Why valve timing determines engine performance
Whether a four stroke petrol engine pulls from low revs or delivers power at the top end depends on the valve timing. It defines when the intake and exhaust valves open and close. A shift of 10 to 20 degrees changes cylinder filling, torque and emissions. This is why production engines use variable valve timing.
The most common form is a camshaft phaser. It shifts the timing while the engine runs.
The animation below shows a single cylinder four stroke engine in section with a spiral valve timing diagram. The crank mechanism and valve train move with the crank angle. The gas exchange appears in colour. A marker links each crank position to the corresponding point on the diagram.
A slider advances or retards the camshaft by ±30° of crank angle. The timing diagram, its dimensions and the gas exchange follow the adjustment. A detail view shows the valve train magnified.
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Description of the animation
On the left, the engine appears in section. Flywheel with crank web, connecting rod and piston form the crank mechanism. Above them sits the valve train with camshaft, two rocker arms and the intake and exhaust valves. Spark plug and injector are in the cylinder head, with the oil sump indicated below.
The cylinder contents appear as coloured areas. Fresh charge flows in blue through the open intake valve. With fuel injection it takes on a yellow tint. During compression, the colour shifts to red.
A flame front burns downwards through the chamber. The burnt gas leaves the cylinder as grey exhaust through the exhaust valve. A blue spark at the plug and a yellow spray cone at the injector mark the moments.
The coloured areas are not scripted. They are calculated from the valve openings and piston speed. Adjusting the camshaft changes when and how the cylinder fills and empties.
At the top left, a viewport shows the valve train at twice the size. Valves lifting off their seats, cams rolling against rocker arms and the gap during valve overlap are visible here. The full view does not show these details.
The timing diagram runs alongside on the right. Below the crank angle slider, a coloured bar shows the four strokes in a linear view. It adapts to any adjustment.
How to read the timing diagram
One working cycle takes two crankshaft revolutions. In a circular diagram, 720° would overlap. The diagram here is therefore drawn as a spiral: the crank angle runs around the centre while the radius grows over two revolutions.
The first revolution lies on the inside, the second on the outside. Every crank angle of the working cycle has its own point.
How to find your way around:
- Coloured sections: blue = intake, red = compression, yellow = power, grey = exhaust. Each section begins and ends where the valves open and close, not at 0°, 180°, 360° and 540°.
- TDC and BDC lines: two dashed lines mark top and bottom dead centre. All angular dimensions refer to them.
- Valve overlap: the grey exhaust section runs past the start of the blue intake section. This region is the overlap. Two angles describe it: “intake opens before TDC” and “exhaust closes after TDC”.
- Start of compression and exhaust: two further dimensions state how many degrees after BDC the intake valve closes and how many degrees before BDC the exhaust valve opens (blowdown).
- Ignition marker: a blue spark symbol at the red to yellow transition marks the ignition point.
- Marker: a dot travels along the spiral with the crank angle. In the overlap region a second, blue marker appears. The same crank angle then lies on both the grey and the blue branch.
Component overview
The animation shows mechanical components working together. The table below explains each part:
| Component | Function |
|---|---|
| Crankshaft | Converts the piston’s reciprocating motion into rotation. |
| Flywheel | Stores rotational energy and bridges the three strokes that produce no power. Visible as the large wheel on the crankshaft. |
| Connecting rod | Links the piston to the crankshaft and transmits force between them. |
| Piston | Moves up and down inside the cylinder. It compresses the charge and absorbs combustion pressure. |
| Cylinder | The space in which the piston travels. Its volume determines the displacement. |
| Cylinder head | Closes the cylinder at the top. It houses valves, spark plug and injector. |
| Camshaft | Rotates at half crankshaft speed. Through its cam profile, it controls when and how far the valves open. In section, both cams lie on top of one another. Their angular offset creates the gap between intake and exhaust timing. |
| Camshaft phaser | Rotates the camshaft relative to the crankshaft and shifts all valve timings together. Operated via the slider in the animation. |
| Rocker arm | Lever that transfers the cam lift to the valve. |
| Intake valve | Opens around top dead centre and admits fresh charge. It closes after bottom dead centre. |
| Exhaust valve | Opens before bottom dead centre (blowdown) and releases burnt gas from the cylinder. |
| Spark plug | Produces the spark that ignites the compressed mixture. |
| Injector | Injects a metered quantity of fuel. Shown as a yellow spray cone during the intake stroke. |
| Oil sump | Collects lubricating oil in the crankcase. Indicated as the dark area below the crankshaft. |
Interactive controls
The sliders set the following parameters:
- Crank angle (0–720°): rotational position of the crankshaft. A working cycle covers two revolutions. The coloured bar below the slider shows the active stroke.
