Re: How bad is too bad?

===Engine cooling===
--------------
From Wikipedia, the free encyclopedia
Jump to: navigation, search

==Basic principles==

Cooling of internal combustion heat engines is most often
accomplished by a fluid coolant whose heat is transferred to ambient
atmosphere or water, as for instance with marine engines.

Heat engines generate mechanical power but, as with a waterwheel,
there must be an exit for the working fluid, and that appears as waste
energy. For example, a waterwheel cannot run unless its water has
remaining velocity to exit the wheel as relatively dead water, which
in a way is what the exhaust and radiator does for an automobile
engine. In combustion engines, some heat goes out the exhaust and
some through the cooling system, because without cooling, machine
components would overheat and fail.

Some high thermal efficiency engines run without explicit cooling
and with only accidental heat loss, using a design called adiabatic
process. Such engines can achieve high efficiencies but are
impractical for most applications.

=Basics=

Most internal combustion engines are fluid cooled, meaning
directly by atmospheric air, or by water that is in turn most often
cooled by atmospheric air. With a large supply of water at convenient
temperatures, such as with marine engines, the body of water on which
the boat operates is usually used as a heat sink. Air and water
cooling, each have preferred applications, such as light aircraft or
chain saws, that use direct air cooling while most automotive and
larger engines use water or similar liquid.

Most liquid cooled engines use water with additives such as antifreeze
and rust inhibitors, while some special purpose engines use water
substitutes, such as propylene glycol or the combination of propylene
glycol and ethylene glycol. Not to be overlooked, most direct air
cooled engines also achieve some cooling through an oil cooler
(radiator) that is used to keep lubricating oil at a useful
temperature in an otherwise too hot environment.

The main advantage of water cooling is that the temperature of several
heat sources immersed in water can reasonably be estimated, while the
same cannot be said with a passing air draft. That in the simplest
form characterizes the problems of direct air and water cooling that
arise from heat capacity and conductivity of the coolant.

Because heat transfers by temperature gradient, similar to water
flowing downstream, temperature difference between the hot and cold
governs the rate of heat transfer from engine to coolant, although
this can be aided by pumping more air or water past the heat
source. The same holds true for the exchange from water to ambient
atmosphere (radiator). For this reason motor vehicles use fans to
insure rapid air flow even when the vehicle is standing or traveling
at a slow rate where ram air alone is insufficient.

Specific heat capacity is a major criterion that favors water, in
contrast to oil, or air as a coolant. For example, one gram of oil can
absorb about 55% of the heat for the same rise in temperature as water
while having about 90%, its density. A given volume of oil can absorb
about 50% of the heat as the same volume of water so this makes heat
transfer area and volume of coolant an issue. Thermal conductivity
of water is about 4 times that of oil, whose viscosity can be ten
times greater than water, demanding more energy for circulating oil
coolant making oil an inferior coolant for heat engines.

Among air, oil, and water, air has the lowest heat specific heat
capacity (or per volume) and less than a tenth the thermal
conductivity, although having a much lower viscosity. Therefore, air
cooling relies on great volume to move heat, and conducting heat of an
engine to large surface area (fins) for cooling presents difficulties
and limitations. Distributing air cooling too keep components at the
same temperature presents a major problem. In contrast water, with its
higher heat capacity accomplishes this readily, besides which, with
local overheating water boils, tuning liquid to gas at a useful
temperature for engine cooling. The phase change of water to steam,
takes up 540 times as as much heat as raising the same amount of water
1°C besides occurring at 100°C, areasonable temperature for such an
engine.

Internal combustion engines operate with a wide range of
temperatures throughout their operating cycle. Intake air is as cool
as possible, which occurs naturally from evaporating fuel, while
cylinder walls should not heat intake air significantly before
compression, but also not cool heated air after combustion. A cylinder
temperature of 90°C, that is additionally insulated by a boundary
layer, is a reasonable compromise. Motor oil has its viscosity
designed to operate at this temperature in water cooled engines whose
temperature is controlled by thermostats.

Other concerns may also affect engine design. For example, although
air is a relatively poor coolant, its cooling systems are simple, such
as in aircraft, where high velocity ram-air is abundant, and from
which small engines can be reliably cooled. Larger engines are
generally water cooled today, the radial engine being a design of
the past. Because light aircraft engines run at propeller speeds
(<2500rpm), they do not reach high performance such as automobile
engines and can operate with air cooling.

Minimize the number of heat transfer interfaces is desired in order to
maximize the temperature difference at each stage. However, engines
with oil to water and water to air cooling transfers have been built.

The benefits of water cooling are fundamental to conductivity, heat
capacity and economic availability of water, besides which the solid
engine block with water jacket suppresses noise. Because large air
cooled engines are difficult to design and even more difficult to cool
uniformly, most air cooled engines are under 500 kW, whereas water
cooled engines exceed 80 MW

Wärtsilä-Sulzer RTA96-C 14-cylinder diesel:

http://people.bath.ac.uk/ccsshb/12cyl/.



Jobst Brandt
.

Relevant Pages

  • Re: Thrift shop distiller $9
    ... with a heat exchanger device to heat what? ... Water to make steam or oil ... I want to turn the exhaust manifold into ... drinking it....maybe with further seawater cooling, ...
    (rec.boats.cruising)
  • Re: radiator leak
    ... I agree with Ritz advice and also disagree with SnoMan. ... antifreeze/water will be less effective in cooling the engine than a 50/50 ... Water is a better conductor of heat than antifreeze. ...
    (rec.autos.makers.saturn)
  • Re: Hot water question for Larry
    ... Here's a bunch of thoughts on your hot water: ... Never use electric heat when you are burning fuel anyway. ... if put in the freshwater cooling closed circuit. ... is without risking to overheat the engine at high load. ...
    (rec.boats.cruising)
  • Re: OT - Six stroke engine
    ... was wondering if there was some obvious "gotcha" factor that I hadn't ... but I'm wondering how much of the heat of the ... head after the water has cooled these things off. ... The amount of cooling derived from this water ...
    (rec.crafts.metalworking)
  • Re: Is liquid cooled superior to air cooled
    ... Boulevard is water cooled. ... This is why you don't see many modern 140bhp+ engines with air cooling. ... For smaller low-powered bikes (and big low-powered bikes like ...
    (rec.motorcycles)