Heat Transfer in I.C. Engines

BY –SATYAM KUMAR UPADHYAYAMITY SCHOOL OF SCINCE & TECHNOLOGYAMITY UNIVERSITYNOIDA, UTTAR PRADESHINDIA

upadhyay.satyam.satyam@gmail.com

IntroductionThe internal combustion engine is a rich source of examples

of almost every conceivable type of heat transfer. There are a wide range of temperatures and heat fluxes in the various components of the internal combustion engine. Internal combustion engines come in many sizes, from small model airplane engines with a 0.25 " (6 mm) bore and stroke to large stationary engines with a 12" (300 mm)

About 25 % of the air/fuel mixture energy is converted to work, and the remaining 75% must be transferred from the engine to the environment. The heat transfer paths are many, and include many different modes of heat transfer.

In this module, we will discuss the heat transfer processes in the engine components, then consider the engine parameters and variables which affect the heat transfer processes.

Engine Heat Transfer1. Impact of heat transfer on engine

operation2. Heat transfer environment 3. Energy flow in an engine 4. Component temperature and heat flow 5. Engine heat transfer (i) Fundamentals

(ii)Spark-ignition engine heat transfer

(iii) Diesel engine heat transfer

Engine Heat Transfer-cont.Heat transfer is a parasitic

process that contributes to a loss in fuel conversion efficiency.

The process is a “surface” effect Relative importance reduces

with: – Larger engine displacement – Higher load

IntroductionInternal combustion engines use heat to

convert the energy of fuel to power. Engine temperature is not consistent

throughout the cycle.Not all of the fuel energy is converted to power.Excess heat must be removed from the engine.In engines, heat is moved to the atmosphere

by fluids‐‐water and air.If excess heat is not removed, engine

components fail due to excessive temperature.Heat moves from areas of high temperature to

areas of low temperature.

Introduction-cont. Additional heat is also generated by

friction between the moving parts.This heat must also be removed.When fuel is oxidized (burned) heat is

produced.Only approximately 30% of the energy

released is converted into useful work.• The remaining (70%) must be

removed from the engine to prevent the parts from failure/seizure/melting.

Heat Transfer Peak burned gas temperature ≈ 2500 K Combustion period heat fluxes may reach to 10

MW/m2, whereas at the other parts of the cycle it is essentially zero.

Maximum metal temperature fo rthe inside of the combustion chamber is much lower values due to

– Cracking on materials (cast iron ‐ 400°C, aluminum alloys ‐ 400°C

– Prevent deterioration of lubrication oil (keep below ‐ 180°C)

– Spark plugs and valves must be kept cool to avoid knock and pre‐ignition problems

Should maintain the combustion temperature Effects for emissions Heat transfer to inlet manifold reduces the airflow

Energy Flows in Engines

There are three overall paths for energy flow: shaft work, coolant, and exhaust. They are approximately equal, each about 1/3 of the energy of the incoming fuel/air mixture.

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Energy Flows in Engines-cont.Why is heat transfer in engines

important ?There is a need to keep the temperatures of two critical areas below material design limits. These areas are the piston crown and the exhaust valve

Emission levels and octane requirements are a function of engine temperature.

Energy Flows in Engines-cont. How do we determine engine heat transfer ?

The calculation of engine heat transfer is difficult, due to the periodic air and fuel flow and the complex geometry of the engine. We rely primarily on experimental results. With recent advances in computational fluid dynamics, computation of engine heat transfer is becoming more possible.

What are typical heat transfer rates in engines ?

The majority of engines produced are automotive six cylinder engines, with about a 4" (100 mm) piston diameter (bore) and 4" (100mm) piston stroke, producing about 100 hp (75 kW). Since the heat transfer to the coolant and the heat convected from the exhaust are about equal to the power produced, the heat transfer to the coolant and to the exhaust will also be about 75 kW.

For this typical automotive engine, the total cylinder volume or displacement is typically about 300 cubic inches (0.005 m3), and the total cylinder area is about 0.2 m3. Therefore the power density is about 75 kW/ 0.005 m3 or 15 MW/m3 of displacement. The heat transfer per unit cylinder area will be 75kW/0.2 m3 or 375 kW/m3.

Engine Heat Transfer: Impact Efficiency and Power: Heat transfer in the inlet

decrease volumetric efficiency. In the cylinder, heat losses to the wall is a loss of availability.

