Valves for Gaseous and/or Alternative Engines
The constant increase of worldwide interest in the
use of COMPRESSED NATURAL GAS (CNG) and/or LIQUIFIED GAS
as an alternative to GASOLINE, GAS OIL or FUEL OIL is
1) A necessity of diversifying the sources of supply,
so that more countries use fuels like CNG.
2) The reduction of pollutant emissions through the use
of "cleaner" fuels.
3) A reduction in operational costs, through a decrease
in the price of fuels.
4) An improvement in the engine's performance, achieving
lower levels of consumption and greater power.
5) The extension of the engine's utility and a decrease
in the periodicity of its maintenance.
This globalized tendency, led BASSO S.A. to the
creation of a Research and Development program meant to
offer valves for COMPRESSED NATURAL GAS or LIQUIFIED
GAS. In this sense, the "EXTRA GAS" line was launched in
1991 for Argentinian engines, in addition to the valves
exported for FORD, GM and DODGE engines- among others,
turned into used with gaseous fuels in several countries
around the world.
¿What are the characteristics of CNG as fuel, that
make it necessary to manufacture special parts such as
valves, inserts, etc?.
NATURAL GAS is a combination of gaseous hydrocarbons; it
is mainly composed of Methane (85 - 90%) but it also
contains Propane, Butane, Carbon Dioxide, Nitrogen, etc.
It is obtained both from oilfields and from gas
formations and it must be compressed at high pressures
(200 kgs/cm2) in order to store significant quantities
in reduced spaces.
LIQUIFIED PETROLEUM GAS (L.P.G.) results from the
combination of two gaseous hydrocarbons -Propane and
Butane. It is obtained either as a subproduct of
processes in oil refineries, or by separating NATURAL
GAS at very low temperatures. Being at room temperature
and at relatively low pressures, it can be preserved in
The characteristics that affect or influence the design
of the combustion chamber, valves and guides are the
1) It is a "LEAD FREE" fuel, which means it does not
Some years ago, a compound called Lead Tetraethylene was
added to conventional gasoline "WITH LEAD". In this way,
its antidetonating conditions were improved, making its
use in more "compressed engines" possible without risk
of detonation and with the advantage of producing more
power with less consumption.
The lead added to the gasoline was deposited on the
seats from the cylinder head, protecting them from wear
since it worked as a "SOLID LUBRICANT" that stands in
the way between the valve and its seat. In the case of
GAS, the non- existence of such protective "layer" might
cause wear due to abrasion, as the valve keeps on
rotating during the successive openings and closures. If
they do not belong to the hardened-type, both the seat
and the insert suffer from a progressive loss of
material, which is known as seat "RECESSION". In the end,
a serious failure is produced in the valve and seat, due
to burning, stress breakage, loss of sealing, etc.
In general terms, this failure mechanism is produced in
exhaust valves, since they are exposed to a corrosive
environment and to extremely high temperatures (up to
870ºC) resulting from the burning of gases.
Furthermore, these valves endure great efforts resulting
from combustion pressure and violent closures against
their seat, several times per second (at an engine speed
of 3000 R.P.M., a valve opens and closes 25 per second).
Likewise, the lead contributed to the lubrication of the
valve guides, acting as a solid lubricant placed between
the stem and the guide.
2) There is a further important characteristic that has
not been properly considered during the first
developments of engines "turned" to CNG: It burns more
slowly and has greater difficulties to start combustion
than the conventional liquid fuels.
At the same time, the engine's exhaust cam was designed
to allow the corresponding valve to remain settled for a
while, thus evacuating the heat from the previous
When it is open again, the temperature from the gases
burnt in the interior are supposed to have lowered
enough to remain below the limits allowed for the
Given that gas burns more slowly than gasoline, when the
exhaust valve opens in a CNG engine, it is in contact
with gases which have not "cooled" enough and, as a
consequence, it starts to overheat.
On the other hand, once it enters the combustion chamber
-not as little drops of liquid fuel but as gas-, there
is no evidence of the heat absorption produced when
these drops, which are suspended in the air stream that
enters the cylinder, vaporize.
Because of these two factors, many users consider that
the combustion of Natural Gas is "HOTTER" than that of
gasoline. Although this is a conceptual mistake, the
exhaust valve presents, as a consequence, a higher
working temperature, even to the point of surpassing the
sealing capacity of the valve and its seat due to a
failure in the head's or seat's material in the cylinder
head. This might be the result of one or several factors:
- Lack of hot hardness.
- Inadequate endurance to corrosion.
- Insufficient endurance to wear.
In order to tackle this problem, an improvement in the
valve's material is necessary, or even a high-hardness
coating in the seat's face, through the use of special
alloys welded using cutting-edge plasma welding. This is
the case of the valves covered in "STELLITE" -or similar
ones -manufactured by BASSO S.A.
The main advantages these materials present for the
valve's seat face are the ability to keep a good hot
hardness and their excellent endurance to corrosion and
wear. All this is achieved due to the combination of
Cobalt, Iron, Nickel and Chrome.
As regards big engines, it is recommended that, in the
case of Heavy Duty trucks, the seat angle of the exhaust
valve be modified, reducing it from 45º to 30º (or until
15º in the case of compromised valves).
What can be achieved through this? In the following page,
the picture shows a diagram of the forces acting on the
valve and its seat due to the work of the spring, the
inertia produced by the valve’s alternative movement and
the resultants from the combustion pressure.
Although not all these forces act simultaneously, it can
be said that its resultant F acts according to the
valve’s axis and is balanced with the reactions that act
in the seat of the cylinder head (equal and opposite to
f30 or f45 respectively).
