VALVES FOR GASEOUS FUEL
EThe constant increase of the worldwide
interest in the use of COMPRESSED NATURAL GAS (CNG) and/or LIQUIFIED
GAS as alternative fuels of the GASOLINE and FUEL OIL is because
1)The need of diversify the supply resources,
allowing many countries the use of fuel such as GAS.
2)To reduce the pollution ignitions, with “cleaner”
3)To reduce fuel costs.
4)To increase engines efficiency, getting more power with less consumption.
5)To extend the useful life of engines.
Some years ago, this tendency, generalized now
all over the world, produced a program of research and development
in BASSO S.A. in order to offer valves which could be used with
COMPRESSED NATURAL GAS or LIQUIFIED GAS. It was made concrete in
1991 with the throwing of the “EXTRA GAS” valve series
in Argentina for domestic engines, adding to the export valves for
FORD, GM, DODGE and other engines, which were turned to be used
with gaseous fuels in several countries.
What are the characteristics of GAS as a fuel,
that oblige people to manufacture valves, valve seat inserts and
other special elements?
NATURAL GAS is a mixture of gaseous hydrocarbons,
(principally Methane 85% to 90%) and others, such as Propane, Butane,
Carbon Dioxide, Nitrogen, etc. It is obtained from oil fields associated
with petroleum, otherwise it can be found in formations which only
contain gas. Gas must be compressed to high pressures (200 kg/cm2)
in order to accumulate outstanding quantities in reduced places.
LIQUIFIED PETROLEUM GAS (or L.P.G.) is a mixture
of two gaseous hydrocarbons (Propane and Butane), which is obtained
as a subproduct of the processes of petroleum refineries, or else
is obtained by separation (at very low temperatures) of NATURAL
GAS. It can be kept in liquid condition with room temperature and
relatively low pressures.
Characteristics, which affect or influence on
the design of the combustion chamber, valve and guides, are the
1)The first important characteristic is that it
has not lead, that is to say it is a “LEAD FREE” fuel.
In conventional gasolines “WITH LEAD”
is added a compound called Tetraethyl Lead in order to improve their
antidetonation conditions, that is to say, they can be used in more
“compressed” engines without risks of detonation and
getting more power with less consume.
The lead added to gasoline is deposited over the seats in the cylinder
heads, protecting them from wearing when they are working as “SOLID
LUBRICANTS” which are put between the valve and its seat.
Using GAS, this protective “layer” does not exist, it
can produce a wear due to abrasion when valve is turning during
the succeeding openings and closings. If the seat or the insert
are not hardened, the seat loses material, what is called seat “RECESION”.
It produces an important failure in the assembly seat-valve because
of burning, weariness breakage, sealed losing, etc.
In general, this mechanism of failure takes place
in exhaust valves, because they have to work in the corrosive environment
and with high temperatures (up to 780º C) of the burnt gases.
We must also remember that they support the big efforts of the combustion
pressure and the violent closing against its seat several times
a second (at an engine speed of 3000 R.P.M., a valve opens and closes
25 times a second).
In the same way, the presence of lead helps to lubricate the valve
guides, putting between stem and guide as solid lubricants.
2)Another very important characteristic, which
have not been correctly appreciated in the first developments of
engines “converted” to GAS, is that GAS burns more slowly
and with more difficulties to begin combustion than the conventional
Moreover, the engine's exhaust cam was designed for the corresponding
valve be settled a certain time, evacuating the heat of the previous
When the valve opens again, the inside burnt gases must already
have lowered their temperature enough in order to not pass the acceptable
limit for the material.
When the exhaust valve opens in an engine which was converted to
CNG, will “find” gases which are not “cold”
enough. So, it begins heating more than normal levels, because gas
burns more slowly than gasoline.
On the other hand, when in the combustion chamber enters gas instead
of small drops of liquid fuel (as in gasolines), heat absorption
does not take place which is produced when these small drops stay
in the air current that enters the cylinder and vaporize themselves.
These two factors have induced many users of Natural Gas to believe
that its combustion is “HOTTER” than gasoline combustion.
Although it is a wrong idea, the consequence is that the working
temperature of the exhaust valve is higher. This temperature exceeds
the seal's capability of the valve and its seat because of material
failure on the head or on the cylinder head. All this may be caused
due to one or more of these factors:
- Lack of hardness when heating
- Inaccurate resistance to corrosion
- Not enough resistance to wearing
In order to give a solution to this situation,
it is necessary an improvement in the valve's material, or to cover
the face of the seat's valve, by special alloys. These alloys are
approved by plasma systems. This special process is done by Basso
S.A. stellite face valves.
