Diesels were there first
Diesels (compression-ignition piston engines) are among the earliest players in internal combustion, yet they win at LeMans, the premier 24-hour auto race. They are effective in sizes from tiny car use — maybe 30 cubic inches — to the oceangoing container ships that boast bores and strokes of 1 by 1.5 meters. They power about half of Europe’s cars, most trains, virtually all the world’s heavy long-haul trucking and ocean freight shipping, and … hardly any airplanes. What’s up with that?
Rudolph Diesel’s first practical engine was built in 1897 and weighed 9,000 pounds; it produced 18 reliable horsepower. By 2008, the South Koreans built diesels that make 115,000 horsepower — at 88 rpm. (All the supergiant oceangoing diesel engines are today built in South Korea, by Hyundai or MAN.)
Diesels deliver horses
Efficiency, too, is a trademark, and comes with high reliability. Without spark ignition, entire systems of spark generation and management are eliminated, but diesels can be hard to start, typically requiring “glow plugs” until the engine runs on its own.
Boosted intake air and fuel injection are required; a carburetor can’t force the fuel/air mix into a pressurized cylinder. Sturdy components are required: pistons, cylinders, heads, connecting rods, and their crankshafts — all must be larger, heavier, and stronger than gasoline engines require.
High pressures generate a lot of heat. In today’s high-efficiency diesels, which operate near 50 percent efficiency (two-stroke diesels are a couple points higher; four-strokers are a couple points lower), the exhaust gas carries off about 54 percent of the wasted energy; the cooling system (air or water) takes care of nearly 40 percent; the oil cooler handles about 6 percent. This screams for exhaust-powered performance-enhancers, otherwise known as turbochargers.
Allan Lockheed, who consults on high-performance engine designs, notes, “It did not take too long before [piston-engine] designers realized that turbochargers were the real power-makers, so they developed better and better ways to drive the turbochargers, until they finally got rid of the piston engine altogether, and made the turboprop, which is basically a sophisticated, self-powered turbocharger with a gearbox.”
Still, diesel operation is intrinsically simpler than spark-ignition operation. Diesels need no controls, sensors, and circuits for making sparks, and there is no requirement for ignition electric power when running. A glow plug circuit for starting is not a big engineering challenge. A diesel’s power is controlled, not by a throttle on its air supply, but by the amount of fuel it receives. Therefore, a diesel with a functioning fuel-delivery system will not over-rev, a welcome “safety valve” in the case of, say, a runaway prop. The “fuel injection vs. carburetion” debate will not be resolved here, but the general advantage of fuel injection is well-accepted, and all diesels are fuel-injected.
Diesels, relying as they do on compression for ignition, necessarily run high compression ratios. The stronger components needed to run 20:1 and up all day long weigh more than their typical gasoline/spark-engine counterparts. Heavier crankshafts, rods, and pistons are expected. Not always anticipated are heavier valves and seats, valve springs, cylinders, heads, and engine cases. Also because the compression is higher and efficiency is greater, diesel power pulses are sharper and more powerful. These nastier pulses have fragged many an unsuspecting propeller and cracked unspecialized motor mounts, exhaust systems, and anything else that attaches to the engine, including starters, alternators, and intake components. The easiest response to all this destruction has been to add weight; but aero-designers at least are aware of the reduced usefulness added weight brings to an aircraft, and many novel approaches have been brought about to obviate the snappiness of diesel pulses.