Why the four stroke engine matters
The four stroke Otto engine is the most widely used internal combustion engine in the world. Billions of these engines power passenger cars, motorcycles, lawnmowers, portable generators and boats. Whether you are commuting to work, mowing the lawn or running a backup generator – chances are a four stroke engine is doing the work. Understanding how it converts fuel into motion means understanding a piece of technology that has shaped modern transport, industry and everyday life for over a century.
The following animation shows a single cylinder four stroke engine in section. The crank mechanism and the valve train move in step with the crank angle, the gas exchange and combustion are shown in colour, and a pV diagram running alongside links every crank position to the thermodynamic cycle. Engine speed and crank angle can be changed in real time.
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Description of the animation
The animation shows a single cylinder four stroke engine with overhead valves in section. The crank mechanism – flywheel, connecting rod and piston – converts the linear motion of the piston into rotation. Above it sits the valve train with camshaft, rocker arms and the two inclined valves for intake and exhaust. The spark plug and the fuel injector are arranged at the top of the cylinder head.
The cylinder content is made visible through coloured areas: the cold fresh charge flows in blue through the opened intake valve, changes colour during compression via a yellowish intermediate tone to the red of the hot mixture, burns in a flame front travelling downward and finally leaves the cylinder as grey exhaust gas through the exhaust valve. A blue ignition spark at the plug and a yellow injection spray at the nozzle mark the corresponding moments.
On the right a pV diagram runs alongside. A marker travels along the curve and shows the state the working gas is currently in. The coloured sections of the curve – intake, compression, combustion and power – follow the actual valve timings and the speed dependent ignition point.
The Play button starts the engine. The displays at the bottom left show the ignition point, the flame travel time and the flame path.
Four checkboxes below the diagram toggle the work areas of the individual strokes: intake, compression, power and exhaust. The area under the pV curve for a stroke equals the work integral of that stroke. Strokes in which the volume increases (intake, power) contribute positive work. Strokes in which the volume decreases (compression, exhaust) require negative work.
When several strokes are activated at the same time, the areas superimpose with correct signs. Activating all four strokes yields the effective net work per cycle – the cycle integral \( \oint p \, dV \).
Component glossary
The animation shows several mechanical components working together. The table below gives a brief explanation of each part:
| Component | Function |
|---|---|
| Crankshaft | Converts the reciprocating motion of the piston into continuous rotation. |
| Flywheel | Stores rotational energy and smooths out the gaps between power strokes. |
| Connecting rod | Links the piston to the crankshaft and transmits force between them. |
| Piston | Moves up and down inside the cylinder; compresses the charge and receives the combustion pressure. |
| Cylinder | The chamber in which the piston moves. Its volume defines the engine displacement. |
| Cylinder head | Closes the cylinder at the top; houses the valves, spark plug and fuel injector. |
| Camshaft | Rotates at half crankshaft speed and controls when and how far the valves open via its cam profiles. |
| Rocker arm | Lever that transfers the cam lift to the valve, amplifying or reducing the motion. |
| Intake valve | Opens during the intake stroke to let fresh air–fuel mixture into the cylinder. |
| Exhaust valve | Opens during the exhaust stroke to let burnt gases leave the cylinder. |
| Spark plug | Produces the electric spark that ignites the compressed air–fuel mixture. |
| Fuel injector | Sprays a metered quantity of fuel into the intake port or directly into the cylinder. |
Interactive controls
The following parameters can be set with the sliders:
- Crank angle (0–720°): rotational position of the crankshaft for manual positioning. One complete working cycle spans two revolutions (720°).
- Engine speed (3000–4500 min-1): rotational speed of the engine. It affects the ignition point, since the ignition advance grows with engine speed.
The Play/Pause button starts or stops the engine.
Four checkboxes below the pV diagram toggle the work areas of the individual strokes (intake, compression, power, exhaust). Activated areas superimpose with correct signs, so that the sum of all four areas shows the effective net work per cycle.
