Knowledge

What Is Oil Evaporation and Why Does Oil Evaporate?

Engine oil isn’t just a lubricant—it’s a complex mixture of molecules that protect your engine from heat, friction, and wear. But did you know that engine oil can evaporate over time? Understanding oil evaporation is important for maintaining engine performance, saving money, and preventing damage.

What Is Oil Evaporation?

Oil evaporation is the process by which lighter oil molecules turn into vapor when exposed to heat.

Think of it like water boiling—but with oil, it happens gradually inside the engine at high temperatures.

Why Oil Evaporates?

Several factors cause oil to evaporate in your engine:

1- Temperature

• Engine oil is exposed to high heat near piston rings, turbochargers, and exhaust valves

• Typical oil film temperature near the piston rings: 150–300 °C

• Turbo and exhaust areas can get hotter, but oil itself doesn’t reach 600 °C (combustion gases are that hot)

• High heat causes lighter molecules in the oil to vaporize. 

2- Oil Composition

• Mineral oils have more light fractions → evaporate faster

• Synthetic oils have uniform molecules → evaporate less

• Additives can also affect volatility

3- Air Flow

• Crankcase ventilation (PCV system) moves oil vapor away

• This is normal, but it also increases evaporation

4-Pressure and Shear

• Oil passes through tight clearances and high-pressure zones

• Shear can break large molecules into smaller ones → easier evaporation

5- Oil Age and Oxidation

• Old oil breaks down, forming smaller molecules

• Evaporation rate increases with oil age

6- Fuel Dilution

• Fuel mixing with oil reduces viscosity

• Makes it easier for oil to evaporate

7- Hot Climate

• In Australia, high summer temperatures and long drives accelerate evaporation

• Synthetic oils perform better under these conditions

Why Oil Evaporation Matters?

Oil evaporation isn’t just a “technical detail”—it has real consequences for engine performance, efficiency, and longevity. Here’s why it matters:

1-Increased Oil Consumption

• When oil evaporates, the engine oil level drops faster than normal.

• Drivers notice: “My car keeps burning oil” or “I have to top up oil frequently.”

• High evaporation oils (high NOACK) are a common hidden reason for oil loss.

Impact:

• Frequent oil top-ups → higher running cost

• Risk of running the engine with low oil → serious engine damage

2- Engine Deposits and Sludge

• Evaporated oil molecules burn inside the combustion chamber.

• This form:
  • Carbon deposits on pistons
  • Ring sticking → reduced compression
  • Sludge inside the engine

Impact:

• Reduced fuel efficiency

• Harder engine start

• Long-term engine wear and tear

3- Turbocharger Damage

• Turbos operate at very high temperatures (250–350 °C at bearings, even hotter at the turbine).

• Evaporated oil leaves hard carbon deposits on turbo components.

• This can cause:
  • Reduced turbo efficiency
  • Bearing damage → turbo failure

Impact:

• Expensive repairs or replacements

• Reduced engine performance

4- Oil Thickening

• When light molecules evaporate, remaining oil becomes thicker.

• Thick oil:
  • Flows slower → poor lubrication
  • Increases friction → higher fuel consumption
  • Accelerates engine wear

Impact:

• Less efficient engine

• Higher fuel bills

• Shortened oil life

5- Emission System Problems

• Oil vapors can enter the exhaust system.

• This affects:
  • Catalytic converters
  • Diesel Particulate Filters (DPF)
  • Exhaust Gas Recirculation (EGR) system

Impact:

• Higher emissions

• Possible legal compliance issues

• Expensive emission system repairs

6- Shortened Oil Life

• Evaporation changes oil composition.

• Lighter fractions evaporate first → oil loses its protective qualities.

• Additives may evaporate → oil loses detergents, dispersants, and anti-wear properties.

Impact:

• Oil drains become shorter

• Higher cost due to frequent oil changes

7- Fuel Economy Impact

• Oil viscosity changes when light molecules evaporate → engine friction increases.

• Higher friction = more fuel consumed.

Impact:

• Even small evaporation differences can affect fuel efficiency by 1–3%, which is significant for fleets or long-distance drivers.

What Is the NOACK Test?

One key factor affecting engine health is oil evaporation, measured by the NOACK test.

he NOACK test (ASTM D5800) measures how much oil evaporates under high temperature.

How It Works:

1. Oil is heated to 250 °C

2. Air is passed over it for 60 minutes

3. Evaporated oil is measured as a percentage

Example:

• NOACK = 8% → 8% of oil evaporated

• NOACK = 15% → more evaporation, less stability

Lower NOACK = less evaporation = better high-temperature performance.

Why NOACK Matters?

Oil with high NOACK:

• Burns faster → more top-ups

• Forms carbon deposits → reduces engine life

• Thickens remaining oil → reduces lubrication efficiency

• Harms turbochargers and emissions systems

Oil with low NOACK:

• Reduces oil consumption

• Protects pistons, rings, and turbo bearings

• Improves fuel economy

• Lowers emissions

• Extends oil drain intervals

Synthetic vs Mineral Oil and NOACK