Spark plugs
Spark plugs are probably the most important part of the ignition system and selectingthe correct spark plug is critical.
• LPG produces higher spark plug temperatures than gasoline. Part of the reason for this is the fact that gasoline enters the engine as liquid droplets. When these liquid droplets vaporize they have a cooling effect on the combustion chamber valves and spark plug (this is referred to as “heat of vaporization”).
• In heavy-duty applications it may be advisable to install spark plugs that are one heat range colder than the ones for gasoline.
• One way of choosing the correct spark plug heat range is to install the standard gasoline spark plug and run the engine for 100 hours. Remove and inspect the plugs. If the heat range is correct the plugs will be a light tan incolor and the gap will only have increased from .001 inches to .003 inches. If the heat range is to hot the plug will be white in colorand the gap will have increased more than .003 inches. In this case a colder plugs hould be installed and the 100-hour test procedure repeated.
• Care must be taken not to use a plug with a heat range that is too cold. Colder spark plugs require a higher secondary voltage than hotter ones. If the spark plug used is to cold misfires and/or backfires and increased exhaust emissions will result. Also fouling of the plugs may increase because they will not be hot enough to burn off contaminants.
LPG
• Ignition timing may be advanced 10% - 20% above the gasoline specification.
• Initial timing plus centrifugal advance timing should not be more than 28 degrees at2500 RPM (or max governed no load RPM if less than 2500 RPM).
• If vacuum advance is used, initial timing plus centrifugal advance timing plus vacuum advance timing should not be more than 45 degrees total advance.
CNG
• Ignition timing may be advanced 20% - 30% above the gasoline specification.
• Initial timing plus centrifugal advance timing should not be more than 32 degrees at2500 RPM (or max governed no load RPM if less than 2500 RPM).
• If vacuum advance is used, initial timing plus centrifugal advance timing plusvacuum advance timing should not be more than 45 degrees total advance.
EFFECTS OF AIR TEMPERATURE ON POWER OUTPUT
The temperature of the air entering an engine is very important for two reasons :
1. Hot air entering an engine can lead to detonation and preignition, which will injure or destroy an engine in short order. The cooler the temperature of the incoming air the healthier it is for the engine.2. As the temperature of air entering an engine increases it expands becoming less dense and lighter. This reduces the volumetric efficiency and therefore the horsepower output of the engine. For every 10-degree increase of engine intake air temperature the horsepower output drops 1%. Since under hood air temperature caneasily reach 200 degrees it is very important that the engine air intake be ducted outside the engine compartment. As an example an engine that makes 100 HP breathing air at 60 degrees F. will only make 86 horsepower breathing air at 200 degrees F. This decrease in power can be explained by the fact that an engine requires 7lbs. of air to make 1 horsepower for 1 hour. As air is heated it expands and becomes less dense and lighter (as in a hot air balloon). A greater volume of air isrequired to weight 7lbs. An engine running at rated full load RPM can only breath a fixed volume of air. The number of available pounds of air is reduced by using hot air(a 100 cubic inch displacement 4 stroke engine will only pass 100 cubic inches of airand fuel for every 2 revolutions of the crankshaft. The displacement is fixed by thebore and stroke. The displacement cannot increase to allow for the high temperatureand lower density of the incoming air).
Spark plugs are probably the most important part of the ignition system and selectingthe correct spark plug is critical.
• LPG produces higher spark plug temperatures than gasoline. Part of the reason for this is the fact that gasoline enters the engine as liquid droplets. When these liquid droplets vaporize they have a cooling effect on the combustion chamber valves and spark plug (this is referred to as “heat of vaporization”).
• In heavy-duty applications it may be advisable to install spark plugs that are one heat range colder than the ones for gasoline.
• One way of choosing the correct spark plug heat range is to install the standard gasoline spark plug and run the engine for 100 hours. Remove and inspect the plugs. If the heat range is correct the plugs will be a light tan incolor and the gap will only have increased from .001 inches to .003 inches. If the heat range is to hot the plug will be white in colorand the gap will have increased more than .003 inches. In this case a colder plugs hould be installed and the 100-hour test procedure repeated.
• Care must be taken not to use a plug with a heat range that is too cold. Colder spark plugs require a higher secondary voltage than hotter ones. If the spark plug used is to cold misfires and/or backfires and increased exhaust emissions will result. Also fouling of the plugs may increase because they will not be hot enough to burn off contaminants.
LPG
• Ignition timing may be advanced 10% - 20% above the gasoline specification.
• Initial timing plus centrifugal advance timing should not be more than 28 degrees at2500 RPM (or max governed no load RPM if less than 2500 RPM).
• If vacuum advance is used, initial timing plus centrifugal advance timing plus vacuum advance timing should not be more than 45 degrees total advance.
CNG
• Ignition timing may be advanced 20% - 30% above the gasoline specification.
• Initial timing plus centrifugal advance timing should not be more than 32 degrees at2500 RPM (or max governed no load RPM if less than 2500 RPM).
• If vacuum advance is used, initial timing plus centrifugal advance timing plusvacuum advance timing should not be more than 45 degrees total advance.
EFFECTS OF AIR TEMPERATURE ON POWER OUTPUT
The temperature of the air entering an engine is very important for two reasons :
1. Hot air entering an engine can lead to detonation and preignition, which will injure or destroy an engine in short order. The cooler the temperature of the incoming air the healthier it is for the engine.2. As the temperature of air entering an engine increases it expands becoming less dense and lighter. This reduces the volumetric efficiency and therefore the horsepower output of the engine. For every 10-degree increase of engine intake air temperature the horsepower output drops 1%. Since under hood air temperature caneasily reach 200 degrees it is very important that the engine air intake be ducted outside the engine compartment. As an example an engine that makes 100 HP breathing air at 60 degrees F. will only make 86 horsepower breathing air at 200 degrees F. This decrease in power can be explained by the fact that an engine requires 7lbs. of air to make 1 horsepower for 1 hour. As air is heated it expands and becomes less dense and lighter (as in a hot air balloon). A greater volume of air isrequired to weight 7lbs. An engine running at rated full load RPM can only breath a fixed volume of air. The number of available pounds of air is reduced by using hot air(a 100 cubic inch displacement 4 stroke engine will only pass 100 cubic inches of airand fuel for every 2 revolutions of the crankshaft. The displacement is fixed by thebore and stroke. The displacement cannot increase to allow for the high temperatureand lower density of the incoming air).
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