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questa è la prima versione del libro che ho recuperato anni fà via internet ma la nuova versione ha molte informazioni in più
Chapter 8
Lubrication
and Cooling
THE RELIABILITY of any engine is closely related to adequate lubrication and
efficient cooling. Unfortunately, it is in these areas that the two-stroke engine is most
vulnerable. It has to rely on just the scantiest supply of lubricating oil to resist piston
seizure in a cylinder badly distorted by the steep temperature gradient existing between
the hot exhaust side and the much cooler inlet side of the barrel.
The lubricating oil must be able to prevent metal to metal contact of moving
engine parts and at the same time assist in conducting heat away from the piston crown
to the cylinder wall. Additionally, it must form a seal between the piston rings and
cylinder wall to contain the pressure of combustion effectively. If the oil film is too
thin, blow-by will result, reducing the amount of energy available to power the piston
down.
There are basically three types of oil: mineral oil derived from crude stock;
vegetable oil from the castor bean plant; and synthetic oil, which is man-made or man
modified and used straight or blended with mineral or vegetable oil.
Most motorcycle oils are mineral-based, with a variety of additives used to
improve them in certain functions. I would recommend mineral oils for all except
competition two-stroke engines. My favourite mineral oil is Castrol Super TT. It will
provide very good lubrication and wear resistance, better I believe than any other
mineral oil available and better than most synthetic and castor oils. Like all mineral
oils, Super TT will dirty the plug, and leave some carbon on the piston crown and in the
combustion chamber.
In all of my competition engines, I specify Castrol R40 or R30 castor oil (R40 for
air-cooled engines, R30 for water-cooled). This oil provides the best anti-wear
protection of any oil that I know. The mere fact that my engines produce top
horsepower testifies that it must be doing an excellent job of reducing friction by
keeping moving parts separated. When you strip a 12,000rpm road racing motor after
300 racing miles and find the ring gaps opened up by only 0.007inch, and hone marks
still visible on the cylinder walls, then you know the oil you are using is good.
Many tuners do not like castor oil or blended castor/synthetic because of some
problems associated with the use of an oil of this type. Some claim that castor gums up
the rings and causes ring sticking, but I have never found this problem, even on engines
that are required to run 500 miles between rebuilds.
There is one area for concern, and this is the main reason why oil companies try to
discourage the use of castor oil. Castor-based oils are hygroscopic, which means they
will absorb moisture from the atmosphere. Therefore, once a container is opened, its
entire contents should be used, or if oil is left over this should be poured into a smaller
container so that no air space is left above the oil from which to absorb moisture.
Remember, too, that castor oil will also absorb moisture after it has been mixed with
fuel. Therefore, do not use fuel more than three days old, and don't forget to drain the
fuel from the tank and carburettor bowl.
While we are on the subject of castor bean oil, don't think for a minute that all
castor oils are as wear resistant as Castrol R. This all depends on how well the
manufacturer de-gums the basic castor stock and on what additives are used. Some
castors provide wear protection no better than average mineral and synthetic oils.
Today, more and more people are turning to and advocating the use of synthetic
lubricant. There are several points in favour of synthetics, namely: less exhaust smoke,
less incidence of plug fouling and less build-up on the piston crown and in the
combustion chamber. Some also claim better wear protection and more power as a plus
in favour of synthetic lubes, but in general my research has produced an opposite
result. I have found some synthetic oils to have a wear factor twice as high as the better
castor and mineral oils and I have never found a synthetic to allow an engine to
produce as much power as Castrol R. For these reasons, I could not recommend the use
of synthetic oil in highly stressed competition engines.
The manufacturers of synthetic oils claim their oils will give better power because
the amount of oil in the fuel can be reduced (eg: Bel-Ray MC-1 is mixed 50:1 as
compared to 20:1 for most mineral oils). But why should it ever be imagined that a
smaller quantity of lubricant entering the engine will give a power increase? At the
races it almost seems as if there is as much glory to be gained from running a fuel/oil
ratio of 60:1 as there is in actually winning the race; by the pit bragging going on it
would seem to be so!
