A recurring topic at the technical service of MAHLE Aftermarket: the oil consumption of four-stroke engines. The “standard values” depend here on the engine design. Increased consumption can be due to wear and tear of the engine and the auxiliary equipment, the operating conditions or even the actual engine oil. In addition to oil consumption during operation, the oil level can also drop due to losses through damaged sealing elements.

The path of the engine oil is via the piston rings into the combustion chambers, via the valve guides into the intake and exhaust channels and via vaporization/atomization into the crankcase – with venting through the crankcase ventilation. Most of this oil is then combusted or oxidised in the catalyst. When the piston moves downwards an ultra-thin film stays behind in the cylinder, which partly evaporates or burns off. The remaining oil serves to lubricate the upper piston ring. A certain oil consumption is therefore normal or even desired. In today’s smaller passenger car engines, this is about 0.05 to 0.5 litres per 1,000 km. Engines with larger displacement and commercial engines have accordingly higher oil consumption. During the running-in period – up to about 5,000 km running distance – a new/reconditioned engine may consume a little more oil.

BANAL, BUT STILL A SOURCE OF ERRORS: MEASURING THE OIL LEVEL
The oil level must always be measured about 3 minutes after the engine is switched off. Under cold conditions, condensates and unburned fuel in the engine oil can distort the measuring result – and when measuring immediately after turning off, the oil level can be found too low, as the engine oil still needs to collect in the oil sump.

WORN PISTON RINGS AND CYLINDER WORKING SURFACES – OFTEN THE CAUSE OF INCREASED OIL CONSUMPTION.
As can be seen clearly in the pictures, the running partners piston rings and cylinder working surface are extremely worn. However, the parts had only been running for a few thousand kilometres. At the same time the engine became conspicuous through blue smoke and increased oil consumption.

Signs of wear: the chromium plated bearing ridges of the DSF ring have been completely removed ...

… and no honing is left in the running area of the rings.

The heavy oil consumption was therefore due to the extreme wear of the components. But what caused this wear?

FROM PRACTICAL EXPERIENCE THE FOLLOWING CAUSES ARE POSSIBLE:

• The load-bearing ability of the lubrication oil film has been diluted by unburned fuel.
• Abrasive particles (dirt) have been in the engine oil.
• The oil film has been impaired by warming up the engine for too long – a high risk, especially at initial start-up of a new or overhauled engine.

TO AVOID THIS TYPE OF DAMAGE THE FOLLOWING IS RECOMMENDED:

• Do not warm-up the engine but operate it immediately at medium revs and medium load.
• Check that the injection system, the cold-start enrichment and if applicable, the ignition system is working properly.
• All parts of the engine should be carefully cleaned and well oiled before assembly.
• Make sure that all components of the intake system are free from contaminations when the engine is re-fitted.
• Never attempt to clean an old air filter with compressed air, but always use a new filter element. On the one hand only few coarse particles can be removed with compressed air. On the other hand the filter paper is subject to natural embrittlement and can rip due to the compressed air without noticing.
• Check the complete intake system for leaks, so that no (contaminated) bypass air can be drawn in.

TWO SIGNIFICANT TYPES OF DAMAGE

WEAR AT A COMPRESSOR CYLINDER
A special case: the compressed-air system of a commercial vehicle
When this type of damage is found in a compressor, the cause can be a defective valve plate (i.e. leaky membrane valve). Pistons and cylinders are cooled down fast when compressed air flows from the tank pass a leaky membrane valve into the compressor, after the engine is turned off. The moisture in the compressed air condenses and dilutes the lubrication oil. The condensate reduces the load-bearing ability of the oil, resulting in mixed friction and therefore in heavy wear of the rings and the cylinder working surface. Increased oil consumption and oil contamination of the complete compressed-air system is the consequence. First indication of this type of damage is an oil-contaminated air dryer, which can be regarded as a warning sign for a defect in the safety-relevant brake system. The compressor should then be repaired immediately.

Wear at a compressor cylinder

WORN VALVE GUIDES
Valve guide in the cylinder head
The cause for increased oil consumption can also be located in the cylinder head. If the valve guides are worn, oil can reach the combustion chambers or the exhaust manifold trough the enlarged gaps between guide and valve shaft. Older, brittle valve stem seals also contribute to increased oil consumption.

When valves are replaced, we recommend exchanging the valve guides at the same time. Experience shows that new valve stem seals in combination with old, worn guides only reduce oil consumption for a short time. This is because the strong radial movements of the valve shaft damage the seals after a few thousand kilometres, which leads again to increased oil consumption.