The Complete Guide to Intercooling Page 1 of 5
If you run a turbo or blown car, you need an intercooler for best performance.
By Julian Edgar When a turbo or supercharger compresses air, the air is
heated up. While this hot air can be fed straight into the
intake of the engine (and often is), there are two disadvantages in taking this approach.
Firstly, warm air has less density than cool air - this
means that it weighs less. It's important to know that it's
the mass of air breathed by the engine that determines
power, not the volume. So if the engine is being fed warm,
high pressure air, the maximum power possible is significantly
lower than if it is inhaling cold, high pressure air. The
second problem with an engine breathing warm air is that the
likelihood of detonation is increased. Detonation is a process
of unstable combustion, where the flame front does not move
progressively through the combustion chamber. Instead, the
air/fuel mixture explodes into action. When this occurs,
damage to the pistons, rings or head can very quickly happen.
If the temperature of the air can be reduced following the
turbo or supercharger, the engine will have the potential to
safely develop a higher power output. Intercoolers are used to
cause this temperature drop.
Temperature Increase:
There are a number of factors that affect the temperature
increase that occurs when the air is compressed. Firstly, the
higher the boost pressure, the greater will be the temperature
increase. As a rule of thumb, if you are using a boost
pressure level of more than about 0.5 Bar (~ 7 psi), an
intercooler is generally a worthwhile investment.
Secondly, the lower the efficiency of the compressor, the
higher the outlet air temp. However, it is difficult to
accurately estimate the efficiency of the compressor and even
if such a figure is available, it doesn't necessarily apply to
all the different airflows that the compressor is capable of
producing. In other words, there will be some combinations of
airflow and boost pressure where the compressor is working at
peak efficiency - and other areas where it isn't. While a
well-matched compressor should be at peak efficiency most of
the time, in some situations it will be working at less than
optimum efficiency. This will change the outlet air
temperature, usually for the worse.
Thirdly, the turbo- or supercharged car engine is not
working in steady-state conditions. A typical forced induction
road car might be on boost for only 5 per cent of the time,
and even when it is on boost, it is perhaps for only 20
seconds at a stretch. Any decent forced induction road car
will be travelling at well over 160 km/h if given 20 seconds
of full boost from a standstill, meaning that longer periods
of high boost occur only when hill-climbing, towing or driving
at maximum speed. While all of the engine systems should be
designed with the maximum full load capability in mind, in
reality very few cars will ever experience this. This factor
means that the heat-sink ability of the intake system must be considered.
If the inlet air temperature of the engine in cruise
condition is 20°C above ambient, then on a 25° day the inlet
air temp will be 45°C. After 30 minutes or so of running, all
of the different components of the intake system will also
have stabilised at around this temperature. If the engine then
comes on boost and there is a sudden rise in the temp of the
air being introduced to this system, the temperature of the
turbo compressor cover (or blower housing), inlet duct,
throttle body, plenum chamber, and inlet runners will all
increase. These components increase in temp because they are
removing heat from the intake air, limiting the magnitude of
the initial rise in the actual intake air temperature. As a
result, the infrequent short bursts of boost used in a typical
road-driven forced-induction car often produce a lower initial
intake air temperature than expected. This doesn't mean
that intercooling is not worthwhile - it certainly is - but
that the theory of the temperature increase doesn't always match reality.
