How an Internal Combustion Engine Works

An internal combustion engine (ICE) turns the chemical energy of fuel into mechanical work by burning a fuel–air mixture inside cylinders; the expanding hot gases push pistons that rotate a crankshaft, producing power. Most road engines follow a four-stroke cycle—intake, compression, power, exhaust—coordinated by valves and, in gasoline engines, a spark; diesels ignite fuel by compression heat. Modern ICEs add precise electronic control, turbocharging, and emissions after-treatment to improve efficiency and reduce pollution.

The Core Cycle That Produces Power

At the heart of a typical automotive ICE is the four-stroke cycle, which describes the repeating sequence that fills the cylinder with air (and sometimes fuel), compresses it, releases energy via combustion, and expels the exhaust. Each stroke is half a crankshaft turn; together they make one full cycle per two revolutions.

• Intake: The intake valve opens as the piston travels down, drawing in air (gasoline engines often mix fuel either here or directly in-cylinder; diesels draw air only).
• Compression: Both valves close; the piston moves up, compressing the charge to raise temperature and pressure.
• Power (Combustion/Expansion): Gasoline engines spark the compressed mixture; diesels inject fuel into hot, compressed air. Rapid combustion forces the piston down, delivering work.
• Exhaust: The exhaust valve opens; the piston rises to push out combustion products, readying the cylinder for the next cycle.

This sequence converts expanding gas pressure into linear piston motion that the connecting rod turns into crankshaft rotation, ultimately driving the wheels or other machinery.

Key Mechanical Components

The ICE relies on a coordinated set of moving parts and supporting systems to turn intermittent combustion events into smooth, continuous output.

• Cylinders, pistons, rings: Form the combustion chamber and seal gases while minimizing friction and oil loss.
• Connecting rods and crankshaft: Convert reciprocating motion into rotary motion.
• Valves, camshaft(s), and timing drive: Control intake and exhaust events; modern engines may use variable timing and lift.
• Fuel system: High-pressure pump and injectors meter fuel; direct injection places fuel straight into the cylinder.
• Ignition system (gasoline): Coils and spark plugs precisely ignite the mixture.
• Turbocharger/supercharger: Compress incoming air to increase torque and power without enlarging the engine.
• Cooling system: Pump, radiator, thermostat, and passages manage heat to prevent knock and component damage.
• Lubrication system: Oil pump, galleries, and filter reduce friction and wear, carrying away heat and contaminants.
• Engine control unit (ECU) and sensors: Monitor airflow, temperature, oxygen, knock, and more to adjust fueling, spark, and boost in real time.

Together, these systems synchronize airflow, fuel delivery, ignition, and exhaust timing to keep combustion stable, efficient, and clean across operating conditions.

Gasoline vs. Diesel: Two Ways to Ignite

While the mechanical cycle is similar, gasoline and diesel engines differ in how they mix and ignite fuel, affecting efficiency, power delivery, and emissions.

Gasoline (Spark-Ignition, Otto/Atkinson-Miller)

Gasoline engines typically run near a stoichiometric air–fuel ratio for catalytic converter efficiency. They mix fuel via port injection or in-cylinder direct injection. A spark plug initiates flame propagation; variable valve timing, cooled EGR, and sometimes Atkinson/Miller valve timing reduce pumping losses and knock, boosting efficiency. Peak brake thermal efficiency (BTE) in modern passenger gasoline engines can exceed 40% in optimized hybrid-oriented designs.

Diesel (Compression-Ignition, Diesel Cycle)

Diesels draw air only, compress it to high ratios (often 15:1–20:1), and inject fuel into the hot air, achieving autoignition. They operate lean over much of the map, yielding higher efficiency (passenger/light-duty peaks ~40–45%; heavy-duty highway diesels ~45–46%). Combustion and after-treatment strategies—high-pressure common-rail injection, variable geometry turbochargers, high EGR rates, and selective catalytic reduction with urea—control NOx and particulates.

From Heat to Work: Thermodynamics and Efficiency

Efficiency depends on compression ratio, combustion timing, heat losses, friction, and how fully expansion captures combustion energy. Idealized cycles (Otto, Diesel, Atkinson/Miller) describe limits; real engines optimize toward those limits with hardware and control.

• Compression ratio: Higher ratios raise theoretical efficiency but are limited by knock (gasoline) and mechanical stress.
• Boosting: Turbo/supercharging increases charge density for more torque from smaller engines; intercooling reduces knock and NOx formation.
• Valve strategies: Variable timing/lift and Atkinson/Miller reduce pumping losses and enable high-efficiency operation in hybrids.
• Friction and heat management: Low-tension rings, advanced coatings, and optimized cooling reduce losses.
• Hybridization: Electric motors cover transients and low-load zones where ICEs are inefficient, letting the engine run closer to peak efficiency.

