Radial Engine Vs Inline

Harley-Davidson: keepin’ it old school before the phrase “old school” existed.

Radial engines may seem like the remnants of a bygone era, but they’ve actually got quite a lot going for them. Lighter, more powerful and more durable than most in-lines, the radial engine would probably still be in common use today were it not for its aerodynamic drawbacks. But that shouldn’t stop you from putting a vintage radial into your car, because that would just be too awesome for words.

Basic Construction

While radial engines might look a bit strange to the modern eye, they’re actually a fairly logical evolution of the in-line. Image a four-cylinder in-line engine; now imagine cutting a vertical line all the way down through the middle-two cylinders on the engine block and cutting the block in half. Now, pull those halves to the side and lay the pieces flat on a table; you’ve almost got a horizontally opposed, “flat” or “boxer” four-cylinder. Cut those block pieces between the cylinders again so you’ve got four separate pieces of engine block; rotate two of them at a 90-degree angle to the others — making a “+” shaped when viewed from the front — and you’ve almost got a basic radial engine. To finish the change, you need all the block cylinders to lie on the same plane and all four rods to run on the same crankshaft journal. You can do this by connecting one rod — the “master rod” — to the crankshaft and connecting all of the other “articulating” rods to the base of the master rod around the rod journal.

Radial Engines

Radial engines may look a bit antiquated, but they’ve got a lot going for them. Radial engines — particularly two-stroke radial engines — are very light for the amount of power they put out and the displacement they’re capable of. The radial engine’s opposed-cylinder design and short, stubby crankshaft make it literally bulletproof; one of the radial’s best attributes is that it can operate even if one or more of the cylinders gets blown off by an anti-aircraft round. The radial’s shape exposes each cylinder to its own individual stream of cooling air, making it well-suited for air-cooled applications. All of these traits — the excellent power-to-weight ratio, durability and suitability for air-cooled operation — made the radial very popular among aircraft engineers and combat pilots. If radial engines have any serious dynamic drawbacks, it’s that they’re typically limited in upper rpm potential and are more prone to vibration than inline engines. To ward off vibration, most radials used an odd cylinder count to ensure that at no time were two pistons firing on the same side of the engine.

In-line Engines

In-line engines were for some time problematic for use in aircraft. The configuration didn’t lend itself well to air-cooling because the heat from one cylinder would saturate the cooling air before it got to the next. The required liquid-cooling system didn’t endear itself to pilots, who felt it just added more weight and gave enemy pilots one more thing to put a hole in. The fact that the smoother-running in-line could reach higher engine speeds probably had a lot less to do with its rise in popularity than its actual shape. In-line engines were about half as tall and one-third as wide as radial engines of the same displacement, which allowed engineers to build aircraft with smaller-diameter, aero-efficient fuselages. The in-line engine’s slightly higher weight and durability issues fell by the wayside when pilots realized these new, sleek fighters could reach speeds that no radial could dream of. Ultimately, the in-line fighter’s advantage in speed, and subsequently fuel efficiency, won out over its few faults.


You don’t have to look far to find an in-line engine of some sort — your driveway will likely suffice. The V-12 engine’s European “twin six” nickname should give you some idea of how in-line design affected later V-engine designs, and practically every economy car on Earth comes with some sort of in-line engine as its base powerplant. Modern piston-driven aircraft engineers still prefer the in-line not just for its aero-efficiency, but for the fact that after a century of development, it comes out as the more sophisticated and refined design. But don’t take that to mean radials are extinct — oh, no. If you want to see 25 percent of a radial in action, you need look no further than your local biker dive or Harley dealership: Harley Davidson’s classic V-twin engine is essentially just two cylinders hacked off of an eight-cylinder radial. The V-twin’s signature rumble is a result of its even cylinder count, which, you’ll recall, doesn’t work well with this engine design. This coincidence of origin should come as little surprise, since Harley rose to fame primarily by catering to returning World War I veterans who were already largely comfortable with the radial design.