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Basic Engine Dynamics
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Four Stroke Fundamentals
Your engine’s a self-driven air pump; all internal combustion (IC) engines are. However, there are
several designs of IC engines: two-stroke, four-stroke, Wankel or rotary engines, even turbine
engines. For the sake of brevity, we’re restricting our discussion here to four-stroke engines.
A four-stroke engine has valves and a cam that opens these valve in relation to a piston traveling
up and down a cylinder bore. By manipulating the timing of the valve openings we get the four-
strokes by which this engine design is sometimes known.
Briefly, they are: 1) intake; 2) compression; 3) power; 4) exhaust.
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Your engine’s camshaft determines its power curve. By opening and closing the valves with precise
timing in relation to the position of the piston and crankshaft, the cam determines where in the rpm
range an engine makes the most power and torque.
Of course the cam is dependent on the intake and exhaust tract design efficiency, i.e., how well
these channels flow throughout the rpm range of the engine as well as the pressure curve in the
cylinder dictated by the motion of the piston. The cam can’t flow any more than these tracts allow.
What it can do is optimize the physics of the air moving through these tracts within an rpm window in
order to best fill the cylinder with air and fuel.
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The physics of the intake and exhaust tracts come down to accelerating columns of air and using
the inertia of that column to increase the volumetric efficiency of the cylinder. As the piston recedes
to bottom dead center (BDC) it creates a low-pressure area that’s filled by atmospheric pressure (in
an normally aspirated engine) forcing air through the intake manifold into the cylinder. What the
manifold does is make the air into columns. When you get a column of air moving it doesn’t want to
stop, so more of it packs in the cylinder. By the way, air columns in the intake runners behave more
like a Slinky than a column of liquid. By timing the intake and exhaust valve opening and closings—
let’s call these events from now on—you can catch a little more air and fuel and have a cleaner mix
for each power stroke.
The amount of air you can stuff into a cylinder per intake stroke is a measure of the engine’s
volumetric efficiency. If it can capture its full cubic capacity, it’s said to have 100 percent
volumetric efficiency. By tuning the intake dimensions, head port configuration, and cam profile and
timing, you can, within a certain rpm window, get more than 100 percent volumetric efficiency.
(Depending on the efficiency of the intake and exhaust systems.) Generally the torque peak occurs
at peak volumetric efficiency. It should be obvious that volumetric efficiency of the cylinder is a
primary factor in determining power output of an engine. Turbos, superchargers, and nitrous all
work because they increase, by mechanical—or in the case of nitrous—by mechanical as well as
chemical means, the volumetric efficiency of your engine. These techniques literally squeeze more
air into the cylinder per cycle, allowing the engine to burn more fuel and make more pressure.
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Volumetric efficiency falls off as engine rpm increases because there’s less time to fill the cylinders.
That’s why a cam is designed to work with specific intake and exhaust system flow characteristics
within an rpm window. Cams designed to work best at high rpm have a long duration, meaning they
hold the valve open longer. Holding the valves open longer gives the cylinder more time to fill, so
the power band is moved up the rpm range. In addition, high-rpm cams have a lot of what is called
overlap. Much of the work of designing and tuning a cam has to do with how the intake charge and
exhaust charge interact in the “overlap zone.”
Overlap is when both the intake and the exhaust valves are open. This occurs when the piston is
near TDC on the exhaust stroke. Figures 2.1-A through E walk you through the valve events of the
four-stroke engine cycle.
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We mentioned that it’s important to get an uncontaminated charge into the cylinder. That means we
have to expel the charge that was burned in the power stroke. That’s what the exhaust stroke does.
It forces the hot and expanding gases out the exhaust tract. But the real exhaust cycle begins a few
degrees back on the power stroke before the piston reached BDC. Because the gas is a gas, it’s
compressible, so it behaves sort of like a spring. During the power stroke, the gas is expanding and
forcing the piston down the bore, generating power, so it’s under really high pressure. One of the
things engine tuners have discovered is that, if you open the exhaust valve near the bottom of the
power stroke, you can use some of that energy to get the exhaust gases out of the cylinder.
(Opening the exhaust valve while the gases are still burning gives an internal combustion engine its
characteristic exhaust sound.) Then the piston comes up the bore and forces most of the rest of
the spent gas from the cylinder.
The exhaust stroke by itself can’t get all the spent gas out of the cylinder because the piston doesn’
t completely fill the combustion chamber at the top. Ashes don’t burn, and that’s basically what’s left
after the power stroke; removing them so the next charge is as uncontaminated as possible helps
make power. Engine tuners use the same strategy at this end of the four-stroke cycle as they do on
the exhaust cycle. That is, they open the intake valve before the piston reaches TDC and before
the exhaust valve closes.
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Engine tuners also discovered that, by using the inertia of the escaping exhaust gases, they could
pull in a little extra fresh charge air and use it to push out the residual exhaust gas, or what’s called
“scavenging” the combustion chamber of spent gases. It’s a pretty cool trick that's totally dependent
on proper timing of the opening of the intake valve and the closing of the exhaust valve.
Overlap is a critical area of four-stroke engine tuning. Lots of things happen here. The intake valve
is opening and the exhaust valve is closing as the piston is rushing toward it, so valve-to-piston
conflict can occur here. But most important is how the phasing of the valve events at overlap
influence the performance of your engine—mainly in shifting peak power at different rpm levels.
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If the intake is opened too soon, you get too much exhaust gas pulsing into the intake tract. Let's
face it: the intake manifold has much lower pressure than the cylinder at this point. You always get
some if you have overlap, but too much hurts engine performance. If too much exhaust gas gets
into the intake, you have a contaminated intake charge. Because of the scavenging effect we
talked about earlier, most of the contaminated intake air is drawn through the cylinder and out the
exhaust as it should be. Keep in mind that if you have too much scavenging, a lot of the intake
charge air will rush through the cylinder and out the exhaust leaving you with a lean cylinder. Again,
tuning is always a balancing act.
