Why Do Car Engines Have An Even Number Of Cylinders, While Giant Ship Engines Happily Run Odd?

Car engines favor even cylinder counts because a four-stroke cylinder pushes only once every two turns of the crankshaft, and pairing cylinders up is the cheap way to make their shaking cancel and to fill both banks of a V. A two-stroke ship engine pushes every cylinder once per turn, so any count from 5 to 12 fires in a perfectly even rhythm, and its enormous mass, crankshaft counterweights and dampers soak up what is left. Even firing and balance are two different things: an inline three-cylinder fires perfectly evenly yet rocks end to end, while a single-row radial aircraft engine must have an odd number of cylinders to fire evenly at all.

Picture the engine in your driveway. Four cylinders in a hatchback, six in a pickup, eight in anything that rumbles at a stoplight. Ask any car person and you get the same tidy rule. Even is smooth. Odd is buzzy.

Now walk into the engine room of a container ship. The engine there is the size of a house. Often it is a seven-cylinder. Or a five. Or an eleven. Nobody on board finds this strange. The maker sells it that way, in a catalog, with a straight face.

Then there are the radial engines on World War II fighters, with cylinders in a circle. Those were never even. Five, seven, nine, always odd, by design. So the tidy rule has a problem. Two of the most successful engine families ever built ignore it. To see why, start with what an engine is doing in there.

What Does A "Stroke" Mean, And Why Does A Four-Stroke Engine Need Two Turns?

Push a bicycle pump down and let it spring back. That trip is what engineers call a stroke. A piston makes the same trip inside a sealed tube called a cylinder. Every trip turns a crank, the way your legs turn the pedals.

Almost every car engine runs a four-stroke cycle. Trip one: the piston slides down and sucks in air and fuel. Trip two: it rises and squeezes the mix. Trip three: a spark lights it, and the burning gas shoves the piston down hard. Trip four: the piston rises and pushes the spent gas out. Britannica's summary ends with the line that matters. Four strokes, "and two revolutions of the crankshaft."

Read that again, because the whole article hangs on it. Two turns of the crank. Only one of the four trips produces any push. The other three coast on momentum stored in a heavy flywheel. Engineers measure crank position in degrees, like a clock face. One turn is 360 degrees. A full four-stroke cycle is 720 degrees.

Hold on to this: in a four-stroke engine, each cylinder pushes once every 720 degrees. That one number decides who gets to be odd.

What Is A Firing Interval, And Why Do Engineers Want It Even?

Now put several cylinders on one crank. Each pushes once per 720 degrees. The obvious move is to spread those pushes out, so the crank never waits long for the next shove. The gap between pushes is the firing interval. The arithmetic is one line:

firing interval = 720° ÷ number of cylinders

Four cylinders: 720 ÷ 4 = 180 degrees, one push every half turn. Six: 720 ÷ 6 = 120 degrees. Eight: 720 ÷ 8 = 90 degrees. Britannica's engine entry puts a six-cylinder crank's throws 120° apart. The cylinders then "fire at equal intervals in two full rotations of the shaft."

An even interval gives a smooth flow of power, which is why designers chase it. The classic six-cylinder firing order is 1-5-3-6-2-4. Britannica says it is chosen "to minimize vibration."

Notice what the formula does not say. It does not say the count has to be even. Try three: 720 ÷ 3 = 240 degrees, a dead even beat. Try five: 720 ÷ 5 = 144 degrees. Also even. Any whole number works. So where does "even is smoother" come from?

Is Even Firing The Same As A Balanced Engine?

No. This is the confusion the whole myth rests on. Even firing is about when the pushes arrive. Balance is about the shaking. A piston is a lump of metal that stops dead at the top of its trip, reverses, and stops dead at the bottom. Each stop yanks on the engine. Unless something yanks back at the same instant, the block shakes.

The cheapest way to cancel a yank is with a twin. One piston slams to a stop at the top while another slams to a stop at the bottom. The two tugs fight each other, and the block hardly notices. That needs pistons in pairs, and pairs need an even count. Britannica states it as a rule for the standard crankshaft layout. "There must be an even number of cylinders," so that there are "pairs of cylinders whose pistons move in unison."

