Three cylinders vs four, and why one grunts at idle

01 · The job

A push in a straight line has to become spin

Burning fuel pushes a piston down. Wheels need to turn. Everything in an engine exists to bridge that gap.

02 · The analogy

You already know this machine: a bicycle

Your leg pushes in a straight line. The pedal crank turns that into spin. Same colours = same job.

Where the analogy breaks: a cyclist pushes on every half-turn. A cylinder pushes on only one stroke out of four, once every two turns. That gap is the whole 3-vs-4 story.

03 · Parts

Inside one cylinder

A sealed tube, a plug that sits in it like a sliding lid (the piston), two doors at the top (valves), and a spark.

04 · The cycle

Suck, squeeze, bang, blow

Four up-or-down trips of the piston. Only bang actually pushes. The other three are paid for by momentum.

Each trip is half a crank turn, so one full cycle = 2 turns (720°). Every cylinder fires once per 2 turns, no matter how many cylinders there are.

05 · Filling the gaps

More cylinders = fewer silent moments

One power stroke lasts about 180° of crank rotation. Line them up over one full 720° cycle:

3 cylinders: a push every 240°, each lasting ~180° → a 60° gap after every push, with nobody pushing and the next cylinder busy squeezing (which resists).
4 cylinders: a push every 180° → pushes hand off end-to-end. No gap.

06 · The crankshaft

How the crank is shaped decides the rhythm

Looking down the crankshaft from the front, here's where each cylinder's rod attaches.

720° ÷ 3 = 240°. 720° ÷ 4 = 180°. The 4-cylinder's outer pair (1 & 4) and inner pair (2 & 3) always move as mirror images. Remember that, it matters in step 09.

07 · Watch them run

Same 1.2-litre engine, cut into 3 or 4

Side view, front of engine on the left. Drag the RPM. Watch the gaps in the torque trace and the 3-cylinder block see-saw.

Block tilt is exaggerated so you can see it. The torque trace under each engine is a simplified model of the twisting force on the crankshaft; the moving dot is "now".

08 · The torque pulse

Three big kicks with holes vs four smaller overlapping ones

Twisting force on the crankshaft over one full cycle. Same engine size, same average.

The crankshaft speeds up on every kick and slows in every hole. Newton's third law: the engine block twists the opposite way on its rubber mounts each time. Bigger kicks, deeper holes → more twisting of the block. That rolling, felt through the mounts, is a big part of the 3-cylinder idle shudder.

09 · Balance

The see-saw a 4-cylinder doesn't have

Pistons are heavy and slam to a stop at the top and bottom of every stroke, tugging the block each time. Arrows = that tug, one moment in time.

Engineers call the 3-cylinder's see-saw a rocking couple. It happens once per crank turn: 13 times a second at an 800 rpm idle.

10 · Why idle, specifically

At idle, the pulses are slow enough to feel

Pulses per second = rpm ÷ 60 × cylinders ÷ 2. At idle the 3-cylinder's rhythm sits low, where cars and bodies shake.

Put together: bigger kicks + longer holes + see-saw rocking + slow, low rhythm + weak flywheel. That's the idle grunt. Rev it to 3000 and the same pulses come 75 times a second and blur into a hum.

11 · The sound

Why it sounds off-beat

Ticks = each crank turn. Dots = each bang. Over two turns:

The 4-cylinder bangs exactly twice per turn: tidy, even, a buzz. The 3-cylinder bangs 1.5 times per turn, so the bangs fall on and off the crank's own once-per-turn rocking. That syncopation, at a low pitch, is the thrummy "half a six-cylinder" growl people either love or hate.

12 · The gotcha

The 4-cylinder isn't perfect either

Because the rod tilts, a piston covers more distance in the top half of the crank's turn than the bottom. It moves faster up there, so its tug is stronger at the top.

In a 4-cylinder, all four pistons hit that "stronger at the top" moment together, so the extra tugs add up: a vertical buzz at twice engine speed that grows with rpm². That's why a 4-cylinder gets coarse near the redline, and why big ones (2.0 L+) often carry balance shafts. In a 3-cylinder those tugs cancel. Each layout trades one vibration for another.

13 · The fixes

How carmakers tame a 3-cylinder

A good modern 3-cylinder uses one or more of these. That's why some are barely noticeable at idle and some cheaper ones visibly shake the gear lever.

14 · So what

Why anyone builds a 3-cylinder

Fewer cylinders means fewer moving parts rubbing, and less cylinder wall to leak heat into. You pay for that in smoothness at low rpm.

15 · The numbers · cc

What "cc" actually measures

Part 1 was about rhythm. Now the spec sheet. cc is cubic centimetres: the space a piston sweeps between the top and bottom of its stroke. That is the gulp of air and fuel one cylinder can swallow in one down-stroke. Add every cylinder together and you get the engine's "size".

Volume = π ÷ 4 × bore² × stroke (in mm, divide by 1000 for cc). Both presets land on ~1.2 L. The 3-cylinder gets there with big, long-stroke cylinders; the 4-cylinder with small, short ones.

cc is size, not power. It says how much air the engine can breathe per cycle. What you get from that air depends on how hard it is squeezed, how fast the engine spins, and whether a turbo stuffs in extra. The next sections build that up.

The squeeze: compression ratio

The piston doesn't sweep out the whole space. At the top a small pocket is left, the clearance. Compression ratio compares the two.

16 · Torque

Torque is twisting force, not speed

Try loosening a tight bolt. A longer spanner makes it easy: same push, more twist. Torque = force × length of the lever, measured in newton-metres (Nm).

In an engine the spanner is the crank arm, half the stroke long. The push is gas pressure × piston area. So torque comes from two things: how much air the engine swallows (cc) and how hard each bang pushes (pressure). Over a full cycle, the engine's average push is called BMEP:

Torque (Nm) ≈ BMEP (bar) × cc ÷ 125.7. A natural petrol engine manages roughly 10–13 bar at its best; a turbo packs in more air and reaches 18–25 bar. Same cc, nearly double the twist. Drag the push to 20 bar to see why a turbo 1.2 L can out-pull a plain 2.0 L.

17 · Power · BHP

Power is torque, delivered fast

Torque says how hard you can twist. Power says how much twisting you can do per second. Think of carrying bags up stairs: a heavy bag now and then, or a light bag very often. Same work per minute, same power.

For an engine, "bags per minute" is rpm. So power = torque × rpm. The practical formula:

bhp = Nm × rpm ÷ 7121 · kW = Nm × rpm ÷ 9549 · PS = Nm × rpm ÷ 7023.

18 · Rev it up, rev it down

What changes as rpm rises or falls

Here is a typical (illustrative) 1.2 L petrol engine. Torque climbs, peaks and falls again. Power keeps rising because it is torque × rpm, until the torque drop wins. Drag the slider.

A short stroke is how 4-cylinders rev higher: at the same rpm its pistons move slower than the 3-cylinder's long ones (see the readout). Slower pistons, less wear and stress.

19 · Gears

Gears trade speed for twist

The engine can only work well in a narrow rpm band, but the wheels need to go from standstill to highway. The gearbox is a torque and speed exchange: a big gear driven by a small one turns slower but pushes harder.

Illustrative 5-speed with a 4.1:1 final drive and a 1.85 m tyre. Losses ignored. The engine's power stays the same; the gears just choose how it is split between push and speed. First gear: lots of push, little speed. Fifth: little push, lots of speed, and engine rpm stays low (which is why a 3-cylinder lugs if you stay in it too long).