A Turbo Diesel engine has two main components - the Diesel fuel pump and the Turbocharger.
The Diesel Fuel Pump has two objectives to achieve:
First, to supply fuel to the engine when no boost is being produced and, secondly, to give additional fuel when boost is being produced. The pump has two separate systems to do this. When no boost is being produced, the Main Fuel facility supplies fuel in response to accelerator position. The On-boost Fuel facility adds to the main fuel as boost pressure increases.
In a Petrol engine, as the throttle butterfly is opened more air is gradually allowed to enter the engine. On full throttle, the air passage into the engine is essentially unrestricted.
A Diesel engine has no throttle butterfly and therefore there is no inlet restriction to air and it operates with an excess of air almost all the time. Hence, any boost pressure produced, even at a small accelerator setting - giving a small input of fuel - is transmitted to the engine. As the accelerator setting is increased, the increase in fuel will cause the boost to increase to a level which is only limited by the wastegate actuator setting. Many standard vehicles will never reach the maximum actuator setting because even on full throttle they are not set up to supply enough fuel.