The inline three shows what happens without a twin. Its crank pins sit 120 degrees apart, so it fires every 240 degrees. Dead even. But the end cylinders never move as a pair. A study of three-cylinder balance for the Society of Automotive Engineers spells it out. Cylinders 1 and 3 "cause a primary moment about the No.2 cylinder." In plain words, the block rocks end to end, like a seesaw. Even firing, and still a shaker.

So an engine can fire evenly and shake, or fire evenly and sit still. The cylinder count decides the second part, not the first.

Why Do Car Engines Have An Even Number Of Cylinders?

Put the two ideas together and the car's habit stops looking like a law. It starts looking like a bargain. An even count gives you pairs, and pairs cancel the biggest shakes for free. A four-cylinder cancels its main up-and-down shake this way. A smaller wobble survives, twice per turn, because the rising pistons and the falling ones are not perfect mirror images. That wobble was tamed in 1912. The British engineer Frederick Lanchester patented a fix: paired weights spinning at "twice the periodicity" of the crank. It is the ancestor of the balance shaft in a modern four.

Then there is shape. Six or eight cylinders in a line make an engine as long as a sofa. Car makers fold the row into a V instead, two banks sharing one crank. Britannica notes the banks sit 60° or 90° apart, with V-8s at 90°. Two matched banks means an even total, every time. Nobody builds a V-7.

An odd inline count is not forbidden. It just costs extra. The FAA handbook says an inline engine "generally has an even number of cylinders," though some threes have been built. The three needs its own balance shaft to stop the seesaw. The car's rule is: even is cheaper to make smooth.

Why Can A Two-Stroke Ship Engine Have Any Number Of Cylinders?

So far, so reasonable. This is where the ship walks in and changes the rules.

The giant engines in container ships are not four-strokes. They run a two-stroke cycle. Britannica's description is blunt. Keep the compression and power strokes, drop intake and exhaust, and the cycle needs "only one revolution of the crankshaft." The two housekeeping trips vanish. Every cylinder pushes on every turn.

That changes the arithmetic in one stroke, so to speak:

two-stroke firing interval = 360° ÷ number of cylinders

Seven cylinders: 360 ÷ 7 = 51.4 degrees. Five: 360 ÷ 5 = 72 degrees. Nine: 40 degrees. Every one of them is dead even. The cylinder count never made firing even. The stroke count did.

This is why a marine engine catalog reads like a hardware store. MAN Energy Solutions' marine engine program lists its biggest two-stroke, the G95ME-C10.6. It comes in every count from 5 to 12 cylinders. The five makes 34,350 kW (about 46,000 hp). The twelve makes 82,440 kW (about 110,000 hp). Need more power? Bolt on another cylinder. Seven is not a compromise. It is one more than six.

How Do Giant Ship Engines Deal With The Shaking?

Even firing does not make the shaking vanish. So what does the ship do about a seven-cylinder's leftover wobble? Mostly, it ignores it, because it can.

Start with the scale. One cylinder of WinGD's X92 engine has a bore of 920 mm (36 in) and a stroke of 3,468 mm (11.4 ft). The installation manual rates it at 6,450 kW per cylinder at 80 rpm. The seven-cylinder version is 13.3 m (44 ft) long. Its net mass is 1,260 tonnes before any oil or water goes in. That is about 800 family cars, bolted to a steel hull.

Then comes the engineering. With a regular firing order, the manual says, an engine "will inherently balance" its up-and-down and side-to-side forces. What remains are rocking moments. Those are handled the way a ship handles everything: with more steel. Counterweights on the crank ends bring the main rocking down to "acceptable levels." A built-in damper calms lengthwise shaking of the crank. A second-order balancing device is "likely needed" for the six-cylinder version. For seven through twelve, it is "not relevant." On a ship, the odd count is not the problem child.

And when a shipping line wants more? Wärtsilä announced in 2006 that its 14-cylinder RT-flex96C had entered service in a container ship, making 80,080 kW (108,920 bhp) at 102 rpm. Fourteen is even, of course. It got there by adding cylinders to a 12, one at a time, past 13.

