Engine lubrication systems

Need for lubrication

Lubrication of the sliding parts (piston and rings against liner, bearings, cams and valve gear) is needed to:

• Reduce friction and wear, and so power loss.
• Cool the parts by carrying heat away to the sump.
• Seal the piston ring gap, preventing gas leakage.
• Clean the engine by carrying away carbon and metal particles to the filter.
• Prevent corrosion and reduce noise.

Wet sump pressure (forced feed) system

 Sump --> [Strainer] --> [Gear pump] --> [Relief valve]
                              |
                         [Oil filter]
                              |
                        Main gallery
              +---------+-------+--------+
            Main      Big-end  Cam   Valve gear
           bearings   (via     bearings (rocker)
                      drilled
                      crank)
 Return by gravity to sump.

The oil is in the sump at the bottom of the crankcase. A gear (or rotor) pump draws it through a strainer and delivers it at 2-4 bar through a filter into the main gallery. Drilled passages in the block and crankshaft carry it to main bearings, big-end bearings, camshaft bearings and rocker arms. The cylinder walls and gudgeon pins are lubricated by splash and oil mist. A relief valve limits the pressure. The oil returns by gravity to the sump.

Wet versus dry sump

Hydrodynamic lubrication

When the shaft is at rest it touches the bearing at the bottom. As it begins to rotate, the shaft drags oil into the converging wedge between itself and the bearing because of the viscosity. At sufficient speed the oil in the wedge builds a pressure that lifts the shaft on a full fluid film, with the shaft centre displaced from the bearing centre (eccentricity $e$). The surfaces no longer touch, and the friction is only the viscous shear of the oil.

   bearing
  +-----------+
  |  .--O--.  |   O = journal centre (displaced by e)
  | /  shaft\ |   thin film thickest on the entry side,
  | \_______/ |   thinnest at h_min on the exit side
  +-----------+   pressure peak ahead of h_min

The film thickness and load capacity depend on the Sommerfeld number $S = (\mu N/P)(r/c)^2$, where $\mu$ is the viscosity, $N$ the speed in rev/s, $P = W/(dL)$ the bearing pressure, $r$ the journal radius and $c$ the radial clearance. Higher viscosity and speed and lower load increase the film thickness. The friction coefficient follows the Stribeck curve: high at start (boundary), minimum when the film forms, then slowly rising in the fluid regime.

Petroff's equation

For a concentric journal (light load) the shear stress is $\tau = \mu U/c$ and the friction coefficient is

$$f = 2\pi^2 \frac{\mu N}{P} \frac{r}{c}$$

Calculation

$$\begin{aligned} N &= \frac{1500}{60} = 25\ \text{rev/s} \\ P &= \frac{W}{dL} = \frac{4000}{0.1 \times 0.08} = 5 \times 10^5\ \text{Pa} \\ \frac{r}{c} &= \frac{0.05}{0.00005} = 1000 \\ f &= 2\pi^2 \times \frac{0.025 \times 25}{5 \times 10^5} \times 1000 = 0.02467 \end{aligned}$$ $$\begin{aligned} T &= f W r = 0.02467 \times 4000 \times 0.05 = 4.93\ \text{N m} \\ \text{Power loss} &= 2\pi N T = 2\pi \times 25 \times 4.93 = 775\ \text{W} \end{aligned}$$

Answer: $f = 0.0247$, friction torque $= 4.93$ N m, power loss $\approx 0.775$ kW.

Properties of a good lubricant

• Viscosity: resistance to flow. It must be high enough to hold a film under load, and low enough to keep friction low and allow cold starting.
• Viscosity index (VI): the rate of change of viscosity with temperature. A high VI (above 100) means a smaller change. Multigrade oils have a high VI.
• Oiliness (lubricity): ability to form a film on metal under boundary conditions.
• Flash and fire point: must be higher than the operating temperature (flash point above about 200 degrees C), for safety against fire and evaporation.
• Pour point: lowest temperature at which the oil still flows; must be low for cold climates.
• Cloud point, stability: resistance to oxidation, sludge and carbon formation at high temperatures.
• Detergency, anti-foam, corrosion resistance and low volatility.

Types of lubricants

Additives

SAE grading

The Society of Automotive Engineers classifies oils only by viscosity, not by quality.

• Single grade: SAE 20, 30, 40, 50 (viscosity measured at 100 degrees C). A higher number means a thicker oil.
• Winter grades have a W: SAE 0W, 5W, 10W, 15W (measured at low temperature for cranking and pumping).
• Multigrade: SAE 10W-30 behaves like a 10W oil when cold and like a 30 grade oil when hot, so the same oil serves winter and summer.

Quality is given separately by the API service classification (for example SJ, SN for petrol; CF, CK for diesel).