How a Combustion Engine Works

A combustion engine turns the chemical energy in fuel into mechanical motion by burning a precisely metered air–fuel mixture inside cylinders, pushing pistons that spin a crankshaft to deliver power. In practice, most road vehicles use a four-stroke cycle—intake, compression, power, exhaust—managed by an engine control unit (ECU) that times fuel delivery, ignition, and valve events for performance, efficiency, and emissions compliance.

The Core Idea: From Fuel to Motion

Inside each cylinder, a mixture of air and fuel is compressed and ignited. The resulting hot, high-pressure gases expand, forcing the piston down. This straight-line motion is converted into rotation by the crankshaft and transmitted through the drivetrain to the wheels. Gasoline engines typically ignite the charge with a spark, while diesels compress air until it’s hot enough for injected fuel to self-ignite. The throttle (gasoline) or fuel metering (diesel) determines how much energy is released each cycle, controlling power output.

The Four-Stroke Cycle

The following sequence describes the four strokes in a typical gasoline (Otto cycle) or diesel engine, repeated hundreds of times per second across multiple cylinders.

1. Intake: The intake valve opens and the piston moves down, drawing in fresh air (and, in port-injected gasoline engines, air mixed with vaporized fuel).
2. Compression: The valves close and the piston moves up, compressing the air–fuel mixture (gasoline) or just air (diesel) to raise temperature and pressure.
3. Power (combustion/expansion): For gasoline, a spark plug ignites the compressed mixture near top dead center; for diesel, fuel is injected into hot, compressed air and auto-ignites. Expanding gases push the piston down to deliver work.
4. Exhaust: The exhaust valve opens and the piston moves up, expelling combustion products into the exhaust system.

Together, these strokes form one complete cycle per cylinder. Multi-cylinder engines stagger cycles for smooth power delivery; at typical cruising speeds, each cylinder completes dozens of cycles per second.

What Changes in a Diesel?

Diesel engines run higher compression ratios, inject fuel directly into very hot air, and rely on compression ignition rather than a spark. They usually operate with excess air (lean), often without a conventional throttle, and favor turbocharging for efficiency and torque. Cold starts are assisted by glow plugs. Their thermodynamic efficiency can surpass comparable gasoline engines, but controlling nitrogen oxides (NOx) and particulates requires robust aftertreatment.

Key Components You Should Know

The systems below work together to admit air, meter fuel, time combustion, convert linear motion to rotation, and manage heat and emissions.

• Cylinders, pistons, and rings: Form the combustion chamber; rings seal gases and control oil.
• Connecting rods, crankshaft, and flywheel: Convert piston motion into smooth rotational power.
• Valves, camshaft(s), and timing belt/chain: Open/close intake and exhaust passages; timing is critical for breathing and efficiency.
• Fuel system: Pump, lines, injectors (or carburetor in older designs) deliver precise fuel quantities and spray patterns.
• Air system: Intake manifold, air filter, throttle body (gasoline), and often turbocharger/supercharger with an intercooler.
• Ignition (gasoline): Spark plugs, coils, and ECU-controlled timing ignite the charge.
• Sensors and control: Oxygen (lambda), MAF/MAP, knock, crank/cam position, temperature/pressure sensors feed the ECU for closed-loop control.
• Exhaust and aftertreatment: Manifold, catalytic converter, gasoline particulate filter (in many direct-injection engines), diesel particulate filter (DPF), and selective catalytic reduction (SCR) in diesels.
• Cooling: Water pump, radiator, thermostat, fans, and passages manage temperature.
• Lubrication: Oil pump, galleries, and filter provide low-friction films and heat removal.

In modern engines, the ECU orchestrates these components millisecond by millisecond, continuously adapting to load, speed, temperature, and emissions constraints.

Thermodynamic Cycles and Efficiency

Most gasoline engines follow the Otto cycle, while diesels use the Diesel cycle; some adopt Atkinson/Miller-like valve timing to trade peak power for higher efficiency. Efficiency rises with compression ratio and effective expansion ratio but is limited by knock (gasoline), materials, and heat losses. Contemporary peak brake thermal efficiency typically ranges around 35–41% for advanced gasoline engines (helped by direct injection, high compression, cooled EGR, and variable valve timing) and even higher for modern diesels, especially in heavy-duty applications. Remaining energy is lost as heat to exhaust and coolant, as well as friction and pumping work.

