Petrol and diesel technology

How petrol and diesel engines work

As internal combustion engines, petrol and diesel units generate power by burning fuel inside the engine, with the expanding hot gases produced used to drive a piston. With the piston connected to a crankshaft, this motion rotates an axle via the transmission, turning the wheels and moving the vehicle.

Petrol engines

Most petrol and diesel engines in cars utilise the four-stroke cycle - intake, compression, power and exhaust - but in different ways. For petrol engines, during the induction stroke, fuel and air are drawn into the cylinder, before being compressed by the piston into a small volume. This petrol-air mixture is then ignited by a spark from the spark plug, with the explosion causing the gases to expand, forcing the piston down and turning the crankshaft. The burned gases are then expelled from the cylinder via the exhaust valve and into the exhaust pipe.

Diesel engines

Diesel engines work in the same basic way but, to begin with, only air is drawn into the cylinder and compressed. At the end of the compression stroke, diesel is directly injected into the cylinder where, because of the high air temperature caused by compression, it immediately vaporises and ignites. From there on, it behaves in the same way again as a petrol engine – moving the piston, turning the crankshaft and expelling the exhaust gases.


The four-stroke engine was developed in the late 1800’s but it has come a long way since then. Today, technologies such as fuel injection, variable valve timing, turbocharging and cylinder deactivation are all striving to improve efficiency and performance. These “added” systems combine with the use of lighter, more robust materials and special coatings to reduce the friction within engines to help make petrol and diesel engines more efficient than ever.

Almost all new petrol and diesel engines are now turbocharged to help improve emissions. This sees gases diverted from the exhaust pipe to spin a compressor, which sucks in air before forcing it into the engine’s cylinders. This forced induction means that more air is pushed into the combustion chamber than if it were just drawn in by a normal (naturally-aspirated) engine. This provides more power and/or increased efficiency depending on how the engine is tuned.

Another method of forced induction is by supercharging, which works in the same way as turbocharging, though the compressor is run off the engine rather than exhaust gases. This is slightly less efficient than turbocharging because there is a small loss of power from the engine to drive the supercharger – but the net gain is significantly greater.

In diesels, it is now common to see not only turbocharging but common rail direct fuel injection, which feeds diesel at high pressure into the combustion chamber. The higher the pressure, the more finely the diesel is atomised which makes for better burning. Not only is the fuel used more effectively, emissions are reduced too.

To help clean up diesel engines, a number of after treatments have been developed. Although diesels usually outperform petrols in terms of CO2 emissions, they tend to be worse regarding NOx and particulate emissions. To comply with European standards, diesel particulate filters, NOx traps and selective catalytic reduction agents aim to reduce or break down the pollutants as they exit the engine in the exhaust gases.

There is plenty of evidence to support the claim that these after treatment systems aren't working as effectively as intended though. Vehicle testing procedures haven't done enough to bring real-world emissions down to official targets over the past few Euro standards, meaning the majority of cars on roads exceed pollution limits. Visit our emissions microsite at the button below to find out more.

NGC Emissions Microsite

Chris Lilly

Author:Chris Lilly
Date Updated:13th Nov 2017

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