What is the classification of internal combustion engines based on the type of fuel used?

By fuel, IC engines are petrol (gasoline) engines, diesel engines, gas engines running on CNG, LPG, biogas or hydrogen, alcohol-fuelled engines, and dual-fuel engines that burn gas ignited by a pilot diesel injection. The fuel largely fixes the ignition method: petrol and most gaseous fuels need spark ignition and a high octane number, while diesel and biodiesel self-ignite in compression-ignition engines and need a high cetane number. That in turn sets the compression-ratio range, roughly 8–12 for SI and 14–22 for CI.

Explain the main components of an internal combustion engine.

The stationary parts are the cylinder block with its bores or liners, the cylinder head that closes the top of the cylinder and carries the valves and spark plug or injector, the head gasket, the crankcase and oil sump, and the manifolds. The moving parts are the piston with its rings, the gudgeon pin, the connecting rod, the crankshaft and flywheel, and the valve train: camshaft, followers, springs and valves, driven by a chain, belt or gears. The piston, connecting rod and crank form a slider-crank mechanism that turns the gas force on the piston into crankshaft torque. In a four-stroke engine the camshaft runs at half crankshaft speed so that each valve opens once per cycle.

Why are diesel engines generally more fuel-efficient than petrol engines?

A diesel engine runs at a much higher compression ratio (about 14–22 against 8–12 for petrol), and ideal-cycle efficiency rises with compression ratio. It controls load by varying the fuel quantity rather than throttling the air, so pumping losses at part load are much smaller. It also burns an overall lean mixture, which keeps the ratio of specific heats higher and heat losses lower. Per litre, diesel fuel also carries roughly 10–15% more energy than petrol because it is denser, which flatters litres-per-100 km comparisons further.

Why is a multi-valve engine design preferred over a two-valve design?

Two small inlet and two small exhaust valves give a larger total flow area than one large valve of each type in the same bore, so the engine breathes better and has higher volumetric efficiency at high speed. Smaller valves are lighter, so the valve train can follow the cam at higher rev/min without valve float. A four-valve head also leaves the centre free for the spark plug or a vertical injector, which shortens the flame path and improves knock resistance in SI engines and spray symmetry in diesels. The costs are a more complex head, usually DOHC, and more parts.

Calculate the displacement of a four-cylinder engine with a bore of 80 mm and a stroke of 90 mm.

Swept volume per cylinder is V_s = (π/4)·D²·L = (π/4) × (8.0 cm)² × 9.0 cm = 452.4 cm³. For four cylinders the engine capacity is 4 × 452.4 = 1809.6 cm³, about 1.81 litres or 1.81 × 10⁻³ m³. A common slip is mixing mm and m, which puts the answer out by a factor of 1000.

If an engine's compression ratio is increased, what effect does it have on engine performance and efficiency?

Raising the compression ratio increases thermal efficiency because the gas expands through a larger volume ratio. For the Otto cycle η = 1 − 1/r^(γ−1), so power and fuel economy both improve. The gain shrinks as r grows. In an SI engine the limit is knock, because the end gas gets hotter and more highly compressed; higher-octane fuel, better chamber design or knock control is needed. In a CI engine a higher ratio shortens the ignition delay and helps cold starting, but it raises peak pressure and NOx and adds friction, so practical diesels stop at about 16–20.

Engine classification, components and nomenclature

How IC engines are classified (cycle, ignition, fuel, layout, cooling, air supply), what the main components do, and the bore-stroke-clearance nomenclature with compression ratio, mean piston speed and piston position.

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Why it matters

Every later topic in this subject — air-standard cycles, valve timing, combustion, performance testing — is written in the vocabulary of bore, stroke, swept volume, clearance volume and compression ratio. If you can classify an engine from its specification sheet (four-stroke, CI, turbocharged, inline-4, DOHC) you already know most of its likely behaviour: how it is fuelled, how it is ignited, roughly what compression ratio and speed it runs at, and what its emission problems will be.

Key ideas

Classification. Internal combustion (IC) engines are grouped by several independent criteria; one engine sits in one class under each.

