How critical parts of your motorcycle engine are cooled?
Because engines are driven by the chemical energy of a fuel generated during combustion,
its components are subjected to varied degrees of heat.
Exhaust valves no longer fail frequently, but under extreme conditions, they may lose
sections of their heads or break off where the valve stem begins to flare out to form the
valve head. Whirling turbine blades gradually extend due to high temperature creep,
eventually scraping against nonrotating engine elements. Aluminium piston crowns lose
strength quickly at moderate temperatures and can sag or be punched through by
combustion pressure.
As a result, some type of cooling is required to keep engine parts’ temperatures within the
range that their materials can endure.
Pistons
Pistons must endure the gas pressures of combustion for extended periods of time (as a rule
of thumb, peak combustion pressure at peak torque can be 100 times the compression
ratio) as well as the significantly greater inertia forces of the piston’s stopping and starting
at top and bottom dead centre.
The aforesaid stresses are high, which drives the fatigue process, in which the gradual
rearrangement of metal atoms under strain might eventually lead to fracture formation and
failure. Fatigue is strongly temperature dependant, so the hotter the most stressed parts of
a piston run, the faster fatigue develops to failure. More effective piston cooling delays the
process.
When almost all motorcycle engines were air-cooled, pistons were primarily cooled by
accident:
- By making touch with the colder (you hope!) cylinder wall, particularly through the
piston rings, which have the closest contact with the wall. - By conduction to the oil that naturally circulates inside the crankcase as it is pushed
out of the main and con-rod bearings. - Don’t expect the air in the crankcase to provide much piston cooling—oil is
approximately 600 times denser than air!
When liquid-cooling was implemented in most motorcycle engines, pistons breathed a sigh
of relief because cylinder walls backed by liquid coolant remained at a lower and more
consistent temperature. They do not run hotter in the summer or colder in the winter.
As engines were pressed to produce power gain that has sold so many new motorcycles,
lighter pistons were required to achieve faster revs without significantly increasing inertia
stresses on bearings. That meant lighter, thinner pistons with less metal to quickly transfer
heat to the cylinder wall. The solution has been to install piston-cooling oil jets at the
crankcase mouth and aim them up at the bottom of the piston dome. Most production engines have only one jet per piston, however race engines can have many, or even “many” such jets in order to achieve a more consistent piston temperature.
Cylinder Head
The combustion chamber’s “other half” is the cylinder head, which receives the same
temperature as the piston. However, experimental study has demonstrated that half of the
heat entering the cylinder head passes through the walls of the exhaust port. The cause for
this is the high velocity of exhaust gas, which speeds up heat transfer from gas to metal.
Some people are astonished by this, because the old belief that “the gas goes through there
so fast, there’s no time for heat transfer” has died hard.
High gas speed promotes heat transfer because the turbulent and fast-moving heat
significantly thins the stagnant gas boundary layer, which has lost energy due to numerous
impacts with the surface. This, along with the turbulence and speed, ensures that every
square millimetre of port surface is continually in contact with fresh hot stuff, while cooled
gas is quickly carried away.
This is why the exhaust ports on modern engines are made as short as possible, and of
minimum possible diameter (minimizing its surface area).
The top few millimetres of the cylinder bore, while not a part of the head, are intimately
connected. Exposure to combustion, combined with contact with the hot piston at TDC, can
endanger the oil film on this important area (not only does the oil lose viscosity, it can also
evaporate). As a result, designers make extra efforts to get coolant as close as possible to it.
Exhaust Valves
Aren’t both intakes and exhausts subjected to hot combustion gases? They are, however
exhaust valves are intensely heated by high-velocity combustion gas exiting the cylinder,
heating from both sides. High gas speeds improve heat transmission.
Because the valve head has a large surface area, it gathers the greatest heat; however, the
valve seat, on which it typically rests two-thirds of the time, is part of the head, which is
usually liquid-cooled in the modern era. This contact handles the majority of the necessary
valve cooling. If you’ve ever worked on an engine or seen one disassembled, you’ll note that
the exhaust valve’s seating area is wider than the intake. This is done to increase the
amount of contact surface available for transmitting valve heat to the seat and cylinder
head.
Engine Bearings
Instead of the heavier and more fatigue-prone rolling element bearings used in the past,
modern engines use simple bearings. Because the clearances in plain bearings are measured
in thousandths, the volume of oil in them is minuscule, and if it were not regularly supplied
by fresh, cool, and filtered oil from the oil pump, it would rapidly overheat, lose viscosity,
and fail to lubricate.
Valve Train
Valve train friction accounts for only a minor portion of overall engine friction. However, the
pressure between the cam lobe and tappet is significant. As the spinning cam lobe fills the
clearance to the valve tappet and begins to accelerate the valve up off its seat, the oil film
between the lobe and tappet must carry the valve’s inertia as well as the force of the valve
spring(s). Modern designs usually have a hollow camshaft that is filled with oil from the
pump. Each cam lobe has a drilled hole to ensure that it and its tappet are supplied with
enough oil for lubrication and cooling..