The four strokes
One working cycle of the four stroke engine takes two crankshaft revolutions and is divided into four strokes:
- Intake (0–180°): the piston moves downward, the intake valve is open, and fresh mixture flows into the cylinder.
- Compression (180–360°): both valves are closed, the piston moves upward and compresses the charge. Pressure and temperature rise.
- Power (360–540°): shortly before top dead centre the mixture is ignited. Combustion drives the piston downward – this is the only stroke in which work is delivered to the crankshaft.
- Exhaust (540–720°): the exhaust valve opens and the piston pushes the burnt gas out of the cylinder.
Otto engine vs. Diesel engine
Both the Otto and the Diesel engine are four stroke engines, yet they differ in several ways:
| Feature | Otto engine | Diesel engine |
|---|---|---|
| Ignition | Spark ignition (external ignition source) | Compression ignition (self ignition) |
| Fuel | Petrol (gasoline) | Diesel fuel |
| Compression ratio | Approx. 8 : 1 to 13 : 1 | Approx. 14 : 1 to 24 : 1 |
| Mixture formation | Homogeneous air–fuel mixture | Fuel injected into hot compressed air |
| Thermal efficiency | Approx. 25–35 % | Approx. 35–45 % |
| Power to weight ratio | Higher – lighter construction | Lower – heavier due to higher pressures |
| Typical applications | Cars, motorcycles, small engines | Trucks, buses, ships, heavy machinery |
The higher compression ratio gives the Diesel engine a better thermal efficiency, but the Otto engine is lighter, quieter and often less expensive to manufacture. In vehicles, both types are combined with turbocharging and hybridisation to improve fuel economy.
Physical background
The Otto engine is a heat engine: through combustion it converts the chemical energy stored in the fuel into heat, and this in turn into mechanical work. The thermodynamic cycle becomes visible in the pV diagram, in which the pressure is plotted against the cylinder volume. The area enclosed by the closed curve corresponds to the work gained per working cycle:
\[ W = \oint p \, dV \]
The camshaft determines, through the cam profile, when and how far the valves open. It runs at half crankshaft speed, since each valve opens only once per working cycle (two crank revolutions). The valve lift results from the cam lift via the lever arm ratio of the rocker arm.
An important detail is the speed dependent ignition point. The flame front spreads through the combustion chamber at a constant speed. For combustion to develop its full effect, the peak pressure should be reached shortly after top dead centre – where the lever arm of the crank is largest. Since the flame speed stays the same but the crankshaft turns faster at higher engine speed, the ignition point must be advanced further the higher the engine speed is (ignition advance):
- \( n \) – engine speed
- \( v_F \) – flame speed (approximately constant)
- ignition advance – crank angle before TDC at which ignition occurs
At the transition between exhaust and intake, both valves are open at the same time for a short range of crank angle. This valve overlap improves the gas exchange, because the outflowing exhaust gas draws fresh charge in behind it.
Ideal cycle vs. real engine cycle
In thermodynamics the idealised Otto cycle consists of four reversible steps:
- Isentropic compression (1 → 2): the piston compresses the gas adiabatically – no heat exchange with the surroundings.
- Isochoric heat addition (2 → 3): at constant volume the gas receives heat instantaneously – modelling the combustion.
- Isentropic expansion (3 → 4): the hot gas expands adiabatically and pushes the piston down.
- Isochoric heat rejection (4 → 1): at constant volume the remaining heat is released – modelling the exhaust.
The thermal efficiency of the ideal Otto cycle depends only on the compression ratio \( r \) and the heat capacity ratio \( \gamma \):
\[ \eta_{\text{Otto}} = 1 – \frac{1}{r^{\,\gamma – 1}} \]
A real engine falls short of this ideal for several reasons:
- Friction: piston rings, bearings and the valve train consume energy.
- Heat losses: the cylinder walls, head and piston absorb heat that is therefore unavailable for work.
- Incomplete combustion: not all fuel burns completely, especially at high speed or under rich conditions.
