What is a combustion engine? How it works, why it matters, and where it’s headed

A combustion engine is a machine that converts the chemical energy of fuel into mechanical work by burning the fuel with air to create high-pressure gases that push pistons or spin turbines; in most vehicles, this is an internal combustion engine (ICE) using a four-stroke cycle. Put simply, fuel + air burn inside the engine, the expanding gases move parts, and that motion ultimately turns the wheels or a shaft.

Definition and core principle

A combustion engine transforms the heat released by an exothermic reaction into useful motion. In an internal combustion engine, the fuel–air mixture burns inside the engine’s cylinders (or chambers), and the rising pressure drives a piston, rotor, or turbine. In an external combustion engine—like a steam engine—the fuel burns outside, heating a working fluid that then expands to do work. Today’s cars, motorcycles, small aircraft, and many generators use internal combustion; large power plants and some ships rely on turbines or very large diesels.

Key components and what they do

To understand how an internal combustion engine functions, it helps to know its main parts and their roles, especially in a typical piston (reciprocating) design.

• Cylinder and piston: The chamber where air and fuel are compressed and burned; the piston moves up and down under gas pressure.
• Crankshaft and connecting rod: Convert the piston’s up–down motion into rotational motion, delivering torque to the drivetrain.
• Valves and camshaft(s): Control the timing of air–fuel intake and exhaust outflow; variable valve timing adjusts this on the fly.
• Fuel system: Stores and meters fuel; modern engines use electronic fuel injection—port or direct.
• Ignition system (spark-ignition engines): Coil(s) and spark plugs ignite the mixture; timing is controlled electronically.
• Air system: Intake tract, throttle (or variable valves), air filter, and often a turbocharger/supercharger and intercooler to increase air density.
• Lubrication and cooling: Oil pump and galleries reduce friction and wear; water jackets, a pump, and radiator carry heat away.
• Exhaust and aftertreatment: Manifold, catalytic converter(s), particulate filter(s), and, for diesels, selective catalytic reduction (SCR) to cut pollutants.
• Engine control unit (ECU) and sensors: Manage fueling, spark, boost, and emissions using data from oxygen, knock, temperature, and pressure sensors.

Together, these systems admit and prepare air and fuel, burn them under controlled conditions, convert pressure into motion, and clean up the exhaust while protecting the engine from wear and overheating.

The four-stroke cycle (spark-ignition gasoline)

Most modern gasoline engines use a four-stroke process per cylinder, coordinated by the crankshaft and camshafts.

1. Intake: The intake valve opens; the descending piston draws in air and fuel (direct-injection engines add fuel inside the cylinder).
2. Compression: Both valves close; the piston rises, compressing the mixture to raise temperature and promote efficient burning.
3. Power (combustion): Near top dead center, the spark plug ignites the mixture; rapidly rising pressure drives the piston down, making torque.
4. Exhaust: The exhaust valve opens; the rising piston expels combustion products to the exhaust system.

This repeatable sequence turns chemical energy into rotational energy. Engine speed (RPM) indicates how many times per minute this cycle occurs for each cylinder’s power stroke.

Diesel (compression-ignition) and other cycles

Diesel engines compress only air to very high pressures; fuel is injected into the hot air near top dead center and ignites without a spark. This enables higher compression ratios and often better peak thermal efficiency than spark-ignition engines. Beyond the familiar Otto (gasoline) and Diesel cycles, designers use variations to trade power, efficiency, and emissions.

Here are common thermodynamic cycles and architectures you’ll encounter:

• Otto cycle (gasoline SI): Near-stoichiometric mixture and spark ignition; widely used in cars and small engines.
• Diesel cycle (CI): Higher compression, lean burn, and direct injection; dominant in heavy-duty vehicles and many generators.
• Atkinson/Miller: Intake or effective compression shortened to reduce pumping losses and improve efficiency; common in hybrids.
• Two-stroke: Power every crank revolution with scavenging instead of dedicated intake/exhaust strokes; prevalent in small engines and some marine diesels.
• Rotary (Wankel): Triangular rotor in an epitrochoid housing provides smooth power but challenges sealing and emissions.

