What are the four strokes in a four-stroke engine?

The four strokes are Intake (fresh charge enters), Compression (charge is compressed), Power (combustion pushes piston down), and Exhaust (burnt gases are expelled). One complete cycle requires 720 degrees of crankshaft rotation.

What is the difference between Otto and Diesel cycles?

The Otto cycle (petrol engine) uses constant-volume heat addition with spark ignition, while the Diesel cycle uses constant-pressure heat addition with compression ignition. Diesel engines have higher compression ratios (14-22) compared to petrol engines (8-12) and are generally more thermally efficient.

How do you calculate the thermal efficiency of a four-stroke engine?

For the Otto cycle: efficiency = 1 - 1/r^(gamma-1), where r is the compression ratio and gamma is 1.4 for air. For the Diesel cycle: efficiency = 1 - (1/r^(gamma-1)) x [(rho^gamma - 1)/(gamma x (rho - 1))], where rho is the cutoff ratio.

Four Stroke Engine Simulator

Otto & Diesel Cycles — Animate • Learn • Practice • Quiz

Mode

Fuel

RPM 10

Comp. Ratio 10

Stroke Intake

Crank Angle 0°

Pressure 101 kPa

Temperature 300 K

Efficiency 0%

Cycle Otto

How a Four-Stroke Engine Works — Interactive Simulator

The four-stroke internal combustion engine is the most widely used power source in automobiles, motorcycles, generators, and industrial machinery. Each complete power cycle requires four distinct strokes of the piston—Intake, Compression, Power, and Exhaust—corresponding to two full revolutions (720°) of the crankshaft. This simulator lets you visualize every stroke in real time, switch between petrol (Otto cycle) and diesel (Diesel cycle) operation, and adjust parameters like compression ratio and RPM to observe their effects on pressure, temperature, and thermal efficiency.

Understanding the Otto and Diesel Cycles

In a petrol engine, the Otto cycle governs the thermodynamic process. The air-fuel mixture is compressed isentropically, then ignited by a spark plug at top dead center (TDC), producing a rapid constant-volume pressure rise. The thermal efficiency depends only on the compression ratio: η = 1 − 1/r^γ−1, where r is the compression ratio and γ is 1.4 for air. Typical petrol engines operate at compression ratios of 8:1 to 12:1, yielding efficiencies of 56–63%.

The Diesel cycle differs in its heat addition process: air alone is compressed to a high temperature (600–900 K), and fuel is injected under pressure, igniting spontaneously. This constant-pressure combustion introduces the cutoff ratio ρ, making the efficiency formula more complex. Diesel engines use higher compression ratios (14:1 to 22:1) and achieve better fuel economy, though the Diesel cycle efficiency for the same compression ratio is slightly lower than Otto due to the cutoff ratio penalty.

Key Engine Parameters

The compression ratio (r = V₁/V₂) is the ratio of cylinder volume at bottom dead center to top dead center. Higher compression ratios improve efficiency but increase mechanical stress and require higher octane fuel (petrol) or stronger construction (diesel). The mean effective pressure (MEP) represents the average pressure acting on the piston during the power stroke and directly relates to engine torque output. The simulator calculates these values in real time as you adjust the sliders.

Reading the PV Diagram

The pressure-volume (PV) diagram shows the thermodynamic state of the gas throughout the cycle. The enclosed area represents the net work output per cycle. In the Otto cycle, you’ll see two vertical lines (constant-volume processes) connected by two isentropic curves. In the Diesel cycle, the upper horizontal line represents the constant-pressure heat addition phase. Watch the PV diagram trace in real time as the piston completes each stroke.

Who Uses This Simulator?

This interactive four-stroke engine simulator is designed for mechanical engineering students, automotive technology trainees, TVET instructors, and anyone studying thermodynamics or internal combustion engines. The Explore mode provides concept cards with formulas and worked examples, while Practice and Quiz modes test your understanding of cycle efficiency, compression ratios, and engine performance calculations.

Explore Related Simulators

If you found this four-stroke engine simulator helpful, explore our Two Stroke Engine Simulator, Thermodynamics Simulator, Heat Transfer Simulator, and Stress-Strain Diagram for more hands-on practice.