Working cycle: four-stroke (one power stroke per cylinder every two crankshaft revolutions) or two-stroke (one power stroke every revolution, gas exchange through ports while the piston is near BDC).
Method of ignition: spark ignition (SI) — a premixed charge is ignited by a spark; compression ignition (CI) — fuel is injected into air hot enough from compression to self-ignite. This is the most important classification because it fixes the compression ratio range (SI about 8–12, CI about 14–22), the load-control method (SI throttles the mixture quantity, CI varies the fuel quantity at nearly unthrottled air flow) and the fuel property that matters (octane number for SI, cetane number for CI).
Fuel: petrol, diesel, gaseous (CNG, LPG, hydrogen), alcohols, biodiesel, or dual-fuel.
Mixture preparation: carburetted, port fuel injection, gasoline direct injection, diesel direct or indirect injection.
Cylinder arrangement: inline, V, horizontally opposed (flat/boxer), W, radial, opposed-piston. Arrangement affects length, balance and firing intervals.
Valve arrangement: overhead valve (pushrod), single or double overhead camshaft (SOHC/DOHC); older head layouts are named I, L, T and F.
Cooling: air-cooled (fins, common on two-wheelers) or liquid-cooled.
Air supply: naturally aspirated, supercharged or turbocharged.
Speed and application: low/medium/high speed; automotive, marine, stationary, locomotive.

Main components.

Stationary: cylinder block (with cylinders or liners), cylinder head (forms the top of the combustion chamber and carries valves, spark plug or injector), head gasket, crankcase and oil sump, intake and exhaust manifolds.
Moving: piston with compression rings and oil-control ring, gudgeon (piston) pin, connecting rod, crankshaft with counterweights, flywheel, camshaft, valves with springs, and the timing drive (chain, belt or gears) that turns the camshaft at half crankshaft speed in a four-stroke engine.
The slider-crank mechanism (piston, connecting rod, crank) converts the reciprocating gas force on the piston into crankshaft torque; the flywheel smooths the cyclic torque.

Nomenclature.

Bore (D): cylinder inside diameter. Stroke (L): distance the piston travels between the two dead centres, equal to twice the crank radius.
TDC / BDC: top and bottom dead centres, the piston positions nearest to and farthest from the cylinder head, where piston velocity is momentarily zero.
Swept (displacement) volume V_s: volume swept by the piston in one stroke. Engine capacity is V_s times the number of cylinders, usually quoted in cm³ (cc) or litres.
Clearance volume V_c: volume left above the piston at TDC.
Compression ratio r: total cylinder volume at BDC divided by clearance volume. It is a ratio of volumes, not of pressures.
Stroke-to-bore ratio: L/D < 1 is oversquare (short stroke, high-revving), L/D = 1 square, L/D > 1 undersquare (long stroke, good low-speed torque, typical of diesels).
Mean piston speed: average piston speed 2LN/60; it limits engine speed because inertia loads, friction and flow losses all rise with it. Typical automotive values are about 8–20 m/s.
Firing order: sequence in which cylinders fire (for example 1-3-4-2 in an inline-4), chosen for balance and even crankshaft loading.

Connections. Compression ratio feeds directly into the Otto and Diesel efficiency expressions; swept volume and speed give the air flow used in volumetric efficiency and in brake mean effective pressure; the piston-position relation below is used again in combustion and valve-timing analysis.

Formulas

V_s = (π/4)·D²·L

V_s swept volume of one cylinder (m³), D bore (m), L stroke (m). Valid for any reciprocating engine.

V_total = k·V_s

k number of cylinders; V_total engine capacity (m³; 1 L = 10⁻³ m³ = 1000 cm³).

r = (V_s + V_c) / V_c = 1 + V_s / V_c

r compression ratio (dimensionless), V_c clearance volume (m³). Rearranged: V_c = V_s / (r − 1).

L = 2·a

a crank radius (m).