These strategies shape how and when heat is added and removed, influencing efficiency, power density, and emissions.

Air, fuel, and ignition management

The engine targets a mixture and burn profile the ECU can control precisely. Gasoline engines typically operate near a 14.7:1 air–fuel mass ratio for catalytic converters to work best, with stratified or lean modes used selectively. Direct injection in gasoline engines boosts knock resistance and can inject at up to roughly 200–350 bar; diesels use common-rail injection frequently exceeding 2,000 bar (some heavy-duty systems exceed 2,500 bar) for fine atomization and multiple injection events. Turbochargers and superchargers increase intake air mass; intercoolers reduce charge temperature to improve knock resistance and efficiency. Knock (uncontrolled end-gas autoignition) limits compression ratio and boost in gasoline engines; algorithms, knock sensors, high octane fuels, cooled EGR, and water or water–methanol injection in some applications mitigate it. Advanced systems like variable compression ratio, variable valve timing/lift, and Mazda’s spark-controlled compression ignition (SPCCI) extend efficient operating zones.

Efficiency and performance

Only part of the fuel’s energy becomes wheel work; the rest is lost to heat, exhaust, and friction. Modern naturally aspirated gasoline engines peak around 35–40% thermal efficiency in optimized hybrid applications; advanced diesels can exceed 45% in light-duty and approach or surpass 50% in heavy-duty use. Racing hybrid power units (e.g., Formula 1) have demonstrated over 50% system efficiency. Key gains come from higher compression, downsizing with turbocharging, direct injection, friction reduction, better combustion phasing, and waste-heat recovery. Hybridization (from 48‑V mild hybrids to full hybrids and plug-ins) lets engines run closer to their sweet spots and shut off at idle, cutting fuel use and emissions.

Emissions and aftertreatment

Combustion produces carbon dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (HC), and particulate matter (PM). Gasoline engines use three-way catalytic converters (most effective at stoichiometric mixtures) and, with direct injection, often gasoline particulate filters (GPFs). Diesels rely on high EGR rates to temper NOx formation, diesel particulate filters (DPFs) to trap soot, and SCR systems with urea (AdBlue/DEF) to convert NOx to nitrogen and water. Cold starts are the dirtiest phase until catalysts heat up, which is why rapid light-off strategies and close-coupled catalysts are crucial. Tight standards—such as Euro 6/7 in Europe, Tier 3/LEV III in the U.S., China 6b, and Bharat Stage VI—require on-board diagnostics and real driving emissions (RDE) conformity.

Lubrication, cooling, and durability

Engines survive by controlling heat and friction. A pressurized oil system delivers lubricant to bearings, cylinder walls, and valvetrain surfaces; synthetic oils and low-viscosity formulations cut drag while maintaining protection. A liquid cooling system (water–glycol, pump, thermostat, radiator, sometimes electric pumps and split cooling circuits) carries away heat. PCV systems manage blow-by gases. Regular oil and filter changes, proper coolant, air filtration, and attention to timing belts or chains are fundamental to longevity.

Variants and fuels

Combustion engines can run on several fuels and take different forms tailored to use cases and infrastructure.

• Gasoline (petrol): Spark-ignition, high-RPM capable, widely used in light-duty vehicles and small engines.
• Diesel: Compression-ignition, high torque and efficiency, common in trucks, buses, and marine.
• Natural gas and propane: Cleaner-burning alternatives with lower CO2 per unit energy; often spark-ignited.
• Ethanol and flex-fuel: Higher octane enables higher compression; energy density is lower than gasoline.
• Hydrogen ICE: Burns hydrogen with near-zero CO2 but can form NOx; requires special mixtures, injection, and backfire control.
• Synthetic e-fuels: Drop-in hydrocarbons made from green hydrogen and captured CO2; can leverage existing engines and infrastructure but are currently costly and energy-intensive to produce.
• Gas turbines (Brayton cycle): Continuous-combustion engines for aircraft and some power generation, distinct from reciprocating ICEs.
• External combustion (steam): Fuel burns outside the working cylinder; now niche but foundational historically.

