The 2-megawatt set fired up beautifully on test day. Cranked four seconds, caught clean, took the load like it was nothing. Eighteen months later we opened it for overhaul and the main bearings looked like they’d done ten thousand hours. The hour meter said 214. The killer wasn’t running. It was starting — three to four seconds of zero oil pressure at the beginning of every start, because the pre-lube pump had been dead for two years and nothing on the panel said a word about it.
That job is the reason I get asked about pre-lube systems more than you’d expect. The idea is simple: a small electric pump circulates engine oil through the galleries before the starter ever turns, so the bearings see oil pressure before they see rotation. Here’s the thing, though — the three questions I hear about it all have answers that surprise the person asking. So let’s do this the way the questions actually come to me.
Short answer: on a smaller set, no. And I mean that honestly, not as a setup for the next section.
On a 200 or 400 kVA set, the engine’s own oil pump builds pressure within a couple of seconds of cranking. The rotating mass is modest. The bearings were designed for exactly this — a brief dry moment, then oil. I’ve torn down twenty-year-old 400 kVA sets that never had prelube and found bearings still comfortably in spec. For that size, a prelube pump is jewelry. Spend the money on oil and filters instead.
Prime-power sets are a different animal, by the way — they run for hours at a time, so starts are a tiny fraction of their life. Prelube matters least where the engine runs most. It’s the standby sets, the ones that sit and wait for months, that pay the dry-start tax.
The math changes on big standby sets — call it 1.5 megawatts and up. Now you’re cranking an enormous rotating assembly, and the set has been sitting for months while every drop of oil drained back to the sump. Each start begins with three, four, sometimes five seconds of metal running on a thinning film of nothing. One start? Nothing. Dozens of starts a year, year after year? That’s invisible wear, and you don’t get to see it until overhaul day — when everybody argues about oil brands and nobody mentions the first three seconds.
Turbochargers make the case stronger. A turbo’s bearings ride on a film of engine oil, and the turbo is the last stop in the oil circuit — the last thing fed on every start, the first bearing to complain. I wrote once that the turbocharger is the most abused part on a diesel generator. Dry starts are about half of that abuse.
A customer of mine burned through a replacement turbo eight months after the first one failed. The supplier blamed oil quality — easy thing to blame, hard to disprove. I blamed the four to six seconds of zero oil pressure on every cold start. That set’s prelube pump had never worked, not once, not even from commissioning day. New turbo, same dry starts, same dead turbo. The third turbo went in only after we fixed the prelube pump. It’s still running. Draw your own conclusions.
So my honest rule: big standby set that sits for months — a prelube pump is cheap insurance, a few hundred dollars of pump and plumbing guarding a six-figure engine. Small set that exercises every week — skip it. And if someone tries to sell you a prelube system for a 100 kVA rental unit, they’re selling, not engineering.
Here’s the nasty part. A prelube pump can seize. Its motor can burn out. The timer relay driving it can die quietly. And the controller will still run its little “prelube” cycle before every start, timing nothing, alarming about nothing — because most controllers only time the cycle. They don’t check that oil pressure actually rose. The panel says prelube complete. The bearings say otherwise.
I found the dead pump on that 2-megawatt job completely by accident. I was standing next to the set during its weekly exercise, and out of habit I laid my hand on the prelube pump housing while the controller’s prelube light was on. Cold. Silent. It should have been humming, faintly warm. I pulled the maintenance log: two years of weekly tests, every single one marked “passed.” The pump hadn’t turned once in any of them. The hour meter read 214 hours. The bearings had lived a much harder life than that number suggested.
The good installations wire an oil-pressure switch as a crank permissive — no oil pressure, no crank, and now a dead prelube pump announces itself instead of hiding for two years. If your set doesn’t have that interlock, adding it is the upgrade worth doing. A fancier pump is not.
Most prelube timers are set somewhere around 30 seconds to a couple of minutes — long enough to push oil to the farthest gallery. If yours is set to ten seconds because “the weekly test takes too long,” that’s not a setting, that’s a confession.
And the check itself is embarrassingly simple. Once a month, during the prelube cycle:
You’ve probably seen it: kill the fuel, crank the engine for ten or fifteen seconds to “build oil pressure,” then restore fuel and start for real. Does it move some oil? Sure. Is it a substitute for prelube? No — and the price is higher than it looks.
Starters are built for short cranking bursts with cool-down time between them. Three fuel-off cranks to coax up pressure is three times the starter abuse and three times the battery drain, and at cranking speed the engine’s own oil pump moves a fraction of what a dedicated prelube pump pushes through the galleries. On electronic engines you’ll also collect overcrank faults in the event log, which is a fun thing to explain later.
An old mechanic I respect still does the fuel-off crank on his own sets. We argued about it over tea for a solid twenty minutes. He’s not wrong that it moves oil — he’s wrong about what it costs the starter to do the oil pump’s job. I told him so. He told me I’m soft. We’re both probably right about something.
The sane version: on a small set, just start the thing. Two seconds of dry cranking is what those bearings were designed for. On a big set, fix the prelube pump instead of inventing rituals around a broken one.
Next time your big set runs its monthly exercise, walk over during the prelube cycle and put your hand on the pump. Ten seconds, that’s the whole test. Humming and warm — walk away happy. Cold and silent — congratulations, you just found the cheapest repair that engine will ever need. That’s the kind of check we do here before any test we sign off on.
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