08 Feb Starting a Diesel After Rebuild
We’ve learned a lot about diesel engines during this rebuild. On one hand, it’s probably good to know more . . . but some of these things I really never felt the need to know.
Such as what to expect when starting a diesel after a rebuild.
Now, if the rebuild was done in a shop, the shop would have “bench started” it and this won’t apply. Or rather, it will all be done out of sight.
Our rebuild, however, was done in the cockpit and the first start was done right in the boat. What we didn’t know was to expect it to be hard to start the first time. Very hard. To be honest, we were convinced that it wasn’t going to start and all the time and money was for naught.
To be fair, our mechanic (and friend and Gemini owner) kept telling us that it was going to start. Only as it got close to starting did we understand what was going on.
To back up a bit — and I’m keeping this somewhat simple for people like me who don’t know a heck of a lot about internal combustion engines — a diesel runs when the atomized fuel in the cylinder reaches a temperature of about 600 degrees F. To do this on our Westerbeke, the fuel is heated up a bit by a glow plug, while the air is compressed by the piston in the cylinder. As air compresses, its temperature increases — to get it to 600-ish degrees, the pressure needs to be somewhere over 300 psi (the exact numbers depends on the exact diesel fuel, outside air temp, etc.). The injectors squirt the warm fuel into this hot air and it ignites. Diesel fuel combusts solely due to the temperature (there’s no spark as in gas engines) and thus if there is insufficient pressure to get the temperature rise, it just won’t fire.
To get the necessary pressure for the temperature rise, the pistons are pushed into the cylinders — the shrinking space in the cylinder is the combustion chamber where the fuel is sprayed by the injector. The pistons have to fit very tightly into the cylinders or else the air already in the cylinder will just escape and there won’t be enough pressure to raise the temperature to point where the fuel will ignite. That’s where the “rings” come in (part of what was replaced during the rebuild). There are grooves that go around the piston and the rings fit into them and against the cylinder wall to seal off the combustion chamber so that the pressure won’t just escape.
Brand new rings are said not to be “seated” — that is, not 100% sealed against the piston and cylinder wall. It takes running a diesel some to get everything hot for the parts to mate completely.
And here’s the rub — until the rings are seated, the engine won’t have sufficient compression to start. Sort of a Catch-22, huh? The engine won’t start until the rings are seated, and the rings won’t seat until the engine has run.
The solution: keep trying to start it (in other words, pseudo-run it), using the attempts to slowly seat the rings.
When a rebuild is done in a shop, you never see this part. They typically use a large starter and power supply and keep cranking until there’s compression and the engine fires. Then they let it run a while to fully seat the rings (and probably do some final adjusting on valves and other things) and when the engine is put back into the boat, it just starts.
But for a cockpit rebuild, there’s no shop with a big starter and unlimited power supply. There’s the engine’s own starter and the boat’s batteries. You can’t turn it over forever at one time as you don’t want to burn out the starter . . . and the batteries won’t allow this, anyways.
I know, under normal conditions you’re told not to just keep trying to start an engine. “If it won’t start, figure out what’s wrong and don’t try again until it’s fixed.” How many times have I heard some variation on that? Well, in this one not-normal case, the attempts to start the engine are “fixing” the problem.
With our rebuild, it took several days of trying to start the engine before it fired. At first, the starter was the limiting factor — we cranked less than 30 seconds at a time, but then had to let the starter cool. We watched as compression slowly rose from about 100 psi in each of the three cylinders to 200 (normal is 398). Here’s how it sounded . . . time after time. NOTE: The first two videos are sideways, but there isn’t really anything to see, just hear, so don’t worry.
Okay, good, an increase in pressure is a step in the right direction. The mechanic felt that we weren’t spinning the engine fast enough to build up the heat needed to seat the rings further. We’d been running the generator to give the batteries a bit extra oomph (and to recharge after each effort), but it just wasn’t enough. A load test revealed that one of our three batteries was nearing the end of its life, so we replaced it (we have a single battery bank; the engine is small and does not require a dedicated starting battery).
Now we could crank longer and faster. You couldn’t see it, but you could very faintly smell a bit of exhaust on each try. And occasionally the engine would cough. A bit of oil was added to each cylinder to seal up the minute gap between the ring and the cylinder wall and the compression climbed again. Finally, you could feel the engine heating up as the start attempts continued. And then finally . . .
Okay, it’s running . . . but that sounded pretty rough starting. After letting it run maybe 5 minutes and then restarting it, it started right up.
We let it run about 30 minutes, then changed the oil. This is really important with a freshly rebuilt engine as there may be metal shavings in the engine — and again it’s something that you don’t have to worry about with a shop rebuild as they should take care of it.
Starting up again the next morning (with our mechanic present, thankfully), the engine started vibrating badly and the alternator belt squealed as well. Again, the mechanic expected this as the valves needed to be adjusted as they, too, seated themselves. The alternator had simply loosened slightly due to the vibrations and it just needed tightening.
Again, I want to repeat that if you have an engine removed and rebuilt in a shop where it can be started, you shouldn’t face these problems. But if you — like we — have a cockpit rebuild, it’s important to know that hard starting isn’t a sign that the rebuild was bad. We didn’t know what a bear it would be to start the first time, and really stressed out as it didn’t happen immediately. Knowing that it’s normal brings the anxiety level down several notches.
Barefoot Gal starts easily now, and just purrs!