Last reviewed 1 Oct 2026 · 9 min read
Heat engines and IC engines
A heat engine converts the heat energy of fuel into mechanical work. In an internal-combustion (IC) engine the fuel is burnt inside the cylinder, and the hot gases push a piston; the reciprocating motion is turned into rotation by the crank mechanism. Tractors and farm engines mostly use diesel (compression-ignition) engines; small machines and some power tillers use petrol or kerosene engines.
Classification
| Basis | Types |
|---|---|
| Ignition | Spark-ignition (SI) — petrol, gas; Compression-ignition (CI) — diesel |
| Strokes per cycle | Four-stroke (two crank revolutions per cycle); two-stroke (one revolution) |
| Cylinders | Single, two, three, four, six; in-line, V, flat (opposed) |
| Cooling | Water-cooled (most tractors); air-cooled (small engines) |
| Fuel | Petrol, diesel, kerosene, LPG, CNG, biogas, biodiesel, ethanol blends |
| Valve arrangement | Overhead (OHV), side valve (L-head) |
| Speed | Low (< 1000 rpm), medium, high (> 2500 rpm); tractor engines typically 1800–2400 rpm |
| Application | Automotive, tractor, stationary, marine |
Engine terminology
| Term | Meaning |
|---|---|
| Top dead centre (TDC) | Highest position of the piston |
| Bottom dead centre (BDC) | Lowest position |
| Bore (D) | Cylinder diameter |
| Stroke (L) | Piston travel between TDC and BDC (= 2 × crank radius) |
| Swept (displacement) volume | per cylinder |
| Clearance volume | Volume above the piston at TDC |
| Compression ratio | |
| Engine displacement | number of cylinders |
| Firing order | The sequence of firing in a multi-cylinder engine (e.g., 1-3-4-2 for a four-cylinder) |
Typical compression ratios: SI engines 6–10; CI engines 14–22 (tractor diesel engines 15–18).
The four-stroke cycle
- Suction (intake) stroke — piston moves TDC → BDC, inlet valve open; air (diesel) or air–fuel mixture (petrol) is drawn in.
- Compression stroke — piston BDC → TDC, both valves closed; the charge is compressed (and heated). In the petrol engine a spark ignites the mixture near TDC; in the diesel engine fuel is injected near TDC into the hot compressed air and self-ignites.
- Power (expansion) stroke — the burnt gases push the piston TDC → BDC; both valves closed; work is produced.
- Exhaust stroke — piston BDC → TDC, exhaust valve open; the burnt gases are expelled.
- Only one stroke in four produces work; the flywheel stores energy to carry the engine through the other three. Valve timing — the inlet opens a little before TDC and closes after BDC; the exhaust opens before BDC and closes after TDC (valve overlap helps scavenging).
The two-stroke cycle
In a two-stroke engine the cycle is completed in one crank revolution (two piston strokes); there are no valves, but ports in the cylinder wall, opened and closed by the piston. Upward stroke: compresses the charge above the piston while fresh mixture is drawn into the crankcase below. Downward stroke: power; the piston uncovers the exhaust port, then the transfer port, and the fresh charge flows from the crankcase into the cylinder, scavenging the burnt gases.
- Advantages: simple, light, cheap, one power stroke per revolution (higher power for a given size), no valve gear. Disadvantages: poor scavenging, loss of fresh mixture into the exhaust (low fuel economy and high emissions), oil must be mixed with the petrol for lubrication, overheating, shorter life.
- Used in small petrol engines (power sprayers, chain saws, brush cutters, motorbikes in the past) and in large marine diesels.
Comparison: four-stroke vs two-stroke
| Feature | Four-stroke | Two-stroke |
|---|---|---|
| Power strokes per two revolutions | 1 | 2 |
| Valves | Yes | Ports |
| Lubrication | Separate oil system | Oil mixed with fuel (petrol) |
| Thermal efficiency | Higher | Lower |
| Fuel economy and emissions | Better | Poorer |
| Weight per kW | Higher | Lower |
| Cost and complexity | Higher | Lower |
Spark-ignition (petrol) vs compression-ignition (diesel) engines
| Feature | SI engine (petrol) | CI engine (diesel) |
|---|---|---|
| Cycle | Otto (constant-volume combustion) | Diesel (constant-pressure combustion) (dual cycle in practice) |
| Fuel | Petrol (volatile, high octane) | Diesel (higher cetane) |
| Air–fuel preparation | Carburettor or petrol injection — mixture drawn in | Air only compressed; fuel injected at the end of compression |
| Ignition | Spark plug (ignition system) | Self-ignition by heat of compression |
| Compression ratio | 6–10 | 14–22 |
| Thermal efficiency | 25–30 % | 30–40 % |
| Speed | Higher | Lower |
| Weight | Lighter | Heavier (stronger construction) |
| Fuel economy | Lower | Better |
| Torque | Lower, high rev | Higher at low speed |
| Emissions | CO, HC | NOx, particulates (smoke) |
| Fuel cost | Higher | Lower |
| Use | Cars, small engines | Tractors, trucks, pumps |
Ideal cycles
Otto cycle efficiency: ( for air). Diesel cycle efficiency: where is the cut-off ratio ().
For the same compression ratio the Otto cycle is more efficient; but a CI engine uses a much higher compression ratio, so it is more efficient in practice.
A petrol engine has a compression ratio of 8: (ideal air-standard). Actual brake thermal efficiency is only about 25–30 % because of heat loss, friction and imperfect combustion.