Engine Cooling and Lubrication

About 30 % of the fuel energy goes to the coolant and walls. A cooling system must remove this heat and keep the cylinder wall at 80 to 100 degrees C for best performance.

(a) Air cooling vs liquid cooling

 Liquid system:
 Engine jacket -> thermostat -> Radiator -> Pump -> Engine
        (closed thermostat: bypass directly to pump)

The thermostat stays closed during warm-up and opens at about 82 to 90 degrees C to send hot coolant to the radiator. A pressure cap (about 1 bar) raises the boiling point.

(b) Overheating and overcooling

Overheating (too hot)

• Lubricating oil thins and burns; film breaks, causing wear, seizure and scuffing.
• Pre-ignition and knock occur, reducing power.
• Valve and piston distortion, cracked head or blown head gasket.
• Lower volumetric efficiency as intake air is heated.

Overcooling (too cold)

• Fuel condenses on the wall and washes away the oil film, giving more wear.
• Oil becomes viscous, giving higher friction and fuel consumption.
• Poor vaporisation, incomplete combustion and more CO, HC and carbon.
• Condensed acids and water corrode the cylinder and form sludge.
• Lower thermal efficiency since more heat is lost to the coolant.

(a) Lubrication

Functions: reduce friction and wear, cool the parts (piston underside, bearings), seal the piston rings, clean (carry away dirt to the filter) and protect from corrosion.

1. Splash system: a dipper on the connecting rod big end splashes oil from the sump onto the cylinder walls and bearings. Simple, used in small engines (lawn mower); oil supply varies with speed and level.

2. Pressure (wet-sump) system: a gear pump picks oil from the sump, passes it through a filter, and feeds it at 2 to 4 bar through the main gallery to the main bearings, big-end bearings (through the crankshaft), camshaft, and valve gear. A relief valve keeps the pressure safe and the oil drains back to the sump.

 Sump -> strainer -> pump -> filter -> main gallery
                        |  (relief valve)   |-> crank bearings
                        +<------------------ |-> cam, valves

3. Dry-sump system: oil drains to a small collector; a scavenge pump moves it to a separate external tank and a pressure pump feeds the engine. Used in racing and sports cars and aircraft because the engine is low, and there is no oil starvation in high cornering or inclination.

(b) SAE grade and viscosity index

SAE viscosity grade: the Society of Automotive Engineers rates oil viscosity. In 15W-40, the "15W" (W for winter) shows that the oil flows like a SAE 15 oil at cold temperature (it can be pumped down to about -20 degrees C), while "40" shows that at 100 degrees C it behaves like an SAE 40 oil. It is a multigrade oil, used all year, and thin when cold for easy starting but thick enough when hot to protect.

Viscosity index (VI): a number that shows how little the viscosity changes with temperature. A high VI means that the oil thins less when hot. Multigrade oils use polymer additives to give a high VI.

Oil must also meet quality grades such as API SN (petrol) and CK-4 (diesel).

Given: $\dot m_c = 1.5$ kg/s, $c_{pc} = 3.6$ kJ/kg K, $\Delta T_c = 8$ K, $\rho_c = 1030$ kg/m³, $\Delta T_a = 25$ K, $c_{pa} = 1.005$ kJ/kg K.

(i) Heat rejected

$$Q = \dot m_c c_{pc} \Delta T_c = 1.5 \times 3.6 \times 8 = 43.2\ \text{kW}$$

(ii) Coolant volume flow (water pump capacity)

$$\dot V = \frac{\dot m_c}{\rho_c} = \frac{1.5}{1030} = 1.456\times10^{-3}\ \text{m}^3/\text{s} = 1.456\ \text{L/s} = 87.4\ \text{L/min}$$

(iii) Air flow (heat balance: heat lost by coolant = heat gained by air)

$$\dot m_a = \frac{Q}{c_{pa} \Delta T_a} = \frac{43.2}{1.005 \times 25} = 1.72\ \text{kg/s}$$

For air density 1.1 kg/m³ at the radiator, this is $1.72/1.1 = 1.56$ m³/s.

Fuel energy rate

$$\dot E_f = \frac{43.2}{0.28} = 154.3\ \text{kW}$$

Answer: $Q = 43.2$ kW; coolant flow $= 87.4$ L/min; air flow $= 1.72$ kg/s; fuel energy $= 154$ kW.