How a Combustion Engine Works, Step by Step

In brief: a piston engine breathes air, compresses it, ignites a fuel–air charge to release heat and pressure, converts that pressure into piston motion, and exhausts the burnt gases—then repeats this cycle rapidly to turn a crankshaft and drive a vehicle. What follows explains the sequence inside a typical four-stroke engine, how electronics manage it, how power reaches the wheels, and how gasoline and diesel designs differ—along with the technologies that improve efficiency and cut emissions.

The Big Picture: Converting Fuel to Motion

Internal combustion engines turn the chemical energy in fuel into mechanical work. Gasoline engines usually follow the Otto cycle (spark ignition), while diesel engines follow the Diesel cycle (compression ignition). Most road vehicles use four-stroke engines, where each complete cycle takes two crankshaft rotations; smaller tools and some motorcycles may use two-stroke engines, which combine steps to deliver a power stroke every rotation.

Core Components and Their Roles

Understanding the parts clarifies what happens at each step of the cycle. The following list outlines the key components and their jobs.

• Cylinder and piston: The chamber and moving plug where compression and expansion happen.
• Connecting rod and crankshaft: Convert the piston’s up-down motion into rotation.
• Valves (intake and exhaust) and camshaft(s): Time the breathing of the engine.
• Spark plug (gasoline) or injector (diesel): Initiate combustion—spark for gasoline, hot-compressed air for diesel.
• Fuel system: Pump, rail, and injectors meter fuel; modern engines are electronically controlled.
• Air path: Intake, filter, throttle (gasoline), turbo/supercharger and intercooler (if equipped).
• Exhaust path: Manifold, catalyst(s), particulate filter (diesel and some gasoline), muffler.
• Lubrication and cooling: Oil pump and coolant loop prevent wear and overheating.
• Engine control unit (ECU) and sensors: Measure, decide, and actuate for performance, efficiency, and emissions.

Together, these systems allow precise metering of air and fuel, controlled ignition, and reliable conversion of pressure into rotational power.

Four-Stroke Cycle: Step-by-Step Inside One Cylinder

Below is the classic four-stroke sequence. Each stroke corresponds to roughly 180 degrees of crank rotation; a full cycle is 720 degrees (two turns). Notes indicate differences between gasoline and diesel engines.

1. Intake (piston down): The intake valve opens; the descending piston draws in air. In gasoline engines, air flow is moderated by a throttle and fuel is added either in the intake (port injection) or directly in the cylinder (direct injection). Diesels draw unthrottled air only.
2. Compression (piston up): Both valves close; the piston squeezes the charge, raising pressure and temperature. Typical compression ratios: gasoline ~10:1 to 13:1 (higher with knock control), diesel ~14:1 to 22:1.
3. Power/combustion (piston driven down): Near the top of compression, gasoline engines fire a spark slightly before top dead center; the flame front rapidly increases pressure and pushes the piston down. Diesels inject fuel into the hot, compressed air near top dead center; the fuel auto-ignites and sustains combustion as injection continues.
4. Exhaust (piston up): The exhaust valve opens; the rising piston expels spent gases into the exhaust system. A brief valve overlap at the end helps scavenging at higher speeds.

This cycle repeats every two crankshaft revolutions per cylinder. At 2,400 rpm, each cylinder completes 1,200 cycles per minute; only the power stroke produces torque, with the flywheel smoothing output between strokes.

What the ECU Does Each Cycle

Modern engines rely on an electronic control unit to coordinate fuel, spark, air, and aftertreatment. The steps below summarize the control loop that occurs hundreds of times per second.

1. Sensing: Read crank/cam position, intake air mass/pressure, throttle position, temperature, oxygen sensors, knock, and more.
2. Calculating: Determine load and required torque; compute fuel quantity and spark timing (or diesel injection timing/pressure) from calibration maps and real-time feedback.
3. Actuating: Command injectors, ignition coils, electronic throttle, variable valve timing/lift, EGR valves, and turbo wastegate/variable geometry.
4. Correcting: Trim fueling from oxygen sensor feedback, adjust ignition to avoid knock, and manage aftertreatment temperatures and regeneration (DPF/SCR) as needed.

This closed-loop control maximizes efficiency and power while meeting emissions limits under changing conditions.

From Cylinder to Wheels: Mechanical Power Flow

The engine’s torque must be modulated and delivered to the road. The following sequence shows how power travels from the pistons to the tires.

