Engine Cycles

Engine Cycles

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What are engine cycles?

Internal combustion engines, also called motor engines, work using a four-stroke cycle, also known as the engine cycle. This cycle has four stages: intake, compression, combustion expansion, and exhaust. These four stages are repeated over and over again to create power and convert chemical energy into mechanical energy. In simpler terms, engines use four to turn gasoline into energy that powers your car. These four steps are really important to understand how engines work.

Engine cycle analysis

The engine cycle has four stages: intake, compression, combustion, and exhaust. These stages are illustrated in figure 1, which shows how a four-stroke diesel or petrol engine works. It's important to understand the key components of an engine cylinder. The combustion happens inside the cylinder, which is moved vertically by a piston connected to a rod. There are two valves at the top of the cylinder: an intake valve and an exhaust valve. Between these valves, you'll find the fuel injector or spark plug. These components work together to make the engine run smoothly.

Four-stroke engine cycle

In petrol or diesel engines each vertical movement of the piston either upwards or downwards is called a stroke. Hence in four-stroke engines, the piston does 4 total upward and downward movements in total which are usually divided into four different stages to complete an engine cycle.

Intake Stroke

The first stroke of a petrol engine is the intake stroke. During this stroke, the piston travels down the cylinder from the upper maximum position to the lower minimum position. At the same time, premixed air and fuel are suctioned into the cylinder via the open inlet valves, increasing the volume inside the cylinder and keeping the pressure in the cylinder below atmospheric pressure. The fuel is pre-mixed with air before it reaches the inlet valve, usually with a device called a carburetor. However, more recently, an electronic control unit (ECU) is used to carefully evaluate the quantity of fuel injected in the air inlet port just above the inlet valves.


After the intake stroke, the valves are closed, and the piston moves upward from the minimum vertical position to the maximum position, decreasing the volume and increasing the pressure inside the cylinder. The mixture is compressed towards a spark plug, and work is done on the air during compression. This is the second stroke.

To ensure maximum efficiency, it's important that the spark occurs right before the end of the stroke. This timing allows the mixture to reach the top of its stroke, allowing maximum pressure to operate on the descending piston. The heated fuel powers the turbine, and then it is injected into the combustion chamber, where it is burned to produce energy.


As the piston nears the top maximum position towards the end of the second stroke, the pressure in the cylinder is at its highest. The high pressure and temperature cause the fuel mixture to ignite when a spark from the spark plug is introduced. During this stage, the volume inside the cylinder remains almost constant.


The high pressure from the expanded gases forces the piston to move downwards. Work is done by the expanding gases. The exhaust valve opens at the minimum position, and the pressure reduces to nearly atmospheric. This is the third stroke.


After the combustion stage, the piston moves upwards, expelling the burnt gases through the open exhaust valve. During this fourth and final stroke, the pressure in the cylinder drops to just above atmospheric pressure.

The process you've described is known as the Otto cycle, which is the basic operating cycle for most modern car engines. The cycle involves adding and rejecting energy in the form of heat during the combustion and exhaust stages, while work is done by the compression and expansion stages. This cycle is repeated over and over again to power the engine and keep the vehicle moving.

What are the two types of cycles for petrol and diesel engines?

Excellent explanation! You are absolutely right that there are two types of engines: diesel and petrol/gasoline engines, which operate according to different theoretical engine cycles. The Otto cycle is the principle behind how petrol/gasoline engines operate, while diesel engines operate according to the Diesel cycle.

The ideal or theoretical Otto cycle assumes specific conditions during each of the four strokes. The intake stroke occurs under isobaric conditions (0-1), the compression stroke occurs under reversible and adiabatic conditions (1-2), the combustion stroke occurs under isochoric conditions (2-3), the expansion stroke occurs under reversible and adiabatic conditions (3-4), and the exhaust stroke occurs under isochoric conditions (4-1).

Adiabatic is a thermodynamic process that occurs without transferring heat or mass between the system and its environment. Isochoric is a thermodynamic process that occurs under a constant volume, while isobaric is a thermodynamic process that occurs under constant pressure.

The ideal Otto cycle can also be represented graphically using a pressure vs volume graph, where the four strokes are described with numbers from 1 to 4, completing one engine cycle. The constant volume and constant pressure processes are shown in the graph.

You have an excellent understanding of the principles behind engine cycles and how they work!

 Ideal Otto cycle.
Ideal Otto cycle


The ideal or theoretical diesel cycle is the principle at which the diesel engine operates. It  can be described  assumes the following conditions:

Intake is isobaric(0-1)Compression is  adiabatic(1–2) .Combustion (heat addition)is isobaric (2–3 ).Expansion is adiabatic (3–4)Exhaust(heat rejection) is isochoric ( 4–1).

Ideal diesel cycle
Ideal diesel cycle

An indicating  Otto cycle of a real petrol engine and diesel engine obtained using a pressure sensor in the cylinder and a transducer whose output depends on the angular position of the crankshaft is shown in figure 4.

a) Indicating diesel Engine, b)Indicating petrol engine
a) Indicating diesel Engine, b)Indicating petrol engine

You are absolutely correct that the figures shown in real-life engine cycles are not the same as those in the theoretical cycle. This is because the thermodynamic processes that occur in internal combustion engines are not exactly as assumed in the theoretical cycles. The combustion and expansion stages are constant in terms of volume and pressure as assumed, and they are not reversible in real-life conditions.

In addition to the Otto and Diesel cycles, there are other engine cycles, such as the Carnot cycle, Brayton cycle, and Rankine cycle. The Carnot cycle is the most efficient cycle, while the diesel engine cycle is the least efficient.

Understanding these different cycles and the principles behind them is crucial for engineers and designers to develop more efficient and effective engines that can power our vehicles and machines while minimizing their environmental impact.

What are the equations for engine cycles?

The theoretical efficiency of an engine with a compression ratio of 1.85 can be calculated using the equation: η = 1 - (1/rn), where η is the overall efficiency and rn is the ratio. Substituting the compression ratio of 1.85, the theoretical efficiency of the engine is 0.46 or 46%.

The indicated power of a six-cylinder engine can be calculated using the equation: Pi = (A x ncylinders x ns)/2, where Pi is the indicated power, A is the area under the p-v curve, ncylinders is the number of cylinders and ns is the number of cycles per second. Substituting the values, the indicated power of the engine is (200 x 6 x 5)/2 = 600.

Engine Cycles

What are the different  stages of engine cycles? 

The four different stages that an internal-combustion enine goes through are Intake, Compression, Combustion-expansion, and Exhaust.

What is meant by engine cycle? 

Engine cycles are  four stages in an internal combustion engine that complete a cycle.

What are heat engine cycles? 

Heat engine cycles are repetitive four step sequences  that convert thermal energy into useful work by compressing ,burning and expanding  gases.

What is the most efficient engine cycle? 

The most efficient engine cycle is the Carnot cycle.

What are the five sequences in a four-stroke engine cycle? 

The five sequences of a four-stroke engine cycle are Intake, Compression, Combustion, Expansion and Exhaust.

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