- Engine speed (3000–4500 rpm): affects the ignition point because ignition advance increases with speed. The display at the bottom left shows the advance angle. The ignition marker on the diagram moves accordingly.
- Camshaft phaser (−30° to +30° crank angle): shifts valve timing relative to the crankshaft. Negative values mean “advanced”, positive values “retarded”. Opening durations stay unchanged. Only their position in the working cycle shifts.
The play/pause button starts and stops the engine.
The four strokes
- Intake: the piston moves down with the intake valve open. Fresh mixture flows into the cylinder. The blue filling entering through the left valve shows this. On the diagram, the marker travels along the blue section.
- Compression: both valves are closed, the piston moves upwards. Pressure and temperature rise. The charge changes colour from blue through yellow to red. The stroke begins when the intake valve closes, not at BDC.
- Power: ignition occurs before top dead centre (blue spark at the plug). A flame front travels downwards and drives the piston down. This is the only stroke that delivers work to the crankshaft.
- Exhaust: the exhaust valve opens before bottom dead centre. The piston pushes grey exhaust gas out of the cylinder. The stroke ends after top dead centre. The grey section extends past the blue one on the diagram.
Valve timing in detail
Valve overlap
During the transition from exhaust to intake, both valves are open for a short range of crank angle. This valve overlap improves gas exchange. The outflowing exhaust column has inertia and draws fresh charge in. At the same time, residual gas is scavenged from the combustion chamber.
Set the crank angle slider to about 0° or 720° to observe this. Both valves stand open – visible in the magnified viewport. Grey exhaust gas and blue fresh charge are present in the cylinder at the same time.
A large overlap favours high engine speeds. It worsens idle and part load behaviour because residual gas is pushed back into the intake tract. This is where variable timing comes in.
“Intake closes” – the most important value
The intake valve does not close at bottom dead centre but later. The incoming gas column has inertia and keeps flowing in while the piston already moves upwards (ram effect). At high speeds this effect is strong. At low speeds the piston pushes part of the charge back into the intake port.
The closing point determines the effective compression ratio. Compression begins from this moment on.
Exhaust blowdown
The exhaust valve opens before bottom dead centre. This sacrifices work at the end of the expansion stroke. In return, cylinder pressure drops before the exhaust stroke begins, so pumping work is reduced. The timing is a compromise between these two losses.
What the camshaft phaser does
The engine in this animation has one camshaft for both valves. The phaser rotates it as a whole relative to the crankshaft. All four valve events move by the same amount. Opening durations and overlap size stay constant. Only their position in the working cycle changes.
The camshaft turns at half crankshaft speed. A shift of 30° crank angle corresponds to rotating the camshaft by 15°.
| Setting | Effect on valve timing | Engine behaviour |
|---|---|---|
| Advanced (negative values) | Intake opens and closes earlier; exhaust opens and closes earlier | Better filling at low speeds, more torque; but earlier blowdown and less ram effect at high speeds |
| Base setting (0°) | Designed for the mid speed range | Balanced compromise |
| Retarded (positive values) | Intake opens and closes later; exhaust opens and closes later | Uses the ram effect at high speeds for more peak power; loss of filling at low speeds because charge is pushed back out |
The animation shows this step by step. Move the slider left or right: the coloured sections of the spiral rotate, the angular dimensions change, and the stroke bar shifts. Start the animation and the colour sequence in the cylinder changes as well.
