Introduction

Improper evacuation of combus-

tion byproducts can lead to lost

power, fuel economy and overall

vehicle performance. A fundamen-

tal understanding of the exhaust

process can help lead to improved

engine efficiency.

With all the current interest in

providing engines with properly

timed, correctly conditioned and

rpm-related air/fuel mixtures,

focus can be lost on the impor-

tance of efficient cylinder evacua-

tion. As pointed out in an earlier

n2performance.com lecture, inter-

nal combustion engines are sur-

rounded by atmospheric pressure,

externally acting on both the intake

and exhaust system. Despite there

are times when atmospheric pres-

sure is higher than cylinder pres-

sure, the ability of exhaust gas

dynamics to effectively remove

unburnable residue is key to high

percentages of fresh mixtures and

power.

The following discussion con-

templates many of the major areas

of engine component design and

function that relate to an efficient

exhaust system. The subject of

mufflers is being withheld for a

future lecture. There’s sufficient

exhaust system material to review

that does not include these parts.

This approach will also make it

easier to tie mufflers into the over-

all system, once that discussion

unfolds.

Basic Information

The exhaust "problem"

Simply stated, exhaust gas will

not "burn a second time." It also

occupies space, so its removal

from an engine’s cylinders is nec-

essary to achieve good combustion

efficiency, following the successive

induction of fresh air/fuel mixtures.

The "problem," if you will, amounts

to creating a set of conditions

whereby the greatest possible

quantity of combustion byproducts

(exhaust gas) is removed from the

engine. Unfortunately, the "prob-

lem" is never completely resolved,

even in the best of cases. But the

objective is still to achieve the high-

est degree of evacuation obtain-

able from firing cycle to firing

cycle.

Purely from the standpoint of air

pollution, exhaust gas that remains

in an engine’s cylinders tends to

reduce combustion temperature.

Of the combustion byproducts that

pertain to emissions concerns,

oxides of nitrogen (NOx) is heat

related. By the reduction of com-

bustion heat, a corresponding

reduction in oxides of nitrogen can

be accomplished. Exhaust gas that

is intentionally put back into an

engine’s cylinders, especially dur-

ing times of high heat production

(engine under load or experiencing

increased power output), is called

"exhaust gas re-circulation" or

EGR. The introduction of EGR is

regulated by a vacuum- or load-

sensing device that opens or closes

a valve directly connected to the

engine’s exhaust system, typically

from a passage in the intake man-

ifold that is provided exhaust gas.

From the standpoint of combus-

tion efficiency, EGR is a deterrent to

power and fuel economy, although

of emissions benefit. Depending

upon the efficiency of an engine’s

overall exhaust system, some

amount of "residual EGR" can also

reduce power and economy. So in

cases where outright power

(and/or fuel economy) is the objec-

tive, optimizing combustion

byproducts removal is necessary.

Exhaust gas evacuation

Consider the sequence of events

leading up to and during the

removal of combustion byprod-

ucts. During combustion, cylinder

pressure rises to its highest value,

turning this pressure into work

done on the pistons, which results

Jim McFarland

1 Exhausting Systems

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.

Exhausting Systems