Internal combustion engine, explained

An internal combustion engine is a heat engine that generates mechanical power by burning fuel inside the engine itself, converting the energy released from combustion into motion via pistons or turbines. It remains the dominant power source for road transport, many ships and small aircraft, even as electric powertrains and low‑carbon fuels rapidly advance.

What it is and what it covers

In technical terms, an internal combustion engine (ICE) releases chemical energy by combusting a fuel–air mixture within a confined space, increasing pressure and temperature; that high‑pressure gas then does work on moving parts. The term broadly includes reciprocating piston engines (gasoline and diesel) and gas turbines, though in everyday use it most often refers to piston engines in cars, trucks, motorcycles, and machinery.

How it works

The four-stroke gasoline (Otto) cycle

Most passenger cars use a four‑stroke piston engine operating on the Otto cycle, where intake, compression, combustion/expansion, and exhaust happen in sequence for each cylinder.

1. Intake: The intake valve opens, the piston moves down, and a fuel–air mixture (port‑injected or directly injected) enters the cylinder.
2. Compression: Valves close and the piston moves up, compressing the mixture to raise temperature and pressure.
3. Power (combustion/expansion): A spark plug ignites the mixture near top dead center; expanding gases push the piston down, delivering torque to the crankshaft.
4. Exhaust: The exhaust valve opens, the piston moves up, and spent gases exit to the exhaust system and catalytic converter.

This repeating sequence happens thousands of times per minute across multiple cylinders, with timing, injection, and valve events coordinated by the engine control unit to balance power, efficiency, and emissions.

The diesel (compression-ignition) cycle

Diesel engines compress only air to a higher ratio than gasoline engines, then inject fuel into the hot, high‑pressure air so it self‑ignites without a spark plug. They deliver strong low‑end torque and higher peak efficiency, especially under steady loads.

Key differences from gasoline engines include the ignition method, higher compression ratios, and a different emissions profile that requires distinct aftertreatment systems.

• No spark ignition: Fuel auto‑ignites from heat of compression.
• Higher compression: Typical ratios 14:1 to 20:1 for light‑duty diesels.
• Different controls: High‑pressure common‑rail injection, exhaust gas recirculation (EGR), diesel particulate filters (DPF), and selective catalytic reduction (SCR) for NOx control.

Together, these design choices make diesels efficient and durable for heavy transport and marine uses, while necessitating robust emissions controls to meet regulations.

Two-stroke and rotary variations

Two‑stroke engines combine the strokes to produce power every crankshaft revolution, achieving high power density for small engines (chainsaws, some motorcycles) and very large low‑speed marine diesels. Rotary (Wankel) engines use a triangular rotor in an epitrochoidal housing to achieve smooth, high‑rev power with fewer moving parts; they saw niche automotive use and are experiencing a revival as compact range extenders.

Core components you’ll find in most ICEs

Although designs vary, most piston engines share common subsystems for air handling, fueling, combustion, and heat management.

• Block, cylinders, pistons, connecting rods, and crankshaft (the mechanical core)
• Cylinder head with intake/exhaust valves, camshafts, and spark plugs or injectors
• Intake system with air filter, throttle (gasoline), and often turbocharger/supercharger and intercooler
• Fuel system: tank, pump, rails, and injectors (port or direct injection)
• Engine control unit (ECU) with sensors (oxygen/λ, MAF/MAP, knock, temperature)
• Lubrication: oil pump, galleries, filter; Cooling: water pump, radiator, thermostat
• Exhaust aftertreatment: three‑way catalytic converter (gasoline), DPF and SCR (diesel)
• Starter, alternator, and, in hybrids, an integrated motor‑generator

Together, these systems meter air and fuel precisely, control combustion timing, manage heat, and clean exhaust to meet performance and emissions targets.

Performance, efficiency, and emissions

Efficiency in practice

Thermal efficiency varies widely by design and operating point. Modern gasoline engines typically reach peak brake thermal efficiency around 35–41% in optimized hybrid‑style Atkinson/Miller configurations, while conventional non‑hybrid gasoline engines often peak in the high‑20s to low‑30s. Light‑duty diesels can exceed 40–45% under favorable conditions, and the largest slow‑speed marine diesels surpass 50%. Real‑world average efficiency is lower because engines rarely operate at their peak point across a drive cycle.

Pollutants and how they are controlled

ICEs emit both climate and air‑quality pollutants. Understanding the major pollutants helps explain modern control technologies.

