It can be a tricky process to start a marine diesel engine from cold with different techniques required for different engines. Callum Smedley bring us his guide

Starting a diesel engine from cold can be a tricky business. Many direct-injection engines use extra fuel for cold starting. On some, such as small Yanmars, this extra fuel is supplied automatically, and the owner need do nothing except set the ‘throttle’ correctly. On others, however, special actions have to be taken prior to starting from cold. For instance, on old Volvos and Listers, there’s a cold-start button at the back of the engine that must be pressed down for each cold start attempt. This action allows the fuel control rack to run to a ‘cold-start’ position by removing a block on the fuel rack, a system also known as ‘over fuel’. On the Petter Mini 6 and Mini Twin, a wire loop will be found on the front of the engine. This is pushed down and released after the ‘throttle’ has been set before starting.

Compression Ratio

The engine’s compression ratio can be increased for cold starting on some engines by the addition of lubricating oil to the cylinder above the piston. This cold, thick oil gets around the piston rings and valves, helping to seal them better. Less air is lost during compression, which means the final pressure will be higher and, of course, hotter. This is done on the Petter Mini 6 and the Mini Twin, and some Sabb engines.

An electric heater fitted inside an air manifold


Engines which have indirect injection are fitted with pre-heating as an aid to cold starting, and some direct engines may be fitted with a form of pre-heating as an option for use in low-temperature situations.

Glow plugs

Electric pre-heater plugs (glow plugs) are fitted permanently in the combustion chamber and powered prior to start from the engine’s electrical system. The heater pre-heats the whole combustion chamber. Its tip, which extends into the combustion chamber, is still hot when the fuel is injected, aiding ignition. Therefore a three-cylinder engine has three glow plugs, one for each cylinder.

On cars, an orange light on the instrument panel is illuminated when pre-heat is selected and goes out when the required temperature has been reached, indicating that the starter motor can now be engaged. On most marine engines, however, this indication is not given, so the operator needs to be aware of how long the pre-heat should be applied for.

A diesel oil burner and igniter heat the air intake

Electric coils

An electrical coil, mounted in the air intake manifold, is supplied with electric power during cold starting to pre-heat the air as it passes into the cylinder.

Ignition burner

A fuel reservoir supplies a burner in the air intake manifold. This burner is ignited during a cold start to pre-heat the air and was standard on Perkins 4-107, 4-108 and 4-109 engines. It should be noted that smoke can come out of the air intake filter during heating.

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