How a four-stroke engine works: interactive 3D guide

All 7 steps

1. 1The intake stroke

   The piston goes down from the top. The intake valve opened 15° before the top and the exhaust valve shuts 10° after it, so as the piston falls it draws fresh air in through the throttle and the runner. On the way, the injector sprays a fine mist of petrol at the back of the intake valve, and the air and the fuel go in together as the charge. The pressure in the cylinder stays a little under the air’s outside.

2. 2The compression stroke trap

   The piston comes back up. The intake valve stays open until 50° past the bottom, while the charge rushing in keeps on coming, then shuts. With both valves shut, the piston squeezes the charge into the chamber under the head, ten and a half times smaller than the space it filled, and its pressure and temperature climb: here to about 14 bar by the time the spark fires, an illustrative figure. Avoid this trap: Drive into water that reaches the engine’s air intake, or fast enough to push a bow wave over it, and the intake stroke draws water into the cylinder with the air. Air can be squeezed; water cannot. On the compression stroke the piston comes up against the water and stops short of the top, and an engine turning at speed drives on against it: the connecting rod bends or breaks, or the crankshaft, the head or the block cracks. The AA warns that an egg cup of water sucked in can wreck an engine. Turn back from water that is moving or more than 10 cm (4 in) deep; if you must cross shallower water, know where your car’s air intake is and go slowly and steadily, so that no bow wave builds. If the engine stops in the water, do not start it again: the water has to come out first. How much water, where the piston stops and how the rod bends are our picture.

3. 3The power stroke

   About 15° before the top, the spark plug fires across its gap. A flame front spreads out from it through the squeezed charge, the burning gas gets hotter and its pressure jumps, here to a peak of about 46 bar just after the top (illustrative). That pressure drives the piston down, and through the rod it turns the crankshaft: of the four strokes, this is the only one that makes work. Near the bottom the exhaust valve starts to open.

4. 4The exhaust stroke

   The exhaust valve opened 55° before the bottom, so most of the pressure has already blown down into the exhaust. Now the piston rises and pushes the burnt gas out past the open valve, through the port and down the pipe, against a pressure only a little over the air’s. Just before the top, the intake valve starts to open again for the next cycle.

5. 5The whole cycle, at speed

   Run on, the four strokes follow each other without a pause: intake, compression, power and exhaust, two turns of the crank for every stroke that makes power. It slows down at the top between the exhaust and the intake strokes to show the overlap: for 25° of crank the exhaust valve is still closing while the intake valve is already opening, so the gas rushing out helps to draw the fresh charge in. Then the ignition is switched off, and it coasts to a stop at the top of the compression stroke, ready for the timing belt.

6. 6Why the camshafts turn at half speed trap

   The stand turned round to show the back: the crank sprocket has 20 teeth and each cam sprocket 40, so the belt turns the camshafts once for every two turns of the crankshaft, once for each cycle of four strokes, in which every valve opens once. At the top of the compression stroke the three timing marks line up: the mark on the flywheel under its pointer, and the marks on the two cam sprockets facing the pointer between them. Turn the crank two full turns and they line up again. Avoid this trap: Put on with its marks three teeth out, here with the cam sprockets ahead of the crank, the belt makes every valve open and close 54° of crank too early. Turned over, the engine comes up to the top of its exhaust stroke with the intake valve already well open, and on an interference engine the two meet: the valve stem bends, the valve no longer seals, and the head has to come off. Line up all three marks before the belt goes on, and turn the crank two full turns by hand before the engine is started: any resistance means stop.

7. 7The engine taken apart

   Every part lifted out along the way it comes off: the coil and the cam cover, the flywheel off the back of the crankshaft, the belt and its tensioner, the cam sprockets, the camshafts with their bearing caps, the bucket tappets and the spark plug, the intake and the exhaust, the cylinder head with its valves and springs, the head gasket, the gudgeon pin, the piston and its rings, the big-end cap and the connecting rod, the oil pan, the main bearing cap, the crankshaft and its sprocket. Explode spreads them further apart.

What is in it

• Piston, rings and gudgeon pin · an 86 mm bore
• Connecting rod and crankshaft · an 86 mm stroke, its counterweight
• Flywheel · its ring gear and TDC mark
• Cylinder head · two valves, their springs and bucket tappets
• Two camshafts and the timing belt · a 20-tooth and two 40-tooth sprockets
• Spark plug and coil, port injector, throttle
• Exhaust pipe

Questions

Safety

Work on a cold engine with the battery disconnected: the exhaust stays hot long after it stops, the ignition gives a shock, and a fan or a belt can start moving on its own. Never turn the crankshaft or a camshaft with the timing belt off, and never run an engine in a closed garage.

