The animation shows the processes that take place when controlling a four-stroke petrol engine. The animation shows the control diagram of a four-stroke petrol engine. A realistic cross-sectional representation can be used to illustrate how the control diagram must be read.
Note on Use
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Description of the Animation
The timing diagram shows the valve timing of an internal combustion engine as a function of the angle of the camshaft. The length of the respective phases depends directly on how the cams are aligned on the camshaft. The timing diagram is therefore sometimes also referred to as a camshaft diagram.
It is not only the orientation of the cams that determines the valve timing, but also their shape. A steep cam profile opens the valve quickly and keeps it open longer — this favours high cylinder filling at high RPM. A shallower profile opens the valve more gently and for a shorter duration, providing smoother engine operation at low RPM. Asymmetric cam profiles, where the opening flank is steeper than the closing flank, allow the valve to open quickly while closing gently — reducing mechanical stress and noise. The cam shape therefore directly determines the appearance of the timing diagram: wider cams produce longer arcs, while steeper cams create more abrupt transitions.
The ignition timing is shown in the timing diagram.
The air-fuel mixture is ignited even before the piston reaches top dead centre. This is the point at which the piston is at the top of the cylinder.
The ignition point shifts depending on the engine speed. In the animation, the engine speed can be changed interactively, as happens when the accelerator pedal is pressed.
At low RPM (e.g. 1,000 RPM), the ignition advance angle is typically around 10° before TDC. At higher speeds (e.g. 5,000 RPM), it shifts to 35–40° before TDC. The reason: at higher RPM, the flame front has less time to propagate, so ignition must occur earlier to ensure peak combustion pressure still falls in the optimal range of 10–15° after TDC. You can observe this relationship in the animation by changing the engine speed and watching the ignition point move in the timing diagram.
The cross-section of the four-stroke petrol engine can be zoomed in. The valve overlap then also becomes visible. Valve overlap means that both valves are open at the same time when expelling.
Valve overlap offers several advantages:
- More efficient gas exchange: exhaust gases are expelled more efficiently. More fresh air enters the combustion chamber.
- Improved cooling: The engine is cooled better thanks to the improved air exchange.
The duration and position of valve overlap varies from engine to engine and depends on the design features and performance targets. Careful optimisation of valve overlap can influence engine performance, fuel consumption and emission levels.
The Four Strokes Explained
A four-stroke petrol engine completes one power cycle in four distinct piston strokes, corresponding to two full revolutions (720°) of the crankshaft.
1. Intake Stroke (Induction)
The piston moves downward from top dead centre (TDC) to bottom dead centre (BDC). The intake valve opens, and the downward motion creates a partial vacuum that draws a fresh air-fuel mixture into the cylinder.
2. Compression Stroke
Both valves are closed. The piston moves upward, compressing the mixture to about 1/8 to 1/12 of its original volume (compression ratio 8:1 to 12:1). This raises temperature and pressure significantly. Near the end, the spark plug fires.
3. Power Stroke (Combustion)
The ignited mixture burns rapidly, producing expanding gases that push the piston downward. This is the only stroke that produces mechanical work. Peak pressures can reach 40 to 60 bar, and temperatures can exceed 2000 °C.
4. Exhaust Stroke
The exhaust valve opens, and the piston moves upward, pushing burnt gases out. The exhaust valve opens slightly before BDC to use remaining gas pressure for a head start on expelling the exhaust.
Why Is Ignition Before Top Dead Centre?
The timing diagram shows that the spark plug fires before the piston reaches TDC. The reason lies in the flame propagation speed: the flame front takes 1–2 milliseconds to travel across the combustion chamber. If ignition occurred at TDC, peak pressure would be reached too late, wasting energy.
By igniting early, maximum pressure occurs around 10–15° after TDC, pushing the piston downward with maximum force at the optimal moment.
Consequences of Incorrect Ignition Timing
- Too early: Causes engine knock — uncontrolled detonation that can damage pistons and bearings.
- Too late: Reduced power, higher exhaust temperatures, and increased fuel consumption.
Modern engines use electronic control units (ECUs) to adjust ignition timing continuously based on RPM, load, and temperature. The animation lets you see this shift interactively as you change the engine speed.
Valve Overlap in Detail
The intake valve opens before TDC at the end of the exhaust stroke because gas inertia from the fast-moving exhaust creates a scavenging effect that pulls fresh mixture in. The exhaust valve closes after TDC, allowing exhaust momentum to continue evacuating residual gases. This improves volumetric efficiency.
Fixed overlap is a compromise: too much causes rough idling at low RPMs; too little limits high-RPM performance. The reason for rough idling lies in the low gas velocity at low engine speeds. The intended scavenging effect fails to materialise because the exhaust gas flow lacks the energy to draw fresh mixture into the cylinder. Instead, exhaust gases can flow back through the already-open intake valve into the intake manifold. This worsens the mixture quality in the cylinder, combustion becomes uneven, and the engine runs roughly. At high RPM, the situation reverses: gas velocity is high enough to fully exploit the scavenging effect — here, greater overlap would be beneficial to draw more fresh air into the cylinder. Modern variable valve timing systems (Honda VTEC, BMW VANOS/Valvetronic, Toyota VVT-i) adjust overlap dynamically, optimising for every operating condition.
Comparison: Petrol Engine vs. Diesel Engine
| Feature | Petrol (Otto) | Diesel |
| Ignition | Spark plug | Self-ignition |
| Compression ratio | 8:1 to 12:1 | 14:1 to 25:1 |
| Fuel delivery | Manifold or direct injection | Direct injection |
| Throttle | Air is throttled | No throttle — fuel quantity controls power |
| Timing diagram | Shows ignition point | Shows injection point |
| Efficiency | 25–35% | 35–45% |
The diesel engine achieves higher efficiency due to its higher compression ratio and absence of throttling losses. The petrol engine runs more smoothly, is quieter, and starts more easily in cold conditions.
