1986/06/23 Caterpillar

G398, G399, 3512, 3516 Spark Ignited Engines Equipped With Turbochargers And Aftercoolers

Low emission, special arrangements are available for the above engines. Low emission engine technology has been developed by Caterpillar to respond to clean air standards in the United States. When operated under specific and controlled conditions, the above engines are capable of an emission level of oxides of nitrogen (NOx) in the range of two grams per horsepower per hour (2 g/hp/h).

The low exhaust emission levels of these special arrangement engines is achieved by operating with a very lean air/fuel ratio. Engines operating at very lean air/fuel ratios are especially sensitive to those variable factors which affect the ratio of fuel-to-air. Changes in inlet air temperature, gas temperature and/or gas BTU content will have an effect on the air/fuel ratio and engine performance. If the air/fuel ratio is too rich, NOx emissions will increase and the engine will approach detonation. If the air/fuel ratio is tool lean, NOx emissions may decrease very slightly, but the engine will become unstable and begin to misfire. Therefore, as conditions change and stabilize, the fuel system must be readjusted to return to the correct air/fuel ratio.

The air/fuel ratio must be set precisely, in order to produce low exhaust emissions, while maintaining correct engine operation. Initial adjustments made on the engine, will be made for the fuel and conditions available at the factory. Carburetor adjustments must be made with an oxygen meter, after site installation, to make sure the correct air/fuel ratio is established for the conditions in which the engine will actually operate.

After engine installation, use an oxygen meter to sample and analyze exhaust gas and determine the percent of oxygen present during operation. Low emission engines are equipped with sample lines or ports in the engine exhaust system from which an oxygen sample can be drawn. Low emission engines will have six to eight percent free oxygen in the exhaust, depending on engine configuration and the amount of NOx reduction that is required. Refer to "Low Emission Service Manual Supplement", Form SENR3052-01 (G398, G399 Engines), or "Operation And Maintenance Guide", Form SEBU6010 (3512, 3516 Spark Ignited Engines), for detailed instructions on air/fuel ratio adjustment procedures.

At this time, Caterpillar does not offer an oxygen meter through the parts distribution system. Caterpillar also does not specify requirements for oxygen meter models or manufacturers. However, it should be remembered that an oxygen meter is a sensitive instrument and such factors as durability, serviceability and ease of operation should be considered before a final selection and purchase is made. In addition, special consideration must be given to filtering out any water that might condense in the sample lines or ports of the engine. Severe damage can result if all water vapor is not filtered out of the exhaust sample before it enters the oxygen meter.

Two brands of oxygen meters have been tested by Caterpillar, and are listed with a brief explanation and the addresses of the manufacturers. When contacting any manufacturer of oxygen meters, be sure to state the intended application and use of the meter and specify that a water vapor trap is necessary.

The Teledyne oxygen meter/analyzers, Model 320-P Analog Meter Readout or Model 320-PD Digital Display Readout, use a battery operated sampling pump and an electrochemical sensor to measure the oxygen concentration in a gas sample containing 0 to 100% oxygen. These units must be modified to add a water trap on the inlet tubing. A water trap of the type commonly used on air compressors is suitable for this application.

The Neotronics Fuel Efficiency Monitor (F.E.M.) is a microprocessor based unit that uses an electrochemical cell and battery operated sampling pump to measure the oxygen concentration in a gas sample containing 0 to 20.9% oxygen. It also measures exhaust stack temperatures from 25 to 999°C (77 to 1830°F). A special water trap filter is available as an accessory and must be used with this tool for this application.

An oxygen meter is REQUIRED to adjust low emission engines. It is also very useful in making air/fuel ratio adjustments to standard engines. To better understand the effects of different air/fuel settings on various engine functions, refer to the engine data sheet 195.0, Form LEKQ2364, of the Caterpillar Technical Manual. The graphs included in that publication are for 6.25" Bore Natural Gas Engines, but the general shapes of the curves are similar for all spark ignited engines.

The measurement of free oxygen in the exhaust gas is an accurate indicator of air/fuel ratio. The oxygen level (air/fuel ratio) desired in the exhaust will vary with engine configuration and operating conditions. Engines equipped with catalytic converters, for exhaust emission control, require an air/fuel ratio which JUST eliminates oxygen in the exhaust (0 to 0.5% oxygen). However, if optimum fuel consumption is of prime importance, an oxygen level of 2% will be required. In applications where maximum oil life and minimum maintenance are required, an air/fuel ratio of 0% oxygen is preferred. Some oxygen levels require close monitoring of oil performance, and may require more frequent oil changes. The article "Use The Correct Oil In Natural Gas Engines", Service Magazine, March 4, 1985, gives guidelines for evaluating oil performance in these engines.

Spark ignited engines with an air/fuel ratio that has been carefully adjusted, so that all fuel and all oxygen is consumed (stoichiometric), will have low levels of NOx and provide minimum oil sensitivity. These types of air/fuel ratios are common when operating on well-head gas.

Some oxygen meters may lose their sensitivity to accurately measure oxygen levels below 0.5%. To adjust the air/fuel ratio with this type of meter, enrich the air/fuel ratio until the oxygen level falls to the minimum reading. When the oxygen level reaches the minimum level that the meter can read, continue to enrich the air/fuel ratio until a slight increase in manifold pressure is detected. This should be the point of 0% oxygen in the exhaust gases.

NOTE: To minimize the possibility of sample line leaks affecting the oxygen reading, it is best to sample the exhaust on dual carburetor turbocharged engines, ahead of the turbochargers.

In theory, oxygen levels can be adjusted or set in either of two ways: setting the power screw and adjusting the "A" regulator or setting the "A" regulator and adjusting the power screw. Either method can be used to obtain the correct oxygen level in the exhaust gas. However, setting the "A" regulator and adjusting the power screw could result in response or cold starting problems. The normal methods for adjusting the air/fuel ratio on spark ignited engines should be followed as published in the "Gas Engine Application And Installation Guide", Form LEBH2363, or the appropriate Engine Service Manual. The final step in each adjustment procedure should be to fine tune the air/fuel ratio to achieve the desired exhaust oxygen level as defined by the application.