How a Combustion Engine Is Powered

A combustion engine is powered by the chemical energy stored in fuel, which burns with air to create high-pressure gases that push on pistons or turbine blades, turning that force into useful mechanical work. In practice, the engine mixes fuel and air, ignites the mixture (by spark or compression), converts the resulting heat into pressure, and uses that pressure to spin a crankshaft or turbine connected to a vehicle’s drivetrain or a machine’s load.

The Core Idea: Fuel, Air, and Ignition Create Pressure

At the heart of every internal combustion engine is a controlled burn. Fuel vapor and oxygen react rapidly in a combustion chamber, releasing heat that raises gas temperature and pressure. That pressure does the work—either driving a piston down a cylinder (reciprocating engines such as those in most cars and trucks) or propelling hot gas across blades (gas turbines in aircraft and power generation). The engine’s design manages timing, quantity of fuel, mixture quality, and heat removal to turn a series of fast, tiny explosions or rapid burns into smooth, reliable power.

The Four-Stroke Piston Engine

The majority of road vehicles use a four-stroke, spark-ignition (gasoline) or compression-ignition (diesel) piston engine. The following sequence explains how one cylinder produces power repeatedly.

1. Intake: The intake valve opens and the piston moves down, drawing in air (and, in older designs, air-fuel mixture). Modern engines typically inject fuel directly into the cylinder or the intake port.
2. Compression: Valves close and the piston moves up, compressing the mixture to raise temperature and pressure in preparation for combustion.
3. Power (Combustion): Ignition occurs—by a spark plug in gasoline engines or by heat of compression in diesels—creating high-pressure gases that force the piston down and do work on the crankshaft.
4. Exhaust: The exhaust valve opens and the piston moves up, expelling spent gases to make room for a fresh charge.

By staggering these cycles across multiple cylinders, the engine delivers nearly continuous torque, which is smoothed by a flywheel and transmitted to the drivetrain.

Key Components That Make Power Possible

Several core parts collaborate to turn combustion into mechanical output. Here are the main components and what they do.

• Cylinders, pistons, connecting rods, and crankshaft: Convert gas pressure into rotary motion.
• Valves and camshaft(s): Control the timing of intake and exhaust flow; variable valve timing optimizes performance and efficiency.
• Fuel system: Tank, pump, lines, injectors (or carburetor in older engines) meter and deliver fuel precisely.
• Ignition system (gasoline): Coils, plugs, and electronic control provide accurate spark timing.
• Air handling: Throttle (gasoline), turbocharger/supercharger, intercooler, and air filter manage airflow and boost.
• Engine control unit (ECU) and sensors: Monitor oxygen, airflow, temperature, knock, and more to adjust fueling and timing in milliseconds.
• Cooling and lubrication: Radiator, water pump, thermostat, oil pump, and passages remove heat and reduce friction.
• Exhaust and aftertreatment: Manifold, catalytic converter, diesel particulate filter (DPF), and selective catalytic reduction (SCR) cut emissions.

Together, these systems ensure the engine breathes, burns, cools, and cleans effectively while delivering power under varying loads and conditions.

Spark-Ignition vs. Compression-Ignition

Gasoline (spark-ignition) engines ignite a premixed air-fuel charge using a spark plug, favoring higher engine speeds and smoother operation. Diesel (compression-ignition) engines compress only air until it’s hot enough, then inject fuel that auto-ignites, delivering strong low-speed torque and higher efficiency but requiring higher injection pressures and robust emissions control for NOx and particulates.

From Combustion to the Wheels: The Energy Path

Power must travel from the flame front to the road. The following sequence maps the energy conversion steps inside a typical vehicle.

1. Chemical to thermal energy: Fuel oxidizes, releasing heat.
2. Thermal to pressure: Hot gases expand, raising pressure in the cylinder.
3. Pressure to mechanical: Pressure pushes the piston; the connecting rod turns the crankshaft.
4. Mechanical to usable torque: The crankshaft’s rotation is smoothed by a flywheel and sent through a clutch or torque converter.
5. Gearing and delivery: A transmission and differential adjust torque and speed, delivering power to the wheels.

Every step imposes losses—friction, pumping, heat—which engineers minimize with design and control strategies.

Starting, Idling, and Throttle Control

Before an engine can be self-sustaining, an electric starter motor (powered by a 12V or 48V battery) cranks it to a speed where fuel, air, and ignition can produce continuous power. Once running, an idle control system maintains minimal speed. In gasoline engines, a throttle regulates air (with the ECU matching fuel); in diesels, torque is mainly set by fuel quantity while airflow is managed by boost and, sometimes, a throttle for emissions. Turbochargers harness exhaust energy to increase intake pressure, improving power density and efficiency.

