Cool air is good for making power, but
could hot air be even better?
By
C.J. Baker Most
people know that engines make more power when the inlet
air is cooler. Let’s take a look at why this is
true — at least in most applications. We’ll
also tell you right up front that this article might
leave you with more questions than answers. Then again,
you might be the one that provides the additional answers
and takes the world to the next generation of internal
combustion engines. To
understand what goes on during both the intake cycle
and the power cycle when inlet air temperature is reduced,
we need to consider both normally-aspirated and supercharged
gasoline engines, as well as turbocharged diesels.
We’ll also limit this discussion to four-cycle
engines. Before
we go any further, let’s define a couple of terms.
For this article, we’ll say that supercharging
is anything that increases the amount of oxygen available
in the cylinder to support combustion of fuel above
what could be expected from cylinder filling due to
atmospheric pressure only. We’ll assume atmospheric
pressure at sea level to be 14.7 PSI and that “normal” air
contains approximately 21 percent oxygen. We’ll
also exclude oxygen-bearing fuels, such as nitromethane,
as a form of supercharging. This means that any form
of mechanical compressor that pumps more air into an
engine, such a belt- or gear-driven “supercharger”,
or an exhaust- or turbine-driven “turbocharger” is
included, as well as the injection of nitrous oxide,
or even oxygen itself. Gale
Banks has a favorite saying, “It’s all about
airflow.” Airflow helps engines make power in
many ways, as explained in other articles on this site,
but it is also true that the more air you can flow
through an engine, the more oxygen that will be available
for burning fuel. More oxygen means more fuel can be
burned, and that means more power. Maybe his saying
should be refined for this article to be “It’s
all about oxygen content.” This is most evident
when dealing with an ordinary normally-aspirated gasoline
engine. Many hot rodding tricks relate to getting more
air(read oxygen) into the cylinder. Whether it’s
by installing a less restrictive fuel injection system
or carburetor, a freer flowing intake manifold, porting
the cylinder head(s), increasing camshaft lift or duration,
the purpose is still the same — get more oxygen
into the cylinder. Now in all fairness, the hot rodder
is looking at getting maximum oxygen into the cylinder
at wide open throttle for peak power (to beat the other
guy). This is partly why nitrous oxide (an oxygen-rich
gas) injection is so effective. Nitrous oxide effectively
increases the percentage of oxygen in the working fluid
(which becomes a mixture of air, nitrous oxide, and
fuel) above the 21 percent oxygen in air alone. That
means more fuel can be mixed into the working fluid
too for greater combustion heat to expand the working
fluid and increase pressure in the cylinder. Additionally,
when the compressed nitrous oxide, which is stored
in its pressurized container as a liquid, is injected,
it depressurizes and changes state from a liquid to
gas, cooling the working fluid for an accompanying
density increase. Of course, it would take an incredible
amount of nitrous oxide to be able to use it at all
times, so as you would expect, nitrous oxide injection
is only used on demand at wide open throttle. But what
if we could get more oxygen into the engine at all
throttle positions all the time? Then what happens? In
another article on this site, “Airflow – the
Secret to Making Power”, we explain that the
air throttle on a gasoline engine controls the density
of the intake charge that enters the cylinder. We also
explain how superchargers and turbochargers increase
the density under boost conditions. In some regards,
we can look at density as the amount of oxygen crammed
into a given volume of air (the working fluid). Increased
density means the molecules in the air are closer together
in the same space — more air mass (and oxygen)
in the same space. Here’s where things can get
a little muddy. We have to consider increased air density
in both unconfined and confined spaces. Let’s
look at an unconfined space, such as the atmosphere,
because that’s the world of the normally-aspirated
engine. Two things affect air density in the atmosphere — pressure
and temperature. As atmospheric pressure goes up, indicated
by higher barometric pressure on a barometer, the density
increases if the temperature stays the same. In other
words, at any given temperature, if the barometric
pressure rises, so does the air density. By the same
token, as temperature goes down, the density increases
if the atmospheric pressure stays
the same. Atmospheric air density is very important
to normally aspirated engines. Obviously, you can’t
do much to increase the atmospheric air pressure in
regard to a normally-aspirated engine, but you can
enhance it slightly with some form of ram air taken
either from the front of the vehicle or from a dynamically
high pressure area, such as the base of the windshield.
More importantly, in most cases you can do something
about the temperature of the inlet air. The object
is to get the coolest air possible to the engine’s
intake system. Many engines induct air that has passed
through the radiator or over other warm areas of the
engine, significantly heating the air and reducing
its density. By relocating the air intakes to duct
outside air that hasn’t been warmed into the engine,
density is significantly increased. For example, it
is not uncommon for air to increase up to 50º F. passing
through the radiator and air conditioning condenser
on a late model vehicle. The general rule of thumb
is that for every 10º of temperature drop, the density
(and oxygen content) increases 1 percent. It’s
actually more like 1.8 percent. Similarly, power increases
by an equal amount. So, in this example, if you can
intake air that hasn’t been heated, you can gain
as much as 5 to 9 percent more power. Happily, the
best places to collect cool air are the same places
that work for ram air, so you can get the density gains
from both pressure and temperature using the same intake
ducting. To
get back to our earlier question of what happens when
we have cooler, or higher density, air at all throttle
positions, it means that the engine is capable of producing
given amounts of power at lesser throttle openings.
This generally equates to better fuel economy. It also
means the engine has greater power potential for accelerating
or climbing grades. Cooler intake air also suppresses
detonation since the working fluid doesn’t reach
as high a temperature on the compression stroke — again,
a plus for accelerating or climbing grades. Both
gasoline and diesel engines that use superchargers
and turbochargers face their own unique problems with
intake air temperature. Superchargers and turbochargers
significantly heat the intake air as they compress
it to create boost. The higher boost pressure increases
the air density, but the increased temperature of the
air can largely offset this density gain. In this case,
we’re talking about the affects of pressure and
temperature in a confined space, the intake system.
Consequently, it is desirable to cool the compressed
air before it enters the engine. In most cases, especially
where boost levels exceed 7 PSI, cooling the compressed
air with a charge air cooler, often called an intercooler,
increases the air density more than any density losses
that occur due to the accompanying pressure drop due
to cooling or flow restrictions through an intercooler.
In other words, intercooling results in a net density
increase for the air entering the cylinder. Intercooling
also provides other benefits. For supercharged or turbocharged
gasoline engines, reducing the intake air temperature
suppresses detonation, just as it does for normally-aspirated
gasoline engines. For diesel engines, intercooling
not only increases charge density, it also results
in lower exhaust gas temperature. Excessive exhaust
gas temperature, above 1300º cannot be sustained in
a diesel without eventual engine and/or turbocharger
failure. Lowering intake temperature results in an
almost equal reduction in exhaust temperature. For
example, the air exiting the turbocharger on the Banks
Sidewinder pickup was approximately 500º F. under full
power. Dual air-to-water marine intercoolers, connected
to a reservoir of ice water, were then used to reduce
the air temperature to 100º F. before it entered the
engine. With the intercooling, exhaust temperatures
remained manageable for the duration of the Bonneville
World Speed Record runs. Without intercooling, the
exhaust temps would have been in the 1800º-1900º F.
range. The
final conclusion is that regardless of whether an engine
is normally aspirated or supercharged, gas or diesel,
the cooler the intake air, the better. Usually.
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