Aircraft engines are designed to satisfy several challenging requirements. They must be:
- light, because any engine "deadweight" just subtracts from payload
- small, to minimize drag (friction)
- powerful, so that the airplane can cruise and climb rapidly
- reliable, since "pulling over" while airborne is not a good option
- repairable, since wholesale replacement is very expensive
Unlike automobile engines those on aircraft run at high power settings for very long times. In general the engine is run at maximum power for a few minutes while taking off, at a slightly reduced power for climb, and then spends the majority of its time at a cruise setting—typically 65% to 75% of full power. In contrast a car engine might spend 20% of its time at 65% power while accelerating away from a green light, followed by 80% of its time at 20% power while cruising.
If a car engine fails you simply pull over to the side of the road. If the same occurs in an single-engine aircraft it will glide but, depending on the terrain and technique, may end in a crunch. For this reason aircraft engine designs tend to favor reliability over performance and even then it was many years before they had the reliability needed to fly over the Atlantic or the Pacific Ocean [].
Long duration at high power combined with the requirement for high reliability means that the engine must have large engine displacement to minimise over-stressing and be built very tough. Aircraft engines tend to be somewhere between 50% and 100% larger than a similarly powered modern car engine for a given power output. The engine also needs to be lifted into the air, powering its own deadweight. The power to weight ratio is one of the most important characteristics for an aircraft engine. A typical 250 hp engine weighs just 15% of the total when installed into a 3000 lb maximum weight light-aircraft.
They also tend to use the simplest parts and include two sets of anything needed for reliability, including ignition system (spark plugs and magnetos) and fuel pumps. Independence of function lessens the likelihood of a single malfunction causing an entire engine to fail. Thus magnetos are used because they do not rely on a battery. Two magnetos were originally installed so a pilot can switch off a faulty magneto and continue the flight on the other—but, later, dual ignition was found to offer some detonation protection too. Similarly, a mechanical engine-driven fuel pump is often backed-up by an electric one.
Two engines are more attractive from a reliability angle than a single one, regardless of the fact that the additional systems decrease the statistical reliability. Many twin-engined aircraft are designed to be capable of at least a marginal climb on one engine, even carrying the maximum load at take-off. But by doubling the number of engines, the chance of one failing is at least doubled. However, the chance of both failing at the same time becomes very small. Frequently, the greater economy and simplicity of a single-engine aircraft is deemed preferable to the added reliability of a twin, especially for non-commercial use.
Another difference between cars and aircraft is that the aircraft spend the vast majority of their time travelling at high speed. This allows aircraft engines to be air cooled, as opposed to requiring a radiator, which can lead to lower weight and complexity. Similarly, aircraft naturally operate at higher altitudes where significantly less air (oxygen) is available than at ground level. As engines need oxygen to burn fuel, an induction-assist mechanism—like a turbocharger or supercharger—is especially appropriate for aircraft use, though with the usual drawbacks of additional cost, weight and complexity.
At one time all engine designs were new and there was no particular difference in design between aircraft and automobile engines. This changed by the start of WWI, however, when a particular class of air-cooled rotary piston engines became popular. These had a short lifespan, but by the 1920s a large number of engine designs were moving to the similar radial engine design. This combined air-cooled simplicity with large displacements and they were among the most powerful small engines in the world.
Both the rotary and radial engine have one drawback however, they both have very large frontal areas (see drag equation). As planes increased in speed and demanded better streamlining, designers turned to water-cooled inline engines. Throughout WWII the two designs were generally similar in terms of power and overall performace but some mature-design radials tended to be more reliable. After the war, in the USA, the water-cooled designs rapidly disappeared.
For the smaller application, notably in general aviation, a hybrid design in the form an air-cooled inline, almost always 4 or 6 cylinders horizontally opposed, is most common. These combine small frontal area with air-cooled simplicity, although they required careful installation in order to be effectively cooled, notably the rearmost cylinders. To make repairs practical, each cylinder is individually replaceable, as are each of the accessories (pumps, generator and magnetos).
Throughout most of the history of aircraft-engine design engines tended to be more advanced than their automobile counterparts. High-strength aluminum alloys were used in these engines decades before they became common in cars. Likewise, those engines adopted fuel injection instead of carburetion quite early. Similarly, overhead valves.
Today the piston-engine aviation market is so small that there is essentially no commercial money for new design work. Almost every engine flying is based on a design from the 1960s, or before, using original materials, tooling and parts. Meanwhile the relentless financial power of the automobile industry has continued improvement A new car design is likely to use an engine designed in the last three years, build of alloys that did not exist more than five years ago and having ignition and other systems features that did not exist then either. Whereas modern car engines require no maintenance at all (other than adding fuel and oil) for over 100,000 km, aircraft engines are now, in comparison and paradoxically, rather heavy, dirty and unreliable.
Over the history of the development of aircraft engines, the Otto cycle, that is, conventional gasoline powered engines have been by far the most common type. That is not because they are the best but simply because they were there first and type-certification of new designs is difficult.
Another promising design for aircraft use was the Wankel engine. The Wankel engine produces less power for any given size of engine, about 2/3rds that of a conventional design, but does so for about 1/2 the weight and complexity. In this role the power to weight ratio is king, and the Wankel makes particularly good sense. Considerable thinking on such designs started in the post-war era, but at the same time the entire industry felt that jets, often in the form of turboprops, would power everything from the biggest to smallest designs. In the end little work was actually carried out, much to the chagrin of many.
The diesel engine is another engine design that has been examined for aviation use. In general diesel engines are more reliable and much better suited to running for long periods of time at medium power settings—this is why they are widely used in trucks for instance. Several attempts to produce diesel aircraft engines were made in the 1930s but, at the time, the alloys were not up to the task of handling the much higher compression ratios used in these designs. They generally had poor power-to-weight ratios and were uncommon for that reason. Improvements in diesel technology in automobiles—leading to much better power-weight ratios—the diesel's much better fuel efficiency (particularly compared to the inefficient old designs currently being used in light aircraft)—the high relative taxation of gasoline compared to diesel in Europe—have all seen a revival of interest in the concept. As of May 2004 one manufacturer, Centurion, is already selling certified diesel aircraft engines for light aircraft, and other companies have alternative designs under development. It remains to be seen whether these new designs will succeed in the marketplace but they potentially represent the biggest change in light aircraft engines in decades.