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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 of:

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” fuels.
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 following:

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 liquid fuels.
Moreover, the engine's exhaust cam was designed for the corresponding valve be settled a certain time, evacuating the heat of the previous explosion.
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 alloy.
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 will sink.

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 usage.

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 better service.

Valves for CNG engines

- Lack of lead
- Very low combustion speed
- High antiknock power
- Corrosiveness due to sulfur compounds
- Low lubrication

- Cylinder head insert recesion
- Valves' guides and stems wearing
- Valves' seats burning
- Seats' surfaces fatigue
- Neck corrosion by sulfates

- 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

- Inserts of accurate hardness with a good contact surface against the cylinder head
- “Premium” materials, as basis materials for the manufacturing of valves
- Hard recoveries in exhaust valves seats (stellite)
- Seat plasma recovering process of last generation






Made by Eng. Fernando Alberto Curello, from MOTOR PARTS INTERNATIONAL, a company for research and development belonging to BASSO S.A. ARGENTINA Group.