State-of-the-art gasoline hybrid engines now achieve BTE just above 40%, while large marine two-stroke diesels exceed 50% under steady loads; Formula 1 hybrid power units surpass 50% when counting energy recovery systems.

Emissions and How They’re Controlled

Combustion creates pollutants that must be minimized in-cylinder and then treated in the exhaust stream to meet regulations.

• Carbon monoxide (CO) and unburned hydrocarbons (HC): Reduced via precise fueling, good mixing, and three-way catalysts (gasoline).
• Nitrogen oxides (NOx): Lowered with cooled EGR, lean-burn strategies, and after-treatment—three-way catalysts (stoichiometric gasoline) or SCR systems (diesel/lean gasoline).
• Particulate matter (PM): Mitigated with optimized injection and gasoline or diesel particulate filters (GPF/DPF), now common with direct injection.
• Carbon dioxide (CO2): Proportional to fuel used; cut by efficiency measures, downsizing/boosting, hybridization, and low-carbon fuels.

Modern ECUs coordinate combustion and after-treatment temperatures through late injections, spark timing, and air–fuel control to keep catalysts active and effective across driving conditions.

Control Electronics: The Engine’s Nervous System

Real-time electronic control enables today’s performance, efficiency, and emissions levels by constantly adapting to conditions.

• Sensors: Mass airflow, manifold pressure, oxygen/air–fuel ratio (lambda), knock, crank/cam position, temperature, and exhaust after-treatment monitors.
• Actuators: Injectors, ignition coils, throttle, variable valve timing and lift devices, turbo wastegate/variable geometry, EGR valves.
• Software: Maps and models manage transient response, cold starts, catalyst light-off, knock avoidance, and adaptive learning for fuel quality and aging components.

This closed-loop system keeps combustion stable and clean from idle to full load, in heat, cold, altitude, and over the vehicle’s lifetime.

Lubrication and Cooling: Keeping It Alive

Continuous combustion and high pressures demand robust thermal and friction management to ensure durability and efficiency.

• Oil system: Pressurized oil forms protective films on bearings and cylinder walls, removes heat, and suspends contaminants for filtration.
• Cooling system: Liquid coolant circulates through the block and head, regulated by a thermostat and often electric pumps; some engines use split cooling for faster warm-up.
• Detonation control: Knock sensors and cooling strategies prevent damaging autoignition in gasoline engines; piston cooling jets in high-load engines manage crown temperatures.

Proper oil viscosity, quality, and change intervals, along with intact cooling components, are critical to minimize wear and sustain efficiency.

Variants and Modern Innovations

Beyond the conventional four-stroke, manufacturers deploy specialized designs and features to meet performance and efficiency goals.

• Two-stroke and rotary (Wankel): Higher power density but more challenging emissions; niche uses persist.
• Cylinder deactivation and variable displacement: Shut valves and fuel to selected cylinders at light loads to reduce pumping and friction losses.
• Variable compression ratio (VCR): Mechanically alters compression (e.g., multi-link crank systems) to balance efficiency and power.
• Homogeneous charge compression ignition (HCCI)/SPCCI: Controlled compression ignition in gasoline engines for diesel-like efficiency at some loads.
• Start–stop and 48V mild hybrids: Cut idling fuel use and assist transients.
• Alternative fuels: Ethanol blends, compressed natural gas (CNG), liquefied petroleum gas (LPG), synthetic fuels, and even hydrogen ICEs reduce fossil CO2 or tailpipe pollutants when infrastructure allows.

These innovations target the ICE’s weak spots—light-load efficiency, transient response, and emissions—while leveraging its high power density and established manufacturing base.

What Happens in Practice

In real driving, the ECU constantly trades off torque demand, efficiency, and after-treatment needs. Cold starts enrich mixtures to light catalysts quickly; turbo boost ramps with load; at highway cruise, modern engines may deactivate cylinders or run Atkinson timing; under heavy load, enrichment or additional EGR and intercooling protect components and control NOx and knock.

Summary

An internal combustion engine burns fuel inside sealed cylinders to create high-pressure gases that push pistons and turn a crankshaft. The four-stroke cycle, managed by valves, fueling, and ignition, converts this pressure into motion, while electronics, turbocharging, lubrication and cooling, and exhaust after-treatment make the process efficient, reliable, and clean. Gasoline and diesel engines differ in how they ignite fuel, but both rely on precise control to meet today’s performance and emissions targets, increasingly alongside hybridization and advanced combustion strategies.