My experience has shown that the more oil you pour into a two-stroke, the harder
it runs. Just how much you should pour in depends on several factors, but it usually
works out that the longer you hold the throttle wide open, the more oil you should use.
This is due to the fact that the fuel/air ratio will be leaner at full throttle than at half
and three-quarter. Therefore, with less fuel entering the engine at full throttle,
proportionally less oil will be available for lubricating the piston at a time when it
requires the most lubrication. Spelled out, it means that on a track with long straights
you will have to use more oil than on a tight twisty track.
Keep in mind that your engine only needs enough oil to lubricate one stroke at a
time and then the excess is burnt up. If your bike is dribbling oil out of the exhaust then
you are running too much oil for its needs, or for your riding speed. A faster rider on
the same machine may need more oil, because he is holding full throttle for longer
periods.
When you start experimenting with oil ratios, always use the engine
manufacturer's recommendation as a reference point and work from there. If you go
too rich, the spark plug will be coated with black soot and the exhaust pipe will be wet.
—« there is not enough oil, the plug could look white or grey, the pipe will be very dry,
the piston crown will be white or light grey, possibly with 'death ash' forming under the
crown. A'ny of these signs indicate that you are bordering on a seize up.
Generally, I would say that road racing engines will work best at a 16:1 to 20:1
fuel/oil ratio, depending on the nature of the course. Desert racers require 16:1 but, if
plug fouling proves to be a problem, try 18:1 or 20:1. For enduro and motocross 20:1
or 22:1 is the best ratio. Go-karts with fixed gearing, without a clutch operating on
short sprint tracks, will usually not tolerate more oil than 25:1 and, if you find that you
are fouling plugs, you may have to drop as low as 30:1. Under no circumstances should
you run leaner than 32:1.
All of the above fuel/oil ratios are for mineral and castor oils. Synthetic oils are an
entirely different kettle of fish. If you choose to use this type of oil you will have to run
it at the ratio the oil manufacturer recommends. This is because the oil people load the
oil up with additives, in an attempt to give it acceptable scuff resistance when mixed at
50:1. Mixed at 25:1 there will be twice as much chemical additive and detergent being
inducted into your engine and this could very easily cause carbon build-up and plug
sooting, serious enough to stop or even damage the engine.
It seems that the trend towards leaner and leaner oil ratios has resulted from the
desire of two-stroke engine manufacturers to eliminate plug fouling completely in twostroke
mower, outboard and chain saw engines. These engines are seldom serviced and
the plug is probably only changed each time the rings are replaced. To cut down on
spark plug deposits, the manufacturers decided on less oil and, unfortunately, this idea
has carried over into competition two-stroke circles.
The results of my most recent oil testing are shown in TABLE 8.1. The engine was
a fully worked Suzuki RM 125C motocross unit. As you can see, reducing the oil
content from 20:1 to 27:1 (I wasn't brave enough to lower it any further) resulted in a
power loss of about 8% at the top of the power range — a heavy price for the sake of a
clean plug. On top of that the piston showed signs of scuffing bad enough to deter me
from testing at 32:1 which, according to a lot of tuners, is the best mix when using R40.
When the fuel/oil mix ratio was raised to 16:1, power was marginally improved by
about 2%, which is almost too small to measure on the dyno. However, the piston was
much cleaner and the rings showed no sign of gumming up.
Accuracy is of utmost importance when blending oil and fuel. It is of no use
mixing one and a half beer cans of oil to each drum of fuel, you have to be precise. For
measuring the oil you need either a laboratory measuring cylinder or a graduated
beaker. Fill the measuring container with the required quantity of oil, and be sure to
allow the oil plenty of time to drain out when you pour it into your drum of fuel. Keep
the measuring equipment clean, preferably in a dustproof plastic bag.
Determining how much fuel is in a drum is not easy. The drum may say that it
contains 20 litres, but this can vary considerably, even when the drums are factory
filled, as in the case of racing fuel. If you mix your own racing fuel the inaccuracy may
be even worse, as a 20 litre drum will actually hold 22 litres filled nearly to the top.