The Throttle and Its Influence on Valve Timing
The comparison table highlights a key difference: petrol engines use a throttle valve, diesel engines do not. The throttle sits in the intake tract and regulates the amount of air entering the cylinder. At part load — the typical driving condition — the throttle is only partially open. This creates a vacuum in the intake manifold that acts as a brake on the engine and reduces efficiency (throttling losses).
This directly affects valve timing: with restricted airflow, the gas velocity at the intake valve changes, and valve overlap behaves differently than at full load. The BMW Valvetronic system is particularly noteworthy here: it varies the valve lift to control air intake directly through the valves — making the conventional throttle valve largely redundant. This significantly reduces throttling losses and improves petrol engine efficiency.
Historical Development of Engine Timing
The earliest four-stroke engines (Nikolaus Otto, 1876) used simple side valves with fixed, conservative timing. The overhead valve (OHV) design moved valves to the cylinder head for better combustion. Overhead camshaft (OHC) and dual overhead camshaft (DOHC) designs enabled more precise valve control at higher RPMs.
Since the 1980s, variable valve timing has revolutionised engine design:
- Honda VTEC (1989): uses two different cam profiles on the same camshaft. At low RPM, the valves follow a mild cam for economical, smooth running. Above a certain RPM, a hydraulic pin locks the rocker arms onto a more aggressive cam — the valves open further and for longer, delivering significantly more power. The transition is noticeable while driving.
- BMW VANOS (1992): continuously rotates the camshaft relative to the crankshaft. This shifts all valve events uniformly earlier or later without changing valve lift or opening duration. In the timing diagram, the arcs move as a whole around the circle. Later versions (double VANOS) control the intake and exhaust camshafts independently.
- Toyota VVT-i (1996): works similarly to VANOS, continuously adjusting the intake camshaft. This increases or decreases valve overlap depending on operating conditions — minimal at low RPM, maximum at high RPM.
- BMW Valvetronic (2001): in addition to camshaft phasing, it continuously varies the valve lift. An intermediate lever between cam and valve is adjusted by an electric motor. At low load, the valves open only a few millimetres — this replaces the throttle valve and significantly reduces throttling losses.
- Fiat MultiAir (2009): replaces the mechanical cam drive on the intake side with an electro-hydraulic system. An oil volume between the cam and valve is controlled by a solenoid valve. This allows every individual valve lift to be freely varied in height and duration — independent of the cam shape and even different from cylinder to cylinder.
How to Read a Timing Diagram
The circular diagram represents 720° of crankshaft rotation. TDC (top dead centre) is at the top, BDC (bottom dead centre) at the bottom. Read clockwise.
Key valve events and typical angles:
- IO — Intake Opens: 5–20° before TDC
- IC — Intake Closes: 30–60° after BDC
- EO — Exhaust Opens: 40–60° before BDC
- EC — Exhaust Closes: 5–20° after TDC
Where the intake and exhaust arcs overlap near TDC is the valve overlap. The ignition advance angle is marked near TDC on the compression-to-power transition — watch it shift in the animation as you change engine speed.
What Happens with Incorrect Valve Timing?
If a timing belt snaps or chain skips, valves lose synchronisation with pistons. In interference engines, pistons can strike open valves — bent valves, damaged pistons, repair costs of several thousand euros.
Shifted timing (e.g. belt installed one tooth off) causes power loss, rough idling, increased emissions, and hard starting.
Worn components — cam lobes and valve springs — gradually alter effective timing. Regular maintenance (valve clearance checks, timing belt replacement at specified intervals) keeps the engine running as designed. The animation’s timing diagram represents the ideal state; deviations from it are a key diagnostic tool in professional engine repair.
Timing Belt vs. Timing Chain
Two systems are used to drive the camshaft, and they differ fundamentally in their wear characteristics. The timing belt is a fibre-reinforced rubber band with a toothed profile. It runs quietly, is lightweight and inexpensive, but degrades over time due to heat, oil, and mechanical stress. An aged timing belt can snap without warning — all valve timing is instantly lost, and in interference engines, pistons will strike the valves. This is why manufacturers specify fixed replacement intervals (typically 60,000–120,000 km or 5–8 years).
The timing chain is a metal chain that is considerably more durable and in many engines is designed to last the lifetime of the vehicle. Its wear manifests differently: the chain links stretch minimally over time, causing the camshaft to rotate slightly relative to the crankshaft. In the animation’s timing diagram, this would mean that all valve events shift slightly — the opening and closing points move by a few degrees. A hydraulic chain tensioner compensates for this stretch up to a point. Once the tensioner reaches the end of its adjustment range, the stretched chain produces a rattling noise on cold starts — a typical early warning sign before the chain skips a tooth.
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Overview and Download
| Title | Control of a four-stroke petrol engine |
| Target audience | Teachers and lecturers |
| Platforms | Microsoft® Windows® Apple® Macintosh® (version dependent) |
| Features | Full screen mode Lossless scaling Large screens and projectors supported |
| License | Freeware |
| Download | Contact |
Contributors
C. Hein, S. Rikowski
Sources
- Technical drawing of the motor: based on Ulrich Rapp (http://www.ulrich-rapp.de/stoff/fahrzeug/motor/index.htm)
- Authoring tool (control elements supplied): Adobe Animate