Grade 10 power mechanics – Classifications of Engines Quiz

An internal combustion engine (like most car engines) burns the fuel–air mixture inside the cylinder where combustion produces pressure to move the piston. External combustion (like a steam engine) burns fuel outside the working cylinder to generate steam or another working fluid.

A four-stroke engine has four distinct strokes—intake (induction), compression, power (expansion) and exhaust—over two revolutions of the crankshaft, which is the common cycle for most petrol and many diesel engines.

A two-stroke engine combines the functions of intake and compression in the first stroke and power and exhaust in the second, producing a power stroke every crankshaft revolution—this is why small engines and some motorcycles use two-stroke designs.

Spark-ignition engines (typically petrol) use a spark plug to ignite the fuel–air mix. Compression-ignition engines (diesel) compress air until it is hot enough to ignite injected fuel without a spark.

Compression-ignition (diesel) engines are designed to run on diesel fuel, which ignites under high compression. Petrol is used in spark-ignition engines; electricity and hydrogen are different energy sources.

Engines are often classified by cooling: air-cooled engines lose heat directly to the air (cooling fins), while water-cooled engines circulate coolant through a radiator. The other options are not standard cooling classifications.

Water (liquid) cooling provides more uniform temperature control across the engine, improving efficiency and reducing noise. It usually requires a radiator and maintenance, and lubrication is still necessary.

An inline engine has cylinders placed in one straight line. Radial engines are circular around the crankshaft, boxer (opposed) engines have opposing pairs, and triangular cluster is not a standard layout.

A V-engine uses two banks of cylinders angled to each other, forming a V profile. This gives a more compact length than an inline engine and is common in many cars and trucks.

The Wankel engine is a rotary engine using a triangular rotor in an epitrochoid housing, providing smooth rotation without reciprocating pistons. Boxer is flat-opposed piston, radial has piston banks in a circle, and opposed-piston is different.

Engines are often grouped by motion: reciprocating engines (pistons moving back and forth) and rotary/turbine engines (continuous rotation). The other pairs are not standard engine-motion classifications.

A naturally aspirated engine relies on atmospheric pressure to fill the cylinders, without forced induction like turbochargers or superchargers that compress intake air for more power.

Engines are classified by application: automotive (cars, matatus), marine (boats), industrial or stationary (generators, pumps), and aircraft engines. Colour-based or fuel-colour groupings are not standard.

The Otto cycle describes the ideal thermodynamic cycle for spark-ignition (petrol) engines operating on the four-stroke cycle. Diesel engines follow the Diesel cycle; steam turbines and electric motors follow different principles.

Diesel engines compress only air to a high pressure and temperature so that injected diesel fuel ignites spontaneously. Petrol engines use a spark and therefore operate at lower compression ratios.

Spark plugs provide the ignition spark for petrol engines. Diesel engines ignite fuel by compression and typically do not use spark plugs (they may use glow plugs to aid cold starts). Fuel injectors, crankshaft and pistons are common to both types.

Valvetrain location classifications include OHV (valves in the head operated by pushrods from a camshaft in the block) and OHC (camshaft(s) in the cylinder head). These are standard technical terms used in engine design.

DOHC uses two camshafts in the cylinder head (usually one for intake and one for exhaust), allowing more precise timing and often more valves per cylinder (e.g., four valves), which improves airflow and performance.

Many traditional two-stroke engines use a pre-mix where oil is mixed with the fuel so the oil lubricates moving parts as the mixture passes through the engine. Larger four-stroke engines usually have a separate oil sump.

Engine displacement is the total swept volume of all cylinders and is commonly shown in cm3 (e.g., 1500 cm3) or litres (e.g., 1.5 L) in vehicle specifications. Area, weight or electric current units are not used for displacement.

Multi-cylinder engines distribute power strokes over multiple cylinders, reducing vibration and improving smoothness. They still need cooling, fuel and ignition systems; fuel use depends on design and size.

Boxer (opposed) engines have opposing cylinder banks so pistons move in and out opposite each other, giving good balance. The other descriptions do not match the boxer layout.

Gas turbines are rotary engines where continuous combustion drives a rotor. They are not reciprocating piston engines (two-stroke or boxer) or Wankel, which are different types of internal combustion engines.

A turbocharger harnesses exhaust gas energy to spin a compressor that forces more air into the engine, allowing more fuel to burn and increasing power. It does not cool the engine directly or replace transmission components.

Engines designed to run on LPG or CNG are gas-fuelled internal combustion engines. They combust gas inside the cylinders. They are not steam engines, electric motors, or hydraulic units.

Engines are commonly classified by strokes per cycle into two-stroke and four-stroke types. The other terms are not standard classifications in engine theory.

An internal combustion engine (ICE) produces power by burning fuel inside its cylinders so the expanding gases act directly on pistons or rotors.

Diesel engines compress air to high pressure and temperature so the injected diesel fuel ignites by compression, not by a spark.

The official names for the four-stroke cycle are intake (suction), compression, power (combustion/expansion) and exhaust.

A two-stroke engine produces one power stroke per crankshaft revolution, while a four-stroke produces one power stroke every two revolutions.

Air-cooled engines use cooling fins and airflow to remove heat directly from the cylinder surfaces rather than using water in a radiator.

Reciprocating engines use pistons moving in a straight line (back and forth) inside cylinders to convert pressure into mechanical motion.

A V engine arranges cylinders in two angled banks that form a 'V' when viewed from the front, common in many cars and trucks.

A boxer or horizontally opposed engine places cylinders in two banks directly opposite each other, giving low center of gravity and good balance.

Radial engines mount cylinders in a ring around a central crankshaft; they were common in older aircraft designs.

A turbocharger uses exhaust-driven turbine to drive a compressor that increases intake air pressure and density, boosting engine power.

Spark-ignition engines (like most car petrol engines) use petrol which is ignited by a spark from the spark plug.

Compression-ignition engines are designed to run on diesel fuel, which ignites under high compression without a spark.

Multiple cylinders allow power strokes to overlap and balance forces, producing smoother operation and reduced vibration.

In an overhead camshaft design the cam(s) sit in the cylinder head above the valves, reducing valve-train complexity and inertia.

The spark plug creates the electrical spark inside the combustion chamber that ignites the petrol-air mixture.

Diesel engines typically operate at higher compression ratios and are more fuel-efficient, producing better thermal efficiency and often more torque.

Wankel rotary engines are compact and produce smooth rotary motion with a high power-to-weight ratio, though they have other trade-offs.

A vertical engine has its crankshaft axis oriented up-and-down; this classification describes the engine's geometric orientation.

Stationary engines are used in fixed installations (generators, pumps) while mobile engines are used in vehicles and movable machinery.

Double-acting engines (common in some steam engines) apply pressure alternately to both sides of the piston so power is produced on both strokes.

Many two-stroke engines rely on oil mixed into the fuel to lubricate the piston and crankcase, unlike four-strokes with separate sump lubrication.

Most small motorcycles and boda-bodas use single-cylinder engines because they are simpler, lighter and cheaper to maintain.

Glow plugs heat the combustion chamber in diesel engines during cold starts to help vaporise fuel and reach ignition temperature.

Sleeve valves use a movable sleeve that slides to align ports for gas flow; poppet valves are the usual mushroom-shaped valves in many modern engines.

Gas engines are spark-ignited engines adapted to burn gaseous fuels like compressed natural gas (CNG) or biogas.