Cylinders, cylinder layouts and firing orders

A cylinder is one of the tubes inside an engine where a piston slides up and down and the fuel and air burn. Every cylinder does the same four-stroke job, and all of them turn the same crankshaft.

Most car engines have three, four or six cylinders; big performance cars may have eight, ten or twelve. An engine’s size is the space all its cylinders sweep in total, so a 2.0-litre four-cylinder has four cylinders of about half a litre each. Most modern engines use cylinders of roughly that size, because it burns fuel efficiently.

More cylinders means more, smaller pushes on the crankshaft. That makes an engine smoother, and smaller pistons can rev higher. The cost is more parts, more friction, more weight and a higher price, which is why everyday cars mostly stick to three or four.

The cylinders can be arranged in three main ways, called the engine’s layout:

• Inline, or straight: all the cylinders in one row.
• V: two rows, called banks, leaning apart in a V shape over one crankshaft.
• Flat, or boxer: two banks lying flat on either side of the crankshaft.

Pick a layout. The top drawing shows the engine from the front, with the first cylinder of each bank in section. The bottom one looks down on all the cylinders, with the front of the engine on the left, and lights each one orange while it is on its power stroke.

• Inline-3. Common in small cars. Firing every 240° leaves short gaps with no power stroke, so triples have a distinctive thrum.
• Inline-4. By far the most common layout. It’s simple, compact and cheap, and fires 1-3-4-2.
• Inline-6. Long, but its pistons balance each other perfectly, so it’s exceptionally smooth. BMW is famous for them.
• V6. Two banks of three. It’s as smooth as most drivers need and short enough to fit sideways in a front-wheel-drive car.
• V8. Two banks of four, packing a lot of engine into a short block. Big American cars and many performance cars use them.
• Flat-4. Wide and low, which lowers the car’s centre of gravity. Subaru and Porsche (with flat-6s) are known for them.

Each cylinder fires once every two turns of the crankshaft, so an engine makes half as many power strokes per turn as it has cylinders. Slow the demo down and count: two per turn for an inline-4, three for a six, four for a V8. With more than four cylinders, a new power stroke starts before the last one has finished, so the push on the crankshaft never stops.

The order the cylinders fire in is the firing order. It is never simply 1-2-3-4, for three reasons.

• Balance. Firing neighbouring cylinders one after the other would rock the engine from end to end. Jumping between the ends and the middle keeps the forces even.
• Spreading the load. Each push twists the crankshaft a little. Sharing the pushes along its length stops one end from being hammered.
• Breathing. Spacing out the cylinders that share an intake or exhaust pipe stops them fighting over the same air or exhaust flow.

The firing order also shapes an engine’s sound. A typical V8’s order makes each bank fire unevenly, so the exhaust pulses bunch up into the deep burble everyone recognises. Racing-style flat-plane V8s fire each bank evenly and scream instead.

The demo shows every engine firing evenly. A few, such as some older V6s, have uneven gaps between firings.

Cylinder 1 is normally at the front of the engine, which is the end with the timing belt or chain, not necessarily the front of the car. In most front-wheel-drive cars the engine sits sideways, so cylinder 1 is often nearest one wheel.

V and flat engines differ by maker. Some number one bank with odd numbers and the other with even, as in the demo; Ford numbers one bank 1 to 4 and the other 5 to 8. The workshop manual for your car will show which is which.

The numbers matter when something goes wrong. If the engine warning light comes on and a code reader shows P0301, P0302 and so on, the last digit is the cylinder that is misfiring. P0300 means several cylinders, or a misfire that moves around.