The majority of new and used cars on UK roads either use
petrol or diesel as fuel. At first thought this difference
in fuel type doesn't seem overly dissimilar. The reality
is however that a petrol and diesel engine operate using
different thermodynamic cycles. These cycles describe the
way in which both engines burn the fuel to create power.
A diesel car adopts the Diesel Cycle, whereas a petrol engine
relies on the Otto Cycle.
Petrol Engines & Spark Plugs
In a petrol car, fuel and air is mixed by either a carburettor
or electronic fuel injection system. This fuel/air mixture
is then either injected or drawn into the engine's cylinder.
Petrol car require spark plugs to actually ignite the fuel
once it has been compressed inside the engine by the cylinder.
Diesel engines are very different.
Diesel Engines & Glow Plugs
Diesel cars do not need spark plugs or complicated electronic
ignition systems. A diesel engine either draws air or has
air forced into the cylinder with a turbo. This air is then
compressed to a much higher degree until it reaches approx
800 °C. Diesel fuel is then injected which ignites on contact
with the highly compressed hot air.
It's not surprising that diesel car ownership has ballooned
over the past decade, petrol taxes and 'miles to the gallon'
have become major topical issues affecting all of us.
Glow plugs in diesel engines assist in cold starting by
heating cold combustion chambers and cylinders during the
initial start up period. A cold diesel engine absorbs too
much heat from the compressed air, preventing successful
ignition. Glow plugs are electrically operated for around
10 seconds prior to start up. Once a diesel engine is running
the glow plug is not used.