What Are Internal Combustion Engines?

Internal combustion engines are machines that burn fuel inside a sealed chamber to convert chemical energy into mechanical motion, powering most cars, trucks, many ships, small aircraft, and generators today. In essence, they mix fuel with air, ignite it to create a rapid pressure rise, and use that force to move pistons or turbine blades, turning a shaft that does useful work. This article explains how they operate, the main types and fuels, their efficiency and emissions, and how policy and technology are shaping their future.

How an Internal Combustion Engine Works

Most road vehicles use a piston-style internal combustion engine with a four-stroke cycle, coordinating valves, fuel injection, and spark or compression ignition to deliver power smoothly and efficiently.

The four-stroke cycle describes the sequence of piston movements and gas exchanges that produce power.

1. Intake: The piston moves down, drawing an air-fuel mixture (gasoline engines) or air alone (diesels) into the cylinder.
2. Compression: The piston moves up, compressing the gases to raise temperature and pressure.
3. Power: The mixture ignites—via spark (gasoline) or self-ignition from compression (diesel)—forcing the piston down and turning the crankshaft.
4. Exhaust: The piston moves up again, expelling combustion gases through the exhaust valve.

Together, these strokes repeat many times per second, producing a continuous output of torque at the crankshaft that can be geared to wheels, propellers, or generators.

Key Components

Understanding the core parts helps explain how durability, efficiency, and emissions are managed in modern engines.

• Cylinders, pistons, connecting rods, and crankshaft: Convert gas pressure into rotary motion.
• Valvetrain (camshaft, valves): Times intake and exhaust; variable valve timing improves efficiency.
• Fuel system (injectors, pump): Meters and atomizes fuel; gasoline direct injection and high-pressure diesel common rails dominate today.
• Ignition system (spark plugs, coils): Triggers combustion in spark-ignition engines.
• Turbocharger/supercharger: Force more air into the engine, boosting power and efficiency.
• Cooling and lubrication: Manage heat and friction; vital for longevity and emissions control.
• Aftertreatment: Catalysts and filters that clean exhaust before release.

These elements work in concert under electronic control, with sensors and engine control units optimizing performance dozens of times per second.

Major Types of Internal Combustion Engines

Internal combustion engines vary by how they ignite fuel, how many strokes they use, and whether they are piston-based or turbine-based.

• Spark-ignition (gasoline): Use a spark plug to ignite a premixed air-fuel charge; typically follow the Otto, Atkinson, or Miller cycles.
• Compression-ignition (diesel): Compress only air until it’s hot enough to ignite injected diesel; renowned for high torque and efficiency.
• Two-stroke: Complete a power cycle every two piston strokes; common in small tools and some marine engines for power density.
• Rotary (Wankel): Use a triangular rotor in an epitrochoidal housing; compact and smooth but challenging for emissions and sealing.
• Gas turbines (jet engines): Continuous-flow combustion spins turbine stages; dominant in aviation and some power generation.

While automotive markets favor four-stroke piston engines, specialized niches—like aviation turbines or marine two-strokes—use different architectures to match operating demands.

Thermodynamic Cycles and Efficiency

Internal combustion engines follow idealized thermodynamic cycles whose practical implementations determine efficiency and drivability. Efficiency is highest under steady, optimized loads and lower during cold starts or stop‑and‑go driving.

• Otto cycle: Common in gasoline cars; balances power and responsiveness with moderate efficiency.
• Diesel cycle: Higher compression and lean combustion yield greater thermal efficiency and torque.
• Atkinson/Miller cycles: Use valve timing or boosting to mimic longer expansion than compression, improving efficiency (popular in hybrids).
• HCCI/SPCCI variants: Homogeneous charge compression ignition strategies aim for diesel-like efficiency in gasoline engines, with limited production use.

Typical peak thermal efficiency ranges around 20–40% for passenger vehicles, exceeding 40% in the latest hybrid-optimized gasoline engines and approaching or surpassing 45% for advanced light-duty diesels; large marine two-stroke diesels can reach about 50% under ideal conditions.

Fuels and Emerging Alternatives

ICEs can run on a broad array of fuels, each with trade-offs in energy density, emissions, cost, and compatibility with existing infrastructure.

• Gasoline: Widely available; high specific power; emissions controlled via three-way catalysts.
• Diesel: Higher energy density and efficiency; requires particulate filters and NOx control.
• Natural gas (CNG/LNG) and LPG: Lower CO2 per unit energy and reduced particulates; used in fleets and some heavy-duty/marine applications.
• Ethanol and methanol: Alcohol fuels with high octane; used in blends (e.g., E10/E85) or dedicated engines.
• Biodiesel and HVO/renewable diesel: Drop-in diesel substitutes from bio or waste feedstocks; cut lifecycle CO2 depending on source.
• Synthetic e-fuels: Hydrocarbons made from captured CO2 and green hydrogen; compatible with existing engines but currently costly and limited to pilot scale.
• Hydrogen (H2) ICE: Burns hydrogen with near-zero CO2 tailpipe emissions; challenges include NOx control, storage, and fuel supply.

