Glow plugs are only used in engines that have 'pre-combustion chambers'. For direct injection engines (where the fuel is injected directly into the cylinder) glow plugs are not used.
Examples of direct injection engines are the Dodge truck's Cummins diesel, and the Hercules engines in the MEP-004 and MEP-005. The Dodge uses a heater grid in the intake instead of glow plugs, while the Hercules engines use an 'ether' starting aid for very low temperatures. I've toyed with a grid heater for the MEP-004, and I'm sure it could be done without too much trouble--but a block heater is certainly the easiest solution.
Glow plugs, though common, are not universal by any means. Some engines that do use glow plugs include automobile diesels (such as VW and Benz) and the 6.2/6.5 engines found in a number of trucks and buses.
There's nothing wrong with glow plugs, but without a pre-combustion chamber it is physically impossible to fit them into the combustion chamber. As well, the pistons in a direct injection engine such as the Hercules are radically different. They do have a small dished out combustion area in the top of the piston, with a very interesting ball like structure in the center. (Think a small ball (1/8 in diameter?) on a very short spike.) The ball gets heated and helps fire off the fuel once the engine is running.
BTW, the MEP-004 is a 4-cylinder engine, the MEP-005 is a 6-cylinder engine. Both engines are virtually identical, the 6-cylinder engine just has two more cylinders 'grafted' on to it... (Or, looking the other way, the 4-cylinder engine has the last two cylinders lopped off...
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The MEP-006 also has a 6-cylinder engine, but it is not a Hercules, and the MEP-006 is turbo charged.
Yes, I do have an MEP-004, have rebuilt it (the engine) so I'm rather familiar with them. I also have a number of diesel trucks (Hummers) with 6.5 engines (which I also rebuild).
Without glow plugs, without any form of heating, the MEP-004 will start reliably at about 20 degrees. Once colder than that, you need to heat the block, and run a good winter grade oil (or a synthetic) that has good cold viscosity. The secret to starting these is cranking speed, so oil in the winter months can be more critical. In my setup, I keep the MEP-004 in the shop in the winter so it is warmer than the outdoors environment. If I need it, I tow it out and crank it up.
