Courses › … › farm machinery power › fmp211 farm power and machinery
Lesson
02 of 16

⚙️ Two-Stroke and Four-Stroke Engines

Learn the working cycles, construction logic, and practical differences between two-stroke and four-stroke internal combustion engines.

This lesson explains how internal combustion engines complete their working cycle and why two-stroke and four-stroke designs behave differently in farm machinery.

What a Heat Engine Does

A heat engine converts heat released from fuel into mechanical work. In farm power, this conversion usually happens inside the engine cylinder, so the engine is called an internal combustion engine.

The basic sequence is:

  • fuel releases heat
  • gas pressure rises
  • the piston moves
  • piston motion turns the crankshaft
  • shaft rotation is used for useful work
The function of an engine is not simply to burn fuel. It is to convert fuel energy into controlled shaft power.

How IC Engines Are Commonly Classified

Internal combustion engines are usually classified in several ways.

Based on fuel used

  • petrol engine
  • diesel engine
  • gas engine

Based on ignition method

  • Spark ignition engine
    • air-fuel mixture is ignited by a spark plug
  • Compression ignition engine
    • only air is compressed first, and fuel ignites due to high temperature of compressed air

Based on working cycle

  • four-stroke cycle engine
  • two-stroke cycle engine

The last classification is the main focus of this lesson.

Basic Construction of an IC Engine

The important engine parts include:

  • cylinder
  • piston
  • piston rings
  • connecting rod
  • crankshaft
  • cylinder head
  • flywheel

The piston moves up and down inside the cylinder. The connecting rod transmits this reciprocating motion to the crankshaft, which converts it into rotary motion.

The flywheel helps smooth the uneven torque produced during engine operation.

Four Events of the Engine Cycle

Every engine cycle is built around four basic events:

  1. suction or intake
  2. compression
  3. power or expansion
  4. exhaust

The main difference between two-stroke and four-stroke engines is how these four events are completed.

Four-Stroke Engine

In a four-stroke engine, the full cycle is completed in:

  • four strokes of the piston
  • two revolutions of the crankshaft

These engines normally use inlet and exhaust valves for charge entry and gas removal.

1. Suction stroke

  • inlet valve opens
  • piston moves downward
  • air or air-fuel mixture enters the cylinder

2. Compression stroke

  • piston moves upward
  • both valves remain closed
  • charge is compressed

At the end of this stroke:

  • in a petrol engine, the spark plug ignites the mixture
  • in a diesel engine, fuel is injected into hot compressed air

3. Power stroke

  • fuel burns
  • gas pressure rises sharply
  • piston is forced downward
  • useful power is transmitted to the crankshaft

4. Exhaust stroke

  • exhaust valve opens
  • piston moves upward
  • burnt gases leave the cylinder
In a four-stroke engine, only one of the four strokes is the actual power stroke.

Two-Stroke Engine

In a two-stroke engine, the full cycle is completed in:

  • two strokes of the piston
  • one revolution of the crankshaft

These engines generally use ports instead of valves, and gas movement is coordinated by piston position.

Upward stroke

During the upward stroke:

  • the charge already in the cylinder is compressed
  • fresh charge enters the crankcase through the suction port
  • ignition occurs near the end of compression

So this stroke combines:

  • suction into the crankcase
  • compression inside the cylinder

Downward stroke

During the downward stroke:

  • combustion drives the piston down
  • exhaust port opens
  • transfer port opens
  • fresh charge enters and helps push out burnt gases

So this stroke combines:

  • power
  • exhaust
  • scavenging or clearing of gases

Why Two-Stroke and Four-Stroke Engines Differ in Practice

The difference in cycle design causes major practical differences.

Four-stroke engines

  • one power stroke in two crankshaft revolutions
  • better fuel utilization
  • higher thermal efficiency
  • cleaner gas exchange
  • heavier for the same power

Two-stroke engines

  • one power stroke in every crankshaft revolution
  • higher power output for a given size
  • lighter construction
  • less complete fuel use
  • lower efficiency
  • more difficult exhaust clearing

Comparison Table

Feature Four-stroke engine Two-stroke engine
Cycle completion Two crankshaft revolutions One crankshaft revolution
Power strokes One power stroke in four piston strokes One power stroke in two piston strokes
Gas control Uses valves Uses ports
Fuel use More complete combustion Less complete combustion
Thermal efficiency Higher Lower
Flywheel size Larger Smaller
Torque uniformity More even Less even
Suitability Broad range, including farm tractors Mostly smaller and lighter applications

Diesel Engine Working Principle

Most agricultural tractors use diesel engines, so the basic diesel principle is important.

In a diesel engine:

  • only air enters during suction
  • air is compressed to high pressure and temperature
  • diesel fuel is injected at the end of compression
  • ignition occurs due to heat of compression

This is why diesel engines do not need a spark plug.

Special features of diesel engines

  • high compression ratio
  • fuel injection system
  • ignition by compression heat
  • good torque at lower speed
  • better fuel economy than petrol engines in many farm uses

Diesel Engine Versus Petrol Engine

The broad exam-level difference is:

  • Diesel engine
    • uses diesel fuel
    • compression ignition
    • higher compression ratio
    • lower running cost
    • heavier construction
  • Petrol engine
    • uses petrol
    • spark ignition
    • lower compression ratio
    • lighter construction
    • generally higher operating cost in farm use

Agricultural Relevance

The working cycle matters because it affects:

  • power output
  • fuel efficiency
  • engine weight
  • maintenance requirement
  • suitability for tractors, power tillers, and stationary engines

Four-stroke diesel engines dominate in agricultural tractors because they offer:

  • better efficiency
  • better durability
  • better torque behavior for field work

Summary Cheat Sheet

  • Internal combustion engines complete four basic events: suction, compression, power, and exhaust.
  • A four-stroke engine completes the cycle in four piston strokes and two crankshaft revolutions.
  • A two-stroke engine completes the cycle in two piston strokes and one crankshaft revolution.
  • Four-stroke engines are generally more efficient and cleaner in operation, while two-stroke engines are lighter and produce more power for a given size.
  • Diesel engines ignite fuel by heat of compression and are widely preferred in agricultural tractors.

References

1 source • [1]

[1]

ICAR e-Courses

Lesson Doubts

Ask questions, get expert answers