Fuel choices increasingly reflect policy and supply chains; drop-in low-carbon options ease transition, while hydrogen and e-fuels target hard-to-electrify sectors.

Emissions and Control Technologies

ICE emissions fall into two categories: greenhouse gases and local air pollutants. Modern engines rely on sophisticated aftertreatment and combustion strategies to minimize both.

• CO2: A byproduct of burning carbon-based fuels; proportional to fuel consumed.
• NOx (nitrogen oxides): Formed at high temperatures; contribute to smog and ozone.
• CO (carbon monoxide): From incomplete combustion; toxic and odorless.
• Unburned hydrocarbons (HC): Fuel that escapes combustion; precursor to smog.
• Particulate matter (PM/soot): Tiny particles harmful to health; notable in diesel and gasoline direct injection.

Regulations worldwide have driven dramatic declines in these pollutants from new vehicles over the past three decades, though CO2 remains tied to fuel use and lifecycle impacts.

How Modern Systems Clean the Exhaust

Controlling emissions requires both in-cylinder strategies and exhaust aftertreatment tailored to the fuel and engine type.

• Three-way catalysts (TWC): Convert NOx, CO, and HC simultaneously in stoichiometric gasoline engines.
• Gasoline particulate filters (GPF): Capture fine particles from direct-injection gasoline engines.
• Exhaust gas recirculation (EGR): Lowers combustion temperatures to reduce NOx formation.
• Diesel oxidation catalysts (DOC): Oxidize CO and HC and aid DPF regeneration.
• Diesel particulate filters (DPF): Trap soot; periodically regenerate to burn off deposits.
• Selective catalytic reduction (SCR): Uses urea (AdBlue) to convert NOx to nitrogen and water.
• Lean NOx traps and ammonia slip catalysts: Supplement NOx control and clean up residual ammonia.

Together with precise fuel injection, turbocharging, and electronic controls, these technologies enable compliance with stringent standards like Euro 6/VI and U.S. Tier 3/HD2027.

Where Internal Combustion Engines Are Used

ICEs dominate sectors requiring high energy density, quick refueling, and long range, though electrification is gaining ground where practical.

• Passenger cars and light trucks: Still common globally, often with turbocharging and hybridization.
• Heavy-duty trucks and buses: Diesel engines provide torque and efficiency on long hauls.
• Marine: Low-speed two-stroke diesels power large cargo ships; LNG and methanol are emerging.
• Aviation (piston) and turbines: Small aircraft use piston engines; most planes use gas turbines.
• Off-road and small engines: Agriculture, construction, lawn equipment, snowmobiles, and generators.
• Hybrid powertrains: ICEs paired with electric motors, improving efficiency and urban emissions.

Application dictates engine design and fuel choice, balancing cost, reliability, and regulatory constraints.

Advantages and Limitations

ICEs offer strengths that made them ubiquitous, but they also carry trade-offs compared with electric and other alternatives.

The benefits of ICEs explain their long-standing role in transportation and industry.

• High energy density and long range with fast refueling.
• Mature, global manufacturing and service infrastructure.
• Wide fuel compatibility and flexible power outputs.
• Proven durability in harsh, remote, and heavy-duty applications.

These advantages keep ICEs relevant where uptime, range, and payload are paramount.

Limitations increasingly shape technology and policy choices.

• Tailpipe emissions (CO2 and local pollutants) without extensive controls.
• Lower efficiency versus electric drivetrains, especially in stop-and-go use.
• Maintenance complexity (fluids, filters, moving parts).
• Noise and vibration, though much improved in modern designs.

As regulations tighten and batteries improve, ICEs face stiffer competition in light-duty markets while remaining critical in harder-to-electrify segments.

Maintenance and Reliability Basics

Routine maintenance preserves performance, efficiency, and emission compliance over an engine’s life.

• Oil and filter changes on schedule to protect bearings and timing systems.
• Air and fuel filter replacements to maintain mixture quality and injector health.
• Spark plugs and ignition components for gasoline engines to ensure clean combustion.
• Coolant and belt/chain service to prevent overheating and timing failures.
• DPF/SCR upkeep on diesels, including correct urea use and regeneration cycles.
• Software updates that refine calibration and emissions performance.

Adhering to manufacturer intervals and using correct fluids/fuels is the simplest way to maximize longevity and minimize emissions.

Safety Considerations

ICEs involve heat, pressure, and combustible fuels, requiring basic precautions.

• Prevent carbon monoxide buildup: Never run engines in enclosed spaces without ventilation.
• Manage fire risks: Address fuel leaks promptly; keep extinguishers rated for flammable liquids.
• Avoid burn and injection injuries: Hot surfaces and high-pressure fuel systems demand proper tools and protection.
• Handle alternative fuels carefully: Hydrogen requires leak detection and ventilation; LNG/CNG involve cryogenic or high-pressure systems.