From the parallelogram of forces, it can be observed
that for the same resultant F, the components f30 are
less than f45, which means that solely with the change
of angle the charges acting in the seats diminish.
Therefore, the wear in its surface is less significant.
In the case of angles lesser than 15º, which are not
advisable, the seat looses its "self-cleaning" capacity.
In other words, any carbon particles, combustion
residues, etc, are likely to get stuck between the valve
and its seat, not sliding outwards as higher "slopes" (seat
angle), which implies a risk of seat and / or valve "burning
Such solution can only be provided through the use of a
fuel as "clean" as gas and it is not advisable for any
type of liquid fuel, diesel or gasoline, since they
contain impurities, particles and post-combustion
3) The third remarkable characteristic
of Gas as a fuel has to do with its elevated
antidetonating power, equivalent to a 120 / 130-octane
gasoline (with no need of Lead Tetraethylene). This
allows us to increase the engine’s compression
relationship up to 11:1 or 12:1, thus, obtaining greater
power with less consumption.
Nonetheless, in most engines "converted" to Gas the
compression relation is not increased, so that its dual
use (Gasoline or Gas) is maintained, according to the
feasibility of supply. In some cases, these "dual”
operations have caused the valves to fail, since the set-up
of the ignition was inadequate for gas. It was suitable
for conventional gasoline yet too "lagged behind" for
Gas. It is precisely due to low combustion speed that it
requires greater spark advances than gasoline, between
10 and 12 degrees more initial advance.
4) A further characteristic that brings about wear in
valve guides is the gas’ insufficient lubricating
capacity. As verified in the tests carried out by BASSO
S.A. in high-mileage vehicles operating under the most
severe conditions, hard chroming is vital for the stems
in that it reduces friction with the guide, increases
the stem’s duration and facilitates the retention of
lubricant oil between stem and guide.
To conclude this brief "guide of gas as a fuel", we are
now providing a series of recommendations that might be
useful in the case of engines turned to Gas:
A) To improve heat transference from the valves to the
cylinder head, it is advisable to make the exhaust seats
wide (from 1/16" to 3/32" that is to say from 1,60 to
B) Do not mechanize the seats from the cylinder head
with angle interference between valves and seats;
prepare them with the same angles.
C) Keep a concentricity of less than 0,025 mm between
guide and seat.
D) Make the seat in the cylinder head deeper, placing
the valves in a deep-set position. The edges of the
closed valves must not stick out inside the combustion
E) Use hardened seats (or inserts), composed by
considerable quantities of chrome and nickel. Modern
engines have sintered seats, suitable for CNG.
Generally, cast-iron heads have induction-tempered seats.
This solution is not acceptable in "HEAVY DUTY" service
since it will produce their sinking (they will not be
able to resist the effort). It will be necessary to "insert".
F) Keep clearances between valves and guides within the
original values, except when hollow valves (sodium-filled)
are replaced for solid valves, in which case it will be
necessary to increase the clearances in order to
compensate the solid valve’s greatest expansion.
G) Do not use rotator cups nor block their free rotation
if there are not any other valve cups available.
H) Do not rectify secondhand valves, for it reduces
their margin. Use only the best valves available,
covered in "STELLITE", chromed stems made of the best
stainless steels, such as the ones manufactured by BASSO
I) Use an ignition advance corrector, which
automatically moves the spark time forward from 10 to12
degrees when turned to GAS.
Bear in mind that when turning engines to natural GAS
you must use the appropriate valves if you want to avoid
future trouble both for yourself as a user and for the
rectifier, who would be forced to do the same job twice.
We hope this guide turns out to be useful. We would
appreciate your spreading its contents for the
achievement of a wider knowledge of the products
manufactured by BASSO S.A. and to contribute in the
provision of a better service.
VALVES FOR GNC
1) CHARACTERISTICS OF CNG AS FUEL
- Lack of lead.
- Very low speed of combustion
- High antidetonating power.
- Corrosion due to sulphurate compounds.
- Low lubricity.
2) PROBLEMS THAT MIGHT APPEAR IN CYLINDER HEADS AND
- Valve seat recession
- Valve and stem wear.
- Burning of valve seat.
- Seat surface strain.
- Fillet corrosion due to sulphates.
3) AGGRAVATING FACTORS
- High speed operation and high charge during long
- Ignition advance suitable for gasoline yet
insufficient for gas.
- Gas equipment with inadequate A/C regulation. Poor or
excessively rich blending.
- Inadequate heat transmission from insert to cylinder
4) REQUIREMENTS OF THE HEAD / VALVE SYSTEM
- Inserts of adequate hardness in cylinder head and good
- "Premium" material as the valve’s basic materials.
- Hard coatings in the seats of exhaust valves (stellite
or the like).
- Application process of cutting-edge plasma coating.
BASSO S.A.’s VALVES FOR CNG
* BASIC INTAKE AND EXHAUST MATERIALS
* SEAT COATING MATERIALS.
* CHROMED STEMS.
* TESTS CARRIED OUT IN DYNAMOMETERS AND HUNDREDS OF
THOUSANDS OF KILOMETERS IN SEVERE FLEET SERVICES.
* FIRST-CLASS KNOWLEDGE OF ENGINEERING AND MANUFACTURING.
Preparado por el Ing. Fernando Alberto Curello, de
MOTOR PARTS INTERNATIONAL, Empresa de investigación
y desarrollos especiales del grupo BASSO S.A. de ARGENTINA.