Basic 4 Stroke/Cycle Engine Operation
Basic Performance Enhancements
Basic 4 Stroke/Cycle Engine Operation
The spark ignited 4-stroke cycle engine requires the following
4 basic operations:
Intake air/fuel is drawn
into the cylinder
The intake charge is
The charge is ignited
The spent mixture (exhaust)
This cycle requires 2 crankshaft revolutions to complete.
The camshaft, which is operating the valves, will only complete one revolution
during this cycle. Therefore, the cam drive is a 2:1 ratio with the crankshaft
(2 crank revolutions per cam revolution).
While there a many factors affecting engine performance,
only the most significant ones are covered here. Other factors include
friction, combustion efficiency, ignition, moments of inertia, and lubrication.
The key to enhancing engine power is to efficiently fill
the cylinder with the maximum amount of intake charge as possible and
efficiently combust it. Spark ignited (SI) engines require a certain air
to fuel mixture ratio to operate properly. More fuel requires more air
to keep the proper mixture and burn completely. An ideal theoretical engine
would operate with a stichomythic (just enough air to allow complete fuel
combustion) air to fuel ratio. The intake charge is this air fuel mixture.
There are several things that affect how much charge you can get into
The camshaft is a key part of getting maximum charge into
the cylinder. By increasing valve lift, the intake charge has less restriction
and will fill the cylinder more completely before the intake valve closes.
This is a fairly simple and intuitive concept. The affect of overlap is
a bit more complicated. Overlap is the time when both the intake and exhaust
valves are open somewhat. The exhaust valve is partially closed and the
intake valve begins to open. If you have good exhaust scavenging (covered
later) this last portion of the exhaust stroke will actually be sucking
the exhaust out of the cylinder. This sucking will help draw the intake
charge into the cylinder sooner if the intake valve is open. Performance
cams will also leave the intake valve open after the piston begins the
compression stroke. If the intake manifold is properly designed to work
with the cam, the intake charge in the intake port will gain inertia when
it is drawn into the cylinder. This means that it is now moving and force
will be required to stop it. When the piston begins the upward stroke,
the intake charge in the manifold runner will need to stop. Because of
the inertia, it pressurizes instead. If you time the valve operation just
right, you can use this pressurizing affect to its full extent before
finally closing the intake valve. This affect is usually maximized at
mid and upper RPM ranges and the cam and intake must be matched to optimize
the same rpm range.
Optimizing camshaft design for mid to upper RPM ranges
will result in poor idle quality. The amount of overlap needed for enhanced
performance at the higher speeds will allow exhaust to flow back into
the cylinder during overlap due to intake manifold vacuum, and lack of
any significant scavenging or inertia affect, at idle. This exhaust gas
is inert in the combustion chamber. The poor idle cannot be eliminated
by air/fuel mixture changes.
As we discussed in the camshaft section above, the intake
system needs to be optimized to match the cam profile. For low RPM engines,
long, small diameter runners will cause the maximum port velocity, and
thus the most intake inertia. These long small runners will become more
and more restrictive as the RPM increases since the airflow will increase
with RPM. Therefore, a midrange RPM manifold will have shorter but larger
diameter runners. As the RPM goes up, the port diameter can increase to
keep the optimum port velocity. Once you get really high RPM, restriction
becomes the biggest problem. At this point, you generally need a short,
large diameter port.
For multiple cylinder engines, and low to mid RPM range
performance, you can further optimize the intake system by combining runners.
This will let some of the inertia from one cylinder be used by another.
Runners can be combined into one final plenum which can also enhance intake
inertia. Back in the carburetor days, the dual plane intake manifold used
Many modern engines use 2 intake runners per cylinder.
This began with multiple intake valves (4 valve engines). Each intake
valve would have itís own intake runner. One intake runner would be long
to enhance low RPM performance. The other intake runner would be much
shorter. The short runner is closed by a small throttle plate on that
individual runner until higher RPM, usually around 3,000 to 4,500 RPM.
This method gives you the best of both worlds. These engines have the
capability of having a much flatter power curve. The valves are usually
the biggest restriction in the intake system. Having 2 somewhat smaller
valves allows more flow. Because the pistons are usually round, the combustion
chamber has limited space to put the valves. If you use more than one
intake valve, you can achieve more total opening area and reduce the restriction
substantially. Because of this, 3, 4, and 5 valve per cylinder engines
build more power higher in the RPM range.
The multiple intake runner concept can also be employed
on normal 2-valve engines. Ford has done this on many engines and they
refer to it as split-port induction. The 1997 F150 V6, the 1997 Escort
2.0L, and the 1997 Windstar 3.8L used this technology.
Sonic affects can also be used to enhance performance.
Then the intake valve slams shut, a sound wave echoes back up the intake
runner. Sound a basically a high frequency pressure pulse. If you time
it right, and use resonance to your advantage, this sonic wave can be
used to pressurize another cylinder. Chrysler used this technology back
on the 1960ís.
