Oversized and Under-Loaded Generators: Wet Stacking, Glazing and What Light Loading Does to a Diesel Engine

In short. A diesel engine is built to work hard. When a generator runs for long periods at a small fraction of its rating, the cylinders run too cool for clean combustion: unburnt fuel and oil collect in the exhaust, carbon builds up on injectors, valves and rings, and the cylinder walls can glaze so the rings no longer seal. The result is black oily drips from the exhaust, smoke, rising oil consumption and, eventually, an overhaul that a correctly loaded engine would not have needed. Many engine makers advise against long running below roughly 30 % of rated load; your engine maker’s guidance is the one that counts.

Who this page is for: For facility managers, plant engineers and owners whose generator is much bigger than the load it actually carries: understand what light loading is doing to the engine, measure it properly, and choose the most economical fix.

What happens inside the engine

At good load, cylinder temperatures and pressures are high, fuel burns completely, and the piston rings are pressed firmly against the cylinder walls by combustion pressure. At light load, much less fuel is injected and much less heat is produced:

• Incomplete combustion: fuel droplets do not fully burn in the cool cylinder. Some pass into the exhaust; some wash past the rings into the oil.
• Carbon deposits: partly burnt fuel forms carbon on injector tips, valves, piston crowns and ring grooves. Rings can stick in their grooves.
• Bore glazing: with low cylinder pressure, rings ride lightly and the fine cross-hatch on the cylinder walls polishes to a glaze. Glazed bores do not hold oil, so the rings never bed in again — oil consumption rises permanently.
• Turbocharger oil leakage: at low boost pressure, oil can pass the turbo seals into the intake and exhaust, adding to the wet deposits.
• Fuel dilution of oil: unburnt fuel thins the lubricating oil, weakening its protection.
• After-treatment trouble: on newer low-emission sets, cool exhaust can prevent exhaust after-treatment from working or regenerating as designed.

“Wet stacking” is the visible end of this: a black, oily liquid weeping from exhaust joints, the turbo and the silencer, and dripping from the stack.

How loaded is “too light”?

Average load (share of rating)	What to expect over time	Action
Under 10 %	Rapid build-up of deposits; wet stacking within weeks of regular running	Act now — this is a sizing or operating problem.
10–30 %	Steady build-up; glazing risk with many hours	Plan regular loaded runs and evaluate the fixes below.
30–50 %	Generally acceptable for most engines, with periodic higher-load runs	Monitor; include a loaded run in the maintenance plan.
50–80 %	The comfortable range for most diesel sets	Normal operation.
Over 80 % for long periods	Within rating if correctly rated, but little margin for steps and summer	Check the rating class and summer derating.

General guidance — the engine maker’s minimum-load recommendation governs.

How sites end up oversized

• Sized from connected load, adding up every nameplate instead of measuring real demand.
• Future expansion that never happened, or tenants who never moved in.
• Sized for one big motor start that occurs for a few seconds a day.
• Load reduced over the years: LED lighting, efficient air conditioning, variable-speed drives, rooftop solar.
• Night and holiday operation: a building set sized for daytime runs all night feeding only lifts, pumps and common lights.
• A bigger rental set sent “to be safe”.

Measuring it properly

1. Use kW, not amps alone. Amps include reactive current; the engine only works for kW.
2. Log for at least a week covering day, night and holiday patterns — from the controller’s trend log, an energy meter, or a temporary power logger.
3. Record running hours in each load band: how many hours below 30 %, 30–50 %, above 50 %.
4. Note the peaks and what causes them — a motor start lasts seconds; a chiller runs for hours.
5. Compare with fuel — litres per kWh rising at the same load is another warning.

Signs the engine is already affected

• Black oily residue at exhaust joints, turbo outlet, silencer or rain cap.
• Blue-white or white smoke on starting and light load that persists after warm-up.
• Oil top-ups getting more frequent.
• Oil level rising or oil looking thin — possible fuel dilution.
• Black smoke when a big load is suddenly applied.
• Low compression or high crankcase blow-by on test.

