Introduction

An efficient conversion of fuel

into heat is a simplified objective

of internal combustion engines.

The complexity of this process can

be reduced to certain fundamen-

tals, essential to the understanding

of or modification to a stock pow-

erplant.

The Combustion Process

(Basic Information)

In common terms, an internal

combustion engine “burns” air

and fuel. While there is a specific

chemical reaction process that

occurs during this so-called

“burn,” that discussion will be held

for this lesson’s Advanced seg-

ment.

Essentially, fuel and air become

mixed during the pre-combustion

process. Once mixed, this blend is

compressed to a pressure much

higher than atmospheric and then

ignited. Ignition can be either by

electrical spark or the result of

“compression ignition” or “self

ignition” by the admission of a

diesel fuel into a high compres-

sion, air-heated environment.

If gasoline is the subject fuel,

spark ignition occurs and a flame

begins to travel through the com-

bustion space. This burning action

creates increased pressure within

the cylinder, resulting in work

(forces) acting on the pistons and

forcing them downward. Such pis-

ton movement creates rotational

forces (torque) on the crankshaft,

culminating motive power for the

vehicle. Ideally, maximum cylinder

pressure is developed immediately

after the piston’s TDC (top dead

center) position during the power

stroke (acting under combustion

pressure).

Gasoline fueled engines

In sequence, the combustion

process involves the introduction

of air and fuel into the cylinder,

compression of this mixture to a

level of increased pressure and

temperature and then ignited by a

timed electrical spark shortly

before the piston reaches top dead

center. Upon ignition, the flame

travels throughout the combustion

space and is only partially com-

plete when the piston passes

through TDC. At TDC, a condition

called “constant volume combus-

tion” occurs, causing both com-

bustion temperature and pressure

to increase quickly.

Because of differences between

mechanical positioning of the pis-

ton and flame speed, peak cylin-

der pressure is developed just

past TDC. This fact is evident in

internal combustion engine study

involving Engine Cycle Analysis, a

subject to be discussed further in

the Advanced section of this les-

son. What is important about this

notion is that peak cylinder pres-

sure and mechanical TDC of a

piston do not occur simultaneous-

ly. When these discussions begin

to include the building and com-

ponent matching for high per-

formance or racing engines, pis-

ton position and spark timing will

be revisited.

The importance

of air/fuel ratios

As “suspended” (or mixed) with

air, visualize fuel droplets as small

beads or spheres. The combustion

process, essentially, begins on the

outer portions of these droplets

and proceeds inward until the

droplet is consumed by the burn.

Large droplets require more time

to be consumed than smaller

Jim McFarland

1 The Combustion Process of an Internal Combustion Engine

Join Jim McFarland as he steps into his role

as the "Performance Professor" and shares

a wealth of knowledge and experience that

will help take you to a higher level of per-

formance.

The Combustion Process

of an Internal Combustion Engine

As “suspended” (or mixed)

with air, visualize fuel

droplets as small beads or

spheres. The combustion

process, essentially, begins

on the outer portions of

these droplets and proceeds

inward until the droplet is

consumed by the burn.