• Hot parts and fumes: brakes, exhausts and engines stay hot long after a drive, and a running engine fills a closed garage with carbon monoxide: let parts cool, and keep the door open.
• Moving parts: keep hands, hair and tools clear of belts, pulleys and the fan, which can start on its own; disconnect the battery before working on the engine.
• Car batteries: a lead-acid battery gives off explosive hydrogen. No sparks, flames or smoking near it, eye protection on, and the last jump lead clamped to bare metal away from the battery.

About this guide

3D Spatial Reconstruction: models are engineered from factual measurements to show the parts, the fixings and the order of the job before you start. Always check the handbook of your own car.

The engine is a generic single cylinder of our own design, not any maker’s: an 86 mm bore and an 86 mm stroke (about 500 cc, the size of one cylinder of many 2.0-litre four-cylinder engines), a 140 mm connecting rod and a compression ratio of 10.5:1, within the 8:1 to 12:1 that petrol car engines have used. Its two valves sit in a pent-roof chamber, each opened by its own camshaft through a bucket tappet, and one toothed belt drives both camshafts.

The piston’s height follows the slider-crank formula, x = r cos θ + √(l² − r² sin² θ), and each valve’s lift follows its cam lobe, whose shape we worked out for a flat tappet. The valve timing is a typical one: the intake opens 15° before top dead centre and shuts 50° after bottom dead centre, the exhaust opens 55° before bottom dead centre and shuts 10° after the top, which gives 25° of overlap. The spark fires 15° before the top here; real engines move it with speed and load.

The pressure in the readings is illustrative: worked out from a simple model of the cycle at full throttle (a polytropic squeeze and a burn over about 50° of crank), not measured, so its 14 bar at the spark and 46 bar peak are only the right size. Real engines differ in the details: many have two intake and two exhaust valves, a chain instead of a belt, or direct injection into the cylinder; this one has port injection, its injector spraying at the back of the intake valve. To show everything at once, its timing belt and flywheel are both at the back and the whole engine is cut down its middle, and the 3D runs far slower than a real engine.

The hydrolock trap in step 2 follows Wikipedia’s article on hydrolock, the AA’s floodwater advice and Start Rescue’s: water drawn in through the air intake cannot be compressed, so the piston cannot finish its stroke, and an engine turning at speed bends a rod or cracks the crankshaft, the head or the block. How much water comes in, where the piston stops (30° before the top here, the crank 12° further) and how the rod bows are our picture, not measurements; the pressure reads “—” while it plays because our model of it does not cover water.

It is an interference engine: its open valves reach into the space the piston sweeps. With the belt on at its marks the intake valve clears the piston by 4 mm at the top; on this engine one tooth out (18° of crank) leaves 1.6 mm, two teeth just touch and three, as in the trap, hit hard enough to bend the valve. Those figures come from this engine’s own geometry, not from any real one: your engine’s workshop manual has its marks and its belt’s interval.

Sources: Wikipedia: four-stroke engine (the four strokes, two turns a cycle, the camshaft at half speed), Wikipedia: piston motion equations (the slider-crank), Wikipedia: compression ratio (8:1 to 12:1 for petrol cars), Wikipedia: Toyota S engine (86 × 86 mm 2.0 litres, one belt driving both camshafts), MathWorks: SI combustion cylinder (a typical valve timing with 25° of overlap), Wikipedia: ignition timing (the spark advanced with speed, peak pressure after the top), Wikipedia: fuel injection (port injection upstream of the intake valve), Wikipedia: piston ring (two compression rings and an oil control ring), Wikipedia: interference engine, Wikipedia: timing belt (camshaft), AA1Car: timing belts and interference engines (bent valves), ALLDATA DIY: timing belts (a piston can meet an open valve), car.co.uk: when a timing belt breaks (3 to 7 hours’ labour; not a DIY job), 2CarPros: a belt procedure (two turns and check the marks; teeth out and the valves touch), Wikipedia: hydrolock (water over the air intake, the piston cannot finish its stroke, bent rods, cracked crank, head or block), The AA: driving through floodwater (keep out of deep or moving water, go slowly with no bow wave; an egg cup of water can wreck an engine), Start Rescue: driving through flood water (find the air intake first; water compressed bends the rod; do not restart a stalled engine).

The 3D models, drawings and text are our own. The device shown is a generic one of our own design, not any one maker’s model.