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Old 12-04-2019, 10:05 AM   #12
Ski in NC
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City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,194
Quote:
Originally Posted by Mischief Managed View Post
My neighbor has a Kubota tractor with basically the same engine as my genset, and it starts just fine after using the glow plugs, even at really low temperatures like -20F. That got me thinking about the differences between the two engines, and I came up with the following theory:

I suspect it's the governor on my genset that makes it start without vigor. When you start a non-governed diesel, the throttle is typically set to idle, which means very little fuel is going in. As long as there's enough heat in the combustion chamber to ignite the fuel going in, it will start right up, just as my CATs do. On a mechanically-governed engine, the throttle is wide open when the engine is off because it's trying to achieve 1800 RPM and it's clearly not any where near 1800 RPM. Thus, when I try to start it, the maximum amount of fuel possible is going in, which cools the combustion chambers and makes it harder to start. I bet if I held the throttle at idle while cranking the engine over, it would start immediately, like my neighbor's tractor. Another thought I have is that the engine may not have any variability to the injection timing since it's designed to run at 1800 RPM only. That would likely mean that the injection timing is too advanced at low speed which would exacerbate the issue by spraying fuel into the combustion chamber before it even reaches it's highest temperature.
No. The Kubota and most small diesels are indirect injection (IDI), larger engines like the Cat are direct injection (DI). DI has the injector squirt right into a bowl on the piston, the bowl forms the combustion chamber. IDI has a combustion chamber in the head, ported to the cylinder. The injector squirts into that chamber (also called a pre chamber or swirl chamber), then the burning gasses flow into the cylinder.

When starting an IDI engine, lots of heat of compression is lost as the hot gasses squirt through the cold metal and into the chamber. That is why almost all IDI engines need some glow plug heat (also in the chamber) to get enough heat to fire.

IDI is used to reduce combustion noise, but there is a penalty in efficiency. On larger engines DI is preferred as efficiency is primary and noise concerns secondary.

DI engines don't need preheating, but there will be a temperature below which they simply do not start. Some Cummins, Yanmar, etc models have an electric grid heater in the intake manifold to help cold starts, but in the marine world how many are boating in zero deg F weather?? The grid heater also reduces cold idle smoke. Many boaters find the grid heaters annoying and disable them. Hard on the rest of the electrical system.

Some automotive DI engines have glow plugs that extend the tips into the piston bowl combustion chamber, but I don't think they are in the marine world.

The governors on DI and IDI engines basically act the same way, with some tuning differences and a wide variety of mechanical gizmos to make it work. But the basic control of the injection by the governors of any type act similar.
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