Why Do Radial Engines Always Have An Odd Number Of Cylinders?

Now for the engine that flips the car's rule on its head. A radial, like the one on the P-47 Thunderbolt, has its cylinders in a circle around one crank. The FAA handbook lists the row sizes: "three, five, seven, or nine." Odd, every time. Not by tradition. By arithmetic.

In a radial, every piston in a row shares a single crank pin. The FAA describes one piston "connected to the crankshaft by a master rod," with the rest hung off it by shorter links. As the crank goes round once, it passes every cylinder in turn: 1, 2, 3, 4, 5. But these are four-stroke engines. Each cylinder can push only once every two turns.

The neat solution is to fire every other cylinder as the crank sweeps past. The FAA gives the order for a five: 1-3-5-2-4. Follow it around. Turn one fires 1, 3, 5. Turn two fires 2, 4, and lands back on 1. Every cylinder gets a push, spaced 720 ÷ 5 = 144 degrees apart. Now try six. Skip-one gives you 1, 3, 5, then 1, 3, 5 again, forever. Cylinders 2, 4 and 6 never get a turn. An even count breaks the trick. An odd count makes it work. The FAA's nine checks out the same way: cylinders 40 degrees apart, and "a space of 80° between firing impulses," which is 720 ÷ 9.

So the radial does not merely tolerate odd. It requires it. "Even is better" was never a law of physics. It was the habit of one layout.

Why Did The Navy Prefer Radial Engines?

If radials shook and stuck out into the wind, why did the US Navy build its air arm around them? Because the alternative leaked. A liquid-cooled engine carries a radiator, hoses and coolant. Any of them can fail, 200 miles from a carrier.

The turning point was a 1921 Navy contract for Charles Lawrance's J-1, a nine-cylinder radial. The Smithsonian's National Air and Space Museum holds one. Its notes say the J-1 met a Navy need for a light carrier engine "not plagued with water leakage problems." NASA's history of the period adds that trouble with heavy engines "prompted the Navy in 1927 to move to air-cooled engines" for its flying boats. Light, simple, tough, and no coolant to lose. The odd cylinder count came along for the ride. The Navy, sensibly, never cared.

What Was The Biggest Radial Engine Ever Built?

The odd rule applies to a single row. Stack rows, and you can pile on cylinders until the cooling gives up. The record holder is the Pratt & Whitney R-4360 Wasp Major. The National Air and Space Museum calls it "the largest piston engine that the company ever produced." It has 28 cylinders "in four rows, arranged in a spiral" for cooling. Later versions made 3,207 kW (4,300 hp). The museum's example weighs 1,538 kg (3,390 lb), about the mass of a family sedan, hung on the nose of an airplane.

Now do the arithmetic. Four rows, 28 cylinders. That is four rows of seven. Even the biggest radial ever built, with its even total, is odd at heart. Britannica spans the family at "five to as many as 28" cylinders and calls the radial "once the dominant piston-engine type."

So, Why Do Car Engines Go Even And Ship Engines Go Odd?

Because they are solving two different problems, and "even" was only ever the answer to one of them.

A car engine is a four-stroke. Each cylinder pushes once every 720 degrees, so it needs a crowd of cylinders to keep the crank fed. Pairing cylinders cancels the worst of the shaking for free. Folding them into a V keeps the engine short. Both tricks want an even number. The car did not choose even because odd fires badly. A three fires as evenly as a six. It chose even because even is the cheapest route to smooth.

A ship engine is a two-stroke. Each cylinder pushes on every turn, so 360 ÷ n is an even rhythm for any n you like. What shaking is left meets a thousand-tonne engine turning 80 times a minute. Counterweights and dampers mop up the rest. Balance is bought with mass and steel, not with pairs. An odd count is unremarkable there because it costs nothing.

And the radial shares one crank pin among a ring of four-stroke cylinders. It has to skip every other one to fire at all, and that only works when the ring is odd. Three engines, three answers, and not one universal rule. (For another pair of machines solving one problem in opposite ways, see why car engines never melt but machine-gun barrels do.) The next time someone tells you even engines are smoother, nod. Then ask them about the ship.