What I recommend is, assuming that you have brought your fuel in a drum that is
supposed to contain 20 litres, drain the fuel from the drum and then refill it with
precisely 20 litres measured with a suitable, accurate, 1 or 2 litre measure. Then take a
rule and measure how many inches the fuel is from the top of the drum. Next make a
gauge from light metal as shown in FIGURE 8.1, to fit in the neck of the drum and
indicate the height of fuel for 20 litres. You can then use your gauge on any other 20
litre drum of fuel that you buy, assuming the drum style doesn't change.
TABLE 8.1 Suzuki RM125C horsepower/oil tests
rpm--------Test 1 (hp)------Test 2 (hp)-------Test 3 (hp)
----------------------------------------------------------------------
8000-----------15.7---------------15.4-----------------16.0
8500-----------18.8---------------18.7-----------------18.7
9000-----------20.4---------------20.5-----------------19.2
9500-----------21.3---------------21.7-----------------19.6
10000---------21.9---------------22.1------------------20.3
10500---------22.6---------------22.9------------------20.7
11000---------23.2---------------23.6------------------21.4
11500---------17.3---------------17.6------------------15.8
Test 1 — Castrol R40 mixed at 20:1 with Shell 115 MB racing fuel. Champion N-57G plug
— no sign of carbon. Light coat of 'varnish^ on sides of piston.
Test 2 — Castrol R40 mixed at 16:1 with Shell 115 MB racing fuel. Champion N-57G plug
— slight trace of carbon on insulator, heavier deposits on plug shell and earth
electrode. Less 'varnish' on piston than with 20:1 mix.
test 3 _ Castrol R40 mixed at 27:1 with Shell 115 MB racing fuel. Champion N-57G plug
— very clean — cleaner than plugs from 20:1 and 16:1 tests. Heavy 'varnish'
coating right around ring lands and down exhaust side of piston.
Two Stroke Performance Tuning
When blending oil and fuel you must be careful not to be confused by volumes
which, on the surface, appear similar. In the Imperial system one pint is 20 fluid
ounces, whereas in the US system one pint is 16 fluid ounces: considerably less. To
assist you with mixing for various fuel/oil ratios refer to TABLE 8.2.
Incompatibility/insolubility of oil and fuel can mean big trouble, so don't take it
for granted that all oils and fuels will mix properly. Mineral oils such as Castrol Super
TT will blend with any of the leaded and unleaded fuels available out of the pump at
the local garage, but it may not blend with some 100 or 115 octane racing fuels without
the addition of 5-15% of benzol, toluol or methyl benzine.
Castrol R will blend with some regular pump fuels, depending on whether they
contain a proportion of benzol or toluol. It will also blend with any 100 or 115 octane
racing fuel containing 15% toluol, methyl benzine or benzol.
When methanol is used, it is necessary to mix it with a specially formulated oil.
Castrol M castor oil and Shell Super M castor oil are both soluble in methanol.
To determine the compatibility of your fuel/oil mix, make up a small sample at
the correct ratio in a clear glass bottle. Shake it well, as you should always do when
blending oil and fuel. Leave it to stand for 24 hours, and check for separation. If there
appears to be some insolubility, try mixing 5% toluol, benzol or methyl benzine. In
extreme cases you may need up to 15% of these fuels added, to maintain solubility.
At times the oil may not completely separate out of the fuel, but, instead, may
form in layers through it. When you find this problem, again try blending 5% methyl
benzine, benzol or toluol with your mix.
No matter what type of oil you run, or how well it is blended, you will still end up
with premature cylinder/piston wear and possibly even risk seizure if you don't allow
the engine to warm up before working it hard. I recommend that you don't ride off
until the barrel is getting reasonably warm. This will ensure that piston and bore wear is
kept to a minimum. I have seen engines seized by being operated too hard right after
being started. This occurs because the piston gets hot first and expands at a faster rate
than the barrel, which takes much longer to warm up and expand the correct amount to
provide the proper piston to cylinder working clearance.
Most two-stroke engines are air-cooled, but even water-cooled engines rely
indirectly on air to stabilise the temperature of the cylinder head and barrel. The
cooling arrangement of every internal combustion engine performs the vital function
of dissipating heat in order to maintain normal engine operation.