The efficiency of the scavenging effect rises with engine rpm, which is reasonable since the gases
have a higher velocity (and therefore less pressure) and more inertial energy to do the work of
pulling the intake air through the cylinder. It’s also influenced by the efficiency of the exhaust port
and related manifolding such as headers, exhaust tube diameter, and muffler. If the exhaust system
is highly efficient, you don’t need to open the exhaust valve as soon as you would if the exhaust
was slightly restricted.
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It’s interesting to note that intake contamination isn’t usually a problem at high rpm because the
tendency for exhaust gas to pulse back into the intake is most pronounced at low rpm. This is why
long-duration cams with lots of overlap idle so poorly. The exhaust stroke is, in addition to recycling
spent mixture, pumping pressure into the manifold, so it messes up the vacuum signal to the
carburetor on carbureted cars and sends a confusing manifold absolute pressure (MAP) reading to
the computer. It’s not until you speed up the charge air velocity at higher rpm that the engine starts
to make good, smooth power.
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On the power stroke, most of the work is
done in the first 90 degrees of crank
rotation. As the piston gets near BDC, it’s
not putting as much pressure on the crank
to spin it. The gases are still hot and
expanding, but for the most part their job is
done.
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Why waste the heat of the expanding gas?
Why not use its energy to help evacuate the
cylinder? That’s what engine tuners are up to
when they get the cam to open the exhaust
valve while the piston is still on the power
stroke. Near BDC the exhaust valve pops open
and the expanding gases punch into the low
pressure of the exhaust system.
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Just as the exhaust valve opens on the
power stroke, so does the intake open on
the exhaust stroke. At high rpm, a certain
amount of overlap helps improve the quality
of the intake charge by using the exiting
exhaust pulse’s low pressure to draw intake
air through the combustion chamber. At low
rpm, too much overlap results in low intake
pressure and intake charge contamination
because the high-pressure exhaust gases
bleed into the low-pressure intake. This
results in a lopey idle and loss of low-end
power.
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The point-of-intake valve closing influences
how much intake charge the cylinder captures
as well as the dynamic compression ratio.
Closing the intake valve too late causes a loss
of some intake charge because it’s forced
back into the intake manifold. For high-rpm
power, you need a cam that closes the intake
later in the cycle because you need the
duration.
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The dynamic compression ratio of an engine
changes throughout the rpm range. A cam that
closes the intake early and has little overlap
makes good bottom-end power and idles
smoothly. It also has a dynamic compression
ratio closer to that of the static compression
ratio. Closing the intake late means you lower
dynamic compression, but you can gain
high-rpm power.
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Previous | Next
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This has been a sample page from
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How To Install and Use Nitrous Oxide
Injection Systems For Maximum Horsepower
by Joe Pettitt
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Includes information on nitrous basics and advance
nitrous theory. Written with the assistance of Nitrous
Oxide Systems
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Nitrous oxide injection is one of the potentially easiest, least
expensive, and fastest ways to substantially increase engine
horsepower. This new title, authored with the assistance of one of
the industry's largest manufacturer of nitrous equipment, provides
the latest technical information available regarding the proper
installation and use of this high performance, yet potentially
damaging equipment.
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Click below to view sample
pages from each chapter.
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"How to Install and Use Nitrous Oxide is filled with information
on nitrous, including the basics of advanced nitrous theory.
Photos, charts, and graphs accompany the text and illustrate
key points. Hands-on sections of the book cover how to plumb
a nitrous system and how to set up an engine to handle nitrous.
There's information on ignition timing, compression, wiring,
solenoids, octane, and fuel delivery." -- SPORT TRUCK, April
1999
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Chap. 1 - Introduction to Nitrous
Chap. 2 - How Nitrous Works
Chap. 3 - The Nitrous System
Chap. 4 - Installation Tech
Chap. 5 - Operating and Tuning
Chap. 6 - Basic Engine
Chap. 7 - Advanced Tuning
Chap. 8 - Nitrous Fuel Injection
Chap. 9 - Dyno Sessions
Chap. 10 - Real World Project
Chap. 11 - Chemical Reference
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8-3/8 X 10-7/8
128 pages
300 b/w photos
Item: SA50
Price: $18.95
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Click here to buy now!
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This is a great book that anyone using, or considering using a
nitrous oxide system will love!
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Other items you might be interested in
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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Sport Compact Turbos & Blowers
While most owners of sport compacts can afford the simple bolt-ons
available, some owners want to take their modifications a step further.
There is intense competition to be the fastest, and quite often the only
way to win is to go to the next level – by installing a supercharger /
blower or turbocharger on your engine. This book is an enthusiast’s
guide to understanding and using turbochargers and superchargers
on sport compact cars.
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Price:
$18.95
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How to Rebuild and Modify Carter/Edelbrock Carburetors
Author David Emanuel outlines carburetor types, gives a thorough look
at carb selection and carb function, and offers detailed information on
modifications, tuning, and rebuilding Carter/Edelbrock carburetors.
Also features the history of Carter as well as the history of the AFB
and the AVS since the purchase by Edelbrock. Contains more than
300 color photos, illustrations, and diagrams.
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Price:
$22.95
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Shipping is combined and discounted for multiple item purchases!
Buy more and save on shipping!
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Satisfaction is guaranteed. Our store has a NO HASSLE RETURN POLICY within 7 days of
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SHIPPING
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If you have any questions or comments please feel free to contact us. We look forward to serving
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Thanks for your business!
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MRE
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Grinnell, IA 50112
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