Fuels That Power Combustion Engines

Combustion engines run on a range of fuels, each with traits that influence performance, efficiency, and emissions. Here are the most common options.

• Gasoline: High-volatility fuel for spark-ignition engines; octane rating resists knock in high-compression conditions.
• Diesel: Energy-dense fuel for compression-ignition; cetane rating reflects ignition quality.
• Jet fuel/kerosene: Used in gas turbines and some diesel applications.
• Ethanol and gasoline blends (e.g., E10, E85): Raise octane; can reduce petroleum use and some emissions.
• LPG (propane) and CNG: Clean-burning gaseous fuels with lower CO2 per unit energy than gasoline or diesel.
• Biodiesel and renewable diesel: Bio-based diesel substitutes compatible with many diesel engines.
• Hydrogen (ICE): Experimental and niche applications use hydrogen in modified engines; challenges include storage, NOx control, and infrastructure.
• Synthetic “e-fuels”: Hydrocarbon fuels produced using captured CO2 and green hydrogen; currently limited by cost and scale.

Fuel choice affects engine design—compression ratio, injection strategy, aftertreatment—and ultimately shapes the balance of power, efficiency, and emissions.

Efficiency and Losses

Only a fraction of fuel energy becomes useful work. Typical roadgoing gasoline engines convert about 20–35% of fuel energy to crankshaft power in real-world use; modern hybrids can reach around 40% under optimal conditions. Passenger-car diesels often achieve near or above 40%, while large slow-speed marine diesels can exceed 45–50% thermal efficiency. Losses arise from heat rejection, friction, and pumping. Technologies that improve efficiency include direct injection, variable valve timing and lift, turbocharging and downsizing, Atkinson/Miller cycles (common in hybrids), advanced combustion modes (e.g., HCCI/SPCCI), low-friction designs, waste-heat recovery, and mild hybrid systems that recuperate braking energy and assist acceleration.

Emissions and Control Systems

Combustion produces pollutants that must be mitigated. The following list outlines key emissions and how modern engines address them.

• CO2: Proportional to fuel burned; reduced mainly by efficiency gains and low-carbon fuels.
• Carbon monoxide (CO) and unburned hydrocarbons (HC): Cut by three-way catalytic converters and precise fuel/air control.
• Nitrogen oxides (NOx): Managed via exhaust gas recirculation (EGR), lean NOx traps, and SCR systems using urea (diesels and lean-burn engines).
• Particulate matter (PM): Controlled with diesel particulate filters; gasoline particulate filters address GDI soot.
• Evaporative emissions: Limited by sealed fuel systems and charcoal canisters.

Electronic controls, high-pressure injection, and aftertreatment hardware work together to meet increasingly stringent air-quality standards while preserving performance.

Variants Beyond Pistons

Not all combustion engines are alike. Gas turbines use a continuous Brayton cycle—compressing air, adding fuel and burning it, then expanding hot gas through turbine stages to produce shaft power or jet thrust. Two-stroke piston engines, common in small tools and some marine engines, complete a power cycle every revolution for high power density. Rotary (Wankel) engines use a triangular rotor in an oval housing for compactness and smoothness, though with challenges in sealing and emissions.

Summary

A combustion engine is powered by the chemical energy of fuel, which burns with air to generate high-pressure gases that do mechanical work on pistons or turbine blades. Engine systems precisely meter air and fuel, initiate and control combustion, convert pressure to torque, and manage heat and emissions. Fuel type, combustion strategy, and control technologies determine how efficiently and cleanly that energy becomes motion.

How do engines prepare fuel to be combusted in Kia?

Gasoline Engine
Gasoline gets mixed with air easily, so it can produce combustion with just a little spark. As a result, the gasoline engine has a spark plug to ignite the air and fuel mixture.

How did combustion engines work before spark plugs?

Ignition. Internal combustion engines require ignition of the mixture, either by spark ignition (SI) or compression ignition (CI). Before the invention of reliable electrical methods, hot tube and flame methods were used.

How do combustion engines make power?

In a spark ignition engine, the fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, the spark ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke.

Does a combustion engine need electricity?

Any car that uses spark plugs needs electricity to fire them. Diesels use compression to cause ignition, so they can theoretically be designed to run without electricity.