With retarded timing, blue fresh charge flows in longer. With advanced timing, the exhaust valve opens sooner.
Real engines go further. With two camshafts (DOHC), intake and exhaust can be phased independently. The overlap itself then becomes variable. Some valve trains also adjust lift and opening duration.
Physical background
The camshaft runs at half crankshaft speed. Each valve opens once per working cycle, that is, per two crankshaft revolutions:
\[ n_{\text{cam}} = \frac{n}{2} \]
This speed ratio is visible in the animation. While the flywheel completes two revolutions, the camshaft turns once. Drag the crank angle slider from 0° to 720° and follow the cams in the viewport.
The valve lift follows from the cam lift via the lever ratio of the rocker arm:
\[ h_{\text{v}} = h_{\text{c}} \cdot \frac{l_{\text{v}}}{l_{\text{c}}} \]
- \( h_{\text{v}} \) – valve lift
- \( h_{\text{c}} \) – cam lift above the base circle
- \( l_{\text{v}} \), \( l_{\text{c}} \) – lever arms on the valve and cam side
The cam profile consists of a base circle and a tangentially blended nose. While the rocker arm rides on the base circle, the valve stays closed. Only the part of the nose projecting beyond the base circle produces lift.
The opening duration is a property of the profile. Rotating the shaft cannot change it, only shift it.
Speed dependent ignition point
The flame front spreads through the combustion chamber at about 20 m/s. Peak pressure should occur after top dead centre where the crank throw offers a favourable lever arm. Ignition must therefore be advanced further at higher speeds.
The display at the bottom left gives the advance angle, the flame travel time in degrees and the flame path in millimetres. Change the speed from 3000 to 4500 rpm: the flame path stays the same while the advance angle grows. The ignition marker on the diagram moves accordingly.
Classroom use
Tasks that can be worked through with the animation:
- In the base setting, read off all four valve timings. Enter them into a timing diagram of your own.
- Determine the opening duration of the intake valve in degrees. Check whether it changes when the camshaft is phased. Explain the result.
- Set the phaser to −30° and to +30°. Which angle stays constant and which ones change? What does this mean for the valve overlap?
- Step the crank angle through the region around 720°. From which angle are both valves open, and from which angle only one?
- Record the ignition advance at 3000, 3600 and 4500 rpm. Plot the relationship and explain it using the constant flame speed.
Historical background
Alphonse Beau de Rochas described the four stroke principle in 1862. Nikolaus August Otto built the first four stroke engine in Cologne in 1876. For decades, valve timing was fixed and represented a compromise. A camshaft designed for torque cost peak power; one for power ruined idle quality and fuel consumption.
- 1960s/70s: first hydraulic camshaft phasers in racing and commercial vehicles.
- 1983: Alfa Romeo introduces one of the first production cars with electronically controlled intake camshaft phasing.
- 1989: Honda presents VTEC, which switches between two cam profiles. Lift and duration become variable.
- 1990s/2000s: phasing on both camshafts becomes standard. BMW introduces variable valve lift with Valvetronic.
- Today: variable valve timing is a prerequisite for Miller and Atkinson operation, for exhaust gas recirculation and for turbocharged engines.
Practical applications
- Car engines: camshaft phasing is standard on petrol engines.
- Motorcycles: large overlaps and late intake closing enable high output at high speeds.
- Small engines: lawnmowers and generators use fixed valve timing. They run at a constant operating point.
- Hybrid drivetrains: Atkinson operation through late intake closing is a key element of modern hybrids.
Related animations
Web animations
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Overview
| Title | Valve timing and camshaft phasing in the four stroke petrol engine |
| Target group | Teachers and lecturers |
| Features | Full screen mode lossless magnification supports large screens and projection magnified detail view of the valve train timing diagram with automatic angular dimensioning |
| Licence | MIT The technical drawing of the engine included in the software is not covered by the MIT licence. Copyright is held by Ulrich Rapp. |
Contributors
C. Hein, S. Rikowski
Sources
Technical drawing of the engine: based on Ulrich Rapp (http://www.ulrich-rapp.de/stoff/fahrzeug/motor/index.htm)