Fuel Dilution in Diesel Engines

Fuel Dilution Causes, How to Protect your Engine

Fuel dilution is arguably a diesel engines greatest adversary - it occurs normally, however it can cause excessive wear and become the ultimate demise of an engine in severe cases. It is for this reason that the concentration of diesel fuel (and other contaminants such as exhaust soot) in engine oil is monitored in certain industries through periodic oil analyses performed by a qualified laboratory.

What is fuel dilution?

Fuel dilution, or less commonly "crankcase dilution", refers to the dilution of engine oil by diesel fuel. Fuel dilution is an entirely normal phenomenon that occurs naturally in every internal combustion. It is of particular importance in diesel engines because they rely on a compression ignition cycle and therefore combustion efficiency is entirely dependent on engine temperature, specifically that of the combustion chamber. In theory, with an obvious point of diminishing return, the colder the combustion chamber the greater the degree of fuel dilution that occurs. The primary concern with fuel dilution in diesel engines is the lowering of engine oil viscosity, potentially having a significant impact on the oil's role as a lubricant between the various moving parts of an engine in high stress environments.

What Causes Fuel Dilution?

The normal cause of fuel dilution is blowby; the escaping of gases between the piston rings and cylinder bore. In an ideal world, a piston would seal perfectly against the cylinder wall. However, no cylinder is perfectly round, no bore is perfectly smooth, and there is no perfect seal. In fact, your engine has a crosshatch pattern of microscopic grooves purposely etched into each cylinder; it helps with lubrication by giving engine oil something to cling to. Case in point, a certain amount of fuel and exhaust gases will escape past the seal between the piston rings and cylinder. Since fuel dilution is completely normal, it is excessive amounts of fuel in the engine oil that is of concern. There are a number of activities and events that contribute to excessive fuel dilution, as well as ways to prevent it.

Wet Stacking

Wet stacking is a phenomenon that occurs in compression ignition engines. Whereas a gasoline engine, for example, ignites its air-fuel mixture with a spark at the appropriate time to initiate combustion, a compression ignition engine relies solely and entirely on the heat of the air charge in the cylinder during an injection event to initiate combustion milliseconds after the fuel is injected. A "cold" engine (referring to an engine that has not reached operating temperature and not necessarily making any connection to ambient conditions) will have a lower combustion efficiency than a "hot" engine. This is do to the nature of the combustion process; the compressed air charge in an engine at (or near) operating temperature contains more heat than the compressed air charge of an engine well below operating temperature, despite the fact that both conditions will exhibit an air temperature that is high enough to initiate combustion.

In conditions where combustion chamber temperature is low (relatively speaking), injected fuel has a tendency to ignite later in the compression stroke than when the combustion chamber temperature is higher. Under such instances, fuel spraying out of the injector will have a tendency to stick to the cylinder walls, "washing" the engine oil and ultimately sliding past the piston ring and entering the crankcase. Under normal circumstances, the volume of fuel that enters the crankcase is low and has little effect on oil lubrication.

Wet stacking and thus fuel dilution typically occurs most prominently under conditions where combustion temperatures are low. These events can generally be divided into two instances; excessive idling and the period in which a running engine has yet to reach operating temperature. With regard to excessive idling, recall that the diesel cycle is opposite the gasoline cycle with regard to air-fuel ratio. A diesel engine that runs rich will exhibit relatively high combustion temperatures while an engine that runs lean will display much lower combustion temperatures. At idle, a diesel engine does not require much fuel to run and will have a tendency to run lean, thus combustion temperature can drop significantly over a period of time. Under these conditions, the rate of fuel dilution is increased compared to that of an engine operating under load at operating temperature. Wet stacking is the sole reason why every OEM requires more a more stringent maintenance schedule for diesel vehicles that are idled excessively.

The initial start of a diesel engine, especially in cold weather, also presents conditions in which fuel dilution occurs at a greater rate. Although there is adequate heat being produced to permit combustion, the cylinder walls and overall cylinder temperatures are relatively cool. For this reason, it is generally not recommended that a diesel engine be ran at idle in order to bring it to full operating temperature. In fact, a diesel engine will rarely reach operating temperature in any reasonable amount of time without being driven; an engine will warm in much less time if a load is applied. This does not suggest that a diesel engine should be driven immediately after start-up, however it should not be allowed to idle for extended periods of time; 3 to 5 minutes (depending on ambient temperature) of idling is generally sufficient to allow oil pressure to stabilize and engine temperature to reach a suitable range before driving in cold conditions. In warmer weather, 1 to 2 minutes of idle before driving is entirely sufficient.

Poor And/Or Neglected Maintenance

In a normally functioning engine in "good" condition, fuel dilution becomes of significance when oil changes are not performed at the proper intervals. Manufacturers publish oil change intervals in two categories; "normal" and "severe" or "heavy duty" conditions. These intervals are to be followed closely, as the engineers who designed your engine have taken fuel dilution into consideration in producing their service schedule. Recall that fuel dilution is entirely normal and unavoidable. If oil is changed as recommended, the concentration of fuel in the engine oil should be well within the allowable range specified by the engine manufacturer.

In addition to regularly scheduled oil changes, fuel system maintenance can impact the rate of fuel dilution. Dirty, fowled, or otherwise unsatisfactory injectors may display a compromised spray pattern, thus degrading the atomization effect of fuel as it leaves the injector nozzle. This poor atomization contributes to a low combustion efficiency and greater degree of fuel dilution. Injectors are a regular replacement item and are not designed to last the lifetime of your engine. Reduced injection pressure, caused by a worn injection pump, contributes in the same way. Excessive smoke, rough idle, and low power conditions may all signify an injection system related problem. Such problems should be addressed promptly.

