side of the piston.
It is likely that in the future more and more engines will incorporate systems like Delphi’s detonation detector, used on many Harley-Davidsons. Once the ignition spark has passed across the plug electrodes, 800 volts DC are applied, generating a current proportional to the degree of ionization in the combustion gas. When the extra ionization generated by detonation appears, the engine control retards the ignition spark until the detonation ceases. Earlier deto detectors were microphones fastened to the cylinder head. Automatic detonation control is one more step toward the selfoptimizing and self-managing engine. □