• CO2: A direct result of carbon in the fuel; closely tied to fuel consumption.
• NOx (nitrogen oxides): Formed at high combustion temperatures; harmful to lungs and a smog precursor.
• CO (carbon monoxide): From incomplete combustion, especially when rich.
• Unburned hydrocarbons (HC): From incomplete combustion or fuel vapor.
• Particulate matter (PM): Soot and ultrafine particles; notable in diesels and gasoline direct injection engines.

Regulations push these down via engine calibration and exhaust aftertreatment, while CO2 reduction relies on efficiency gains, hybridization, and lower‑carbon fuels.

To meet modern rules, manufacturers use a suite of systems that treat exhaust and optimize in‑cylinder combustion.

• Three‑way catalytic converters (gasoline) to cut NOx, CO, and HC when operated near stoichiometric air–fuel ratio
• Diesel particulate filters (DPF) to trap and regenerate soot
• Selective catalytic reduction (SCR) with urea/AdBlue to reduce NOx
• Exhaust gas recirculation (EGR) and variable valve timing to manage combustion temperature
• Gasoline particulate filters (GPF) for direct‑injection gasoline engines
• Turbocharging with downsizing, cooled EGR, and Miller/Atkinson timing for efficiency

These technologies, combined with precise electronic control and on‑board diagnostics, allow today’s engines to be far cleaner and more efficient than previous generations.

Fuels and emerging alternatives

ICEs can run on a range of fuels, each with trade‑offs in energy density, emissions, and infrastructure needs.

• Gasoline: Widely used; high energy density; spark ignition; increasing use of direct injection and turbocharging.
• Diesel: Compression ignition; high efficiency and torque; requires DPF and SCR to meet NOx/PM limits.
• Ethanol and blends (e.g., E10–E85): Higher octane; can enable more efficient operation; lower tailpipe CO2 per unit energy but lower energy density.
• Liquefied petroleum gas (LPG) and compressed natural gas (CNG): Cleaner combustion with lower NOx/PM; methane slip is a climate concern for natural gas.
• Biofuels (biodiesel, HVO/renewable diesel): Drop‑in options for diesels with potential lifecycle CO2 reductions depending on feedstock.
• Synthetic e‑fuels: Hydrocarbon fuels made using captured CO2 and green hydrogen; compatible with existing ICEs but currently costly and energy‑intensive to produce.
• Hydrogen (H2) ICE: Burns hydrogen in a modified engine; zero CO2 at the tailpipe, but NOx control is required; being piloted in heavy‑duty and motorsport applications.

As policy tightens and decarbonization accelerates, interest is growing in low‑carbon liquids and gases for sectors where batteries are challenging, though scale and cost remain hurdles.

Where ICEs are used today

ICEs power most of the global vehicle fleet, virtually all long‑haul trucks today, many ships (especially large two‑stroke diesels), general aviation piston aircraft, and a vast array of off‑road equipment and small engines. Battery‑electric and fuel‑cell vehicles are gaining share quickly—particularly in light‑duty markets—yet ICEs continue to dominate heavy transport and legacy fleets.

Policy and market signals are reshaping ICE deployment across regions.

• European Union: CO2 standards effectively require 100% reduction for new cars by 2035, with a pathway for vehicles running exclusively on e‑fuels. The Euro 7 emissions regulation was adopted in 2024, with phased application later this decade by vehicle class.
• United States: EPA finalized tougher greenhouse‑gas and pollutant standards for light‑ and medium‑duty vehicles for model years 2027–2032 (2024 rule), steering fleets toward higher efficiency, hybrids, and EVs; California’s Advanced Clean Cars II targets 100% zero‑emission new light‑duty sales by 2035, with several states following.
• China: China VI emissions standards are in force for light‑ and heavy‑duty vehicles, with continued tightening and rapid EV adoption in urban markets.
• Other markets: India’s BS6 Phase 2 and similar frameworks in Japan, South Korea, and Latin America are pushing cleaner ICEs and hybridization.

These trends mean new ICE designs increasingly appear in hybrids, heavy‑duty applications, and niches where alternatives are not yet practical.

Advantages and limitations

ICEs offer several practical benefits that have sustained their widespread use.