The two-stroke engine is, in fact, a heat engine in that it relies on the conversion of
fuel into heat, and then into mechanical energy to produce power at the crankshaft.
Only about 23% of the heat is converted into power, another 33% is lost through the
exhaust, and the rest is eliminated through the cooling system.
Lately, a lot has been said about applying a ceramic insulating coating to the
combustion chamber and piston crown, to reduce to some extent the heat energy which
is lost to the cooling system. It was felt that since it was heat energy, produced by the
burning of a fuel, which heated the gases in an engine and caused them to expand and
force the piston down, then reducing heat conduction to the cooling .system should
increase cylinder pressure, and result in more power.
In theory, ceramic coatings to thermally insulate the combustion chamber and
piston crown sound a logical way to increase power, but in practice it hasn't worked
out. In many instances the reverse has occurred, due to the end gases detonating as a
result of increased pressure and temperature within the combustion chamber. I think
many tuners realise that a liquid-cooled engine operating at a coolant temperature of
75°C will make significantly more power than if operated at 95°C, even though in the
latter case considerably less combustion heat energy is lost to the cooling system. Why
is this? Well, charge density will be superior with a cool engine and the combustion
process will be more controlled, reducing the incidence of detonation.
Ceramic coatings are, I feel, only beneficial in low-speed engines and engines
operated mainly at small throttle openings. Low-speed engines lose much more heat
energy to the cooling system than high-speed engines, because each combustion cycle is
longer. In the case of engines operated at light throttle openings the combustion
process is often retarded, due to excessive dilution of the fuel charge by residual
exhaust gas. With a ceramic coating applied to the combustion chamber and piston
crown, combustion will be faster and more complete, due to the increase in combustion
temperature.
The only other situation in which ceramic coatings may be beneficial is for coating
the piston crown only in engines used for desert racing or those burning exotic fuel such
as nitro and nitrous oxide. Such engines seem particularly prone to piston burning and,
under these circumstances, ceramic coatings appear to offer a degree of protection.
One company which applies ceramic coatings is Heany Industries in America.
Using a plasma-spray system, a ceramic coating 0.012-0.014in. thick is applied. The
plasma coating process, called Heanium coating, utilises an electric arc device, into
which argon gas is injected to generate a plasma stream of high temperature gas (up to
30,000°F). Powdered materials introduced into this plasma stream turn into a molten
spray as they are propelled towards the surface to be coated.
As the Heanium coating is 0.012-0.014in. thick, a piston which has had the crown
coated will cause an increase in the compression ratio and a decrease in the piston to
head (squish) clearance. To overcome both of these problems a thicker head gasket will
have to be used.
Heanium coating may also be applied to the exhaust port and the inlet port of
motors with relatively straight ports. This will not do much to improve power, except
that inlet charge density may increase a little, but cylinder distortion and overheating
will be reduced. Cylinder distortion is not such a problem with liquid-cooled barrels, so
I feel that you would be wasting your money coating the ports of these engines. Aircooled
barrels definitely will benefit from Heanium coating in the exhaust and inlet
ports. Coating the exhaust tract will reduce the amount of heat which the finning on
the exhaust side of the barrel has to dissipate. Hence that side of the motor will be
cooler and, as a result, the cylinder bore will distort less. Conversely, coating the inlet
port will increase the temperature of this side of the barrel because the insulating
barrier will prevent the fuel charge from cooling the metal surround the inlet tract. The
end result will be a lower temperature differential between the exhaust and inlet sides of
the cylinder, and less distortion.
Besides reducing the heat load on the cooling system by using Heanium-coated
ports, air-cooled engines can have the heat radiating area of their cooling fins
increased. It has been found that blasting the head, barrel and crankcase with
aluminium oxide increases the surface area about five times. If these parts are then
sprayed with Kal-Guard coating and oven baked to keep the aluminium oxide on the
metal surfaces, engine operating temperatures will be reduced by 10-13%. Of course,
the cooling fin area can also be enlarged by more direct means. Both DG and Webco
produce a range of replacement cylinder heads for Japanese engines. These heads have
a bigger fin area to improve cooling and reduce any tendency for the engine to detonate
or seize pistons.