Control and Timing

Electronic control units coordinate fuel injection timing and duration, spark timing (gasoline), turbo boost, and variable valve timing/lift. Knock sensors let the ECU advance ignition near the knock limit for efficiency. Oxygen sensors enable stoichiometric combustion with three‑way catalysts in gasoline engines; diesels run lean and rely on DPFs and SCR for emissions. Drive-by-wire throttles, cylinder deactivation, and start–stop systems are common strategies to cut fuel use and emissions in real-world driving.

Forced Induction

To increase power and efficiency, many engines compress the intake air so more oxygen (and fuel) fits into each cycle. The two main approaches are outlined below.

• Turbocharger: An exhaust-driven turbine spins a compressor to pressurize intake air. Wastegates and variable-geometry turbines control boost; intercoolers reduce charge temperature.
• Supercharger: A mechanically driven compressor (Roots, twin-screw, or centrifugal) provides immediate boost but consumes some engine power to operate.

Forced induction increases specific output and can enable engine downsizing, but adds complexity, heat management demands, and transient response trade-offs.

Emissions and Aftertreatment

Combustion produces carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), and particulates (PM). Gasoline engines operating at stoichiometric air–fuel ratios use three-way catalytic converters; direct-injection gasoline engines increasingly add gasoline particulate filters. Diesels combine cooled EGR with DPFs for soot and SCR (urea/AdBlue) for NOx. Current regulatory frameworks (such as Euro 6/7 in Europe and Tier 3/LEV III in the U.S.) drive tighter control of cold-start emissions, on-road (RDE) performance, and durability of aftertreatment systems.

Supporting Systems: Lubrication and Cooling

Pressurized oil forms protective films on bearings and cylinder walls, reduces friction, and helps cool hot components like pistons. Modern oils and variable oil pumps balance protection with efficiency. Cooling systems circulate coolant through the block and head to the radiator, with thermostats and electric fans maintaining optimal temperatures; separate circuits may cool the turbocharger, EGR, or hybrid components. Rapid warm-up strategies reduce fuel use and emissions during the most critical cold-start phase.

Variations: Two-Stroke and Rotary

Two-stroke engines complete one power event per crank revolution using ports instead of valves, yielding high power density at the cost of emissions and efficiency; they remain common in small tools and some marine applications, with modern direct-injected designs improving cleanliness. Rotary (Wankel) engines use a triangular rotor within an epitrochoid housing to create expanding chambers, offering smooth, compact power but challenging sealing and emissions; they’ve recently reappeared in select range-extender roles.

From Ignition to Wheels: Drivetrain Context

Engine torque passes through a clutch or torque converter to a transmission (manual, automatic, dual-clutch, or CVT), then to the differential and axles. Features like cylinder deactivation, variable compression (as in some VC‑Turbo engines), and hybridization (mild 48‑V belt starter‑generators to full hybrid power-split systems) keep engines operating closer to their most efficient zones while meeting real-time power demands.

What Limits Performance and Reliability

The points below highlight common constraints and how engineers or owners mitigate them.

• Knock and low-speed pre-ignition (LSPI) in turbocharged, direct-injection gasoline engines, managed via calibration, hardware, and oil formulations.
• Thermal management challenges at high load; robust cooling and careful ignition/boost control prevent detonation and component stress.
• Friction and pumping losses, reduced by low-viscosity oils, optimized bearing surfaces, and variable valve strategies.
• Intake valve carbon buildup in direct-injection gasoline engines, addressed with dual injection (port + DI), periodic cleaning, or catch cans.
• Oil degradation and contamination; timely oil and filter changes preserve wear protection and turbo health.
• Aftertreatment upkeep: O2/NOx sensor health, DPF regeneration on diesels, and ensuring proper SCR fluid levels.

Design improvements and regular maintenance keep engines efficient, clean, and durable, but the most demanding conditions—cold starts, short trips, and heavy loads—benefit most from attentive care.

Safety and Environmental Footprint

Engines produce toxic exhaust; never run them in enclosed spaces. Fuel handling requires care, and hot components can ignite spills. From an environmental perspective, internal combustion emits CO2 and pollutants; while modern engines and fuels have cut per‑mile emissions drastically, policy and market trends are nudging more duty cycles toward electrification and hybrid systems to reduce total emissions, especially in urban driving.

Summary

A combustion engine works by igniting a compressed air–fuel charge to create high-pressure gases that push pistons and turn a crankshaft, with electronics managing timing, fueling, and emissions. Four coordinated strokes, carefully controlled air and fuel delivery, and supporting systems for lubrication, cooling, and exhaust treatment turn chemical energy into usable motion efficiently and reliably. Variations like diesels, forced induction, and hybridization tailor the same fundamental process to different performance, efficiency, and regulatory goals.