Following safety guidance and regular inspections mitigates most operational hazards.

Policy and Market Outlook (2024–2025)

Regulation and market shifts are reshaping how and where ICEs are used, without an immediate, universal phase-out.

• European Union: A 2035 target effectively requires new cars and vans to be zero tailpipe CO2, with a narrow path for vehicles running exclusively on certified carbon-neutral e‑fuels; Euro 7 will tighten pollutant limits and durability requirements later this decade.
• United Kingdom: The ban on new petrol/diesel car sales is set for 2035, aligning with the EU timeline.
• United States: EPA finalized 2027–2032 light- and medium-duty standards in 2024 that strongly cut fleet emissions; they don’t mandate EVs but anticipate a large EV share by 2032. California’s ACC II targets 100% zero-emission new light-duty sales by 2035.
• China: “Dual-credit” policies and incentives are driving rapid NEV adoption while advanced ICEs and hybrids continue; heavy-duty diesel standards are tightening.
• Shipping and aviation: IMO targets net-zero GHG “by or around 2050,” with EU measures (ETS for shipping, FuelEU Maritime) pushing low-carbon marine fuels; aviation relies on gas turbines with rising sustainable aviation fuel mandates (e.g., EU ReFuelEU Aviation starts in 2025).

Net effect: electrification accelerates in light-duty markets, hybrids bridge the gap, and low-carbon fuels gain importance in heavy-duty, marine, and aviation where ICEs remain essential.

Common Misconceptions

Several myths persist about internal combustion engines; clarifying them helps inform better choices.

• “All ICEs are inefficient”: Modern designs can be highly efficient at steady loads, especially diesels and hybrid-optimized engines.
• “Premium gasoline always adds power”: Higher octane helps only if the engine is designed or tuned for it.
• “Diesels are always dirtier”: With DPF and SCR, new diesels can be extremely clean for local pollutants; CO2 depends on use and fuel.
• “Idling uses little fuel”: Idling wastes fuel and increases emissions; stop-start systems mitigate this.
• “ICEs can’t go low-carbon”: Biofuels, renewable diesel, e-fuels, and hydrogen ICEs can cut lifecycle emissions, though supply and cost matter.

Understanding what technology can and cannot do avoids overgeneralization and highlights the role of operating conditions and fuel pathways.

Summary

Internal combustion engines burn fuel inside a chamber to produce mechanical work, a versatile concept that powers everything from cars to cargo ships. They span gasoline and diesel designs, use diverse fuels, and rely on advanced controls and aftertreatment to meet modern standards. While electric powertrains are rapidly expanding—especially for light-duty vehicles—ICEs continue to evolve through hybridization and low-carbon fuels, remaining vital in applications where energy density, range, and refueling speed are decisive.

What is an internal combustion engine?

Internal combustion engines work by using a piston to compress air and increase the temperature in the cylinder. When fuel comes into contact with the high temperature, it ignites and creates energy through combustion. This energy transfer is repeated at high speeds.

Where are internal combustion engines used?

Internal combustion engines (ICE) are the most common form of heat engines, as they are used in vehicles, boats, ships, airplanes, and trains.

What is the main problem with internal combustion engines?

Internal combustion engines create air pollution in two ways: (1) by releasing primary pollutants directly into the atmosphere and (2) by releasing direct emissions that create secondary pollution when they react chemically with elements of the atmosphere.

What are the three types of internal combustion engines?

The three main types of internal combustion (IC) engines are gasoline (spark-ignition) engines, diesel (compression-ignition) engines, and gas turbine engines. Gasoline and diesel engines use reciprocating pistons to create power, with gasoline engines relying on spark plugs to ignite a fuel-air mixture and diesel engines igniting fuel via the heat of compressed air. Gas turbine engines, often found in aircraft, do not have pistons but instead use a continuous flow of combustion gases to spin a turbine. 
Here’s a breakdown of each type:
1. Gasoline (Spark-Ignition) Engines 

• How it works: A mixture of gasoline and air is drawn into the cylinder, compressed, and then ignited by a spark plug. The resulting explosion of gases expands, pushing the piston and generating power. 
• Key feature: Spark ignition is used to initiate combustion. 
• Applications: Commonly found in cars, known for their quick throttle response and smooth operation. 

2. Diesel (Compression-Ignition) Engines

• How it works: Only air is drawn into the cylinder and compressed to a very high pressure, which raises its temperature. Diesel fuel is then injected into this hot, compressed air, causing it to ignite and burn without a spark plug. 
• Key feature: Self-ignition of fuel due to the heat of compression. 
• Applications: Often used in heavy-duty vehicles like trucks and buses. 

3. Gas Turbine Engines

• How it works: Instead of pistons, gas turbines use a continuous flow of air. Air is compressed, and then fuel is added and ignited in a combustion chamber. The rapidly expanding gases pass through a turbine, causing it to spin and produce power. 
• Key feature: Continuous combustion and a lack of reciprocating parts. 
• Applications: Used in aircraft, power plants, and large ships.