- Gas exchange losses: pumping the charge in and pushing the exhaust out requires work (visible as the small loop at the bottom of the pV diagram).
- Finite combustion time: unlike the ideal cycle, combustion is not instantaneous – hence the need for ignition advance.
The difference between the ideal and the real cycle is visible when the angular shape of the textbook pV diagram is compared with the rounded curve in the animation.
Efficiency and energy balance
Of the chemical energy stored in the fuel, a conventional Otto engine converts only about 25–35 % into useful mechanical work at the crankshaft. The remainder is lost in several ways:
| Energy pathway | Share (approx.) |
|---|---|
| Useful mechanical work | 25–35 % |
| Exhaust gas heat | 30–35 % |
| Cooling system heat (cylinder walls, head) | 25–30 % |
| Friction and auxiliary drives | 5–10 % |
This means that roughly two thirds of the fuel energy leaves the engine as waste heat. Recovering some of that heat – for example by heating the vehicle cabin or in a combined heat and power plant – can raise the overall energy utilisation to 80 % or more, even though the mechanical efficiency stays the same.
A brief history
The four stroke principle was first described by the French engineer Alphonse Beau de Rochas in 1862, but it was the German engineer Nikolaus August Otto who built the first practical four stroke engine in 1876 in Cologne. His “Otto Silent Engine” was a breakthrough: quieter, more efficient and more reliable than the atmospheric gas engines of the day. The principle proved so successful that four stroke engines are still called “Otto engines” in German speaking countries.
Key milestones in the further development include:
- 1885/86: Karl Benz and Gottlieb Daimler independently build the first motor vehicles powered by petrol fuelled Otto engines.
- 1893: Rudolf Diesel patents the compression ignition engine – a thermodynamic cousin of the Otto engine with higher efficiency.
- Early 20th century: mass production (Ford Model T, 1908) makes the Otto engine the dominant power source for personal transport.
- Late 20th century: catalytic converters, fuel injection and electronic engine management reduce emissions and improve fuel economy.
- 21st century: downsizing, turbocharging and hybridisation push the Otto engine towards higher efficiency while electric drives begin to complement or replace it.
Practical applications
- Motor vehicles: the Otto engine is the classic drive of cars and motorcycles.
- Small engines: lawnmowers, chainsaws and generators mostly work as single cylinder Otto engines.
- Marine drives: outboard motors use the same working principle.
- Combined heat and power plants: gas powered Otto engines generate electricity and heat at the same time.
Modern developments and outlook
Even though electric vehicles are gaining market share, the Otto engine continues to evolve:
- Downsizing and turbocharging: smaller engines with a turbocharger deliver the same power as larger naturally aspirated engines but consume less fuel.
- Direct injection: injecting fuel at high pressure directly into the combustion chamber improves mixture preparation and efficiency.
- Variable valve timing: electronically controlled camshaft phasing or fully variable valve trains optimise the gas exchange across the entire speed range.
- Miller / Atkinson cycle: modified valve timing effectively reduces the compression ratio relative to the expansion ratio, raising thermodynamic efficiency at the cost of peak power.
- Hybridisation: combining the Otto engine with an electric motor allows the engine to run in its most efficient operating point more often.
- Synthetic fuels (efuels): produced from renewable electricity and captured CO₂, these fuels can make existing Otto engines carbon neutral without hardware changes.
The four stroke Otto engine is therefore far from obsolete. As a mass produced and understood technology, it will remain part of the powertrain mix for years to come – in combination with electrification and sustainable fuels.
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Overview
| Title | Four stroke Otto engine |
| Target audience | Teachers and lecturers |
| Features | Fullscreen mode lossless magnification large screens and projection supported |
| License | MIT The technical drawing of the engine included in the software is not covered by the MIT license. The copyright is held by Ulrich Rapp. |
Contributors
C. Hein, S. Rikowski
References
Technical drawing of the engine: based on Ulrich Rapp (http://www.ulrich-rapp.de/stoff/fahrzeug/motor/index.htm)
I want more explanation about petrol engine and it’s part with their functions