What is a 2 stroke car engine?

This blog post will answer the question, ‘ What is a 2 stroke car engine?’, and cover topics like two-stroke cycle engines vs four-stroke cycle engines, intake for two-stroke cycle engines, and scavenging process.

What is a 2 stroke car engine?

A two-stroke cycle can be summarized in the following points:

  • It completes the combustion of the air-fuel mixture in one piston stroke
  • A two-stroke engine operates using ports instead of valves
  • Consists of a transfer port that adds a new charge into the combustion chamber from the crankcase
  • The first stroke is the compression stroke that opens the inlet port from the crankcase to let in fresh air while simultaneously opening the exhaust port
  • Two-stroke engines require the oil to be mixed prematurely with the fuel.

Two-stroke cycle engines vs four-stroke cycle engines

Below is a tabular comparison of two-stroke and four-stroke cycle engines.

Completion of cycleCompleted in one crankshaft revolutionCompleted in two crankshaft revolutions
Flywheel weightLighter flywheel requiredHeavier flywheel required
Power producedPower is 1.8 times moreLess power produced for same engine size
Cooling and Lubrication requirementsGreater cooling/lubrication is requiredLesser cooling/lubrication is required
Valve and valve mechanismOnly have portsValve and valve mechanism present
Initial costCheaper initial costHigher initial cost
Volumetric efficiencyLess due to lesser time for inductionMore due to more induction time
Thermal and part-load efficienciesBoth are lower than 4-strokeBoth are higher than 2-stroke
Application Smaller vehicles and two-wheelersCars, buses, tractors, etc.

Intake for two-stroke cycle engines

In two-stroke engines, inlet air needs to be input at a pressure greater than atmospheric pressure. At the start of the intake process, the cylinder is still filled with exhaust gas at atmospheric pressure

There is no exhaust stroke. Air under pressure enters the cylinder and pushes out most of the remaining exhaust residual air out of the open exhaust port. After this the exhaust port is closed and the cylinder is filled with air.

The following methods are used to pump air into the cylinders of a two-stroke engine:

  • Through normal intake valves
  • Through intake slots in the cylinder walls

Use of superchargers

In some automobile engines, standard-type superchargers are used and the air is input through the intake valves with no fuel added. The compressed air scavenges the cylinder and leaves it filled with air and a small amount of exhaust residual.

Crankcase compression

In these engines, the air is introduced at atmospheric pressure into the cylinder below the piston through a one-way valve when the piston is near the top dead center(TDC).

The power stroke pushes the piston down and compresses the air in the crankcase, which was designed for this dual purpose. The compressed air then passes through an input channel into the combustion chambers.

For 2-stroke engines using crankcase compression as an intake method, lubricating oil needs to be added to the inlet air. The crankcase in these engines cannot be used as a reservoir for storing oil like other engines. 

Instead, the surfaces of the engine components are lubricated by oil vapor carried by the intake air.

Scavenging process in two-stroke engines

The process of clearing the engine cylinder after the expansion or power stroke is referred to as the process of scavenging.

The scavenging process is the replacement of the combustion products in the cylinder from the previous power stroke with a fresh air charge to be burned in the next cycle.

The efficiency of a two-stroke engine depends heavily on the scavenging process.

Consequences of a poor scavenging process

  • Low mean indicated pressure (m.e.p) which results in high weight and cost per kW of shaft output for the engine.
  • Low amount of oxygen availability results in incomplete combustion leading to higher fuel consumption.
  • Contamination of the lubricating oil reduces the lubricating qualities and eventually results in increased wear and tear in piston and cylinder liners

Stages of the scavenging process

The scavenging process can be divided into the following four stages.


On opening the inlet port, the gases expanding in the main cylinder tend to escape and pre-discharge into the scavenged air manifold. Pre-blowdown ends with the opening of the exhaust port.


The blowdown process terminates at the moment the gs pressure inside the cylinder attains a value slightly less than the air pressure inside the scavenged air manifold.


The scavenged air sweeps out all the residual gases remaining in the main cylinder at the end of the spontaneous exhaust and replaces them as completely as possible with a fresh charge.

Additional charging

After the completion of the scavenging phase, the fresh charge continues to flow till the scavenging ports are open and pressure in the cylinder rises. This phase results in the better filling of the cylinder.

Research shows the most powerful 2-stroke-powered bikes that have been manufactured.


This blog post addressed the question, ‘What is a 2 stroke car engine’.

We came across a tabular comparison between 2-stroke and 4-stroke engines and understood the ways in which the intake process takes place in a 2-stroke engine. Furthermore, the scavenging process and its various stages have also been discussed in detail. Please feel free to comment on the content or ask any questions in the comments section below.

Frequently Asked Questions (FAQs): What is a 2-stroke car engine?

Are 2-stroke engines legal?

2-stroke engines are prone to cause high atmospheric emissions, however, there are no such plans to prohibit their usage .

Is 2-stroke legal in USA?

2-stroke engines are legal in the USA although they are not being actively produced by companies due to the implementation of stricter emission norms.

Do 2-stroke engines need oil?

Oil needs to be mixed into the fuel at a set ratio to ensure the engine receives optimum lubrication during the combustion process.