• High energy density and fast refueling with liquid fuels
• Mature, global supply chains and service networks
• Wide range of sizes and duty cycles, from handheld tools to massive marine engines
• Proven durability, especially in commercial diesel applications

Those strengths underpin their role in transport and industry, particularly where uptime and range are critical.

They also come with constraints that drive the search for alternatives and cleaner operation.

• CO2 emissions tied to fuel carbon content and efficiency
• Air‑quality pollutants (NOx, PM, CO, HC) requiring complex aftertreatment
• Lower peak efficiency than electric drivetrains, especially in stop‑start urban use
• Mechanical complexity, noise, and vibration relative to electric motors

As regulations tighten, meeting emissions goals raises cost and engineering complexity, particularly for small, low‑cost vehicles.

Maintenance and reliability basics

Proper upkeep prolongs engine life, maintains efficiency, and preserves emissions compliance.

• Use the manufacturer’s oil grade and change intervals; low‑ash oils for DPF‑equipped diesels
• Replace air and fuel filters on schedule to prevent lean/rich running and injector wear
• Monitor cooling systems; overheating accelerates wear and can warp heads
• Address check‑engine lights promptly; oxygen sensors and catalytic converters degrade over time
• For direct‑injection engines, manage intake valve deposits (e.g., periodic cleaning or dual‑injection designs)
• Keep aftertreatment systems healthy: ensure DEF quality for SCR, allow DPF regeneration cycles

Routine maintenance not only reduces breakdowns but also keeps real‑world emissions closer to certified levels.

History at a glance

Key milestones show how ICE technology matured and diversified.

• 1876: Nikolaus Otto’s practical four‑stroke engine demonstrated
• 1885–1886: Gottlieb Daimler and Karl Benz develop early gasoline automobiles
• 1897: Rudolf Diesel’s compression‑ignition engine proves high efficiency
• 1920s–1930s: Mass production, carburetion advances, and early supercharging
• 1960s–1970s: Emissions controls begin; catalytic converters and unleaded fuel
• 1990s–2000s: Electronic engine management, common‑rail diesel, variable valve timing, turbo‑downsizing
• 2010s–2020s: Direct injection pervasive, particulate filters on gasoline DI, hybrid ICE cycles (Atkinson/Miller), and low‑carbon fuels/hydrogen pilots

Each era brought better control of combustion, enabling higher efficiency and drastically lower pollutants per mile.

The outlook

In the near term, ICE development is concentrating on hybrids, efficiency improvements (high‑tumble combustion, cooled EGR, variable compression ratio), and robust aftertreatment to meet Euro 7, U.S. EPA 2027–2032, and comparable rules. Heavy‑duty sectors are testing hydrogen ICEs and renewable fuels to cut lifecycle CO2 with existing platforms. Over the longer horizon, electrification will continue to displace ICEs in light‑duty markets, while e‑fuels and biofuels may sustain specialized ICE applications where batteries or fuel cells face practical limits.

Summary

An internal combustion engine burns fuel within its cylinders to create high‑pressure gases that drive mechanical motion. It encompasses gasoline and diesel piston engines—by far the most common—as well as gas turbines in broader usage. Modern ICEs are far cleaner and more efficient than earlier generations thanks to precise electronic control, turbocharging, and sophisticated exhaust aftertreatment, yet they still emit CO2 and require complex systems to control NOx and particulates. As regulations tighten and electrification expands, ICEs are evolving toward hybrid use, cleaner fuels, and specialized roles where their energy density, refueling speed, and durability remain hard to beat.

What is the difference between an internal combustion engine and a normal engine?

The key difference between an internal and external combustion engine is where combustion occurs. The former are efficient and compact, making them ideal for vehicles, while the latter are quieter and can use diverse fuels but are bulkier and less efficient.

Is a gas engine an internal combustion engine?

Gasoline and diesel vehicles are similar. They both use internal combustion engines. A gasoline car typically uses a spark-ignited internal combustion engine, rather than the compression-ignited systems used in diesel vehicles.

What is the internal combustion engine in simple terms?

internal-combustion engine, any of a group of devices in which the reactants of combustion (oxidizer and fuel) and the products of combustion serve as the working fluids of the engine. Such an engine gains its energy from heat released during the combustion of the nonreacted working fluids, the oxidizer-fuel mixture.

Are all car engines internal combustion engines?

In 2021, modern vehicle engines can be more easily understood once divided into their three primary categories, which include: Internal combustion engines. Hybrid engine (Internal combustion engine + electric engine) Electric engine.