Lubrication and cooling systems

Why it matters

Roughly a quarter to a third of the fuel energy in an engine ends up as heat that must be carried away by the coolant and oil, and every sliding pair inside the engine would seize in seconds without an oil film. Lubrication and cooling decide engine life, friction power (and therefore mechanical efficiency), knock and NOx limits, and warm-up emissions. Service engineers, test-bed engineers and designers all work with these systems daily.

Key ideas

Functions of the lubricant

• Reduce friction and wear by separating surfaces with a fluid film.
• Cool parts the coolant cannot reach: bearings, piston underside (oil jets), turbocharger bearings.
• Seal the ring–liner gap, clean (detergent–dispersant additives keep soot and sludge in suspension until the filter or oil change removes them), protect against corrosion and neutralise acids (alkaline reserve, measured as total base number).

Lubrication regimes (Stribeck curve): boundary (metal asperity contact, at start-up and at TDC/BDC ring reversal), mixed, and hydrodynamic (full film, main and big-end bearings at running speed). Friction coefficient falls from boundary to the minimum at the mixed–hydrodynamic transition, then rises slowly with viscous shear.

Viscosity is the key oil property. It falls steeply with temperature; viscosity index (VI) measures how little it changes. Multigrade oils (for example SAE 10W-30 or 5W-30) use polymer VI improvers: the "W" number describes cold cranking and pumping behaviour, the second number the viscosity at 100 °C. Too thin an oil loses film thickness; too thick an oil raises friction and starves bearings on cold starts.

Lubrication systems

• Petroil (mist) lubrication: small two-stroke engines, oil mixed with fuel; high HC and smoke.
• Splash lubrication: dippers on connecting rods; small, slow engines.
• Pressure-feed (forced) lubrication: the standard automotive system. Pump (gear or gerotor) → relief valve → full-flow filter (with bypass valve) → main gallery → main bearings → drilled crankshaft → big-end bearings → camshaft and valve train; cylinder walls and piston pins get splash or jets. Wet sump (oil stored in the pan) is common; dry sump (separate tank and scavenge pumps) is used in racing and off-road engines.
• Crankcase ventilation (PCV) returns blow-by gases to the intake, reducing HC emissions and oil contamination.

Why cooling is needed: peak gas temperatures exceed 2,000 K. Without cooling the oil film breaks down (above roughly 200–250 °C at the liner), pistons and valves lose strength, the head distorts and SI engines knock. Overcooling is also harmful: higher oil viscosity and friction, poor fuel vaporisation, more HC and CO, condensation of acids and water in the sump.

Air cooling: finned cylinders and heads, with ram air or a fan and cowling. Light, simple, no leaks; used in two-wheelers and small engines. Limitations: uneven cooling, noisier, harder to control temperature in large engines.

Liquid cooling: water jackets around cylinders and head; coolant is water with ethylene glycol (antifreeze; raises boiling point and lowers freezing point) plus corrosion inhibitors.

• Thermosiphon (natural circulation): obsolete in cars; slow, needs a high-mounted radiator.
• Pump circulation: centrifugal pump driven by belt or electrically.
• Thermostat (wax-pellet type): stays shut until about 80–90 °C so the engine warms quickly, then opens to route coolant through the radiator.
• Pressure cap: pressurising the system (roughly 1 bar gauge) raises the boiling point, allowing higher coolant temperature and a smaller radiator.
• Radiator: tube-and-fin heat exchanger; fan (viscous or electric) supplies air at low road speed.

Links: friction power appears in mechanical efficiency and heat balance (performance testing); cooling losses are part of the actual-cycle losses; coolant temperature affects knock and cold-start emissions.

Formulas

Q̇ = ṁ · c_p · ΔT

• Q̇: heat transfer rate (W); ṁ: mass flow rate of coolant, oil or air (kg/s); c_p: specific heat (J/kg·K), about 4,180 for water and about 3,500–3,600 for a 50/50 water–glycol mix (take from data for the actual coolant); ΔT: temperature change of that stream (K). Steady flow, no phase change.

Q̇_c = x · ṁ_f · CV

• Q̇_c: heat to coolant (W); x: fraction of fuel energy to coolant (–); ṁ_f: fuel flow (kg/s); CV: lower calorific value (J/kg).