Ū_p = 2·L·N / 60

Ū_p mean piston speed (m/s), N crankshaft speed (rev/min).

n_p = N / (60·x)

n_p power strokes per second per cylinder (1/s); x = 2 for four-stroke, x = 1 for two-stroke.

s = a·(1 − cos θ) + l − √(l² − a²·sin²θ)

s piston distance from TDC (m), θ crank angle from TDC, l connecting-rod length (m). For an infinitely long rod this reduces to s = a(1 − cos θ); with a real rod the piston is past mid-stroke when the crank is at 90°.

Worked examples

Example 1 (standard). A four-cylinder engine has bore 80 mm, stroke 90 mm and compression ratio 10. Find the swept volume per cylinder, engine capacity, clearance volume per cylinder and the mean piston speed at 5000 rev/min.

Swept volume: V_s = (π/4)·D²·L = (π/4) × (8.0 cm)² × 9.0 cm = 452.4 cm³.
Capacity: V_total = k·V_s = 4 × 452.4 = 1809.6 cm³ ≈ 1.81 L (1.81 × 10⁻³ m³).
Clearance volume: V_c = V_s/(r − 1) = 452.4 / 9 = 50.3 cm³.
Mean piston speed: Ū_p = 2LN/60 = 2 × 0.090 m × 5000 / 60 = 15.0 m/s.

Answer: V_s = 452.4 cm³, capacity ≈ 1810 cm³ (1.81 L), V_c = 50.3 cm³, Ū_p = 15.0 m/s.

Example 2 (GATE level). A single-cylinder engine has bore 100 mm, stroke 120 mm and connecting rod 240 mm. Its compression ratio is to be raised from 16 to 18 by machining the cylinder head face. Treat the clearance space as a cylinder of bore diameter. (a) How much must be machined off? (b) How far is the piston from TDC when the crank is 90° past TDC?

Swept volume: V_s = (π/4)·D²·L = (π/4) × (10 cm)² × 12 cm = 942.5 cm³.
Clearance volumes: V_c = V_s/(r − 1). At r = 16: 942.5/15 = 62.83 cm³. At r = 18: 942.5/17 = 55.44 cm³.
Volume to remove: 62.83 − 55.44 = 7.39 cm³.
Piston area: (π/4) × (10 cm)² = 78.54 cm². Height removed = 7.39 / 78.54 = 0.0941 cm = 0.941 mm. (Check: the height of clearance space is L/(r − 1), so Δh = 120 × (1/15 − 1/17) = 0.941 mm.)
Piston position, a = L/2 = 60 mm, l = 240 mm, θ = 90°: s = a(1 − cos θ) + l − √(l² − a² sin²θ) = 60 × 1 + 240 − √(240² − 60²) = 300 − 232.4 = 67.6 mm.

Answer: (a) about 0.94 mm must be machined off; (b) the piston is 67.6 mm from TDC — more than half of the 120 mm stroke (60 mm), because of connecting-rod obliquity.

Common mistakes

Writing r = V_s/V_c. The compression ratio is (V_s + V_c)/V_c; forgetting the 1 makes every clearance-volume answer wrong.
Mixing units: bore in mm with volume in m³. Work in cm (giving cm³) or convert everything to metres first; 1 L = 1000 cm³ = 10⁻³ m³. An answer off by a factor of 10 or 1000 is the usual sign.
Quoting capacity per cylinder when the question asks for the engine (or the reverse).
Treating compression ratio as a pressure ratio. For isentropic compression the pressure ratio is r^γ, not r.
Assuming a four-stroke engine fires every revolution; each cylinder fires once every two revolutions, which halves the power-stroke count used in power calculations.
Taking piston position as exactly half-stroke at 90° crank angle when the connecting-rod length is given.
Calling every heavy-duty engine "CI": classification by ignition, by cycle and by fuel are independent (a CNG engine is SI; a two-stroke can be SI or CI).

For GATE ME

Expect short numerical questions on swept volume, clearance volume and compression ratio (often combined with an Otto or Diesel efficiency), mean piston speed, and the number of power strokes used in indicated-power calculations. Conceptual one-mark questions test SI versus CI features, four-stroke versus two-stroke, the speed of the camshaft relative to the crankshaft, and component functions. Practise converting cleanly between mm, cm³, litres and m³ and rearranging r = 1 + V_s/V_c in both directions.