1. Crankshaft and flywheel: Convert and smooth pulsating torque from each cylinder.
2. Clutch or torque converter: Connects/disconnects the engine from the drivetrain and cushions engagement.
3. Transmission: Selects gear ratios to keep the engine in an efficient speed range for the task.
4. Driveshaft and differential(s): Carry torque to the axle(s) and split it between wheels while allowing different wheel speeds in turns.
5. Tires: Convert torque into tractive force at the contact patch with the road.

Gearing multiplies engine torque at the wheels, enabling both low-speed pulling power and efficient high-speed cruising.

Diesel vs. Gasoline: Step Differences That Matter

While both engine types share the same basic strokes, their combustion strategies lead to practical differences affecting efficiency, emissions, and feel.

• Ignition: Gasoline uses a spark at controlled timing; diesel relies on compression heat and timed fuel injection.
• Mixture formation: Gasoline aims for a near-stoichiometric mix for the catalyst; diesel runs lean with air excess, adding fuel for load.
• Compression ratio: Diesel higher, improving thermodynamic efficiency and low-speed torque.
• Air control: Gasoline throttled (except some advanced strategies); diesel typically unthrottled and controls torque by fuel quantity.
• Aftertreatment: Gasoline uses a three-way catalyst; diesel adds a particulate filter and often SCR with urea to handle NOx.
• Cold start aids: Gasoline needs strong spark and vaporization; diesel uses glow plugs and high cranking compression heat.

These differences explain why diesels excel at efficiency and towing, while gasoline engines often feel more responsive and are simpler to after-treat for emissions at stoichiometric operation.

Two-Stroke Engines: The Compressed Steps

Two-stroke designs combine events to deliver a power stroke every crank revolution, trading simplicity and power density for emissions and efficiency challenges.

1. Upstroke (compression/ignition prep): The piston rises, compressing the charge in the cylinder while drawing fresh mixture into the crankcase (carbureted) or preparing for direct injection.
2. Downstroke (power/scavenging): Combustion drives the piston down; ports open to vent exhaust and admit fresh charge that scavenges the cylinder. Lubrication often comes from oil mixed in the fuel.

Modern direct-injected two-strokes reduce the traditional loss of unburned fuel during scavenging, but four-strokes dominate road vehicles due to cleaner operation.

Efficiency, Emissions, and Modern Aids

Decades of advances have improved how each step of the cycle performs, extracting more work from each drop of fuel while cutting pollution.

• Higher compression and advanced combustion: Atkinson/Miller timing, cooled EGR, and pre-chamber ignition raise peak efficiency; best production gasoline engines reach around 40–41% peak indicated efficiency, with diesels often 40–45% in light-duty and higher in heavy-duty applications.
• Direct injection and precise timing: Shape the burn, reduce knock, and target mixture where needed (e.g., SPCCI/part-time compression ignition in some gasoline designs).
• Boosting: Turbocharging and supercharging increase air mass; intercoolers lower charge temperature for denser fills.
• Variable valve timing and lift: Broaden the torque curve and improve part-load efficiency.
• Friction and thermal management: Low-viscosity oils, low-tension rings, split cooling, and thermal coatings save energy.
• Aftertreatment: Three-way catalysts (gasoline), gasoline particulate filters, diesel particulate filters, and selective catalytic reduction systems slash CO, HC, NOx, and soot.
• Stop-start, cylinder deactivation, and hybridization: Reduce idle fuel use and operate the engine near efficient load points.

The net effect is cleaner, more efficient engines that meet stringent global standards while maintaining performance.

Safety and Maintenance Steps That Matter

Keeping the cycle healthy depends on simple routines that preserve compression, timing, and clean combustion.

• Oil and filter changes: Maintain lubrication films that protect bearings, rings, and cams.
• Cooling system service: Prevent overheating that can cause knock, pre-ignition, or warping.
• Air and fuel filtration: Guard injectors and cylinders from contaminants that erode efficiency.
• Ignition service (gasoline): Fresh spark plugs and sound coils ensure reliable, complete burns.
• Timing belt/chain integrity: Preserves precise valve timing; interference engines risk valve-piston contact if timing fails.
• Diesel-specific care: Monitor DPF/SCR systems, use correct cetane fuel, and allow proper regen cycles.

Regular maintenance sustains compression, accurate metering, and clean exhaust, extending engine life and performance.

Summary

A combustion engine works by cycling through intake, compression, combustion, and exhaust, using valves, pistons, and careful electronic control to turn fuel’s chemical energy into crankshaft rotation. Gasoline engines ignite a premixed charge with a spark; diesels inject fuel into hot compressed air. Modern technologies—direct injection, turbocharging, variable timing, and advanced aftertreatment—optimize each step for efficiency, power, and low emissions. The cycle is simple in concept, but its precision and supporting systems are what make today’s engines reliable and remarkably capable.