It is essential to ensure that your cooling system is working at 100% capacity. Heat
radiation from the cooling fins is retarded by the presence of oil and mud, so make sure
they are clean. Fins and crankcases painted flat black radiate heat considerably better
than shiny silver surfaces. Anything that is obstructing air flow onto the head and
barrel should, if possible, be relocated elsewhere. On road bikes, check to see that the
horn is not blocking air flow to the head. Also investigate to see if the exhaust can be
better located, as the header pipe always seems to be in the way. Every move you make
to encourage air flow over the engine will help performance and reliability.
Water or liquid-cooling is now looked on as the answer to two-stroke cooling
difficulties. However, liquid-cooling is not without problems peculiar to itself. The two
major deterrents to proper heat transfer from the combustion chamber and cylinder to
the liquid cooling medium are deposits and air in the cooling system.
Metallic oxides twelve thousandths of an inch thick formed in the water passages
will cut heat transfer by up to 40%. Therefore, in order to maintain optimum heat
transfer, the cooling passages should be cleaned in a special bath that won't attack
aluminium. Additionally, the system should contain an inhibitor that will keep coolant
passage surfaces clean and free of deposits.
172 There are two basic types of inhibitors: chromates and non-chromates. Sodium
chromate and potassium dichromate are two of the best and most commonly used
water-cooling system inhibitors. Both are toxic, so handle them with care.
Non-chromate inhibitors (borates, nitrates, nitrites) provide anti-corrosion
protection in either water or water and permanent anti-freeze systems. Chromates must
not be used with anti-freeze.
If you decide to use a coolant other than water, ethylene glycol is to be
recommended. Methyl alcohol-based anti-freeze should not be used because of its very
low boiling point and its damaging effect on radiator hoses and water pump seals.
When ethylene glycol anti-freeze is used in concentrations above 30%, additional
inhibitor protection against corrosion is not required. I do not recommend the use of
cooling solutions composed of more than two-thirds ethylene glycol and one-third
water, as heat transfer is adversely affected.
Anti-freeze containing cooling system sealer additives should not be used, as the
sealer may plug the radiator core tubes and possibly even coolant passages in the
engine. Stop leak or sealer of any description is not to be recommended, except in an
emergency to get you home or to finish a race. Then, as soon as possible, it should be
cleaned out by a cooling system specialist, using a high pressure air and water flusher.
Petroleum-derived products such as soluble oil, often used as a water pump
lubricant and corrosion inhibitor, should never be used. A 2% concentration of soluble
oil can raise the cylinder head deck temperature by up to 10%, due to reduced heat
transfer efficiency of the coolant. One popular radiator stop leak contains a high
proportion of soluble oil, which is an additional reason for staying clear of radiator
sealers. Soluble oil turns water milky when it is added.
The presence of air bubbles in the coolant reduces the heat transfer capacity of the
coolant by acting as an insulator. Water pump efficiency is also reduced. Air can be
sucked into the system through a leaking hose and gas bubbles can form due to
localised boiling around the combustion chamber. In the first case, air can be kept out
of the cooling system by ensuring that there are no air or water leaks, and by keeping
the coolant at the proper level.
Gas bubbles or steam pockets are prevented by pressurising the system to the
degree necessary to prevent the water boiling. By pressurising the radiator to 14psi the
boiling point of water is raised from 100°C to about 125°C. Normally, the water
around the combustion chamber should not reach this temperature, but this gives a
safety factor to permit normal operation at higher altitudes. Periodically the radiator
pressure cap should be inspected for deterioration of the seal and the 'blow-off
pressure should be tested.
As the actual heat exchange between the cooling medium and air takes place at the
radiator, it is important that it is free of bugs or any other debris that would restrict air
flow and hence reduce cooling efficiency. The radiator should be painted matt black to
provide the best radiating surface, and also to minimise the effects of external
corrosion.
It is a mistake to run the cooling system without some form of restrictor or
thermostat, as the engine could be over-cooled. If too much heat is transferred to the
coolant, power will be lost. Therefore, do not let the engine run cooler than about
75-80°C. Below about 70°C, cylinder wear increases to a level as serious as operating
the engine at too high a temperature.
ciauzzzz
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