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Home » What is a petrol engine? How does a petrol engine work?

What is a petrol engine? How does a petrol engine work?

petrol engine

There are a variety of petrol engines such as internal combustion engines, spark-ignition engines powered by gasoline (petrol), and similar volatile fuels.

Petrol engines usually have fuel and air pre-mixed before compression (although some modern petroleum engines may utilize cylinder-direct fuel injection). Historically, pre-mixing was done in a carburetor. Still, today it is done by an electronically controlled fuel injection system, except in small engines where the added engine efficiency does not justify the cost/complication of electronics.

The method of mixing fuel and air differs from a diesel engine (a reciprocating engine), as does the use of spark plugs to ignite the combustion process. During the compression stroke of a diesel engine, only air is compressed (and thus heated), and the fuel is injected into the hot air, and it burns on its own. Petrol engines are also fueled by liquefied petroleum gas.

History of the petrol engine

In 1876, Nicolaus August Otto built the first practical petrol engine in Germany, but earlier attempts had been made by Étienne Lenoir, Siegfried Marcus, Julius Hock, and George Brayton.

The compression ratio of the petrol engine

It is possible for both air and fuel in a closed cylinder to auto-ignite – or behave like a compression-ignition engine – if you compress the mixture too much. Petrol engines are designed differently from diesel engines to account for their different burn rates.

If they auto-ignite, the gas expansion inside the cylinder will reach its most significant point before it reaches the top dead center. A static spark plug is typically set before the piston reaches TDC at a minimum crankshaft rotation of 10 degrees.

Still, the spark plug is set much higher at higher engine speeds to allow time for the fuel-air charge to permit substantial combustion before too much gas has expanded – gas expansion occurs as the piston moves down in the power stroke.

Because higher octane petrol burns slower, it is less likely to auto-ignite and has a lower expansion rate. As a result, engines using only high-octane fuel can experience higher compression ratios.

Speed and efficiency of the petrol engine

The higher rotation speed of petrol engines is partly due to their lighter pistons, connecting rods, and crankshaft (an efficiency made possible by lower compression ratios) because petrol burns more quickly than diesel.

The pistons in petrol engines are much shorter than those in diesel engines, which means they usually complete their strokes faster than pistons in diesel engines. On the other hand, petrol engines have a lower compression ratio than diesel engines, lowering their efficiency.

A typical petrol engine has a thermal efficiency of about 20% (on average), nearly half that of a diesel engine. Some newer engines are reported to be more efficient than previous ones (thermal efficiency up to 38%).

Working cycles of a petrol engine

There are two types of fuel cycles for gasoline engines – two-stroke and four-stroke.

Four-stroke cycle
A four-stroke engine (also called a four-cycle engine) is an internal combustion engine in which the piston completes four strokes while the crankshaft rotates. The entire travel of the piston or the cylinder in either direction is known as a stroke.
Two-stroke cycle
Two-stroke engines are internal combustion engines that complete a power cycle with two-piston strokes (up and down) during one process, completed in a single crankshaft revolution. The piston must travel four strokes during two crankshaft rotations to complete a power cycle in a four-stroke engine. An engine that runs in two strokes undergoes combustion and compression simultaneously, with intake and exhaust (or scavenging) functions.
The power band of a two-stroke engine is a narrow range of rotational speeds where the machine has the most incredible power-to-weight ratio. There are fewer moving parts in two-stroke engines than in four-stroke engines.

Cylinder arrangement of a petrol engine

The most common arrangement is either 2 to 12 cylinders in V-formation or 1 to 6 cylinders in line. It used to be a Volkswagen trademark, and flat engines were standard in small planes and motorcycles during the 1990s. There are still many modern Porsches and Subarus that use flat-six engines. These engines are typically air-cooled. A W formation is less common but notable for high-speed vehicles, similar to two V engines side-by-side. These engines are also called rotary and radial. The latter is typically seven to nine cylinders or 10 or 14 cylinders divided into two rings.

The cooling system of a petrol engine

In petrol engines, the cylinders and cylinder head may be air-cooled with fins; or liquid-cooled with a water jacket and radiator. As a result, the cooling fluid is usually a mix of water and either propylene glycol or ethylene glycol. Modern types of antifreeze also contain lubricants and other additives to protect water pump seals and bearings. They have lower freezing points and higher boiling points than pure water. A little pressure is usually added to the cooling system to increase the boiling point of the coolant.

Power measurement of a petrol engine

It is commonly known as brake power, measured at the flywheel, and given in metric horsepower or kilowatts (metric) or horsepower. It is the actual mechanical power output of the engine in a form that can be used.

“Brake” comes from a dynamometer test in which a brake is used to load the machine. You must understand what usable and complete means to make accurate interpretations. For example, in a car engine, aside from friction and thermodynamic losses inside the machine, power is absorbed by the water pump, alternator, and radiator fan, reducing the force available at the flywheel that moves the car forward.

As well as power steering pumps and air conditioner compressors (if fitted), but these are not tested or calculated during power output tests. There is a slight difference in power output depending on the fuel’s energy value, the ambient temperature and humidity, and altitude.


A power rating that does not account for losses in the alternator and radiator fan is deemed incorrect. The International Organization for Standardization (ISO) and the Society of automotive engineers (SAE) publish standards on exact practices and how to adjust for non-standard conditions, such as elevation above sea level.

Most workshop technicians are familiar with chassis dynamometers or “rolling roads.” They measure drive wheel brake horsepower, which is generally 15%-15% less than brake horsepower measured at the crankshaft or flywheel when a motor dynamometer is used.

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