What is mechanical fuel injection? PH Explains

Mechanical fuel injection is a type of fuel delivery system that, as the name implies, relies primarily on mechanical methods to inject fuel into an engine. It preceded electronic fuel injection and was designed to offer a more consistent method of delivering fuel.

How does mechanical fuel injection work?
Early mechanical fuel systems typically relied on a complicated fuel pump that would provide pulses of high-pressure fuel to the injector feeding each cylinder - and the pressure of the fuel arriving at the injector would cause it to automatically open, spraying fuel into the combustion chamber or intake.

This engine-driven pump would feature a series of pistons, operated by cams, which would generate the burst of fuel required. Outside of regulating the output from each piston, few adjustments to the mixture could be made. Consequently, the mixture was often not ideal for the engine's load or operating conditions.

Later automotive systems relied on more advanced metering units, also called 'fuel distributors', which could modulate fuel flow more accurately based on airflow into the engine. A mechanical or electric pump would generate the required fuel pressure, which would then be routed to the required injector at the right moment by the metering unit.

Do diesels use mechanical fuel injection?
Diesels were the first engines to benefit from mechanical fuel injection, as the system made it easy to directly inject the correct amount of high-pressure fuel into the combustion chamber - a distinguishing feature of diesel engine operation.

It's not uncommon to encounter mechanically injected diesels, particularly in older industrial applications. Modern automotive diesels typically use systems that feature electronically controlled injectors, however - but most still rely on a primary high-pressure pump, or in some cases a cam-pressurised injector, to generate the required fuel pressures.

Different types of mechanical fuel injection

  • Central: One or more injectors is placed at the opening of the intake manifold
  • Direct: The fuel is sprayed directly into the combustion chamber
  • Indirect: Fuel is injected into the inlet runners in the intake manifold

Why did manufacturers change to electronic fuel injection?
Mechanical fuel injection systems require precisely machined components, making them expensive. The systems were also often limited in their range of adjustments, which meant they could struggle to deliver the correct amount of as engine load and conditions changed. Consequently, in some cases, users would experience poor running and high fuel consumption.

The pumps and injectors could also wear, which would cause similar issues and require an expert to rebuild them, and problems could be dangerous - primarily due to the high fuel pressures generated by the systems - and costly to fix.

As electronic fuel injection systems advanced and decreased in cost, and the introduction of tighter emissions regulations necessitated more accurate fuel delivery, mechanical injection systems were phased out by most building petrol engines.

A brief history of mechanical fuel injection
The concept was originally introduced in the early 1900s, for use in diesel engines, but was rapidly adopted - with the introduction of a throttle and different metering systems - to petrol engines as engineers sought ways to more precisely deliver fuel.

Mechanical injection rose to prominence in World War II, as the demands made on fuel systems in combat aircraft rapidly outstripped the capabilities of carburettors. Following the war, mechanical fuel injection systems were gradually introduced into race cars - then road cars - as performance requirements increased. The Mercedes-Benz 300 SL, introduced in production form in 1954, featured Bosch mechanical direct injection, for example. Several others followed suit, including Chevrolet, Porsche, Alfa Romeo and Triumph.

The advent of more accessible, precise and self-tuning electronic fuel injection saw the system's popularity fade by the late 1970s. That said, many specialists exist today to maintain older systems and the systems are popular in racing where maximum power is required.

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