What is oil sludge and how can it be avoided?
Oil “sludge” is a common lubrication topic but few people really understand the nature of the problem and how to address it. It is an extreme lubricant condition and a variety of adverse factors need to happen for engine oil to thicken to a mud-like or gel-like consistency which can clog oil channels, filters, valves and screens and result in machine failure.
All modern quality oils used under acceptable running conditions should not result in sludge. It is normal for any engine oil to darken and thicken over time. Darkened oil that flows out during oil drain is a good sign that the detergent and dispersant additives have effectively captured the contaminants so they can be flushed out when you change your oil.
It is the sticky, grainy, thick and extremely discolored gunk that we should watch out for. Google Dictionary defines sludge as a viscous mixture of liquid and solid components. The main ingredients for oil sludge are highly oxidized base oil and contaminants such as water, metal wear particles, dirt and unburnt fuel.
Oxidation is a chemical process where the molecules of a substance react with oxygen. It is the process that turns iron (Fe) into rust (FeO) and causes apples, pears, bananas and eggplants to brown once they are cut up. Similarly, engine oil molecules react with oxygen during the combustion process and when antioxidant additives are depleted, the base oil begins to oxidize. It thickens (viscosity increases) and become acidic (Total Base Number decreases).
The oxidized oil can form many different types of sludge-like compounds in combination with water, fuel and solid contaminants. Extreme engine operating temperatures can accelerate sludge formation. Below are some underlying conditions that can contribute to sludge formation.
Short trips – Frequent short trips (less than 20 minutes) where the engine does not get up to optimal running temperature can cause sludging. A cold engine will attract condensation as soon as it is started – the difference in temperature from the combustion process and the cooler ambient temperature of the engine will cause water to form and mix with the oil. A properly warmed up engine will heat the engine oil to a temperature where the water can evaporate and be released through the PCV valve. Short trips can cause water contamination build up over time.
Stop and go traffic – Driving in congested city roads is extremely demanding on engine oils. Even when engine cooling systems are working properly, the slow speeds mean that the engine runs longer to cover a certain distance. Since most people rely on mileage as a basis for their oil change interval (normally 5,000 kms for mineral based oils), a city driver will spend more time on the road compared to those normally running on highway speeds. In EDSA where average speeds can drop below 30 kms per hour at peak times, it takes a car two to three times longer to run its entire 24 kilometers length compared to the same distance on NLEX. This means the lubricant additives and base oils work two to three times harder to run the same distance. Or put another way, the engine oil properties get depleted two to three times faster. Stop and go traffic oxidizes oil faster. Blow-by gases – A certain amount of combustion gas gets into contact with engine oils while the engine is running under normal conditions. Combustion gases contain soot and unburnt fuel and both of these can combine with oxidized oil to form sludge compounds.
Any condition that can increase blow-by such as damaged piston rings can worsen soot and fuel mixture into the engine oil. Certain components of fuel can undergo chemical transformation and act as binding agents for oxidized oil, water and insoluble particles that form sludge. Similarly, any engine condition that results in incomplete combustion can increase the amount of unburnt fuel which can result in sludge build up. Faulty ignition coils, worn spark plugs and broken computer boxes and fuel injection systems are the primary root causes of unburnt fuel accumulation. Dirt and other solid particles – The crankcase and valve assemblies on engines are sealed systems.
Dirt from air sucked into the engine is filtered by the air cleaner before it can flow into the intake manifolds for fuel mixture. Poor quality or overused air filters can let solid particles get into the combustion chamber and slip into the oil with combustion gases. These solid particles can combine with unburnt fuel, water and other solids and form sludge. Driving on dusty roads and a dirty engine compartment can worsen the situation. Any combination of the above factors increases the risk of sludge formation in your engine.
While the oil is normally the first component to be suspected, sludge needs other factors for it to develop. To avoid sludge, you should use a good quality engine oil and be mindful of your driving conditions and maintenance practices. Avoid repeated short trips. Change oil more frequently when driving through
heavy traffic every day. Change air filters regularly and keep your engine bay clean. Avoid excessive blow- by by using good engine oils with superior anti-friction properties to protect cylinder ring sealing.