Approximately 10 years ago, air- and water-cooled engines held
approximately equal portions of motor vehicle production. This
investigation compared these two types in terms of emissions, fuel
consumption, and operation.
The greatest thermodynamic difference between the two types is
the amount of heat conducted away by the intended cooling medium;
water removes twice as much heat as does air in their respective
engines, with the balance of heat removal being born by exhaust and
oil. However, tests show no difference in efficiencies between the
two, although some sources have speculated that higher air-cooled
engine-compartment temperatures would permit higher air/fuel ratios
through improved carburetion. However, these higher temperatures do
require greater engine component quality. Expected tendencies in
exhaust and emissions failed to appear during tests, however, even
during start and warm-up operation.
Choice of cooling depends on noise levels, engine compartment
temperatures, and costs of development and production, but
especially on water-cooled engines' greater capacity for
temperature control. These factors have led to a present market
share for air-cooled engines of less than 10%