Why does a diesel engine need no spark?
A diesel engine lights its fuel with nothing but squeezed air, which gets hot enough to set the fuel off by itself.
▶ Start the storyA diesel engine needs no spark because squeezing air hard enough makes it hot enough to set fuel alight on its own. In a diesel, only air is drawn into the cylinder and then compressed. That compression raises the air's temperature, and when atomised fuel is injected into the hot air near the top of the compression stroke, it ignites. This is why the diesel is also called a compression-ignition engine. A petrol engine, by contrast, squeezes a mixture of air and fuel and needs a spark plug to light it.
Step 1: Intake
Only air is drawn into the cylinder.
Step 2: Compression
The piston squeezes the air, 15:1 to 23:1, and it heats up.
Step 3: Injection
Fuel is sprayed in near the top of the stroke.
Step 4: Ignition
The hot air lights the fuel by itself.
The squeeze is severe: the is typically between 15:1 and 23:1, and that high compression causes the air temperature to rise.
The invention began in a lecture hall. In 1878 the student Rudolf Diesel heard Carl von Linde explain that steam engines convert just 6 to 10% of heat into work, and that more was possible. Linde also showed him a , a traditional fire starter that lights tinder by rapid compression. His prototype first fired in 1893, and by 1895 it reached an effective efficiency of 16.6%.
Diesel engines were first used to replace stationary steam engines, and since the 1910s they have powered submarines and ships.
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Recap
Petrol engines are lit by a spark; diesel engines are lit by squeezing.
💡 A trick to remember it · Spark for petrol, squeeze for diesel: one is lit, the other is cooked.
Surprising fact · Its inventor was inspired by an old fire piston and died mysteriously on a ship in 1913.
Sources (1)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.