engine-cooling-system

name	engine-cooling-system
description	Internal combustion engine cooling system design — heat rejection, coolant flow, radiator sizing (NTU-effectiveness), thermostat, water pump, fan, pressure cap, EGR cooler.
metadata	{"priority":7,"promptSignals":{"phrases":["engine cooling","radiator design","coolant flow","water pump engine","engine heat rejection","cooling system design"],"minScore":3}}

IC Engine Cooling System — Complete Skill

Heat Rejection Budget

Total fuel energy: Q_fuel = ṁ_fuel × Q_LHV [kW; Q_LHV = 43–44 MJ/kg for gasoline/diesel]

Energy balance (approximate): Q_brake_power + Q_coolant + Q_exhaust + Q_radiation + Q_oil = Q_fuel

Typical distribution (diesel at full load): Brake power: 35–45% Exhaust: 30–35% Coolant: 20–30% Oil cooler: 5–10% Radiation: 5%

Coolant heat rejection Q_c: Q_c = ṁ_fuel × Q_LHV × f_coolant f_coolant ≈ 0.25–0.33 (fraction rejected to coolant)

For a 200 kW engine (35% efficiency): Q_fuel ≈ 570 kW; Q_c ≈ 140–190 kW

Coolant Flow Rate

Energy balance on coolant: Q_c = ṁ_coolant × c_p × (T_out - T_in)

ṁ_coolant = Q_c / (c_p × ΔT_coolant)

c_p (50% EG/water) ≈ 3500 J/kg·K ΔT_coolant = T_out - T_in ≈ 10–15°C (typical engine) T_out (radiator inlet) ≈ 90–105°C; T_in (radiator exit) ≈ 80–90°C

Example: Q_c = 150 kW; c_p = 3500 J/kg·K; ΔT = 12°C → ṁ = 3.57 kg/s → Q = 3.57/1060 = 3.4 L/s

Water Pump

Flow rate from engine: Q_pump (from above) Head required: pressure losses through block, head, thermostat, hoses, radiator ΔP_system = Σ friction losses ≈ 0.3–0.8 bar (typical)

Centrifugal pump: Q = N × D³ × φ (flow coefficient) H = N² × D² × ψ (head coefficient) P = ρgQH / η_pump

Water pump driven from crankshaft (belt or gear): N_pump = N_engine × drive ratio Belt drive ratio: typically 1.0–1.5× engine speed Electric water pump: fixed N, independent of engine speed; better warm-up; VW/BMW use extensively

Radiator Design (NTU-Effectiveness)

Air-side flow: ṁ_air = ρ_air × V_air × A_frontal [kg/s] V_air = car speed + fan-induced velocity A_frontal = radiator face area

NTU-Effectiveness for cross-flow, single-pass: C_min = min(ṁ_coolant × c_p,coolant, ṁ_air × c_p,air) [W/K] C_max = max of the two; C_r = C_min/C_max

ε = 1 - exp{(1/C_r)×NTU^0.22×[exp(-C_r×NTU^0.78) - 1]} (cross-flow, both fluids unmixed)

Required ε: Q_c = ε × C_min × (T_coolant,in - T_air,in) ε = Q_c / (C_min × (T_c,in - T_air))

NTU from ε: invert equation numerically NTU = UA / C_min → UA = NTU × C_min → size radiator for this UA

U for automotive radiator: U_radiator ≈ 30–80 W/m²K (based on air-side area; air-side resistance dominates) Louvered fins: U ≈ 60–80 W/m²K; Plain fins: U ≈ 30–50 W/m²K

Radiator area: A = UA / U_radiator [m²] Compact radiator: 3–8 m² per m² frontal area (with folded fins)

Thermostat

Function: blocks coolant flow through radiator until T_coolant > set point → faster warm-up

Operating temperature: T_open ≈ 80–92°C; fully open at T_open + 10°C Bypass circuit: coolant circulates through engine only during warm-up (no radiator) Modern: electronically controlled thermostat (map-controlled opening; different T at load vs. idle)

Cooling Fan

Fan at idle (no airflow from vehicle motion): Fan must provide minimum airflow for Q_c at idle Q_air,idle = Q_c / (ρ c_p ΔT_air) [m³/s; ΔT_air = T_exit - T_ambient ≈ 20–40°C]

Electric fan: on-demand; activated by temperature switch or ECU Clutch fan (mechanical): viscous coupling disengages at high speed (self-regulating) Twin fans: independent control; one runs at low speed, both at high demand

Fan power: P_fan = Q_air × ΔP_fan / η_fan [kW] ΔP_fan: pressure drop through radiator ≈ 50–200 Pa η_fan ≈ 0.35–0.55 (automotive axial fan)

Pressure Cap and Overflow

Pressure cap: maintains system at 1.0–1.5 bar gauge → raises boiling point of coolant Boiling point: 100% water at 1.5 bar → ~112°C; 50% EG at 1.5 bar → ~125°C Cap pressure rating stamped on cap; must match system rating

Expansion tank (overflow reservoir): accommodates coolant volume change with temperature Volume change ≈ 4–6% of total coolant volume (from cold to hot)

EGR Cooler (if applicable)

EGR (Exhaust Gas Recirculation): reduces NOx; mixed with intake air EGR cooler reduces EGR temperature before intake mixing

EGR cooler design: Q_EGR = ṁ_EGR × c_p,gas × (T_EGR,in - T_EGR,out) ṁ_EGR ≈ 10–30% of total engine air flow T_EGR,in ≈ 200–700°C; T_EGR,out ≈ 60–120°C

Shell-and-tube or plate-type with engine coolant as cooling medium

Coolant Specification

ASTM D3306 (Type I, Type II): ethylene glycol + inhibitors for aluminum and cast iron 50% EG/water: boiling point 108°C (1 atm); 136°C (1.5 bar); freezing –37°C

Inhibitor package: silicate, phosphate, or OAT (Organic Acid Technology) Service life: silicate 2 years/50,000 km; OAT 5 years/150,000 km (GM Dex-Cool)

Output

Provide: Q_c [kW], ṁ_coolant [kg/s or L/s], radiator UA [W/K], radiator frontal area [m²], ε (effectiveness), T_coolant,out [°C] at design condition, water pump flow and head [bar], fan power [W], pressure cap rating [bar], coolant specification.