The most outstanding advantage of this kind of materials for its
use on the face of the valve's seat , is the maintenance of a high
temperature and an excellent resistance to corrosion and wear, thanks
to the combination of Cobalt, Nickel, Iron, and Chromium in their
In the case of big engines, for example a truck of the kind of “Heavy
Duty” (of heavy service), it is also suggested the modification
of the seat's angle of the exhaust valve, decreasing it from 45º
to 30º (or 15º in very involved valves).
What can be achieved with this practice? In the figure shown in
the following page, it is can be seen an .....
3)The third worthy characteristic of GAS as a fuel is its
high antiknock power, equivalent to a 120/130 octane gasoline (without
the using of tetraethyl lead). It allows to increase the compression
relationship of engines up to 11:1 or 12:1 values, getting a bigger
power with less consumption.
However, in most of the engines which were “converted”
to GAS, the compression relationship does not increase, in order
to keep the dual use (gasoline or gas) according to the possibilities
of supplying. In some cases, this “dual” operation has
originated valve's failures by operating with an inaccurate ignition
for gas. That is to say, it fits the conventional gasoline , but
it is too much “retarded” for GAS. Because of its low
combustion speed, it requires higher spark timing advances than
gasoline, between 10 and 12 more degrees of initial advance.
4)A characteristic which sometimes brings wear problems in valve's
guides, is the low capability of lubrication, because it is a GAS.
The tests made by BASSO S.A. in vehicles which run thousands of
kilometers a month under the most severe conditions, show the need
of hard chroming of stems which reduce the friction against the
guide, increase the stem' useful life and helps the movement of
lubricant oil between stem and guide.
As a corollary of this brief “Guide of GAS
as a fuel”, some useful suggestions will be given for the
conversion of engines to GAS:
A) In order to improve the passage of heat from
valves to the cylinder head, it will be profitable to do wider exhaust
seats (from 1/16” to 3/32” i.e. 1,60 to 2,40 mm.).
B) Do not machine the cylinder head's seats with
angle interferences among valves and seats. Prepare them all with
the same angles.
C) Keep the concentricity between guide and seat
lower than 0.025 mm.
D) Machine the seats in the cylinder head, in
a way of putting valves more deeply than usual. Closed valve's edges
must not be seen inside the combustion chamber.
E) Use hardened seats (or inserts), which must
have good chromium and nickel contents in their composition.
Some cylinder heads of cast iron often have their seats hardened
by induction. This solution is unacceptable in applications of “HEAVY
DUTY”, because they simply will not hold up the effort and
F) Keep the clearances between guides and valves
with their original values. Except in the cases where stem sodium
filled valves be replaced with solid ones. It will be necessary
to increase clearances so as to compensate the solid valve's expansion.
G) Do not grind used valves, because their margin's
width decreases. Only use the best available valves, covered with
“STELLITE”, chromed stems (with the best stainless steels)
as the ones manufactured by BASSO S.A. for this special kind of
H) Use a spark timing advance corrector to go
forward 10 to 12 degrees the spark automatically when you pass the
engine to run with gas.
Remember that in every engine converted to Natural
GAS, the use of inaccurate valves will only bring problems to the
user and the rectifier shop, who should do the job twice for the
price of one.
We hope this guide could be useful, and please
help us to make known its content because a better knowledge about
the products manufactured by BASSO S.A. will allow you to give a
Valves for CNG engines
1)CHARACTERISTICS OF GAS AS FUEL
- Lack of lead
- Very low combustion speed
- High antiknock power
- Corrosiveness due to sulfur compounds
- Low lubrication
2)PROBLEMS WHICH MAY APPEAR IN CYLINDER HEADS
- Cylinder head insert recesion
- Valves' guides and stems wearing
- Valves' seats burning
- Seats' surfaces fatigue
- Neck corrosion by sulfates
3) AGGRAVATING FACTORS
- High speed and high load operation during long periods
- Ignition advances fitted for gasoline but not enough for gas
- Gas equipments with deficient Air / Gas regulation. Poor mixture
- Bad heat transmission from insert to cylinder head
4) CYINDER HEAD / VALVE SYSTEM'S REQUESTS
- Inserts of accurate hardness with a good contact surface against
the cylinder head
- “Premium” materials, as basis materials for the manufacturing
- Hard recoveries in exhaust valves seats (stellite)
- Seat plasma recovering process of last generation
BASSO S.A. CNG VALVES
* INTAKE AND EXHAUST RAW MATERIALS
* SEATS COVERING MATERIALS
* CHROMED STEMS
* TESTS MADE IN RUNNING DYNAMOMETERS, WITH THOUSANDS
OF SEVERE SERVICE KILOMETERS
* ENGINEERING AND MANUFACTURING WORLDWIDE LEVEL
Made by Eng. Fernando Alberto
Curello, from MOTOR PARTS INTERNATIONAL, a company for research
and development belonging to BASSO S.A. ARGENTINA Group.