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FUEL DILUTION: The Danger of Fuel Dilution in Smaller Displacement Underground Mining Engines (Part 7 of 10)
Article by William Gillette (LogiLube, LLC)
A failed fuel injector is never a minor maintenance issue. In an underground mine, however, its consequences can develop faster and carry greater operational and safety significance than the same fault on a large open-pit haul truck.
The reason is not simply that one machine is smaller. It is the combination of cylinder count, lubricant volume, duty cycle, ventilation constraints and the speed at which an injector problem can contaminate the engine oil.
Consider an underground haul truck powered by a 16.1-liter inline six-cylinder diesel. Sandvik, for example, lists this engine configuration for its Toro
™ TH663i. By comparison, the Caterpillar 797F open-pit haul truck uses a 105.8-liter Cat® C175-20 V-20 engine and carries approximately 390 liters or 103 U.S. gallons of oil in its crankcase.
If one cylinder in a six-cylinder engine stops contributing useful power, it represents approximately 16.7% of the engine’s cylinder population. In a 20-cylinder engine, one cylinder represents 5%. Actual power loss depends on the injector failure mode, engine-control response, turbocharger behavior and whether the electronic control system derates the engine. Nevertheless, the six-cylinder machine has much less cylinder-level redundancy.
Injector failures are not limited to a completely “dead” cylinder. A nozzle may dribble, leak after injection, deliver too much fuel, produce a distorted spray pattern or fail to atomize fuel correctly. These conditions can cause incomplete combustion. Fuel that does not burn may wash down the cylinder liner, pass the piston rings and enter the crankcase.
This is the definition of fuel dilution as diesel fuel mixes with the lubricating oil.
Why the Smaller Sump Matters
Fuel dilution is a concentration problem. Severity is determined not only by how much fuel enters the crankcase, but also by the volume of oil available to absorb it. The same volume of diesel entering a smaller sump will produce a much greater percentage increase than it would in the 390-liter crankcase of a Cat 797F.
That difference can compress the warning window. A scheduled oil sample may show acceptable fuel dilution, while an injector fault developing soon afterward drives the concentration upward before the next sample is collected. The oil may still look normal on the dipstick, and the engine may continue running, but its lubrication margin can already be deteriorating.
Diesel fuel reduces lubricant viscosity and can reduce the thickness and load-carrying capacity of the oil film separating moving surfaces. Research has linked fuel dilution with viscosity loss, lower oil pressure, increased wear and, at sufficiently high levels, engine damage or failure.
Components at risk include crankshaft and connecting-rod bearings, cylinder liners, piston rings, camshaft surfaces and turbocharger bearings. Dilution can also interfere with the lubricant’s ability to control deposits, suspend contaminants and protect surfaces across changing loads and temperatures.
Underground Duty Creates a Second Risk
In an open pit, exhaust gases disperse into a large outdoor atmosphere. Underground, every diesel engine operates inside a managed ventilation environment. Airflow is a life-support system used to dilute and remove diesel gases and particulate matter. NIOSH identifies diesel engines as sources of carbon monoxide, carbon dioxide, nitrogen oxides, hydrocarbons and diesel particulate matter in underground mines.
A malfunctioning injector can change combustion and increase smoke, unburned hydrocarbons, carbon monoxide and particulate emissions. The response of nitric oxide and nitrogen dioxide is more complex: NOx may rise or fall depending on injection timing, cylinder temperature, air-fuel ratio, after-treatment condition and the specific failure mode. It is therefore more accurate to treat abnormal exhaust chemistry—not an automatic increase in every pollutant—as evidence that combustion quality has deteriorated.
Underground mines control these contaminants through equipment standards, maintenance practices and ventilation plans. In some jurisdictions and mine types, carbon monoxide and nitrogen dioxide are specifically monitored while diesel equipment is operating. An abnormal-emissions event may require the machine to be derated, removed from service or investigated immediately. If mine-air limits are exceeded, site-specific procedures could restrict access or require evacuation.
One injector problem can therefore create two exposures. The first is mechanical: fuel dilution weakens the lubricant and accelerates engine risk. The second is environmental: poor combustion increases the contaminant load imposed on the ventilation system.
Why Periodic Sampling May Not Be Enough
Traditional used-oil analysis remains essential. A laboratory can measure fuel dilution and evaluate viscosity, wear metals, soot, oxidation and other indicators. The limitation is not the quality of the result; it is the time between samples.
Fuel dilution caused by an active injector leak can develop faster than a calendar-based or machine-hour sampling schedule. Manual sampling also depends on the machine being available, safely positioned and accessible to a technician. Underground, that may require travel to a remote heading, interaction with hot surfaces and pressurized fluid, and coordination with production.
A stronger strategy combines laboratory analysis with continuous onboard condition monitoring.
Moving from Scheduled Detection to Exception Detection
SmartOil® G3 applies Autonomous Fluid Intelligence™ to the engine-oil compartment. Rather than waiting for the next manual sample, onboard sensing can monitor changes in lubricant properties and operating conditions while the machine is working. The SmartOil® G3 Edge-AI Brain™ evaluates trends, rates of change and relationships among fluid condition, oil temperature, engine load, operating hours and other machine data.
The purpose is not to claim that one sensor reading is a laboratory fuel-dilution result. It is to recognize when the oil moves away from its established baseline in a pattern consistent with contamination or abnormal combustion.
When a trend exceeds an engineered threshold, Exception Sampling™ can automatically collect a representative oil sample for laboratory confirmation. The maintenance team receives both the anomaly history and a physical sample taken close to the event—not days or weeks later, after the condition may have changed.
This closes an important diagnostic gap. The team can correlate the fluid anomaly with cylinder-balance data, fuel-rate changes, exhaust temperature, fault codes, operator observations and ventilation measurements. The likely injector or combustion problem can then be inspected before diluted oil produces secondary wear.
A Fleet-Level Safety and Reliability Tool
The value extends beyond one engine. Edge-based monitoring allows each machine to establish its own normal behavior while fleet teams compare similar assets across the mine. A rapidly changing viscosity or dielectric trend on one truck can be distinguished from a temperature-related shift affecting the entire fleet.
This supports more focused maintenance decisions. Instead of changing components solely by calendar interval or waiting for a high-fuel laboratory result, the mine can prioritize the machine showing the strongest evidence of an active fault. Parts can be staged and the engine removed from service before the problem becomes a bearing failure, a ventilation event or an unplanned underground recovery.
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Smaller-displacement underground engines package substantial power into compact machines. That same compactness can reduce the time available to identify fuel entering the oil. With fewer cylinders, a single-cylinder problem represents a larger share of engine output. With a smaller sump, a given quantity of leaked fuel creates a higher concentration. With confined ventilation, deteriorating combustion has consequences beyond the engine compartment.
The practical response is earlier detection.
Autonomous Fluid Intelligence™, the SmartOil® G3 Edge-AI Brain™ and Exception Sampling™ create a bridge between continuous onboard awareness and laboratory precision. For underground mine operators, that bridge can protect engines, reduce manual sampling exposure, support ventilation management and give maintenance teams time to act before a developing injector fault becomes a production or safety event.
SmartOil G3™ technology is protected by U.S. Patent No. 10,466,152; 11,761,946; 12,681,003; International Patents, and other U.S. and International Patents Pending
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