To better understand the inertia effects, you can use
water flows. Consider water hammer. Then you flow water through a pipe
at high speed, and then slam the valve shut rapidly, the inertia of the
flowing water will hammer the valve causing a bang sound. If you put a
pressure gage on the tube just before the valve while you do this, you
will see a huge pressure spike. Inertia affects any mass whether it is
fluid or solid. When you throw a ball it requires force to accelerate
the ball from rest. Once the ball is in motion, it requires force to stop
it (catching the ball).
It may seem intuitive that bigger, more open exhaust would
be the best for engine performance. Some believe that there is a proper
amount of exhaust back-pressure for optimum performance. Both of these
theories are wrong. There are a few key things that make exhaust flow
different than other fluid flows. First of all, the exhaust is coming
out in pulses. When each cylinder exhausts, a large surge of flow enters
the exhaust system. Then it stops again until the next exhaust stroke.
True, larger exhaust ports and tubes will flow better but flow is not
the only thing to consider here. If you size the exhaust tubing properly,
you can take advantage of the inertia of the exhaust pulses similar to
what can be done on the intake side. When the cylinder exhausts, the pulse
builds a pressure in the exhaust tube. Once the valve closes, this exhaust
pulse is traveling down the tube with velocity. Because of this, a slight
vacuum is created at the valve. This pressure-then-vacuum pulse will travel
all the way through the exhaust system assuming the restriction is low
and the tube diameter does not change much.
The tubes for each cylinder to the collector are called
primary tubes. These primary tubes can be sized, both diameter and length,
to optimize the exhaust inertia while keeping minimum restriction. It
is a trade-off and will only be optimum at a specific RPM and load. This
is very similar to optimizing the intake system and the same key things
apply. For low RPM performance, a small diameter, long tube will give
the best port velocity, and exhaust inertia. For midrange performance,
a shorter length, larger diameter runner is best. For very high RPM, a
very short, large runner is needed because restriction is the biggest
problem at high RPM.
By grouping primary exhaust tubes into a collector, the
vacuum pulse from one cylinder can be used to create a vacuum on the other
exhaust ports. This vacuum will cause the exhaust to begin flowing more
quickly as the exhaust valve opens. The collector must be long enough
and small enough to keep the inertia of the exhaust pulse optimized while
keeping back-pressure low.
Back-pressure is something we are stuck with, not something
we want. By optimizing the inertia affect, some back-pressure will be
realized during the peak of the exhaust pressure pulse. It is not something
you want but rather simple physics.
Once you get past the end of the collector, you want to
release the exhaust with as little restriction as possible. The end of
the exhaust header is not really the end of the collector. The collector
area in the exhaust should usually be about 18" to 24". If you have 2
banks of cylinders with each bank having itís own collector, you will
also have alternating pulses from each bank. In order to reduce restriction,
you should place a tube between the banks at the end of the collector
area. This will allow pressure pulses to swap back and forth which will
even them out and allow smoother flow through the rest of the exhaust
At very high RPM, the restriction becomes a much bigger
problem. Because of this, attempts to utilize exhaust inertia are generally
abandoned in favor of reducing restriction.
Compression ratio is also an important factor in engine
performance and efficiency. The higher the compression ratio, the more
thermally efficient the engine. Thermal efficiency is the ratio of energy
in divided by energy out. Fuel has a certain amount of energy that is
released during complete combustion referred to as heating value). This
heating value varies depending on the products of combustion and the state
of those products. Because engine exhaust is significantly above the boiling
point of water (one of the primary products of hydrocarbon combustion),
only the lower heating value is realized. The limiting factor for compression
ratio is the fuel. If your compression is too high, the heat caused by
compressing the intake charge will actually ignite it (pre-ignition).
When air is compressed it heats up. This heat of compression concept is
what makes diesel engines run. Sometimes the intake charge will not pre-ignite
but, due to high temperatures, it will detonate when ignited by the spark
plug. Common gasoline can only handle a certain amount of compression
or it will pre-ignite, usually about 10.5:1. Higher octane rated fuel
can handle more compression, up to around 12:1. Alcohol can handle even
more, up to about 14:1. Diesels use compression ratios from 16.5:1 to
A high overlap camshaft will generally let you run a higher
compression ratio without pre-ignition or detonation due to the EGR affect.
Detonation can also be reduced or eliminated by retarding the ignition
timing. Detonation can also be reduced by combustion chamber design. By
making the combustion chamber have more surface area, more quench area
is available to cool the charge. A domed piston will increase the piston
surface area and allow for increased compression ratio. A closed style
combustion chamber will also give more quench area. Generally, an open
combustion chamber with a flat top piston is the most detonation prone
but it is more difficult to get very high compression that way.
Compression is affected by the piston design, head gasket
thickness, and the combustion chamber volume. The crankshaft and the connecting
rod length can also affect the compression ratio by moving the piston
travel up in the cylinder.
The cylinder heads are part of both the intake system
and the exhaust system. The heads are usually the weakest link in these
systems and therefore your best area to improve them. The runner size
should be optimized just like the rest of the intake and exhaust systems
covered above. In addition to runner size, the area just behind the valve
(the bowl) is critical. The bowl area should be larger than the runner.
