Glow Plugs: A success story

Starting a diesel engine can be a challenging process. As I mentioned in Engine Drama, running a diesel engine is rather simple: fuel, lubrication and cooling. Starting is another matter. Since diesels run without electricity (no spark plugs), the fuel is ignited by heating the air inside the cylinder. Much of this heat is provided by compressing the air when the piston moves up the cylinder – a simple thermodynamic process (it’s the opposite of letting a gas out of a cylinder: like dive or propane tanks, whipped cream cartridges, etc. – high to low pressure transitions chill the gas). To achieve this, diesel engines have significantly higher compression than standard gasoline engines.

My engine also has an intercooler, which transfers some of the exhaust heat into the intake air, boosting the efficiency of the engine (higher combustion temperatures cause more complete fuel burning). Heat in the cylinder walls and pistons (from previous combustion cycles) helps the process, too. But these processes can’t help start a cold engine. Enter the lowly glow plug…

Its job is to preheat a small chamber where fuel combustion begins, just on the side of the main “cylinder”. To start most diesels, one turns the ignition to “run” then waits a few seconds until a small yellow light extinguishes – that’s the time it takes for the glow plugs to adequately heat this small chamber where the injectors pump the fuel into the cylinder. When the light goes out, turn the ignition switch to “start”, crank the engine and it fires up.

So in NZ, while at a remote anchorage, when the yellow indicator stopped lighting and the engine wouldn’t start, I figured the glow plugs were not getting power. Confirmed with a quick check of the voltmeter, I found a rather fat (AWG #8), short wire and connected the glow plugs directly to the battery.  After a few seconds, remove the wire, crank the engine, and voila! A running engine.

For many years, I’ve studied (and corrected) the wiring diagram for this engine, so I knew that what I was doing was safe. I also knew that replacing the engine’s “starter module” would probably take a few more weeks to order the $300 part. I wasn’t averse to starting the engine with a wire for a while.  Later, while underway, I took the module apart, found and fixed a bad solder joint on the timing capacitor and now it’s working again.

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