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 due to
:
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?.

CHARACTERISTICS 

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 liquid state.

The characteristics that affect or influence the design of the combustion chamber, valves and guides are the following: :

1) It is a "LEAD FREE" fuel, which means it does not contain lead.

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 explosion.

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 material.
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 out".
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 residues.

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.

RECOMMENDATIONS
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 2,40 mm.).
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 chamber.
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 S.A.
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 VALVES
- 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 periods.
- 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 head.

4) REQUIREMENTS OF THE HEAD / VALVE SYSTEM
- Inserts of adequate hardness in cylinder head and good contact surface.
- "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.