Engine

Definition and Core Role

An automotive engine is the powerplant of a motor vehicle, widely regarded as the "heart" of the car. Its fundamental purpose is to convert the chemical energy of fuel into thermal energy, which is subsequently converted into mechanical energy to propel the vehicle. Modern automotive engines are primarily reciprocating piston internal combustion engines (ICE), which combust fuel inside the cylinders to generate high-temperature and high-pressure gases. These gases drive the pistons in reciprocating motion, which is then translated into rotational motion via the connecting rods and crankshaft to drive the wheels. An engine's performance directly determines a vehicle's power delivery, fuel economy, driving stability, and environmental emissions. A typical engine comprises between 300 and 600 precision components, integrated across two core mechanisms and five key systems.

Development History

The history of the automotive engine mirrors the evolution of the motor industry itself. In 1769, French military engineer Nicolas-Joseph Cugnot mounted a steam engine onto a wooden three-wheeler, creating the world's earliest motor vehicle. In 1860, French technician Étienne Lenoir built the first practical gas engine. In 1866, German engineer Nikolaus Otto developed the first four-stroke reciprocating internal combustion engine. In 1876, Otto built a prototype horizontal four-stroke cycle engine, establishing the foundational architecture for modern petrol engines. In 1892, German inventor Rudolf Diesel patented the diesel engine. In 1936, Mercedes-Benz produced the first passenger car powered by a diesel engine. In 1956, German engineer Felix Wankel invented the rotary engine. In 1967, Germany's Bosch introduced the first electronically controlled petrol fuel injection system. In 2001, FAW Jiefang rolled out its first proprietary engine developed with independent intellectual property. In April 2025, the Side-Yoke horizontally opposed engine made its global debut at Auto Shanghai.

Classification

Automotive engines can be categorised across several dimensions.

By power source, engines are classified into petrol engines, diesel engines, electric vehicle (EV) traction motors, and hybrid powertrains. Petrol engines run on petrol, using spark plugs to ignite the air-fuel mixture to create expanding gases for work; they are characterised by high revving capability, simple construction, lightweight build, and smooth operation. Diesel engines operate on diesel fuel and rely on compression ignition from high-temperature compressed air, delivering high low-end torque and superior fuel economy, albeit with relatively higher noise, vibration, and harshness (NVH) levels.

By operating cycle, they are divided into four-stroke engines and two-stroke engines. A four-stroke engine requires four piston strokes (intake, compression, power, and exhaust) to complete one operating cycle, representing the prevailing mainstream format in modern passenger vehicles.

By cylinder configuration, engines are classified into in-line engines, V-engines, boxer (horizontally opposed) engines, and rotary engines. In-line engines feature a straightforward layout and lower production costs, making them widely used in everyday passenger cars; V-engines offer a more compact longitudinal length and superior running refinement, commonly found in premium and executive models.

By cooling method, they are split into liquid-cooled (water-cooled) and air-cooled variants. Liquid cooling has become the industry standard in modern vehicles thanks to its stable thermal management and precise temperature regulation.

Basic Construction

Taking a conventional petrol engine as an example, its overall construction can be broken down into two core mechanisms and five essential systems.

The two core mechanisms comprise the crank-connecting rod mechanism and the valvetrain. The crank-connecting rod mechanism consists of the cylinder block assembly, piston-connecting rod assembly, and crankshaft-flywheel assembly, serving to convert thermal energy from combustion into linear piston motion, which is then translated via the connecting rods and crankshaft into rotational output power. The valvetrain consists of timing gears/chains, valves, and the valve actuation mechanism, tasked with opening and closing the intake and exhaust valves at precise intervals matching the firing order and operating cycle of each cylinder, ensuring timely intake of fresh charge and prompt scavenging of exhaust gases.

The five essential systems comprise the fuel supply system, ignition system, cooling system, lubrication system, and starting system. The fuel supply system meters and atomises fuel with air before delivering the mixture into the cylinders; the ignition system (exclusive to petrol engines) generates a timed electric spark across the spark plug to ignite the compressed mixture towards the end of the compression stroke; the cooling system circulates coolant to dissipate excess engine heat and prevent overheating; the lubrication system pumps engine oil to moving frictional surfaces via the oil pump to minimise mechanical wear; and the starting system cranks the flywheel and crankshaft via a starter motor to initiate engine firing and starting. Because diesel engines utilise compression ignition, they do not require an ignition system, thus operating with only four key systems.

Operating Principles

A four-stroke petrol engine completes one working cycle across four distinct piston strokes.

Intake Stroke: The piston travels downwards from top dead centre (TDC) to bottom dead centre (BDC) with the intake valve open and exhaust valve closed. The vacuum generated by the descending piston draws the air-fuel mixture into the cylinder.

