Buyers’ Guide



An engine-driven air compressor entails the same level of service as a comparable electric motor driven system as far as the air compressor components are concerned. However, an engine that requires different attention than an electric motor. Commitment to a proper engine maintenance program ensures reliable and long successful operations.

Other than major overhauls, engine service is composed of routine inspections/adjustments and periodic replacement of engine oil, coolant, and spark plugs. To ensure that the maintenance is performed in a timely manner, detailed records should be kept. The records should indicate the service performed and any observations noted during the procedure that reflect the engine condition.

In addition to routine maintenance, a preventative maintenance program consisting of checking for leaks, unusual noise, odors, and abnormal operation should be implemented to protect the engine before a major repair is needed. A typical industrial grade engine maintenance schedule is shown in the two figures below. This does not include pollution control requirements, which are addressed in the section on Operation.


Industrial Grade Engine Maintenance Intervals
Source: Waukesha Engine



Industrial Grade Engine Maintenance Interval Time Line


The maintenance intervals for the automotive-derivative type engines are given in the two figures below.


Automotive-Derivative Engine Maintenance Intervals
Source: Tecogen, Inc.



Auto-Derivative Engine Maintenance Interval Time Line

Routine sampling and testing of the engine oil is strongly recommended as an indication of engine condition. Routine tests should generally consist of three (3) types: chemical/physical, oil condition, and wear analyses.

The chemical and physical analysis evaluates oil integrity by measuring the presence of water, coolant, and entrained solids. The analyses should be compared with previous results to alert the operator of any significant changes in engine condition and to establish optimum service intervals. If concentrations in the test samples exceed limits recommended by the analyst or the engine manufacturer, service work is recommended to correct the situation. This may be as simple as performing an oil change.

The oil condition analysis measures:

  • Concentration of combustion by-products,
  • Sulfur, and
  • Products of oxidation and nitration.

Excessive nitration of the engine oil is often caused by the air-fuel ratio setting. An air-fuel ratio that optimizes fuel consumption can promote excessive production of NOx and nitrated engine oil. The goal is to minimize oil nitration while preserving engine power and fuel economy. This test serves to optimize the time intervals between oil changes and fine tuning of the air-fuel ratio.

Wear analyses measures the concentrations of specific metals in the engine oil. The presence of metals in the oil indicates bearing and component wear. Based upon previously established limits for these materials, test results are compared with control data. Engine wear rates and the need for a service overhaul can be predicted using this technique. Measurements of crankcase “blow-by” and cylinder pressure are important indicators of engine condition as they relate specifically to piston rings and the valve train.

While 25,000 hours indicated in first figure above is a representative value, the time interval for major overhauls varies considerably depending on the engine model and its manufacturer. The actual need for engine overhauls depends on the quality of the preventative maintenance program. Engine condition is more accurately measured by power output, fuel economy, oil consumption and analysis, and the results of crankcase blow-by and cylinder compression tests.

In addition to oil monitoring, engine cooling water must be routinely checked. Improperly treated cooling water can lead to scaling, corrosion, and accumulation of sediments within the cooling water circuit. Since any heat recovery system is an extension of the engine cooling system, proper water treatment is critical to maintain good heat transfer at the tube surfaces.

For industrial grade engines, a top-end overhaul is performed to rebuild cylinder heads and the valve train. Some manufacturers recommend this work after 12,000 – 15,000 full load hours (not including emissions control equipment). The turbocharger and aftercooler for engines so equipped should be inspected and rebuilt at this interval. A major engine overhaul involves disassembling and inspecting the engine block and piston rings, cylinder liners, crankshaft bearings, and seals. Some manufacturers recommend a major overhaul after 24,000 – 30,000 full load hours.

Service contracts are available from the engine-driven compressor manufacturers. These contracts can be written to cover all service procedures, including routine inspections and oil changes, or just for overhauls. Having a single point of contact for this maintenance is essential. Depending on the customer’s commitment to maintaining the engine-driven compressor, a service contract should be seriously considered at the time of purchase to ensure optimum performance and long life. The capabilities of local service representatives may vary greatly and should be taken into account when deciding which service options to pursue.

Costs to maintain an engine can be estimated at roughly $0.01/hp-hr to $0.015/hp-hr ($0.22/100 SCFM-hr to $0.34/100 SCFM-hr). Multiplying this value times the size of the engine and the estimated number of hours of operation per year will give an annualized estimate of maintenance costs. When using this method, one must be cautioned that actual maintenance costs are less than this value in the first few years and larger as the engine requires major overhauls or replacement. This can be seen in the figure below. However, over the life of the engine, this estimate provides a reasonable portrayal of the average annual cost and provides a good starting point for budgeting purposes.


Maintenance Outlay Comparison

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