Aftermarket Performance Devices & Modifications

As a general rule of thumb, more smoke results in a greater degree of fuel dilution in addition to other forms of oil contamination. A portion of anything that exits the tailpipe is passing into your crankcase - period. While exhaust soot technically alters oil viscosity in a different way, its effects are equally, if not more substantially detrimental in terms of oil contamination. If your vehicle is equipped with any device that alters the fuel injection system such that it produces visible smoke, consider more frequent oil changes (at the very least).

Regeneration Post Injection Cycles

Fuel dilution became an even greater concern with the introduction of the diesel particulate filter (DPF), which requires a regeneration or "regen" cycle to periodically clean particulates from the filter. During a regen cycle, a significant amount of heat is required in the exhaust system to allow these particulates to burn off. Fuel is therefore required to be injected into the exhaust stream, raising the temperature and creating conditions in which these particulates can be completely burnt and emitted from the tailpipe. One answer to this dilemma is the post injection cycle; injecting fuel into the cylinder during the end of the exhaust stroke.

As fuel is injected into the cylinder during the exhaust stroke, it follows the path of the exiting exhaust gas stream and enters the exhaust manifold as, for all intents and purposes, atomized raw fuel. Since there is no combustion occurring when this fuel is injected, it is a tendency to stick to the cylinder walls and the "wet stacking" condition previously outlined comes into play. The answer to the problems faced with post injection cycles was to introduce a 9th or 7th injector so that fuel could be injected into the exhaust stream for regen purposes without contributing to fuel dilution. Certain model year DPF equipped trucks suffer from what is generally considered excessive levels of fuel dilution from the post injection process.

Effects of Fuel Dilution

The most profound concerns with fuel dilution are viscosity reduction and oil degradation. Diesel fuel in the oil supply increases the rate at which engine oil breaks down, as well as reduce its viscosity. As viscosity reduces and the oil becomes thinner, it provides a lower degree of lubrication. Engine oil is the single layer of protection between the moving parts of your engine. A thin film of engine oil is subjected to extreme pressure and forces as it provides a barrier between two moving/rotating surfaces. As the oil thins, this barrier may become compromised, increasing the rate of wear. As components wear, the physical tolerances between the two surfaces increase and problem slowly compounds.

Closed Crankcase Ventilation Systems

A closed crankcase ventilation (CCV) is incorporated into your engine to draw crankcase fumes into the intake of the engine. Fuel enters the crankcase in at atomized state, which can loosely be described as somewhere between a gaseous and liquid state. The crankcase ventilation system ensures that at least some portion of blowby products are recycled into the engine. On certain diesel engines, it is popular to delete or modify the crankcase ventilation system. While the CCV serves several purposes, its role in reducing the effects of fuel dilution is particularly important.

Reducing & Protecting Against Fuel Dilution

Fuel dilution is obviously an unavoidable condition, however there are things you can do to reduce fuel dilution and protect against its negative effects.



Avoid excessive idling

Combustion temperatures can drop significantly when an engine is let idle, increasing the magnitude of wet stacking.

Avoid excessive warm up periods

It is important to let an engine idle briefly after an initial start up, especially in cold weather. However, an engine will reach operating temperature much quicker and more efficiency if it is driven. A 3 to 5 minute warm up is entirely acceptable in cold conditions; up to 7 minutes in arctic conditions. Use your block heater to reduce warm up time and aid in starting. Drive the vehicle conservatively until the engine reaches operating temperature.

Avoid stop-and-go driving conditions

Stop-and-go traffic can have a significant effect on fuel dilution primarily do to the extensive idling that it is associated with. While there's nothing we can do about rush hour traffic, consider an alternative route with less frequent traffic stops.

Stick to a strict maintenance schedule

Generally, fuel dilution only becomes a major concern when oil change intervals are extended (or ignored). There's no reason why a routine oil change should be skipped, unless an oil analysis reveals that the engine oil quality is acceptable (this technique is typically reserved for industrial/agricultural applications). Additionally, stick to the "severe" or "heavy duty" service schedule if you're driving activities include extensive idling, towing, etc.

Service cold start aids as needed

An engine may start with one or more bad glow plugs, but it isn't helping your engine. If white smoke is present on start up or during cranking, the problem should be rectified as soon as possible. White/light gray smoke on start up is raw, atomized fuel. With fuel entering and exiting the cylinder without igniting, the rate of fuel dilution is high. Over the course of several thousand miles, this can have a significant impact on oil condition.

Use a synthetic motor oil

Synthetic motor oil provides far superior resilience to and protection from fuel dilution and other forms of oil contamination. Conventional motor oils are becoming a relic of the past and even the OEs are starting to use synthetic oils from the factory.

Address worn fuel system components promptly

Bad injectors, injection pumps, etc. Poor atomization = poor combustion efficiency = likelihood for more contamination.

Avoid excessive smoke

Smoke is indicative of nothing more than an incorrect air-fuel ratio. While a momentary puff of black smoke is entirely normal for most diesel engines, modifications that cause excessive smoke will contribute greatly to engine oil contamination.

Periodically drive DPF equipped trucks on the highway

Certain DPF equipped vehicles will suffer from greater fuel dilution during active regeneration. In addition to avoid stop-and-go traffic, the frequency of active regeneration cycles may be reduced if the vehicle is driven at a constant freeway speed. Additionally, instead of allowing an engine to idle in order to complete an active regen cycle, take it for a drive on the freeway; the regen cycle will be shorter and the rate of fuel dilution may be reduced.