Compression Stroke: Both intake and exhaust valves remain shut as the piston travels upwards from BDC to TDC, compressing the air-fuel charge to a fraction of its initial volume, which significantly raises its temperature and pressure.

Power Stroke (Combustion and Expansion Stroke): As the compression stroke nears completion, the spark plug fires an electric spark to ignite the compressed mixture. Rapid combustion generates high-temperature, high-pressure expanding gases that force the piston downwards from TDC to BDC, rotating the crankshaft via the connecting rod to deliver mechanical work—this is the sole power-generating stroke in the cycle.

Exhaust Stroke: The exhaust valve opens, and the piston sweeps upwards from BDC to TDC, expelling spent exhaust gases out of the combustion chamber into the exhaust manifold. Upon completion of the exhaust stroke, the intake valve reopens to commence the subsequent cycle.

The operating principle of a diesel engine is broadly similar, with the key distinction being that finely atomised diesel is injected under ultra-high pressure directly into the cylinder at the end of the compression stroke, auto-igniting under the intense heat of the compressed air without requiring a spark plug.

Common Faults and Diagnostics

Engines may encounter various operational issues over their service life. The most frequent issues include hard starting or failure to start, loss of power, abnormal fuel consumption, engine overheating, excessive vibration, unusual knocking or rattling noises, oil leaks, coolant leaks, and excessive engine oil consumption. Fault diagnosis typically follows a structured, step-by-step procedure: Driver consultation—gathering service history, mileage, and symptoms preceding the malfunction; Visual and auditory inspection—checking for visible fluid leaks, vacuum leaks, and unusual noises; DTC scanning—plugging a dedicated OBD diagnostic tool into the vehicle's port to retrieve stored diagnostic trouble codes from the engine control module (ECM); and Advanced testing—deploying specialised diagnostic tools like engine analysers for in-depth evaluation. The widespread adoption of On-Board Diagnostics II (OBD-II) has made automotive fault diagnosis significantly faster and more precise.

Maintenance and Repair

Engine repairs are generally categorised into minor repairs and major overhauls based on the depth of the work. An engine overhaul is an extensive rebuilding process involving complete teardown, chemical cleaning, tolerance inspection, replacement of worn components, and precision reassembly. The national standard GB/T 3799-2021 "Technical Specifications for Factory Completion of Automobile Engine Overhaul" took effect on 1 July 2022, establishing fundamental criteria for engine rebuilding, key component tolerances, assembly specifications, post-overhaul inspection, and quality assurance. This standard covers petrol and diesel engines across passenger and commercial vehicles. Additionally, GB/T 19910-2022 "Technical Requirements for Automobile Engine Electronic Control System Repair" came into force on 1 May 2023, standardising diagnostic and repair protocols for electronic engine management systems.

Routine maintenance remains essential to extending engine longevity. Key preventive measures include regular engine oil and oil filter changes, scheduled replacement of air and fuel filters, inspecting coolant level and condition, periodically inspecting spark plugs (for petrol engines) , and cleaning the throttle body and intake plenum.

Industry Development Trends

As of 2026, driven by global decarbonisation policies and the surge in vehicle electrification, the automotive engine industry is accelerating its transition towards zero-carbon fuels, intelligent management, and ultra-high thermal efficiency. Raising thermal efficiency remains the focal point of ICE engineering innovation—between 2025 and 2026, Chinese carmakers achieved a spate of breakthroughs in this domain. In 2025, mass-produced Chinese petrol engines had already achieved thermal efficiencies above 43%, with dedicated hybrid engines surpassing 45%. In June 2026, Chery unveiled its Kunpeng Tianqing dedicated hybrid engine, boasting a peak thermal efficiency of 48.57% and setting a new global benchmark for mass-market internal combustion engines. Around the same time, Geely developed an engine reaching 48.41% thermal efficiency, paired with an AI-driven hybrid powertrain system that further enhances real-world fuel economy. Today, the thermal efficiency of mainstream mass-market petrol engines typically sits in the 38% to 45% bracket.

On the technology front, internal combustion engines are advancing along four primary pillars: Lean engineering—leveraging artificial intelligence to enhance thermal efficiency, power-to-weight ratios, and engineering speed, potentially slashing engine R&D cycles from 36–48 months down to 18–24 months; Platform modularity—facilitating unified, modular architectures capable of supporting varied engine displacements, multi-fuel capabilities, and electrified powertrains; Architectural innovation—reimagining classic formats like V-twin compact range-extender units and rotary engines for hybrid and range-extended EV (REEV) applications; and Frontier applications—capitalising on high energy density and endurance to expand into low-altitude mobility sectors such as flying cars and industrial drones. Internal combustion technology is thus transitioning from a purely ground-based automotive powerplant into a key pillar of multi-dimensional future mobility.