Quick check

An engine has V_s = 500 cm³ and V_c = 50 cm³. What is its compression ratio?
At what speed does the camshaft of a four-stroke engine turn when the crankshaft runs at 3000 rev/min?
Is an engine with bore 86 mm and stroke 86 mm oversquare, square or undersquare?
Find the mean piston speed of an engine with stroke 100 mm at 3600 rev/min.
Which classification decides whether a fuel's octane or cetane number matters?

Answers: 1. 11. 2. 1500 rev/min. 3. Square. 4. 12 m/s. 5. Method of ignition (SI needs high octane, CI needs high cetane).

Interview questions

All Automotive Engines, Emissions and Alternate Fuels interview questions

Try answering each one aloud before you open it.

1.What is the classification of internal combustion engines based on the type of fuel used?Concept
By fuel, IC engines are petrol (gasoline) engines, diesel engines, gas engines running on CNG, LPG, biogas or hydrogen, alcohol-fuelled engines, and dual-fuel engines that burn gas ignited by a pilot diesel injection. The fuel largely fixes the ignition method: petrol and most gaseous fuels need spark ignition and a high octane number, while diesel and biodiesel self-ignite in compression-ignition engines and need a high cetane number. That in turn sets the compression-ratio range, roughly 8–12 for SI and 14–22 for CI.
2.Explain the main components of an internal combustion engine.Concept
The stationary parts are the cylinder block with its bores or liners, the cylinder head that closes the top of the cylinder and carries the valves and spark plug or injector, the head gasket, the crankcase and oil sump, and the manifolds. The moving parts are the piston with its rings, the gudgeon pin, the connecting rod, the crankshaft and flywheel, and the valve train: camshaft, followers, springs and valves, driven by a chain, belt or gears. The piston, connecting rod and crank form a slider-crank mechanism that turns the gas force on the piston into crankshaft torque. In a four-stroke engine the camshaft runs at half crankshaft speed so that each valve opens once per cycle.
3.Why are diesel engines generally more fuel-efficient than petrol engines?Application
A diesel engine runs at a much higher compression ratio (about 14–22 against 8–12 for petrol), and ideal-cycle efficiency rises with compression ratio. It controls load by varying the fuel quantity rather than throttling the air, so pumping losses at part load are much smaller. It also burns an overall lean mixture, which keeps the ratio of specific heats higher and heat losses lower. Per litre, diesel fuel also carries roughly 10–15% more energy than petrol because it is denser, which flatters litres-per-100 km comparisons further.
4.Why is a multi-valve engine design preferred over a two-valve design?Application
Two small inlet and two small exhaust valves give a larger total flow area than one large valve of each type in the same bore, so the engine breathes better and has higher volumetric efficiency at high speed. Smaller valves are lighter, so the valve train can follow the cam at higher rev/min without valve float. A four-valve head also leaves the centre free for the spark plug or a vertical injector, which shortens the flame path and improves knock resistance in SI engines and spray symmetry in diesels. The costs are a more complex head, usually DOHC, and more parts.
5.Calculate the displacement of a four-cylinder engine with a bore of 80 mm and a stroke of 90 mm.Numerical
Swept volume per cylinder is V_s = (π/4)·D²·L = (π/4) × (8.0 cm)² × 9.0 cm = 452.4 cm³. For four cylinders the engine capacity is 4 × 452.4 = 1809.6 cm³, about 1.81 litres or 1.81 × 10⁻³ m³. A common slip is mixing mm and m, which puts the answer out by a factor of 1000.
6.If an engine's compression ratio is increased, what effect does it have on engine performance and efficiency?Application
Raising the compression ratio increases thermal efficiency because the gas expands through a larger volume ratio. For the Otto cycle η = 1 − 1/r^(γ−1), so power and fuel economy both improve. The gain shrinks as r grows. In an SI engine the limit is knock, because the end gas gets hotter and more highly compressed; higher-octane fuel, better chamber design or knock control is needed. In a CI engine a higher ratio shortens the ignition delay and helps cold starting, but it raises peak pressure and NOx and adds friction, so practical diesels stop at about 16–20.

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