Engine cooling

Need for cooling

About 25-35% of the fuel energy goes to the cylinder walls. Gas temperatures reach 2000 degrees C or more, so without cooling:

• The lubricating oil film burns off and the piston and rings seize.
• The metal loses strength; valves and piston crowns burn or crack because of thermal stress.
• Knock and pre-ignition become severe in SI engines, and volumetric efficiency drops since the charge is heated.

The cooling system must remove only the necessary heat. Overcooling lowers thermal efficiency and causes acid corrosion and oil dilution. The wall temperature is kept at about 150-200 degrees C (liner surface), with the oil film below 160 degrees C.

Air cooling system

Heat is taken directly from the cylinder and head to the atmosphere by fins cast on their outer surfaces. The fins increase the surface area. Air flows over them due to the vehicle motion (motorcycles) or a fan and shrouds (tractors, small stationary engines). The cylinders are spaced and finned, and baffles direct the air. The fin area, air speed and material (aluminium has high conductivity) set the cooling.

Air-cooled versus water-cooled

Thermosyphon system

        radiator (top tank)
        ^                 |
  hot water rises     cooled water falls
        |                 v
   cylinder jacket <-- radiator bottom tank

Water heated in the jacket becomes lighter and rises to the top of the radiator. It is cooled by air as it flows down through the tubes, and the denser cold water returns by gravity to the bottom of the jacket. No pump is needed, but the radiator must be higher than the engine and the flow is slow and depends on temperature. It is used on old and small engines.

Pump (forced circulation) system

 Jacket --> [Thermostat] --> Radiator top tank
   ^                              |
   |                          tubes + fan
   +---- [Water pump] <--- bottom tank
         (bypass when thermostat closed)

A centrifugal pump driven by the crankshaft belt drives the water through the jackets and radiator. The flow rate is fast and controlled, giving more uniform cooling. The radiator may be mounted at any level. Used on nearly all automobile engines.

Components

Variation of gas temperature

The gas temperature changes strongly during the cycle.

 T (K)
 2500|        /\
     |       /  \
 1500|      /    \.
     |     /       ..
  700| ___/            ..____
  300|---------------------------> crank angle
     suction comp. comb. exp. exhaust

• During suction and compression the charge temperature rises from about 300-350 K to 700-900 K.
• After ignition it rises quickly to a peak of about 2200-2800 K in an SI engine (about 2000 K in CI), a little after TDC.
• During expansion it falls to about 1200-1500 K at exhaust valve opening and then to about 700-900 K in the exhaust.
• The peak lasts only a very short time. The mean temperature over the cycle is about 1000 K, while the metal wall stays at 400-500 K.

Heat flow

Heat flows from the hot gas to the wall by convection and radiation (radiation matters mainly in CI engines because of soot), then through the wall by conduction, and finally from the outer surface to the coolant by convection:

$$Q = \frac{T_g - T_c}{\dfrac{1}{h_g A} + \dfrac{t}{kA} + \dfrac{1}{h_c A}}$$

The wall temperature varies only slightly through the cycle because the metal thermal inertia averages the gas temperature fluctuations, so heat flow is based on the mean gas temperature. The heat flux is highest at the piston crown, exhaust valve and cylinder head, and depends on gas velocity, turbulence, load and speed. About 25-35% of fuel energy goes to the coolant.

(a) Fuel energy and fuel consumption

$$\begin{aligned} Q_{fuel} &= \frac{BP}{\eta_{bth}} = \frac{60}{0.30} = 200\ \text{kW} \\ \dot{m}_f &= \frac{Q_{fuel}}{LCV} = \frac{200}{44\,000} = 4.545 \times 10^{-3}\ \text{kg/s} = 16.36\ \text{kg/h} \end{aligned}$$

(b) Heat to coolant and water flow

$$\begin{aligned} Q_w &= 0.30 \times 200 = 60\ \text{kW} \\ \dot{m}_w &= \frac{Q_w}{c_{pw}\,\Delta T_w} = \frac{60}{4.187 \times 10} = 1.433\ \text{kg/s} = 86\ \text{kg/min} \end{aligned}$$

(c) Air through the radiator

The radiator rejects all of the heat taken by the water to the air (steady state):

$$\dot{m}_a = \frac{Q_w}{c_{pa}\,\Delta T_a} = \frac{60}{1.005 \times 15} = 3.98\ \text{kg/s}$$

At an air density of about 1.15 kg/m$^3$ this is about 3.46 m$^3$/s.

Answer: (a) 16.4 kg/h of fuel; (b) 1.43 kg/s of cooling water; (c) 3.98 kg/s of air.