Fuel choice affects efficiency, emissions, and practicality; infrastructure and cost often decide what’s feasible at scale.

How power gets to the wheels

The crankshaft’s rotation passes through a transmission that adapts engine speed and torque to road demands. Automatic, dual-clutch, and continuously variable transmissions keep the engine in efficient regions; hybrids may use power-split devices to blend engine and electric motor output seamlessly.

Combustion engines vs. electric motors

Electric motors deliver instant torque and far higher drivetrain efficiency, with lower local emissions. Combustion engines offer quick refueling and high energy density in the fuel, favoring long-range and heavy-duty applications today. Many jurisdictions are phasing in zero-emission sales targets—such as the EU’s 2035 new-car CO2 standard aiming for zero tailpipe emissions with a limited pathway for vehicles running exclusively on e-fuels, and California’s 2035 rule requiring 100% zero-emission vehicle sales with limited plug-in hybrid allowances—pushing a shift toward electrification and low-carbon fuels. Hybrids remain a practical bridge, trimming fuel use and emissions while retaining conventional refueling.

Common misconceptions

A few persistent myths can cloud understanding of combustion engines; here’s what the evidence shows.

• Engines “explode”: Under normal operation, combustion is controlled and rapid, not an explosion; true detonation (knock) is a fault condition.
• Diesels are inherently dirty: With DPFs and SCR, modern diesels can be extremely clean; issues arise when systems are tampered with or poorly maintained.
• Turbochargers always hurt economy: Appropriately sized turbos can improve part-load efficiency by enabling downsizing and reduced pumping losses.
• Higher octane means more energy: Octane measures knock resistance, not energy content; it allows more aggressive tuning without increasing the fuel’s intrinsic energy.

Understanding these points helps explain why engine design choices vary and why maintenance and fuel selection matter.

The bottom line

A combustion engine burns fuel with air to create high-pressure gases that push on a mechanical element—usually a piston—turning heat into motion. Its performance and cleanliness depend on the combustion strategy, control systems, aftertreatment, and fuel. While electrification is growing rapidly, combustion engines remain vital in many sectors, increasingly paired with hybrid systems and cleaner fuels.

Summary

A combustion engine converts fuel’s chemical energy into mechanical work via controlled burning, most commonly inside cylinders in a four-stroke internal combustion engine. Key subsystems—air, fuel, ignition, lubrication, cooling, and exhaust aftertreatment—coordinate to deliver power, efficiency, and low emissions. Variants (Otto, Diesel, Atkinson/Miller, two-stroke, rotary) and fuels (gasoline, diesel, gas, biofuels, hydrogen, e-fuels) tailor engines to different needs. Advances in injection, turbocharging, variable control, and hybridization have pushed efficiency higher and emissions lower, even as policy and market trends accelerate the shift toward electrified transportation.

What is the simple explanation of a combustion engine?

In an internal combustion engine (ICE), the ignition and combustion of the fuel occurs within the engine itself. The engine then partially converts the energy from the combustion to work. The engine consists of a fixed cylinder and a moving piston.

Why are combustion engines being banned?

Reasons for banning the further sale of fossil fuel vehicles include: reducing health risks from pollution particulates, notably diesel PM10s, and other emissions, notably nitrogen oxides; meeting national greenhouse gas, such as CO2, targets under international agreements such as the Kyoto Protocol and the Paris …

What is the simplest explanation of combustion?

Combustion is a type of chemical reaction between a fuel and an oxidant, usually oxygen, that produces energy in the form of heat and light, most commonly as a flame. Because it produces more heat energy than it consumes, combustion is an exothermic reaction.

Why are car makers going back to combustion engines?

Instead of focusing on a purely electric future, it is looking to produce more combustion and hybrid models again. Customers in both the US and Europe have been slower to switch to electric cars than many manufacturers had hoped, due to problems with the charging infrastructure and high purchase prices.