Recovering an affected engine

If deposits are recent, a controlled loaded run often clears them. A common approach — always within the engine maker’s guidance — is to bring the engine up through load steps with a load bank, for example around 30 minutes each at roughly 25, 50 and 75 % and then a longer period at the higher load, watching exhaust temperature, smoke and oil pressure, before stepping back down. Smoke and dripping should reduce as deposits burn off.

If oil consumption stays high afterwards, or compression and blow-by tests show ring and bore wear, the glazing is established and a top overhaul — de-carbonising, re-honing and new rings — is the realistic repair. Running the engine harder after that point does not reverse glazing.

Six ways to fix an oversized set

Option	How it works	Best when	Watch out for
1. Scheduled load-bank runs	A periodic loaded run burns off deposits	The set is only moderately oversized and runs few hours	Treats the effect, not the cause; adds cost each time
2. Permanent automatic load bank	A load bank switches in when site load is below a set level, keeping the engine above its minimum	Long running at light load cannot be avoided	Burns extra fuel to make heat; needs space and ventilation
3. Bring more load onto the set	Move suitable loads to the generator bus — pumps, cooling, process loads	Useful load exists that is currently not backed up	Check motor starting and step loads
4. Add a smaller set for light-load periods	A small set carries nights and holidays; the large set runs only for high demand	Load has a clear day/night pattern	Changeover or paralleling arrangement; two sets to maintain
5. Multi-set load management	On sites with several sets, sets start and stop according to load	Already paralleled systems	Needs tuned load-dependent start/stop settings
6. Replace with a right-sized set	Match the set to measured demand plus sensible margin	The set is far too large and runs many hours	Size for real motor starting and growth — not too small either

Reddy Generators can take an existing set in part exchange where replacement is the right answer.

Worked example (illustrative)

Illustrative figures to show the method. A residential complex has a 320 kVA set (256 kW at 0.8 PF). A week of logging shows:

• Daytime outage load: around 90 kW (35 % of rating).
• Night-time outage load: around 35 kW (14 % of rating) — lifts, pumps, common lighting.
• About 60 % of the set’s running hours are at night.

Symptoms: oily residue at the exhaust flange and frequent oil top-ups.

Fuel comparison at 35 kW, using illustrative specific fuel use for the purpose of the example:

Option at 35 kW	Load share	Illustrative fuel use	Litres per hour
320 kVA set	14 %	about 0.40 L/kWh	about 14
125 kVA set (100 kW)	35 %	about 0.31 L/kWh	about 11

Every 100 night hours on the large set burns roughly 300 litres more than a correctly matched small set would — and does the engine harm. The sensible plan here is option 4: a 125 kVA set for nights and light outages, the 320 kVA set for daytime and high demand, a top overhaul for the large set if tests show glazing, and a monthly loaded run built into the maintenance plan.

Prevention plan

• Monthly test runs under real load — not at no load. Our maintenance planning page sets out the test regime.
• An annual or half-yearly load-bank run at a high share of rating where the site load cannot provide it.
• Trend kW, running hours and litres per kWh; review every service.
• When load changes permanently — a tenant leaves, solar is added — revisit the generator size.
• For new projects, size from measured or properly estimated demand, with motor starting checked separately.

Load profile worksheet

Item	Your site
Set rating (kVA / kW)
Average day load (kW and %)
Average night load (kW and %)
Hours per month below 30 %
Hours per month 30–50 %
Hours per month above 50 %
Largest step load and what causes it
Oil top-up in last 250 hours
Exhaust residue seen? Where?
Litres per kWh trend
Preferred option and reason

Common mistakes

• Running the monthly test at no load.
• Judging loading from amps instead of kW.
• Using permanent load banks to hide a set that is far too large.
• Running a glazed engine harder in the hope of curing high oil consumption.

What to send us

Send the set’s kVA, a week of load readings in kW (or controller trend logs), running hours per month, oil added per 100 hours, and photographs of any residue at the exhaust joints. Use the application engineering enquiry, or WhatsApp the details and photographs.

Related pages

• Load-Bank Sizing
• Load-Bank Testing
• Generator Sizing
• Generator Maintenance Planning
• Top Overhaul
• Upgrade & Replacement Planning
• How to Read a Generator Data Sheet, Field by Field
• White Smoke from a Generator
• Blue Smoke and High Oil Consumption in a Generator

Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.