Fuel injection is fuel injection, whether direct or electronic. The difference is that the latter is controlled by a computer, known as the electronic control unit or module (ECU or ECM), which also controls various electronic components and sensors. In direct fuel injection systems, the injectors are supplied with the fuel through individual high-pressure fuel lines. In electronic fuel injection engines, they are supplied with fuel from a common fuel rail at the cylinder head.

On acceleration, the throttle valve opens to permit more air to enter the engine. The ECM responds by opening more fuel. Sensors monitor the amount of air entering the marine engine and the amount of oxygen in the exhaust. They allow the ECM to fine tune the delivery so the the air-to-fuel ratio is just right.

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The Caterpillar HEUI (Hydraulic Electronic Unit Injection) system supposedly goes one step further in the delivery of ‘just right’ fuel to the marine diesel engine. The fuel injectors are operated ‘electronic-hydraulically’ instead of ‘electronic-mechanically’. The engine lubricating oil (sump oil) is utilized to generate the hydraulic pressure by boosting it with a high-pressure oil pump. It is delivered to the injectors through a manifold between the cylinder head and valve covers. The ECM regulates the oil pressure, which determines the pressure of the fuel to be injected.

Being more fuel efficient and lower in emissions, EFI has become almost universal in modern marine diesel engines.

This is an excerpt from Dick Gandy’s Australian Boating Manual

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