Optimum intake port velocity really aids in cylinder filling. The valve
size is also key. Reducing restriction in the valve opening area with
a 5-angle valve grind can yield some surprising results.
Since engine power is directly related to the amount of
intake charge you can pack into the cylinder during the intake stroke,
force feeding the engine can give you big gains. However, this comes at
a price. Work is required to pressurize the intake charge. This is referred
to as back work. Superchargers are belt driven and do require significant
power when they are pressurizing the intake charge. However, the power
gains are substantially greater than the back work. Therefore, you do
realize a significant net gain. If your engine is set-up properly, your
cylinder will actually be pressurized during the intake stroke. This means
that the cylinder is actually applying a little power to the crank instead
of drawing power from it. Because of this, a misfiring supercharged engine
is much smoother than a misfiring naturally aspirate engine under full
Turbo charging has the same affect on the intake charge
as supercharging. However, turbochargers use exhaust pressure to drive
the compressor. The back work comes from the exhaust pressure. The piston
will draw power from the crank to force the exhaust out into the pressurized
exhaust manifold. The exhaust side of the turbocharger uses this pressure
to spin the exhaust wheel on the turbo. This exhaust wheel is connected
with a shaft to the intake wheel. The intake wheel then compresses the
Turbochargers work very well on diesel engines because
there is no throttle (on most diesels), the exhaust flow rate is high
even at idle. This keeps the turbo shaft speed high so it can quickly
build boost. At highway speeds, the turbo can build boost even when going
a steady speed. This increases the air entering the engine. This additional
pressurized air allows for more complete combustion. Diesels do not use
a fixed air fuel mixture. They will have extra air until full throttle
and sometimes even at full throttle. At full throttle, it can be near
or slightly richer than stoichiometry (this is called over-fueling). The
turbo allows the diesel engine to add more fuel with the additional air
for more power. At steady speeds, increased fuel economy will be realized
due to the additional air, and air pressure, allowing for more complete
combustion and higher affective compression.
On spark ignited engines, the turbocharger does not give
the fuel economy benefit it did on the diesel. SI engines still need the
stoichiometric air/fuel ratio. They also have a throttle to reduce the
intake airflow (and pressure) to control engine speed. Without a throttle,
SI engines would run at full power all the time. Since the throttle limits
the intake air, the exhaust flow is very low at idle. This allows the
turbo to spin very slow (relatively speaking). If you try to apply the
throttle rapidly, the turbo will need to spin up. This takes time and
is referred to as turbo lag.
Nitrous Oxide is not a fuel, it is an oxidizer. Air contains
less than 20% oxygen, which is the only component of air used in combustion.
The amount of fuel must be balanced with that oxygen for proper combustion.
However, if we have a higher concentration of oxygen available for combustion,
we can also add more fuel. The same volume of intake charge can have much
more power. Say for instance we add enough nitrous oxide to bring the
concentration of oxygen in the intake charge up to 40%. Now we can add
twice the fuel and the combustion will produce twice the power to the
crankshaft. The key to nitrous oxide is to make sure you add enough extra
fuel. If you have a high concentration of oxygen available, and you run
the fuel mixture too lean, you can cause major engine damage. You must
add extra fuel when adding nitrous oxide. All nitrous kits have some method
of adding the extra fuel. Most have a fuel solenoid and nozzle to inject
the extra fuel.
Nitrous oxide is very effective. It will produce power
gains across the RPM range and has no back work. Actually, the gains are
bigger at lower RPMís. Nitrous oxide kits generally have a set amount
of nitrous oxide and fuel they add to the engine. Because of this, they
produce huge gains at low RPM because the same amount of nitrous/fuel
is added no matter what the RPM. Some nitrous kits will have multiple
stages to compensate for this. Nitrous kits are rated by the added power
they produce. This added power is generally realized at nearly all RPM
levels in the power-band of the engine.
Nitrous oxide is under high pressure. When it expands,
and changes states from liquid to gas, it cools significantly. This is
the opposite affect as compressing air. Compressing heats, expanding cools.
This cooling affect is also beneficial. By cooling the intake charge,
it becomes denser. That means that a given volume of intake charge contains
more oxygen, again allowing for more fuel, and therefore more power.
Hyper eutectic pistons may have problems with larger nitrous
oxide systems. It is best to use a good forged piston and O-ringed head
when using large nitrous kits.
Performance engine design must consider all aspects of
the camshaft, intake system, and exhaust system. When I say intake and
exhaust systems here I am including the portion of those systems in the
cylinder heads. The engine will only perform as good as the weakest link.
If you have a high RPM camshaft and exhaust design, and a low RPM intake
system, your performance will be sacrificed at both ends of the power
curve. When designing a performance engine, one must consider where in
the rpm range they want the power to be optimized and design the entire
system around that. If you want a real wide power-band, supercharging
or turbo charging is the best option. If you want gobs of power in short
spurts (drag racing or mud bogging for instance) nitrous oxide is an excellent
way to get the job done.