Exhaust temperature: Heat losses to exhaust influence the turbocharger performance. In- cylinder and exhaust system heat transfer has impact on catalyst light up.

Friction: Heat transfer governs liner, piston/ ring, and oil temperatures. It also affects piston and bore distortion. All of these effects influence friction. Thermal loading determined fan, oil and water cooler capacities and pumping power.

Component design: The operating temperatures of critical engine components affects their durability; e.g. via mechanical stress, lubricant behaviour.

Engine Heat Transfer: Impact- cont.Mixture preparation in SI engines: Heat

transfer to the fuel significantly affect fuel evaporation and cold start calibration

Cold start of diesel engines: The compression ratio of diesel engines are often governed by cold start requirement

SI engine octane requirement: Heat transfer influences inlet mixture temperature, chamber, cylinder head, liner, piston and valve temperatures, and therefore end-gas temperatures, which affect knock. Heat transfer also affects build up of in-cylinder deposit which affects knock.

Engine heat transfer environment

Gas temperature: ~300 – 3000°K

Heat flux to wall: Q/A <0 (during intake) to 10 MW/m²

Materials limit: – Cast iron ~ 400°C – Aluminium ~ 300°C– Liner (oil film) ~200°C

Hottest components – Spark plug > Exhaust valve > Piston crown > Head – Liner is relatively cool because of limited exposure to

burned gas

Source – Hot burned gas – Radiation from particles in diesel engines

Heat transfer process in enginesAreas where heat transfer is important

– Intake system: manifold, port, valves – In-cylinder: cylinder head, piston, valves, liner – Exhaust system: valves, port, manifold, exhaust pipe – Coolant system: head, block, radiator – Oil system: head, piston, crank, oil cooler, sump

Information of interest – Heat transfer per unit time (rate) – Heat transfer per cycle (often normalized by fuel heating-value) – Variation with time and location of heat flux (heat transfer

rate per unit area)

Heat Transfer TypesThe three heat transfer

mechanisms are: ConductionConvectionRadiation

Conduction ConductionConduction heat transfer is energy transport due to molecular

motion and interaction. Conduction heat transfer through solids is due to molecular vibration. Fourier determined that Q/A, the heat transfer per unit area (W/m2) is proportional to the temperature gradient dT/dx. The constant of proportionality is called the material thermal conductivity k

Fouriers equation :

The thermal conductivity k depends on the material, for example, the various materials used in engines have the following thermal conductivities (W/m K):

Copper = 400, Aluminium = 240, Cast Iron = 80, Water = 0.6, Air=0.026

The thermal conductivity also depends somewhat on the temperature of the material.

Fig: Conduction through Piston Cylinder Wall

Convection Convection heat transfer is energy transport due to bulk fluid

motion. Convection heat transfer through gases and liquids from a solid boundary results from the fluid motion along the surface.

Newton determined that the heat transfer/area, Q/A, is proportional to the fluid solid temperature difference (Ts-Tf). The temperature difference usually occurs across a thin layer of fluid adjacent to the solid surface. This thin fluid layer is called a boundary layer. The constant of proportionality is called the heat transfer coefficient, h.

Newton's Equation:

The heat transfer coefficient depends on the type of fluid and the fluid velocity. The heat flux, depending on the area of interest, is the local or area averaged.

Convection-cont.For a cylinder block

with a forced convection h of 1000, surface temperature of 100°C , and a coolant temperature of 80°C, the local heat transfer rate is : 

Radiation Radiation heat transfer is energy transport due to

emission of electromagnetic waves or photons from a surface or volume. The radiation does not require a heat transfer medium, and can occur in a vacuum. The heat transfer by radiation is proportional to the fourth power of the absolute material temperature. The proportionality constant s is the Stefan-Boltzman constant equal to 5.67 x 10-8 W/m²K4. The radiation heat transfer also depends on the material properties represented by ε , the emissivity of the material.

Radiation-cont. For a surface with an emissivity of ε(Emissivity)= 0.8

and T = 373 K (100°C) and Stefan Boltzman Constant (5.67x10-8 W/m2-K4), then the radiation heat transfer is:

For moderate (less than 100°C) temperature differences, it should be noted that the radiation and natural convection heat transfer are about the same.

Combustion Chamber Heat Transfer