A turbo-charger for an internal combustion engine
includes an exhaust turbine impeller which is exposed to
and heated by the high temperature of engine exhaust
gas passing through the turbo changer. Therefore, the
shaft bearings of the turbo-charger are also heated be
cause of the heat transmitted from the turbine impeller.
A bearing housing of the turbo-charger is, therefore,
usually provided with cooling water jackets in the vi
cinity of the turbine impeller, and when the engine is
running a part of the engine cooling water is forcibly
circulated therethrough to cool the bearing portions of
the turbo-charger. Namely, a cooling water passage is
branched from the downstream side of a water pump to
introduce the cooling water into the water jackets.
Therefore, when the engine is running, the cooling
water is compulsorily fed into the cooling system of the
turbo-charger. However, when the engine is stopped,
the performance for cooling the turbo-charger depends
largely on a level at which the turbo-charger is posi
tioned, since the bearing portions thereof are then
cooled by a so-called natural or gravity circulation.
Consequently, if the turbo-charger is positioned lower
than the cylinder head of the engine, a natural circula
tion is established through conventional piping for the
cooling water, which cools the the interior of the engine
as well as the turbo-charger. However, if the turbo
charger is positioned higher than the engine cylinder,
the cooling water may be boiled in the water jacket of
the turbo-charger, thus ?lling the jackets with vapor
(gas) which will prevent the circulation of the cooling
water, when the engine is stopped after having been
driven in a heavy load condition. Therefore, positioning
the turbo-charger at a higher level than that of the
engine with conventional piping for circulating cooling
water causes an increase in the temperature of the bear
ing portions and a deterioration in the lubricant.
To avoid the above-mentioned problems, an electric
water pump can be added exclusively for the cooling
water piping of the turbo-charger, and this water pump
is driven after the engine is stopped. This solution may
provide effective cooling, but the electric water pump is
very expensive per se, and the capacities of the alterna'
tor and battery must be increased to cope with the extra
load incurred thereby.