τ = μ · U / h

• Newton's law of viscosity for a thin film: τ: shear stress (Pa); μ: dynamic viscosity (Pa·s); U: sliding speed (m/s); h: film thickness (m).

F = μ · U · (π · d · L) / c, T_f = F · d/2, P_f = T_f · ω

• Petroff's equation for a lightly loaded, concentric journal bearing: d: journal diameter (m); L: bearing length (m); c: radial clearance (m); U = ω·d/2 (m/s); ω = 2πN/60 (rad/s); F: friction force (N); T_f: friction torque (N·m); P_f: friction power (W). It ignores eccentricity, so it underestimates friction in loaded bearings.

ν = μ / ρ

• ν: kinematic viscosity (m²/s; oils are quoted in mm²/s = cSt); ρ: density (kg/m³).

Worked examples

Example 1 (standard): coolant flow needed. Given: brake power 60 kW, brake thermal efficiency 32%, 30% of fuel energy goes to the coolant, coolant c_p = 3.6 kJ/kg·K, permitted coolant temperature drop in radiator = 8 K, cooling air temperature rise = 25 K with c_p,air = 1.005 kJ/kg·K.

1. Fuel energy rate: Q̇_f = BP / η_bth = 60/0.32 = 187.5 kW.
2. Heat to coolant: Q̇_c = 0.30 × 187.5 = 56.25 kW.
3. Coolant flow: ṁ = Q̇_c / (c_p · ΔT) = 56.25 / (3.6 × 8) = 1.95 kg/s.
4. Air flow through radiator: ṁ_air = 56.25 / (1.005 × 25) = 2.24 kg/s. Answer: coolant ≈ 1.95 kg/s; air ≈ 2.24 kg/s.

Example 2 (GATE level): friction power in a main bearing (Petroff). Given: d = 60 mm, L = 25 mm, radial clearance c = 0.03 mm, oil μ = 0.012 Pa·s at running temperature, N = 3,000 rpm.

1. ω = 2πN/60 = 2π × 3000/60 = 314.2 rad/s; U = ω·d/2 = 314.2 × 0.03 = 9.425 m/s.
2. Bearing area: A = π·d·L = π × 0.06 × 0.025 = 4.712 × 10⁻³ m².
3. Friction force: F = μ·U·A/c = 0.012 × 9.425 × 4.712 × 10⁻³ / (0.03 × 10⁻³) = 17.77 N.
4. Torque: T_f = F·d/2 = 17.77 × 0.03 = 0.533 N·m.
5. Power: P_f = T_f·ω = 0.533 × 314.2 = 167 W. Answer: friction power ≈ 167 W per bearing. Doubling viscosity (cold oil) doubles this; halving the clearance also doubles it.

Common mistakes

• Using kJ for c_p and then quoting heat in J (or the reverse); keep one unit system.
• Using diameter clearance in Petroff's equation when radial clearance is required.
• Mixing dynamic viscosity (Pa·s) with kinematic viscosity (mm²/s).
• Thinking the thermostat controls flow through the engine only; it diverts coolant between the bypass and the radiator.
• Reading "10W-30" as viscosity at 10 °C; "W" means winter grade.
• Assuming more cooling is always better; overcooling raises friction, HC and wear.
• Saying the lubricant does no cooling: oil removes a significant share of heat from bearings and pistons.

For GATE ME

This topic feeds heat-balance numericals (coolant energy share from flow and temperature rise), simple energy-balance radiator problems, and viscous friction or Petroff-type bearing problems that link to machine design. Conceptual questions cover lubrication regimes, viscosity index, thermostat and pressure-cap functions, and air versus liquid cooling. Practise Q̇ = ṁ·c_p·ΔT with consistent units and the Petroff chain ω → U → F → T → P.

Quick check

1. Which lubrication regime occurs at the top dead centre ring reversal?
2. Why is a cooling system pressurised?
3. Coolant at 1.5 kg/s with c_p = 3.6 kJ/kg·K cools by 7 K in the radiator. Heat rejected?
4. In Petroff's equation, what happens to friction power if speed doubles?

Answers: 1. Boundary lubrication. 2. To raise the coolant boiling point, allowing higher temperature and a smaller radiator. 3. 37.8 kW. 4. It becomes four times as large (F ∝ U and P = F·U).